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Climate Change Indicators

Climate Change Indicators: Atmospheric Concentrations of Greenhouse Gases

This indicator describes how the levels of major greenhouse gases in the atmosphere have changed over time.

Figure 1. Global Atmospheric Concentrations of Carbon Dioxide Over Time

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This figure shows concentrations of carbon dioxide in the atmosphere from hundreds of thousands of years ago through 2015, measured in parts per million (ppm). The data come from a variety of historical ice core studies and recent air monitoring sites around the world. Each line represents a different data source.

Data source: Compilation of 10 underlying datasets5
Web update: April 2016

Figure 2. Global Atmospheric Concentrations of Methane Over Time

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This figure shows concentrations of methane in the atmosphere from hundreds of thousands of years ago through 2015, measured in parts per billion (ppb). The data come from a variety of historical ice core studies and recent air monitoring sites around the world. Each line represents a different data source.

Data source: Compilation of five underlying datasets6
Web update: AugustÂ 2016

Figure 3. Global Atmospheric Concentrations of Nitrous Oxide Over Time

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This figure shows concentrations of nitrous oxide in the atmosphere from hundreds of thousands of years ago through 2015, measured in parts per billion (ppb). The data come from a variety of historical ice core studies and recent air monitoring sites around the world. Each line represents a different data source.

Data source: Compilation of six underlying datasets7
Web update: AugustÂ 2016

Figure 4. Global Atmospheric Concentrations of Selected Halogenated Gases, 1978â€“2015

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This figure shows concentrations of several halogenated gases (which contain fluorine, chlorine, or bromine) in the atmosphere, measured in parts per trillion (ppt). The data come from monitoring sites around the world. Note that the scale increases by factors of 10. This is because the concentrations of different halogenated gases can vary by a few orders of magnitude. The numbers following the name of each gas (e.g., HCFC-22) are used to denote specific types of those particular gases.

Data sources: AGAGE, 2016;8Â Rigby, 2016;9Â NOAA, 201610
Web update: AugustÂ 2016

Figure 5. Global Atmospheric Concentrations of Ozone, 1979â€“2014

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This figure shows the average amount of ozone in the Earthâ€™s atmosphere each year, based on satellite measurements. The total represents the â€œthicknessâ€? or density of ozone throughout all layers of the Earthâ€™s atmosphere, which is called total column ozone and measured in Dobson units. Higher numbers indicate more ozone. For most years, Figure 5 shows how this ozone is divided between the troposphere (the part of the atmosphere closest to the ground) and the stratosphere. From 1994 to 1996, only the total is available, due to limited satellite coverage.

Data sources: NASA, 2013,11Â 2015,12Â 201613
Web update: AugustÂ 2016

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Figure 1. Global Atmospheric Concentrations of Carbon Dioxide Over Time

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Figure 2. Global Atmospheric Concentrations of Methane Over Time

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Figure 3. Global Atmospheric Concentrations of Nitrous Oxide Over Time

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Figure 4. Global Atmospheric Concentrations of Selected Halogenated Gases, 1978â€“2015

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Figure 5. Global Atmospheric Concentrations of Ozone, 1979â€“2014

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			Key Points

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			Background

About the

			Indicator

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			Indicator Notes

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			Data Sources

Technical

			Documentation

Key Points

Global atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and certain manufactured greenhouse gases have all risen significantly over the last few hundred years (see Figures 1, 2, 3, and 4).

Historical measurements show that the current global atmospheric concentrations of carbon dioxide, methane, and nitrous oxide are unprecedented compared with the past 800,000 years (see Figures 1, 2, and 3).

Carbon dioxide concentrations have increased substantially since the beginning of the industrial era, rising from an annual average of 280 ppm in the late 1700s to 401 ppm as measured at Mauna Loa in 2015â€”a 43 percent increase (see Figure 1). Almost all of this increase is due to human activities.1

The concentration of methane in the atmosphere has more than doubled since preindustrial times, reaching approximately 1,800 ppb in recent years (see the range of measurements for 2014 and 2015 in Figure 2). This increase is predominantly due to agriculture and fossil fuel use.2

Over the past 800,000 years, concentrations of nitrous oxide in the atmosphere rarely exceeded 280 ppb. Levels have risen since the 1920s, however, reaching a new high of 328 ppb in 2015 (average of three sites in Figure 3). This increase is primarily due to agriculture.3

Concentrations of many of the halogenated gases shown in Figure 4 were essentially zero a few decades ago but have increased rapidly as they have been incorporated into industrial products and processes. Some of these chemicals have been or are currently being phased out of use because they are ozone-depleting substances, meaning they also cause harm to the Earthâ€™s protective ozone layer. As a result, concentrations of many major ozone-depleting gases have begun to stabilize or decline (see Figure 4, left panel). Concentrations of other halogenated gases have continued to rise, however, especially where the gases have emerged as substitutes for ozone-depleting chemicals (see Figure 4, right panel).

Overall, the total amount of ozone in the atmosphere decreased by about 3 percent between 1979 and 2014 (see Figure 5). All of the decrease happened in the stratosphere, with most of the decrease occurring between 1979 and 1994. Changes in stratospheric ozone reflect the effect of ozone-depleting substances. These chemicals have been released into the air for many years, but recently, international efforts have reduced emissions and phased out their use.

Globally, the amount of ozone in the troposphere increased by about 3 percent between 1979 and 2014 (see Figure 5).Â â€‹

Water Vapor as a Greenhouse Gas

Water vapor is the most abundant greenhouse gas in the atmosphere. Human activities have only a small direct influence on atmospheric concentrations of water vapor, primarily through irrigation and deforestation, so it is not included in this indicator.4Â The surface warming caused by human production of other greenhouse gases, however, leads to an increase in atmospheric water vapor because warmer temperatures make it easier for water to evaporate and stay in the air in vapor form. This creates a positive â€œfeedback loopâ€? in which warming leads to more warming.

Background

Since the Industrial Revolution began in the 1700s, people have added a substantial amount of greenhouse gases into the atmosphere by burning fossil fuels, cutting down forests, and conducting other activities (see the U.S. and Global Greenhouse Gas Emissions indicators). When greenhouse gases are emitted into the atmosphere, many remain there for long time periods ranging from a decade to many millennia. Over time, these gases are removed from the atmosphere by chemical reactions or by emissions sinks, such as the oceans and vegetation, which absorb greenhouse gases from the atmosphere. As a result of human activities, however, these gases are entering the atmosphere more quickly than they are being removed, and thus their concentrations are increasing.

Carbon dioxide, methane, nitrous oxide, and certain manufactured gases called halogenated gases (gases that contain chlorine, fluorine, or bromine) become well mixed throughout the global atmosphere because of their relatively long lifetimes and because of transport by winds. Concentrations of these greenhouse gases are measured in parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt) by volume. In other words, a concentration of 1 ppb for a given gas means there is one molecule of that gas in every 1 billion molecules of air. Some halogenated gases are considered major greenhouse gases due to their very high global warming potentials and long atmospheric lifetimes even if they only exist at a few ppt (see table).

This indicator looks at global average levels of ozone in both the stratosphere and troposphere. For trends in ground-level ozone concentrations within the United States, see EPAâ€™s National Air Quality Trends Report at: www.epa.gov/air-trends.

Ozone is also a greenhouse gas, but it differs from other greenhouse gases in several ways. The effects of ozone depend on its altitude, or where the gas is located vertically in the atmosphere. Most ozone naturally exists in the layer of the atmosphere called the stratosphere, which ranges from approximately 6 to 30 miles above the Earthâ€™s surface. Ozone in the stratosphere has a slight net warming effect on the planet, but it is good for life on Earth because it absorbs harmful ultraviolet radiation from the sun, preventing it from reaching the Earthâ€™s surface. In the troposphereâ€”the layer of the atmosphere near ground levelâ€”ozone is an air pollutant that is harmful to breathe, a main ingredient of urban smog, and an important greenhouse gas that contributes to climate change (see the Climate Forcing indicator). Unlike the other major greenhouse gases, tropospheric ozone only lasts for days to weeks, so levels often vary by location and by season.Â

About the Indicator

This indicator describes concentrations of greenhouse gases in the atmosphere. It focuses on the major greenhouse gases that result from human activities.

