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Climate Change Indicators

Climate Change Indicators: Drought

This indicator measures drought conditions of U.S. lands.

Figure 1. Average Drought Conditions in the Contiguous 48 States, 1895–2015

This chart shows annual values of the Palmer Drought Severity Index, averaged over the entire area of the contiguous 48 states. 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, 20165
Web update: August 2016

Figure 2. U.S. Lands Under Drought Conditions, 2000–2015

This chart shows the percentage of U.S. lands classified under drought conditions from 2000 through 2015. This figure uses the U.S. Drought Monitor classification system, which is described in the table below. The data cover all 50 states plus Puerto Rico.

Data source: National Drought Mitigation Center, 20166
Web update: August 2016

Categories of Drought Severity

Category
								Description
								Possible Impacts

D0
								Abnormally dry
								Going into drought: short-term dryness slowing planting or growth of crops or pastures. Coming out of drought: some lingering water deficits; pastures or crops not fully recovered.

D1
								Moderate drought
								Some damage to crops or pastures; streams, reservoirs, or wells low; some water shortages developing or imminent; voluntary water use restrictions requested.

D2
								Severe drought
								Crop or pasture losses likely; water shortages common; water restrictions imposed.

D3
								Extreme drought
								Major crop/pasture losses; widespread water shortages or restrictions.

D4
								Exceptional drought
								Exceptional and widespread crop/pasture losses; shortages of water in reservoirs, streams, and wells, creating water emergencies.

Experts update the U.S. Drought Monitor weekly and produce maps that illustrate current conditions as well as short- and long-term trends. Major participants include the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture, and the National Drought Mitigation Center. For a map of current drought conditions, visit the Drought Monitor website at: http://droughtmonitor.unl.edu.

Figure 1. Average Drought Conditions in the Contiguous 48 States, 1895–2015

Figure 2. U.S. Lands Under Drought Conditions, 2000–2015

			Key Points

			Background

About the

			Indicator

			Indicator Notes

			Data Sources

Technical

			Documentation

Key Points

Average drought conditions across the nation have varied since records began in 1895. The 1930s and 1950s saw the most widespread droughts, while the last 50 years have generally been wetter than average (see Figure 1).

Over the period from 2000 through 2015, roughly 20 to 70 percent of the U.S. land area experienced conditions that were at least abnormally dry at any given time (see Figure 2). The years 2002–2003 and 2012–2013 had a relatively large area with at least abnormally dry conditions, while 2001, 2005, and 2009–2011 had substantially less area experiencing drought.

During the latter half of 2012, more than half of the U.S. land area was covered by moderate or greater drought (see Figure 2). In several states, 2012 was among the driest years on record.4 See Temperature and Drought in the Southwest for a closer look at recent drought conditions in one of the hardest-hit regions.

Background

There are many definitions and types of drought. Meteorologists generally define drought as a prolonged period of dry weather caused by a lack of precipitation that results in a serious water shortage for some activity, population, or ecological system. Drought can also be thought of as an extended imbalance between precipitation and evaporation.

As average temperatures have risen because of climate change, the Earth’s water cycle has sped up through an increase in the rate of evaporation. An increase in evaporation makes more water available in the air for precipitation, but contributes to drying over some land areas, leaving less moisture in the soil. Thus, as the climate continues to change, many areas are likely to experience increased precipitation (see the U.S. and Global Precipitation indicator) and increased risk of flooding (see the Heavy Precipitation indicator), while areas located far from storm tracks are likely to experience less precipitation and increased risk of drought. As a result, since the 1950s, some regions of the world have experienced longer and more intense droughts, particularly in southern Europe and West Africa, while other regions have seen droughts become less frequent, less intense, or shorter (for example, in central North America).1

Drought conditions can negatively affect agriculture, water supplies, energy production, and many other aspects of society. The impacts vary depending on the type, location, intensity, and duration of the drought. For example, effects on agriculture can range from slowed plant growth to severe crop losses, while water supply impacts can range from lowered reservoir levels and dried-up streams to major water shortages. Prolonged droughts pose a particular threat to indigenous populations because of their economic and cultural dependence on land and water supplies. Warming and drought can threaten medicinal and culturally important plants and animals and can reduce water quality and availability, making tribal populations particularly vulnerable to waterborne illnesses.2 Lower streamflow and groundwater levels can also harm plants and animals, and dried-out vegetation increases the risk of wildfires.