For carbon dioxide, methane, nitrous oxide, and halogenated gases, recent measurements come from monitoring stations around the world, while measurements of older air come from air bubbles trapped in layers of ice from Antarctica and Greenland. By determining the age of the ice layers and the concentrations of gases trapped inside, scientists can learn what the atmosphere was like thousands of years ago.

This indicator also shows data from satellite instruments that measure ozone density in the troposphere, the stratosphere, and the â€œtotal column,â€? or all layers of the atmosphere. These satellite data are routinely compared with ground-based instruments to confirm their accuracy. Ozone data have been averaged worldwide for each year to smooth out the regional and seasonal variations.Â

Indicator Notes

This indicator includes several of the most important halogenated gases, but some others are not shown. Many other halogenated gases are also greenhouse gases, but Figure 4 is limited to a set of common examples that represent most of the major types of these gases. The indicator also does not address certain other pollutants that can affect climate by either reflecting or absorbing energy. For example, sulfate particles can reflect sunlight away from the Earth, while black carbon aerosols (soot) absorb energy. Data for nitrogen trifluoride (Figure 4) reflect modeled averages based on measurements made in the Northern Hemisphere and some locations in the Southern Hemisphere, to represent global average concentrations over time. The global averages for ozone only cover the area between 50Â°N and 50Â°S latitude (77 percent of the Earthâ€™s surface), because at higher latitudes the lack of sunlight in winter creates data gaps and the angle of incoming sunlight during the rest of the year reduces the accuracy of the satellite measuring technique.

Data Sources

Global atmospheric concentration measurements for carbon dioxide (Figure 1), methane (Figure 2), and nitrous oxide (Figure 3) come from a variety of monitoring programs and studies published in peer-reviewed literature. Global atmospheric concentration data for selected halogenated gases (Figure 4) were compiled by the Advanced Global Atmospheric Gases Experiment, the National Oceanic and Atmospheric Administration, and a peer-reviewed study on nitrogen trifluoride. A similar figure with many of these gases appears in the Intergovernmental Panel on Climate Changeâ€™s Fifth Assessment Report.14Â Satellite measurements of ozone were processed by the National Aeronautics and Space Administration and validated using ground-based measurements collected by the National Oceanic and Atmospheric Administration.

Technical Documentation

Download related technical information PDF

References

1. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

2. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

3. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

4. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

5. [see full list provided below]

6. [see full list provided below]

7. [see full list provided below]

8. AGAGE (Advanced Global Atmospheric Gases Experiment). 2016. ALE/GAGE/AGAGE data base. Accessed June 2016. http://agage.mit.edu/.

9. Rigby, M. 2016 update to data originally published in: Arnold, T., C.M. Harth, J. MÃ¼hle, A.J. Manning, P.K. Salameh, J. Kim, D.J. Ivy, L.P. Steele, V.V. Petrenko, J.P. Severinghaus, D. Baggenstos, and R.F. Weiss. 2013. Nitrogen trifluoride global emissions estimated from updated atmospheric measurements. P. Natl. Acad. Sci. USA 110(6):2029â€“2034. Data updated July 2016.

10. NOAA (National Oceanic and Atmospheric Administration). 2016. Halocarbons and Other Atmospheric Trace Species group (HATS). Accessed June 2016. www.esrl.noaa.gov/gmd/hats.

11. NASA (National Aeronautics and Space Administration). 2013. Dataâ€”TOMS/SBUV TOR data products. Accessed November 2013. http://science.larc.nasa.gov/TOR/data.html.

12. NASA (National Aeronautics and Space Administration). 2015. Tropospheric ozone data from AURA OMI/MLS. Accessed May 2015. http://acdb-ext.gsfc.nasa.gov/Data_services/cloud_slice/new_data.html.

13. NASA (National Aeronautics and Space Administration). 2016. SBUV merged ozone data set (MOD). Version 8.6. Accessed March 2016. http://acdb-ext.gsfc.nasa.gov/Data_services/merged/index.html.

14. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

Atmospheric Concentrations of Greenhouse Gases: Citations for Figures 1, 2, and 3

Figure 1

EPICA Dome C and Vostok Station, Antarctica: approximately 796,562 BCE to 1813 CE

	LÃ¼thi, D., M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T.F. Stocker. 2008. High-resolution carbon dioxide concentration record 650,000â€“800,000 years before present. Nature 453:379â€“382. www.ncdc.noaa.gov/paleo/pubs/luethi2008/luethi2008.html.

Law Dome, Antarctica, 75-year smoothed: approximately 1010 CE to 1975 CE

	Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and V.I. Morgan. 1998. Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores. In: Trends: A compendium of data on global change. Oak Ridge, TN: U.S. Department of Energy. Accessed September 14, 2005. http://cdiac.ornl.gov/trends/co2/lawdome.html.

Siple Station, Antarctica: approximately 1744 CE to 1953 CE

	Neftel, A., H. Friedli, E. Moor, H. LÃ¶tscher, H. Oeschger, U. Siegenthaler, and B. Stauffer. 1994. Historical carbon dioxide record from the Siple Station ice core. In: Trends: A compendium of data on global change. Oak Ridge, TN: U.S. Department of Energy. Accessed September 14, 2005. http://cdiac.ornl.gov/trends/co2/siple.html.

Mauna Loa, Hawaii: 1959 CE to 2015 CE

	NOAA (National Oceanic and Atmospheric Administration). 2016. Annual mean carbon dioxide concentrations for Mauna Loa, Hawaii. Accessed April 14, 2016. ftp://ftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_mlo.txt.

Barrow, Alaska: 1974 CE to 2014 CE
Cape Matatula, American Samoa: 1976 CE to 2014 CE
South Pole, Antarctica: 1976 CE to 2014 CE

	NOAA (National Oceanic and Atmospheric Administration). 2016. Monthly mean carbon dioxide concentrations for Barrow, Alaska; Cape Matatula, American Samoa; and the South Pole. Accessed April 14, 2016. ftp://ftp.cmdl.noaa.gov/data/trace_gases/co2/in-situ/surface.

Cape Grim, Australia: 1992 CE to 2006 CE
Shetland Islands, Scotland: 1993 CE to 2002 CE

	Steele, L.P., P.B. Krummel, and R.L. Langenfelds. 2007. Atmospheric CO2 concentrations (ppmv) derived from flask air samples collected at Cape Grim, Australia, and Shetland Islands, Scotland. Commonwealth Scientific and Industrial Research Organisation. Accessed January 20, 2009. http://cdiac.esd.ornl.gov/ftp/trends/co2/csiro.

Lampedusa Island, Italy: 1993 CE to 2000 CE

	Chamard, P., L. Ciattaglia, A. di Sarra, and F. Monteleone. 2001. Atmospheric carbon dioxide record from flask measurements at Lampedusa Island. In: Trends: A compendium of data on global change. Oak Ridge, TN: U.S. Department of Energy. Accessed September 14, 2005. http://cdiac.ornl.gov/trends/co2/lampis.html.

Figure 2

EPICA Dome C, Antarctica: approximately 797,446 BCE to 1937 CE

	Loulergue, L., A. Schilt, R. Spahni, V. Masson-Delmotte, T. Blunier, B. Lemieux, J.-M. Barnola, D. Raynaud, T.F. Stocker, and J. Chappellaz. 2008. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature 453:383â€“386. www.ncdc.noaa.gov/paleo/pubs/loulergue2008/loulergue2008.html.

Law Dome, Antarctica: approximately 1008 CE to 1980 CE

	Etheridge, D.M., L.P. Steele, R.J. Francey, and R.L. Langenfelds. 2002. Historic CH4Â records from Antarctic and Greenland ice cores, Antarctic firn data, and archived air samples from Cape Grim, Tasmania. In: Trends: A compendium of data on global change. Oak Ridge, TN: U.S. Department of Energy. Accessed September 13, 2005. http://cdiac.ornl.gov/trends/atm_meth/lawdome_meth.html.