About the Indicator

During the 20th century, many indices were created to measure drought severity by looking at precipitation, soil moisture, stream flow, vegetation health, and other variables.3 Figure 1 shows annual values of the most widely used index, the Palmer Drought Severity Index, which is calculated from precipitation and temperature measurements at weather stations. An index value of zero represents the average moisture conditions observed between 1931 and 1990 at a given location. A positive value means conditions are wetter than average, while a negative value is drier than average. Index values from locations across the contiguous 48 states have been averaged together to produce the national values shown in Figure 1.

For a more detailed perspective on recent trends, Figure 2 shows a newer index called the Drought Monitor, which is based on several indices (including Palmer), along with additional factors such as snow water content, groundwater levels, reservoir storage, pasture/range conditions, and other impacts. The Drought Monitor uses codes from D0 to D4 (see table below Figure 2) to classify drought severity. This part of the indicator covers all 50 states and Puerto Rico.

Indicator Notes

Because this indicator focuses on national trends, it does not show how drought conditions vary by region. For example, even if half of the country suffered from severe drought, Figure 1 could show an average index value close to zero if the rest of the country was wetter than average. Thus, Figure 1 might understate the degree to which droughts are becoming more severe in some areas while other places receive more rain as a result of climate change.

The U.S. Drought Monitor (Figure 2) offers a closer look at the percentage of the country that is affected by drought. This index is relatively new, however, and thus too short-lived to be used for assessing long-term climate trends or exploring how recent observations compare with historical patterns. With several decades of data collection, future versions of this indicator should be able to paint a more complete picture of trends over time.

Overall, this indicator gives a broad overview of drought conditions in the United States. It is not intended to replace local or state information that might describe conditions more precisely for a particular region.

Data Sources

Data for Figure 1 were obtained from the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, which maintains a large collection of climate data online at: www7.ncdc.noaa.gov/CDO/CDODivisionalSelect.jsp. Data for Figure 2 were provided by the National Drought Mitigation Center. Historical data in table form are available at: http://droughtmonitor.unl.edu/MapsAndData.aspx.

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. http://www.ipcc.ch/report/ar5/wg1.

2 Gamble, J.L., J. Balbus, M. Berger, K. Bouye, V. Campbell, K. Chief, K. Conlon, A. Crimmins, B. Flanagan, C. Gonzalez-Maddux, E. Hallisey, S. Hutchins, L. Jantarasami, S. Khoury, M. Kiefer, J. Kolling, K. Lynn, A. Manangan, M. McDonald, R. Morello-Frosch, M.H. Redsteer, P. Sheffield, K. Thigpen Tart, J. Watson, K.P. Whyte, and A.F. Wolkin. 2016. Chapter 9: Populations of concern. The impacts of climate change on human health in the United States: A scientific assessment. U.S. Global Change Research Program. https://health2016.globalchange.gov.

3 Heim, R.R. 2002. A review of twentieth-century drought indices used in the United States. B. Am. Meteorol. Soc. 83(8):1149–1165.

4 NOAA (National Oceanic and Atmospheric Administration). 2013. State of the climate: Drought: December 2012. Accessed July 2013. www.ncdc.noaa.gov/sotc/drought/2012/12.

5 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental Information. Accessed January 2016. www7.ncdc.noaa.gov/CDO/CDODivisionalSelect.jsp.

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

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Learn about other indicators in this section

Climate Indicators

Climate Change Indicators

Greenhouse Gases

Weather and Climate

U.S. and Global Temperature

High and Low Temperatures

U.S. and Global Precipitation

Heavy Precipitation

Tropical Cyclone Activity

River Flooding

Drought

A Closer Look: Temperature and Drought in the Southwest

Oceans

Snow and Ice

Health and Society

Ecosystems

Frequent Questions


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Climate Change Indicators: U.S. and Global Precipitation

This indicator describes trends in average precipitation for the United States and the world.