Cape Grim, Australia: 1985 CE to 2015 CE

	NOAA (National Oceanic and Atmospheric Administration). 2016. Monthly mean CH4 concentrations for Cape Grim, Australia. Accessed July 16, 2016. ftp://ftp.cmdl.noaa.gov/data/trace_gases/ch4/flask/surface/ch4_cgo_surface-flask_1_ccgg_month.txt.

Mauna Loa, Hawaii: 1984 CE to 2015 CE

	NOAA (National Oceanic and Atmospheric Administration). 2016. Monthly mean CH4 concentrations for Mauna Loa, Hawaii. Accessed July 16, 2016. ftp://ftp.cmdl.noaa.gov/data/trace_gases/ch4/flask/surface/ch4_mlo_surface-flask_1_ccgg_month.txt.

Shetland Islands, Scotland: 1993 CE to 2001 CE

	Steele, L.P., P.B. Krummel, and R.L. Langenfelds. 2002. Atmospheric methane record from Shetland Islands, Scotland (October 2002 version). In: Trends: A compendium of data on global change. Oak Ridge, TN: U.S. Department of Energy. Accessed September 13, 2005. http://cdiac.esd.ornl.gov/trends/atm_meth/csiro/csiro-shetlandch4.html.

Figure 3

EPICA Dome C, Antarctica: approximately 796,475 BCE to 1937 CE

	Schilt, A., M. Baumgartner, T. Blunier, J. Schwander, R. Spahni, H. Fischer, and T.F. Stocker. 2010. Glacial-interglacial and millennial scale variations in the atmospheric nitrous oxide concentration during the last 800,000 years. Quaternary Sci. Rev. 29:182â€“192. ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/epica_domec/edc-n2o-2010-800k.txt.

Antarctica: approximately 1903 CE to 1976 CE

	Battle, M., M. Bender, T. Sowers, P. Tans, J. Butler, J. Elkins, J. Ellis, T. Conway, N. Zhang, P. Lang, and A. Clarke. 1996. Atmospheric gas concentrations over the past century measured in air from firn at the South Pole. Nature 383:231â€“235. ftp://daac.ornl.gov/data/global_climate/global_N_cycle/data/global_N_perturbations.txt.

Cape Grim, Australia: 1979 CE to 2013 CE

	AGAGE (Advanced Global Atmospheric Gases Experiment). 2015. Monthly mean N2O concentrations for Cape Grim, Australia. Accessed June 5, 2015. http://ds.data.jma.go.jp/gmd/wdcgg/cgi-bin/wdcgg/catalogue.cgi.

South Pole, Antarctica: 1998 CE to 2015 CE
Barrow, Alaska: 1999 CE to 2015 CE
Mauna Loa, Hawaii: 2000 CE to 2015 CE

	NOAA (National Oceanic and Atmospheric Administration). 2016. Monthly mean N2O concentrations for Barrow, Alaska; Mauna Loa, Hawaii; and the South Pole. Accessed June 8, 2016. www.esrl.noaa.gov/gmd/hats/insitu/cats/cats_conc.html.

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A Closer Look: Temperature and Drought in the Southwest

This feature provides a closer look at trends in temperature and drought in the southwestern United States.

Figure 1. Average Temperatures in the Southwestern United States, 2000â€“2015 Versus Long-Term Average

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This map shows how the average air temperature from 2000 to 2015 has differed from the long-term average (1895â€“2015). To provide more detailed information, each state has been divided into climate divisions, which are zones that share similar climate features.

Data source: NOAA, 20162
Web update: AugustÂ 2016

Figure 2. Southwestern U.S. Lands Under Drought Conditions, 2000â€“2015

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This chart shows the percentage of land area in six southwestern states (Arizona, California, Colorado, Nevada, New Mexico, and Utah) classified under drought conditions from 2000 through 2015. This figure uses the U.S. Drought Monitor classification system, which is described in the table in the DroughtÂ indicator.

Data source: National Drought Mitigation Center, 20163
Web update: AugustÂ 2016

Figure 3. Drought Severity in the Southwestern United States, 1895â€“2015

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This chart shows annual values of the Palmer Drought Severity Index, averaged over six states in the Southwest (Arizona, California, Colorado, Nevada, New Mexico, and Utah). Positive values represent wetter-than-average conditions, while negative values represent drier-than-average conditions. A value between -2 and -3 indicates moderate drought, -3 to -4 is severe drought, and -4 or below indicates extreme drought. The thicker line is a nine-year weighted average.

Data source: NOAA, 20164
Web update: AugustÂ 2016

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Figure 1. Average Temperatures in the Southwestern United States, 2000â€“2015 Versus Long-Term Average

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Figure 2. Southwestern U.S. Lands Under Drought Conditions, 2000â€“2015

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Figure 3. Drought Severity in the Southwestern United States, 1895â€“2015

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			Key Points

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			Background

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			Notes

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			Data Sources

TechnicalÂ

			Documentation

Key Points

Every part of the Southwest experienced higher average temperatures between 2000 and 2015 than the long-term average (1895â€“2015). Some areas were nearly 2Â°F warmer than average (see Figure 1).

Large portions of the Southwest have experienced drought conditions since weekly Drought Monitor records began in 2000. For extended periods from 2002 to 2005 and from 2012 through 2015, nearly the entire region was abnormally dry or even drier (see Figure 2).

Based on the long-term Palmer Index, drought conditions in the Southwest have varied since 1895. The early 1900s and the 1950s experienced considerable drought, the 1980s were relatively wet, and the last decade has seen the most persistent droughts on record (see Figure 3).

Background

The American Southwest might evoke images of a hot, dry landscapeâ€”a land of rock, canyons, and deserts baked by the sun. Indeed, much of this region has low annual rainfall and seasonally high temperatures that contribute to its characteristic desert climate. Yet this landscape actually supports a vast array of plants and animals, along with millions of people who call the Southwest home. All of these plants, animals, and people need water to survive.

Water is already scarce in the Southwest, so every drop is a precious resource. People in the Southwest are particularly dependent on surface water supplies like Lake Mead, which are vulnerable to evaporation. Thus, even a small increase in temperature (which drives evaporation) or a decrease in precipitation in this already arid region can seriously threaten natural systems and society. Droughts also contribute to increased pest outbreaks and wildfires, both of which damage local economies, and they reduce the amount of water available for generating electricityâ€”for example, at the Hoover Dam.1

While two indicators in this report present information about unusually high or low temperatures and drought on a national scale (see the High and Low Temperatures indicator and the Drought indicator), this feature highlights the Southwest because of its particular sensitivity to temperature and drought. Parts of the Southwest are also experiencing long-term reductions in mountain snowpack (see the Snowpack indicator), which accounts for a large portion of the regionâ€™s water supply. This feature focuses on six states that are commonly thought of as â€œsouthwesternâ€? and characterized at least in part by arid landscapes and scarce water supplies: Arizona, California, Colorado, Nevada, New Mexico, and Utah. Temperature and drought data come from a network of thousands of weather stations overseen by the National Weather Service.

The map in Figure 1 shows how average annual temperatures in the Southwest from 2000 to 2015 differed from the average over the entire period since widespread temperature records became available (1895â€“2015).

Figures 2 and 3 show two ways of measuring drought in the Southwest: the Drought Monitor and the Palmer Drought Severity Index. The Palmer Index is calculated from precipitation and temperature measurements at weather stations, and has been used widely for many years. The Drought Monitor is a more recent and more detailed index based on several other indices (including Palmer), along with additional factors such as snow water content, groundwater levels, reservoir storage, pasture/range conditions, and other impacts. See the Drought indicator for more information about these indices.

Notes

Natural variability, changes in irrigation practices, and other diversions of water for human use can influence certain drought-related measurements. Soil moisture, ground water, and streamflow are part of Drought Monitor calculations (Figure 2), and they are all sensitive to human activities.