Figure 1. Precipitation in the Contiguous 48 States, 1901–2015

This figure shows how the total annual amount of precipitation in the contiguous 48 states has changed since 1901. This graph uses the 1901–2000 average as a baseline for depicting change. Choosing a different baseline period would not change the shape of the data over time.

Data source: NOAA, 20161
Web update: August 2016

Figure 2. Precipitation Worldwide, 1901–2015

This figure shows how the total annual amount of precipitation over land worldwide has changed since 1901. This graph uses the 1901–2000 average as a baseline for depicting change. Choosing a different baseline period would not change the shape of the data over time.

Data source: Blunden and Arndt, 20162
Web update: August 2016

Figure 3. Change in Precipitation in the United States, 1901–2015

This figure shows the rate of change in total annual precipitation in different parts of the United States since the early 20th century (since 1901 for the contiguous 48 states and 1925 for Alaska). The data are shown for climate divisions, as defined by the National Oceanic and Atmospheric Administration.

Data source: NOAA, 20163
Web update: August 2016

Figure 1. Precipitation in the Contiguous 48 States, 1901–2015

Figure 2. Precipitation Worldwide, 1901–2015

Figure 3. Change in Precipitation in the United States, 1901–2015

Key Points

Background

About the
Indicator

Indicator Notes

Data Sources

Technical
Documentation

Key Points

On average, total annual precipitation has increased over land areas in the United States and worldwide (see Figures 1 and 2). Since 1901, global precipitation has increased at an average rate of 0.08 inches per decade, while precipitation in the contiguous 48 states has increased at a rate of 0.17 inches per decade.

Some parts of the United States have experienced greater increases in precipitation than others. A few areas, such as the Southwest, have seen a decrease in precipitation (see Figure 3). Not all of these regional trends are statistically significant, however.

Background

Precipitation can have wide-ranging effects on human well-being and ecosystems. Rainfall, snowfall, and the timing of snowmelt can all affect the amount of surface water and groundwater available for drinking, irrigation, and industry. They also influence river flooding and can determine what types of animals and plants (including crops) can survive in a particular place. Changes in precipitation can disrupt a wide range of natural processes, particularly if these changes occur more quickly than plant and animal species can adapt.

As average temperatures at the Earth’s surface rise (see the U.S. and Global Temperature indicator), more evaporation occurs, which, in turn, increases overall precipitation. Therefore, a warming climate is expected to increase precipitation in many areas. Just as precipitation patterns vary across the world, however, so do the precipitation effects of climate change. By shifting the wind patterns and ocean currents that drive the world’s climate system, climate change will also cause some areas to experience decreased precipitation. In addition, higher temperatures lead to more evaporation, so increased precipitation will not necessarily increase the amount of water available for drinking, irrigation, and industry (see the Drought indicator).

About the Indicator

This indicator examines U.S. and global precipitation patterns, based on rainfall and snowfall measurements from land-based weather stations worldwide. This indicator starts at 1901 except for the detailed map of Alaska, where reliable statewide records are available back to 1925. The indicator extends through 2015.

This indicator shows annual anomalies, or differences, compared with the average precipitation from 1901 to 2000. At each weather station, annual precipitation anomalies were calculated from total annual precipitation in inches. Anomalies for the contiguous 48 states and Alaska have been determined by calculating average precipitation anomalies for areas within each state based on station density and topography. These regional anomalies are then averaged together in proportion to their area to develop national results. Similarly, global anomalies have been determined by dividing the world into a grid, averaging the data for each cell of the grid, and then averaging the grid cells together.

Indicator Notes

Data from the early 20th century are somewhat less precise because there were fewer stations collecting measurements at the time. To ensure that overall trends are reliable, the data have been adjusted where possible to account for any biases that might be introduced by factors such as station relocation or changes in measurement instruments.

Hawaii and U.S. territories are not included, due to limitations in available data.