Data Sources

Data for Figures 1 and 3 were obtained from the National Oceanic and Atmospheric Administrationâ€™s National Centers for Environmental Information, which maintains a large collection of climate data online at: www.ncei.noaa.gov. Data for Figure 2 were provided by the National Drought Mitigation Center, which gives historical data in table form at: http://droughtmonitor.unl.edu/MapsAndData.aspx.

Technical Documentation

Download related technical information PDF

References

1.Â MacDonald, G.M. 2010. Water, climate change, and sustainability in the Southwest. P. Natl. Acad. Sci. USA 107(50):21256â€“21262.

2.Â NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental Information. Accessed January 2016. www.ncdc.noaa.gov.

3.Â National Drought Mitigation Center. 2016. Maps and data. Accessed January 2016. http://droughtmonitor.unl.edu/MapsAndData.aspx.

4.Â NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental Information. Accessed January 2016. www.ncdc.noaa.gov.

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Climate Change Indicators: Antarctic Sea Ice

This indicator tracks the extent of sea ice around Antarctica.

Figure 1. February and September Monthly Average Antarctic Sea Ice Extent, 1979–2016

This figure shows Antarctic sea ice extent for the months of February and September of each year from 1979 through September 2015 and February 2016. The minimum and maximum sea ice extent typically occur in February and September each year.

Data source: NSIDC, 20164
Web update: August 2016

Seasonal Antarctic Sea Ice

Source: NASA, 20165
Web update: August 2016

Figure 1. February and September Monthly Average Antarctic Sea Ice Extent, 1979–2016

Seasonal Antarctic Sea Ice

			Key Points

			Background

About the

			Indicator

			Indicator Notes

			Data Sources

Technical

			Documentation

Key Points

Antarctic sea ice extent in September and February has increased somewhat over time, although the most recent year was below average. The September maximum extent reached the highest level on record in 2014—about 7 percent larger than the 1981–2010 average—but in 2015 it was slightly below the 1981–2010 average. As for February extent, 2013, 2014, and 2015 were three of the six largest years on record, but extent in 2016 was about 9 percent below the 1981–2010 average (see Figure 1).

Slight increases in Antarctic sea ice are outweighed by the loss of sea ice in the Arctic during the same time period (see the Arctic Sea Ice indicator). Overall, the Earth has lost sea ice at an average rate of 13,500 square miles per year since 1979—equivalent to losing an area larger than the state of Maryland every year.3 This decrease affects the Earth’s energy balance.

Background

The Southern Ocean around Antarctica freezes to form sea ice every year. This sea ice reaches its maximum extent in September or early October, around the end of winter in the Southern Hemisphere. At that point, virtually the entire continent of Antarctica is surrounded by a ring of sea ice (see images below). The ice melts in the summer months (December to February) because of warmer temperatures and long hours of sunlight. Unlike the Arctic, where a large area of sea ice lasts year-round (see the Arctic Sea Ice indicator), the sea ice around Antarctica is thinner and nearly all of it melts in a typical summer. This happens because no landmass is present to stop Antarctic sea ice from breaking up and drifting northward into warmer waters, where it eventually melts.

Like Arctic sea ice, Antarctic sea ice affects global climate, regional climate, and ecosystems. Because ice is more reflective than liquid water, it plays a significant role in maintaining the Earth’s energy balance and helping to keep polar regions cool. (For more information on how the color of the Earth's surface affects the absorption of sunlight, see the Snow Cover indicator.) Sea ice also keeps the air cool by forming a barrier between the cold air above and the warmer water below. Antarctic sea ice is an important part of the habitat for penguins, seals, and other animals.

Warmer air and ocean temperatures are generally expected to reduce the amount of sea ice present worldwide. While warming has already driven a noticeable decline in sea ice in the Arctic, extent in the Antarctic as a whole has not yet declined (and has actually increased slightly), which may reflect influences of wind patterns, ocean currents, and precipitation around the continent.1

About the Indicator

Figure 1 shows Antarctic sea ice extent from 1979, when routine monitoring by satellites started, to early 2016. Sea ice extent is defined as the area of ocean where at least 15 percent of the surface is frozen. This threshold was chosen because scientists have found that it gives the best approximation of the edge of the ice. Data are collected throughout the year, but for comparison, this indicator focuses on the months when sea ice typically reaches its minimum and maximum extent. February is typically when the sea ice extent reaches its annual minimum after melting during the spring and summer. The ice typically reaches its maximum extent in late September or early October after winter freezing, with the largest monthly average extent occurring in September. Data for this indicator were gathered by the National Snow and Ice Data Center using satellite imaging technology and data processing methods developed by the National Aeronautics and Space Administration.

Unlike EPA’s Arctic Sea Ice indicator, this indicator does not show the age of Antarctic sea ice. This is because nearly all of the Antarctic ice melts every summer, unlike the Arctic, where a large portion of ice survives for multiple years.2

Indicator Notes

Compared with the noticeable decrease in Arctic sea ice extent (see the Arctic Sea Ice indicator), the increase in Antarctic sea ice is small. Many factors appear to be influencing Antarctic sea ice extent, including changes in wind patterns, currents, and precipitation.6 Without better ice thickness and ice volume estimates, it is difficult to characterize how the total amount of Antarctic sea ice is responding to climate change.7

Data Sources

The data for this indicator were provided by the National Snow and Ice Data Center. These data are also available online at: http://nsidc.org/data/seaice_index/archives.html. The National Snow and Ice Data Center produces a variety of reports analyzing sea ice data.

Technical Documentation

Download related technical information PDF

References

1. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

2. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

3. Parkinson, C.L. 2014. Global sea ice coverage from satellite data: Annual cycle and 35-yr trends. J. Climate 27(24):9377.

4. NSIDC (National Snow and Ice Data Center). 2016. Sea ice data and image archive. Accessed July 2016. http://nsidc.org/data/seaice_index/archives.html.

5. NASA (National Aeronautics and Space Administration). 2016. NASA’s Goddard Space Flight Center Scientific Visualization Studio. https://svs.gsfc.nasa.gov.

6. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

7. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

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Community Connection: Ice Breakup in Two Alaskan Rivers

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Climate Change Indicators: Ocean Acidity

This indicator describes changes in the chemistry of the ocean that relate to the amount of carbon dioxide dissolved in the water.

Figure 1. Ocean Carbon Dioxide Levels and Acidity, 1983–2015

This figure shows the relationship between changes in ocean carbon dioxide levels (measured in the left column as a partial pressure—a common way of measuring the amount of a gas) and acidity (measured as pH in the right column). The data come from two observation stations in the North Atlantic Ocean (Canary Islands and Bermuda) and one in the Pacific (Hawaii). The up-and-down pattern shows the influence of seasonal variations.

Data sources: Bates, 2016;5 González-Dávila, 2012;6 Dore, 20157
Web update: August 2016

Figure 2. Changes in Aragonite Saturation of the World’s Oceans, 1880–2015

This map shows changes in the aragonite saturation level of ocean surface waters between the 1880s and the most recent decade (2006–2015). Aragonite is a form of calcium carbonate that many marine animals use to build their skeletons and shells. The lower the saturation level, the more difficult it is for organisms to build and maintain their skeletons and shells. A negative change represents a decrease in saturation.

Data source: Woods Hole Oceanographic Institution, 20168
Web update: August 2016

pH Scale

Acidity is commonly measured using the pH scale. Pure water has a pH of about 7, which is considered neutral. A substance with a pH less than 7 is considered to be acidic, while a substance with a pH greater than 7 is considered to be basic or alkaline. The lower the pH, the more acidic the substance. Like the well-known Richter scale for measuring earthquakes, the pH scale is based on powers of 10, which means a substance with a pH of 3 is 10 times more acidic than a substance with a pH of 4. For more information about pH, visit: www.epa.gov/acidrain/what-acid-rain.

Sources: Environment Canada, 2008,9 with additional data from IPCC, 201410
Web update: August 2016

Figure 1. Ocean Carbon Dioxide Levels and Acidity, 1983–2015

Figure 2. Changes in Aragonite Saturation of the World’s Oceans, 1880–2015

pH Scale

			Key Points

			Background

About the

			Indicator

			Indicator Notes

			Data Sources

Technical

			Documentation

Key Points

Measurements made over the last few decades have demonstrated that ocean carbon dioxide levels have risen in response to increased carbon dioxide in the atmosphere, leading to an increase in acidity (that is, a decrease in pH) (see Figure 1).

Historical modeling suggests that since the 1880s, increased carbon dioxide has led to lower aragonite saturation levels in the oceans around the world, which makes it more difficult for certain organisms to build and maintain their skeletons and shells (see Figure 2).

The largest decreases in aragonite saturation have occurred in tropical waters (see Figure 2); however, decreases in cold areas may be of greater concern because colder waters typically have lower aragonite saturation levels to begin with.4

Background

The ocean plays an important role in regulating the amount of carbon dioxide in the atmosphere. As atmospheric concentrations of carbon dioxide rise (see the Atmospheric Concentrations of Greenhouse Gases indicator), the ocean absorbs more carbon dioxide. Because of the slow mixing time between surface waters and deeper waters, it can take hundreds to thousands of years to establish this balance. Over the past 250 years, oceans have absorbed about 28 percent of the carbon dioxide produced by human activities that burn fossil fuels.1

Although the ocean’s ability to take up carbon dioxide prevents atmospheric levels from climbing even higher, rising levels of carbon dioxide dissolved in the ocean can have a negative effect on some marine life. Carbon dioxide reacts with sea water to produce carbonic acid. The resulting increase in acidity (measured by lower pH values) changes the balance of minerals in the water. This makes it more difficult for corals, some types of plankton, and other creatures to produce a mineral called calcium carbonate, which is the main ingredient in their hard skeletons or shells. Thus, declining pH can make it more difficult for these animals to thrive. This can lead to broader changes in the overall structure of ocean and coastal ecosystems, and can ultimately affect fish populations and the people who depend on them.2 Signs of damage are already starting to appear in certain areas.3

While changes in ocean pH and mineral saturation caused by the uptake of atmospheric carbon dioxide generally occur over many decades, these properties can fluctuate over shorter periods, especially in coastal and surface waters. For example, increased photosynthesis during the day and during the summer leads to natural fluctuations in pH. Acidity also varies with water temperature.

About the Indicator

This indicator describes trends in pH and related properties of ocean water, based on a combination of direct observations, calculations, and modeling.

Figure 1 shows pH values and levels of dissolved carbon dioxide at three locations that have collected measurements consistently over the last few decades. These data have been either measured directly or calculated from related measurements, such as dissolved inorganic carbon and alkalinity. Data come from two stations in the Atlantic Ocean (Bermuda and the Canary Islands) and one in the Pacific (Hawaii).

The global map in Figure 2 shows changes over time in aragonite saturation level. Aragonite is a specific form of calcium carbonate that many organisms produce and use to build their skeletons and shells, and the saturation state is a measure of how easily aragonite can dissolve in the water. The lower the saturation level, the more difficult it is for organisms to build and maintain their protective skeletons and shells. This map was created by comparing average conditions during the 1880s with average conditions during the most recent 10 years (2006–2015). Aragonite saturation has only been measured at selected locations during the last few decades, but it can be calculated reliably for different times and locations based on the relationships scientists have observed among aragonite saturation, pH, dissolved carbon, water temperature, concentrations of carbon dioxide in the atmosphere, and other factors that can be measured. Thus, while Figure 2 was created using a computer model, it is based on measurements.

Indicator Notes

This indicator focuses on surface waters, which can absorb carbon dioxide from the atmosphere within a few months.11 It can take much longer for changes in pH and mineral saturation to spread to deeper waters, so the full effect of increased atmospheric carbon dioxide concentrations on ocean acidity may not be seen for many decades, if not centuries. Studies suggest that the impacts of ocean acidification may be greater at depth, because the aragonite saturation level is naturally lower in deeper waters.12

Ocean chemistry is not uniform around the world, so local conditions can cause pH or aragonite saturation measurements to differ from the global average. For example, carbon dioxide dissolves more readily in cold water than in warm water, so colder regions could experience greater impacts from acidity than warmer regions. Air and water pollution also lead to increased acidity in some areas.

Data Sources

Data for Figure 1 came from three studies: the Bermuda Atlantic Time-Series Study, the European Station for Time-Series in the Ocean (Canary Islands), and the Hawaii Ocean Time-Series. Bermuda data are available at: http://bats.bios.edu. Canary Islands data are available at: www.eurosites.info/estoc/data.php. Hawaii data are available at: http://hahana.soest.hawaii.edu/hot/products/products.html.

The map in Figure 2 was created by the National Oceanic and Atmospheric Administration and the Woods Hole Oceanographic Institution using Community Earth System Model data. Related information can be found at: http://sos.noaa.gov/Datasets/list.php?category=Ocean.

Technical Documentation

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References

1. Calculated from numbers in the IPCC Fifth Assessment Report. From 1750 to present: total human emissions of 545 Pg C and ocean uptake of 155 Pg C. Source: IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

2. Wootton, J.T., C.A. Pfister, and J.D. Forester. 2008. Dynamic patterns and ecological impacts of declining ocean pH in a high-resolution multi-year dataset. P. Natl. Acad. Sci. USA 105(48):18848–18853.

3. Bednaršek, N., G.A. Tarling, D.C.E. Bakker, S. Fielding, E.M. Jones, H.J. Venables, P. Ward, A. Kuzirian, B. Lézé, R.A. Feely, and E.J. Murphy. 2012. Extensive dissolution of live pteropods in the Southern Ocean. Nat. Geosci. 5:881–885.

4. Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

5. Bates, N.R. 2016 update to data originally published in: Bates, N.R., M.H. Best, K. Neely, R. Garley, A.G. Dickson, and R.J. Johnson. 2012. Indicators of anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean. Biogeosciences 9:2509–2522.

6. González-Dávila, M. 2012 update to data originally published in: González-Dávila, M., J.M. Santana-Casiano, M.J. Rueda, and O. Llinás. 2010. The water column distribution of carbonate system variables at the ESTOC site from 1995 to 2004. Biogeosciences 7:3067–3081.

7. Dore, J. 2015 update to data originally published in: Dore, J.E., R. Lukas, D.W. Sadler, M.J. Church, and D.M. Karl. 2009. Physical and biogeochemical modulation of ocean acidification in the central North Pacific. Proc. Natl. Acad. Sci. USA 106:12235–12240.

8. Woods Hole Oceanographic Institution. 2016 update to data originally published in: Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

9. Recreated from Environment Canada. 2008. The pH scale. www.ec.gc.ca/eau-water/default.asp?lang=En&n=FDF30C16-1.

10. IPCC (Intergovernmental Panel on Climate Change). 2014. Climate change 2014: Impacts, adaptation, and vulnerability. Working Group II contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg2.

11. Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

12. Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

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Climate Change Indicators

Climate Change Indicators: Arctic Sea Ice

This indicator tracks the extent, age, and melt season of sea ice in the Arctic Ocean.

Figure 1. March and September Monthly Average Arctic Sea Ice Extent, 1979â€“2016

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This figure shows Arctic sea ice extent for the months of September and March of each year from 1979 through 2016. September and March are when the minimum and maximum extent typically occur each year.

Data source: NSIDC, 20164
Web update: NovemberÂ 2016

Figure 2. Age of Arctic Sea Ice at Minimum September Week, 1983â€“2015

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This figure shows the distribution of Arctic sea ice extent by age group during the week in September with the smallest extent of ice for each year. The total extent in Figure 2 differs from the extent in Figure 1 because Figure 1 shows a monthly average, while Figure 2 shows conditions during a single week.

Data source: NSIDC, 20155
Web update: December 2015

Figure 3. Arctic Sea Ice Melt Season, 1979â€“2015

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This figure shows the timing of each yearâ€™s Arctic sea ice melt season. The shaded band spans from the date when ice begins to melt consistently until the date when it begins to refreeze.

Data source: NASA, 20166
Web update: AugustÂ 2016

Dwindling Arctic Sea Ice

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Source: NASA, 20167
Web update: AugustÂ 2016

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Figure 1. March and September Monthly Average Arctic Sea Ice Extent, 1979â€“2016

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Figure 2. Age of Arctic Sea Ice at Minimum September Week, 1983â€“2015

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Figure 3. Arctic Sea Ice Melt Season, 1979â€“2015

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Dwindling Arctic Sea Ice

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			Key Points

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			Background

About the

			Indicator

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			Indicator Notes

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			Data Sources

Technical

			Documentation

Key Points

September 2012 had the lowest sea ice extent ever recorded, 44 percent below the 1981â€“2010 average for that month.

The September 2016 sea ice extent was more than 700,000 square miles less than the historical 1981â€“2010 average for that month (see Figure 1)â€”a difference more than two and a half times the size of Texas. March sea ice extent reached the lowest extent on record in 2015 and hit roughly the same low again in 2016â€”about 7 percent less than the 1981â€“2010 average.

All months have shown a negative trend in sea ice extent over the past several decades. The largest year-to-year decreases have occurred in the summer and fall months.2,3

Evidence of the age of Arctic sea ice suggests that fewer patches of ice are persisting for multiple years (i.e., generally thick ice that has survived one or more melt seasons) (see Figure 2). The proportion of sea ice five years or older has declined dramatically over the recorded time period, from more than 30 percent of September ice in the 1980s to 9 percent in 2015. A growing percentage of Arctic sea ice is only one or two years old. Less old multi-year ice implies that the ice cover is thinning, which makes it more vulnerable to further melting.

Since 1979, the length of the melt season for Arctic sea ice has grown by 37 days (see Figure 3). Arctic sea ice now starts melting 11 days earlier and it starts refreezing 26 days later than it used to, on average (see Figure 3).

BackgroundÂ

Sea ice is an integral part of the Arctic Ocean. During the dark winter months, sea ice essentially covers the entire Arctic Ocean. In summer, some of this ice melts because of warmer temperatures and long hours of sunlight. Sea ice typically reaches its minimum thickness and extent in mid-September, when the area covered by ice is roughly half the size of the winter maximum. The ice then begins expanding again.

The extent of area covered by Arctic sea ice is an important indicator of changes in global climate because warmer air and water temperatures are reducing the amount of sea ice present. Because sea ice is light-colored, it reflects more sunlight (solar energy) back to space than liquid water, thereby playing an important role in maintaining the Earthâ€™s energy balance and helping to keep polar regions cool. (For more information on the effects of surface color on reflecting sunlight, see the Snow Cover indicator.) Sea ice also keeps the air cool by forming a barrier between the cold air above and the relatively warmer water below. As the amount of sea ice decreases, the Arctic regionâ€™s cooling effect is reduced, and this may initiate a â€œfeedback loopâ€? whereby ocean warming caused by more absorption of solar energy leads to an even greater loss of sea ice and further warming.

The age of sea ice is also an important indicator of Arctic conditions, because ice that has accumulated over many years is generally thicker and stronger than younger ice. A loss of older ice suggests that the Arctic ice cover is becoming thinner. Evidence also suggests that the melt season has become longer; the ice is starting to melt earlier in the year and freeze later than it used to.1

Changes in sea ice can directly affect the health of Arctic ecosystems. Mammals such as polar bears and walruses rely on the presence of sea ice for hunting, breeding, and migrating. These animals face the threat of declining birth rates and restricted access to food sources because of reduced sea ice coverage and thickness. Impacts on Arctic wildlife, as well as the loss of ice itself, are already restricting the traditional subsistence hunting lifestyle of indigenous Arctic populations such as the Yupâ€™ik, IÃ±upiat, and Inuit.

While diminished sea ice can have negative ecological effects, it can also present commercial opportunities. For instance, reduced sea ice opens shipping lanes and increases access to natural resources in the Arctic region.

About the Indicator

Figure 1 shows Arctic sea ice extent from 1979, when routine monitoring by satellites started, to 2016. Sea ice extent is defined as the area of ocean where at least 15 percent of the surface is frozen. This threshold was chosen because scientists have found that it gives the best approximation of the edge of the ice. Data are collected throughout the year, but for comparison, this indicator focuses on the months when sea ice typically reaches its minimum and maximum extent. September is typically when the sea ice extent reaches its annual minimum after melting during the spring and summer. Conversely, March is typically when sea ice reaches its maximum extent after winter freezing. Data for this indicator were gathered by the National Snow and Ice Data Center using satellite imaging technology and data processing methods developed by the National Aeronautics and Space Administration.

Figure 2 examines the age of the ice that is present in the Arctic during the one week in September that has the smallest extent of ice. By combining daily satellite images, wind measurements, and data from surface buoys that move with the ice, scientists can track specific parcels of ice as they move over time. This tracking enables them to calculate the age of the ice in different parts of the Arctic. Although satellites started collecting data in 1979, Figure 2 only shows data back to 1983 because it is not possible to know the full age distribution until the ice has been tracked for at least five years.

Figure 3 shows the start and end dates of each yearâ€™s Arctic sea ice melt season, starting in 1979. It is based on the same satellite instruments as Figure 1. Here, the melt season start date is defined as the date when satellites detect consistent wetness on the surface of the ice and snow; the end date is when the surface air temperature stays consistently at or below the freezing point and ice begins to grow in the open ocean.

Indicator Notes

Increasing air and ocean temperatures associated with climate change are not the only factor contributing to reductions in sea ice. Other conditions that may be affected by climate change, such as fluctuations in oceanic and atmospheric circulation and natural annual and decadal variability, also affect the extent of sea ice. Determining the age of ice is an imperfect science, as there are cases where a small amount of older ice might exist within an area classified as younger, or vice versa.

Data Sources

The data for Figures 1 and 2 were provided by the National Snow and Ice Data Center. Data for Figure 1 are also available online at: http://nsidc.org/data/seaice_index/archives.html, while Figure 2 is based on an analysis by the University of Colorado and a map published at: http://nsidc.org/arcticseaicenews/2015/10/2015-melt-season-in-review. The National Snow and Ice Data Center produces a variety of reports and a seasonal newsletter analyzing Arctic sea ice data. Figure 3 data were provided by the National Aeronautics and Space Administration and are available at: https://neptune.gsfc.nasa.gov/csb/index.php?section=54.

Technical Documentation

Download related technical information PDF

References

1. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

2.Â NSIDC (National Snow and Ice Data Center). 2012. Arctic sea ice 101.Â http://nsidc.org/icelights/arctic-sea-ice.

3.Â Comiso, J. 2012. Large decadal decline of the Arctic multiyear ice cover. J. Climate 25(4):1176â€“1193.

4.Â NSIDC (National Snow and Ice Data Center). 2016. Sea ice data and image archive. Accessed October 2016.Â http://nsidc.org/data/seaice_index/archives.html.

5.Â NSIDC (National Snow and Ice Data Center). 2015. Arctic sea ice news and analysis. October 6, 2015.Â http://nsidc.org/arcticseaicenews/2015/10/2015-melt-season-in-review

6.Â NASA (National Aeronautics and Space Administration). 2016. Arctic sea ice melt.Â https://neptune.gsfc.nasa.gov/csb/index.php?section=54

7.Â NASA (National Aeronautics and Space Administration). 2016. NASAâ€™s Goddard Space Flight Center Scientific Visualization Studio.Â https://svs.gsfc.nasa.gov

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Climate Indicators

Climate Change Indicators

Greenhouse Gases

Weather and Climate

Oceans

Snow and Ice

Arctic Sea Ice

Antarctic Sea Ice

Glaciers

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Community Connection: Ice Breakup in Two Alaskan Rivers

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Climate Change Science

Future of Climate Change

On This Page

Increasing greenhouse gas concentrations will have many effects

Future temperature changes

Future ice, snowpack, and permafrost

Future sea level change

Future precipitation and storm events

Future ocean acidification

Increasing greenhouse gas concentrations will have many effects

Key Points

Continued emissions of greenhouse gases will lead to further climate changes. Future changes are expected to include a warmer atmosphere, a warmer and more acidic ocean, higher sea levels, and larger changes in precipitation patterns.

The extent of future climate change depends on what we do now to reduce greenhouse gas emissions. The more we emit, the larger future changes will be.

Related Links

U.S. National Climate Assessment, 2014

USGCRP Global Climate Change Impacts in the United States

IPCC Fifth Assessment Report Exit

NRC America's Climate Choices Reports Exit

IPCC Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation, Summary for Policy Makers Exit(20 pp, 11.8 M, About PDF)

NRC Climate Stabilization Targets Exit

Greenhouse gas concentrations in the atmosphere will continue to increase unless the billions of tons of our annual emissions decrease substantially. Increased concentrations are expected to:

Increase Earth's average temperature

Influence the patterns and amounts of precipitation

Reduce ice and snow cover, as well as permafrost

Raise sea level

Increase the acidity of the oceans

Increase the frequency, intensity, and/or duration of extreme events

Shift ecosystem characteristics

Increase threats to human health

These changes will impact our food supply, water resources, infrastructure, ecosystems, and even our own health.

Future changes will depend on many factors

NRC Climate Stabilization Targets increase in greenhouse gas concentrations

Natural influences on climate (e.g., from volcanic activity and changes in the sun's intensity) and natural processes within the climate system (e.g., changes in ocean circulation patterns)

Scientists use computer models of the climate system to better understand these issues and project future climate changes.

Past and present-day greenhouse gas emissions will affect climate far into the future

Many greenhouse gases stay in the atmosphere for long periods of time. As a result, even if emissions stopped increasing, atmospheric greenhouse gas concentrations would continue to increase and remain elevated for hundreds of years. Moreover, if we stabilized concentrations and the composition of today's atmosphere remained steady (which would require a dramatic reduction in current greenhouse gas emissions), surface air temperatures would continue to warm. This is because the oceans, which store heat, take many decades to fully respond to higher greenhouse gas concentrations. The ocean's response to higher greenhouse gas concentrations and higher temperatures will continue to impact climate over the next several decades to hundreds of years.[2]

To learn more about greenhouse gases, please visit the Greenhouse Gas Emissions page and the Greenhouse Effect section of the Causes of Climate Change page.

Because it is difficult to project far-off future emissions and other human factors that influence climate, scientists use a range of scenarios using various assumptions about future economic, social, technological, and environmental conditions.

This figure shows projected greenhouse gas concentrations for four different emissions pathways. The top pathway assumes that greenhouse gas emissions will continue to rise throughout the current century. The bottom pathway assumes that emissions reach a peak between 2010 and 2020, declining thereafter.

Source: Graph created from data in the Representative Concentration Pathways Database (Version 2.0.5) http://www.iiasa.ac.at/web-apps/tnt/RcpDb

Click the image to view a larger version.

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Future temperature changes

We have already observed global warming over the last several decades. Future temperatures are expected to change further. Climate models project the following key temperature-related changes.

Key global projections

Increases in average global temperatures are expected to be within the range of 0.5°F to 8.6°F by 2100, with a likely increase of at least 2.7°F for all scenarios except the one representing the most aggressive mitigation of greenhouse gas emissions.[2]

Except under the most aggressive mitigation scenario studied, global average temperature is expected to warm at least twice as much in the next 100 years as it has during the last 100 years.[2]

Ground-level air temperatures are expected to continue to warm more rapidly over land than oceans.[2]

Some parts of the world are projected to see larger temperature increases than the global average.[2]

Projected changes in global average temperatures under four emissions pathways (rows) for three different time periods (columns). Changes in temperatures are relative to 1986-2005 averages. The pathways come from the IPCC Fifth Assessment Report: RCP2.6 is a very low emissions pathway, RCP4.5 is a medium emissions pathway, RCP6.0 is a medium-high emissions pathway, and RCP8.5 is the high emissions pathway (emissions are assumed to continue increasing throughout the century). Source: IPCC, 2013 Exit

Click the image to view a larger version.

Observed and projected changes in global average temperature under four emissions pathways. The vertical bars at right show likely ranges in temperature by the end of the century, while the lines show projections averaged across a range of climate models. Changes are relative to the 1986-2005 average. Source: IPCC, 2013 Exit, FAQ 12.1, Figure 1.

Click the image to view a larger version.

Key U.S. projections

By 2100, the average U.S. temperature is projected to increase by about 3°F to 12°F, depending on emissions scenario and climate model.[1]

An increase in average temperatures worldwide implies more frequent and intense extreme heat events, or heat waves. The number of days with high temperatures above 90°F is expected to increase throughout the United States, especially toward the end of the century.[1] Climate models project that if global emissions of greenhouse gases continue to grow, summertime temperatures in the United States that ranked among the hottest 5% in 1950-1979 will occur at least 70% of the time by 2035-2064.[1]

Projected temperature change for mid-century (left) and end-of-century (right) in the United States under higher (top) and lower (bottom) emissions scenarios. The brackets on the thermometers represent the likely range of model projections, though lower or higher outcomes are possible. Source: USGCRP (2009)

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Future precipitation and storm events

Patterns of precipitation and storm events, including both rain and snowfall are also likely to change. However, some of these changes are less certain than the changes associated with temperature. Projections show that future precipitation and storm changes will vary by season and region. Some regions may have less precipitation, some may have more precipitation, and some may have little or no change. The amount of rain falling in heavy precipitation events is likely to increase in most regions, while storm tracks are projected to shift poleward.[2] Climate models project the following precipitation and storm changes.

Projected changes in global annual mean precipitation for a low emissions scenario (left) and high emissions scenario (right). Blue and green areas are projected to experience increases in precipitation by the end of the century, while yellow and brown areas are projected to experience decreases.

Source: IPCC, 2013 Exit

Click the image to view a larger version.

Key global projections

Global average annual precipitation through the end of the century is expected to increase, although changes in the amount and intensity of precipitation will vary significantly by region.[2]

The intensity of precipitation events will likely increase on average. This will be particularly pronounced in tropical and high-latitude regions, which are also expected to experience overall increases in precipitation.[2]

The strength of the winds associated with tropical storms is likely to increase. The amount of precipitation falling in tropical storms is also likely to increase.[2]

Annual average precipitation is projected to increase in some areas and decrease in others. The figure to the right shows projected regional differences in precipitation under two emission scenarios.[2]

Key U.S. projections

Northern areas are projected to become wetter, especially in the winter and spring. Southern areas, especially the Southwest, are projected to become drier.[1]

Heavy precipitation events will likely be more frequent, even in areas where total precipitation is projected to decrease. Heavy downpours that currently occur about once every 20 years are projected to occur between twice and five times as frequently by 2100, depending on location.[1]

The proportion of precipitation falling as rain rather than snow is expected to increase, except in far northern areas.[1]

The intensity of Atlantic hurricanes is likely to increase as the ocean warms. Climate models project an increase in the number of the strongest (Category 4 and 5) hurricanes, as well as greater rainfall rates in hurricanes.[1] There is less confidence in projections of the frequency of hurricanes.[1]

Cold-season storm tracks are expected to continue to shift northward. The strongest cold-season storms are projected to become stronger and more frequent.[1]

The maps show projected future changes in precipitation for the end of this century, compared with 1970-1999, under a higher emissions scenario. For example, in winter and spring, climate models agree that northern areas in the United States are likely to get wetter and southern areas drier. There is less confidence in exactly where the transition between wetter and drier areas will occur. Confidence in the projected changes is highest in the areas marked with diagonal lines. The changes in white areas are not projected to be larger than what would be expected from natural variability. Source: U.S. National Climate Assessment, 2014.

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Future ice, snowpack, and permafrost

Arctic sea ice is already declining.[2] The area of snow cover in the Northern Hemisphere has decreased since about 1970.[2] Permafrost temperatures in Alaska and much of the Arctic [2] have increased over the last century.[1] To learn more about recent changes in snow and ice, visit the Snow and Ice page of the Indicators section.

Over the next century, it is expected that sea ice will continue to decline, glaciers will continue to shrink, snow cover will continue to decrease, and permafrost will continue to thaw. Potential changes to ice, snow, and permafrost are described below.These maps show projected losses of sea ice in the Arctic and Antarctica. The maps in a) show the average ice concentration (the relative area covered by sea ice) from 1986-2005. The maps in b) and c) show climate model simulations of sea ice thickness in February and September near the end of the 21st century under low (b) and high (c) emission scenarios. In the Arctic, February is projected to have less ice (more blue); September is projected to be nearly ice-free (almost all blue). The projected changes in Antarctic sea ice are more subtle. Source: IPCC, 2013

Click the image to view a larger version.

Key global projections

For every 2°F of warming, models project about a 15% decrease in the extent of annually averaged Arctic sea ice and a 25% decrease in the area covered by Arctic sea ice at the end of summer (September).[3] Note that this decrease does not contribute to sea level rise.

The coastal sections of the Greenland and Antarctic ice sheets are expected to continue to melt or slide into the ocean. If the rate of this ice melting increases in the 21st century, the ice sheets could add significantly to global sea level rise.[3]

Glaciers are expected to continue to decrease in size. The rate of melting is expected to continue to increase, which will contribute to sea level rise.[3]

Key U.S. projections

Northern Hemisphere snow cover is expected to decrease by approximately 15% by 2100.[3]

Models project the snow season will continue to shorten, with snow accumulation beginning later and melting starting earlier. Snowpack is expected to decrease in many regions.[3]

Permafrost is expected to continue to thaw in northern latitudes, damaging buildings, infrastructure, and ecosystems in Alaska.[3]

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Future sea level change

Meltwater flowing from the Greenland ice sheet Source: NASAWarming temperatures contribute to sea level rise by: expanding ocean water; melting mountain glaciers and ice caps; and causing portions of the Greenland and Antarctic ice sheets to melt or flow into the ocean.[3]

Since 1870, global sea level has risen by about 7.5 inches.[2] Estimates of future sea level rise vary for different regions, but global sea level for the next century is expected to rise at a greater rate than during the past 50 years.[2] Studies project global sea level to rise by another 1 to 4 feet by 2100, with an uncertainty range of 0.66 to 6.6 feet.[1]

The contribution of thermal expansion, ice caps, and small glaciers to sea level rise is relatively well studied, but the impacts of climate change on ice sheets in Greenland and Antarctica are less understood and represent an active area of research. Changes in ice sheets are currently expected to account for 1.2 to 8 inches of sea level rise by the end of this century.[3]

Past and projected sea level rise from 1800 to 2100. The orange line at right shows the currently projected range of sea level rise of 1 to 4 feet by 2100; the wider range (0.66 feet to 6.6 feet) reflects uncertainty about how glaciers and ice sheets will react to climate change. Source: NCA, 2014.

Click the image to view a larger version.Regional and local factors will influence future relative sea level rise for specific coastlines around the world. For example, relative sea level rise depends on land elevation changes that occur as a result of subsidence (sinking) or uplift (rising). Assuming that these historical geological forces continue, a 2-foot rise in global sea level by 2100 would result in the following relative sea level rise:[4]

2.3 feet at New York City

2.9 feet at Hampton Roads, Virginia

3.5 feet at Galveston, Texas

1 foot at Neah Bay in Washington state

Relative sea level rise also depends on local changes in currents, winds, salinity, and water temperatures, as well as proximity to thinning ice sheets.[2]

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Future ocean acidification

Ocean acidification adversely affects many marine species, including plankton, mollusks, shellfish, and corals. As ocean acidification increases, the availability of calcium carbonate will decline. Calcium carbonate is a key building block for the shells and skeletons of many marine organisms. If atmospheric CO2 concentrations double, coral calcification rates are projected to decline by more than 30%. If CO2 concentrations continue to rise at their current rate, the combination of climate warming and ocean acidification could slow coral growth by nearly 50% by 2050.[5]

Oceans become more acidic as carbon dioxide (CO2) emissions in the atmosphere dissolve in the ocean. This change is measured on the pH scale, with lower values being more acidic. The pH level of the oceans has decreased by approximately 0.1 pH units since pre-industrial times, which is equivalent to an approximately 30% increase in acidity. As shown in the graph and map above, the pH level of the oceans is projected to decrease even more by the end of the century as CO2 concentrations are expected to increase for the foreseeable future.[1][2] Source: IPCC, 2013, Chapter 6.

Click the image to view a larger version.

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References

[1] USGCRP (2014) Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program.

[2] IPCC (2013). Climate Change 2013: The Physical Science Basis Exit.  Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

[3] NRC (2011). Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia Exit. National Research Council. The National Academies Press, Washington, DC, USA.

[4] USGCRP (2009). Global Climate Change Impacts in the United States. Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson (eds.). United States Global Change Research Program. Cambridge University Press, New York, NY, USA.

[5] IPCC (2014). Climate Change 2014: Impacts, Adaptation, and Vulnerability.

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Confidence

Scientists have varying degrees of confidence in the assessment and projection of climate change impacts.

Scientists are confident that humans are contributing significantly to observed warming
Measurements collected by scientists are sufficiently accurate to describe large-scale changes in climate that have occurred, including changes in temperature over the last century. Although it is difficult to say exactly how much warming was caused by humans, many lines of evidence support the conclusion that human activities have caused most observed warming over at least the last several decades.

Climate models simulate future climate change with varying degrees of confidence
Scientists know a great deal about future climate change. For example, there is high confidence that global temperatures will continue to rise and that climate change will significantly affect human and natural systems. However, there are several aspects of climate change that remain more uncertain. These uncertainties stem primarily from (1) uncertainties about future human actions, especially those that affect the sources and sinks of greenhouse gases, and (2) uncertainties about how the climate system will respond to these actions. In turn, a lower degree of confidence in the response of the climate system stems from both the limitations of climate models and the complexity of the climate system.

Confidence varies by geographic scale
Global climate models generally provide consistent and reliable simulations of climate variables only at large continental to global scales. This is because the variability of the climate increases at smaller geographic and shorter temporal scales. Additionally, the grid cells of the models are usually more than 60 miles (100 km) wide, which is larger than important features that matter for local climate like mountains and land cover. Global models and regional techniques are starting to provide useful information about climate changes on local to regional scales. While some of these projections are very well understood, others are more speculative. Generally, confidence decreases as one moves from larger scales to smaller scales. Regional climate modeling is an active area of research to improve our ability to project future changes at smaller spatial scales.

Confidence varies by climate variable
Scientists are more confident in estimates of some climate variables than others. For example, changes in precipitation are more difficult to project than changes in temperature. To accurately estimate precipitation changes, models must correctly project a number of underlying processes, like evaporation, that occur on relatively small spatial scales. Moreover, precipitation is strongly influenced by mountainous terrain and other local or regional geographic features, which are not always well represented in current models. Despite the complexities, some projections and conclusions, particularly at the continental to global scales, can be made.

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Climate Change Science

Climate Change Impacts and Risk Analysis (CIRA)

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What you can do about climate change

The Climate Change Impacts and Risk Analysis (CIRA) project quantifies the physical effects and economic damages under multiple climate change scenarios. The goal of this work is to estimate to what degree climate change impacts and damages to multiple U.S. sectors (e.g., human health, infrastructure, and water resources) may be avoided or reduced in a future under different levels of future global emissions. CIRA advances the estimation of climate change damages by bridging the gap between climate modeling and economic effects, presenting both physical and monetized damages.

The peer-reviewed CIRA reports and analyses demonstrate that global action on climate change will significantly benefit Americans by saving lives and avoiding costly damages across the U.S. economy.

Climate Change in the United States: Benefits of Global Action

The 2015 EPA report estimates the physical and monetary benefits to the U.S. of reducing global greenhouse gas emissions in six broad sectors: health, infrastructure, electricity, water resources, agriculture and forestry, and ecosystems.

Avoiding and Reducing Long-term Risks of Climate Change: A Technical Report for the Fourth National Climate Assessment

This forthcoming Technical Report, currently in peer review,  summarizes and communicates the results of the second phase of modeling under the CIRA project, and is intended to inform the fourth National Climate Assessment (NCA4) of the U.S. Global Change Research Program (USGCRP).

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