Data Sources

The data for this indicator were provided by the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, which maintains a large collection of climate data online at: www.ncei.noaa.gov. The precipitation anomalies shown here were calculated based on monthly values from a network of long-term monitoring stations.

Technical Documentation

Download related technical information PDF

References

1 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental Information. Accessed February 2016. www.ncei.noaa.gov.

2 Blunden, J., and D.S. Arndt (eds.). 2016. State of the climate in 2015. B. Am. Meteorol. Soc. 97(8):S1–S275.

3 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental Information. Accessed February 2016. www.ncei.noaa.gov.

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Learn about other indicators in this section

Climate Indicators

Climate Change Indicators

Greenhouse Gases

Weather and Climate

U.S. and Global Temperature

High and Low Temperatures

U.S. and Global Precipitation

Heavy Precipitation

Tropical Cyclone Activity

River Flooding

Drought

A Closer Look: Temperature and Drought in the Southwest

Oceans

Snow and Ice

Health and Society

Ecosystems

Frequent Questions

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Frequent Questions about Climate Change Indicators

Use the links below to find answers to some common questions about EPA's climate change indicators.

What is an indicator?

Why does EPA compile and publish climate change indicators?

How do the indicators relate to climate change?

Where do the indicator data come from?

What geographic areas and time periods do the indicators cover?

Why do some indicator trends fluctuate over time?

How are EPA's indicators chosen and developed?

Which greenhouse gases do these indicators track?

How do indicators relate to human health?

What is an indicator?

One important way to track and communicate the causes and effects of climate change is through the use of indicators. An indicator represents the state or trend of certain environmental or societal conditions over a given area and a specified period of time. The indicators in this report are designed to help readers understand observed long-term trends related to the causes and effects of climate change. In other words, they provide important evidence of "what climate change looks like." For example, long-term measurements of temperature in the United States and globally are used as an indicator to track and better understand the effects of changes in the Earth's climate.

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Why does EPA compile and publish climate change indicators?

EPA compiles these indicators with the primary goal of informing readers' understanding of climate change. They are designed so that the public, scientists, analysts, decision-makers, educators, and others can use climate change indicators as a tool for:

Effectively communicating relevant climate science information in a sound, transparent, and easy-to-understand way.

Assessing trends in environmental quality, factors that influence the environment, and effects on ecosystems and society.

Informing science-based decision-making.

EPA publishes 37 indicators both online and in a summarized print edition to help readers understand changes observed from long-term records related to the causes and effects of climate change, the significance of these changes, and their possible consequences for people, the environment, and society. The indicators presented here do not cover all possible measures of the causes and effects of climate change, nor all possible climate change indicators found in the full body of scientific literature. Instead, these 37 indicators represent a select but wide-ranging set of high-quality, long-term data that show observed changes in the Earth's climate system and several climate-relevant impacts. Together, these indicators present compelling evidence that climate change is happening now in the United States and globally.

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How do the indicators relate to climate change?

All of the indicators on this website relate to either the causes or effects of climate change. Some indicators show trends that can be more directly linked to human-induced climate change than others. Collectively, the trends depicted in these indicators provide important evidence of "what climate change looks like."

Although each indicator has a connection to climate change, EPA's indicators do not attempt to identify either the extent to which a certain indicator is driving climate change, nor the relative role of climate change in causing a trend in an observed indicator. Connections between human activities, climate change, and observed indicators are explored in more detail elsewhere in the scientific literature. For example, see the U.S. Global Change Research Program's National Climate Assessment.

Some indicators are directly linked to human activities that cause climate change, such as Global Greenhouse Gas Emissions. Changes depicted by other indicators, such as U.S. and Global Temperature, have been confidently linked with the increase in greenhouse gases caused by human activity. Some of the trends in other indicators, such as Wildfires, are consistent with what one would expect in a warming climate but may also be influenced by limitations in the historical data or other factors in addition to climate change. A few indicators, like West Nile Virus, though known to be influenced by climate change, do not yet show any significant trend. In some cases, this could be because the period for which data are available or the geographic scale in which they are presented is limited.

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Where do the indicator data come from?

EPA partners with over 40 data contributors from various government agencies, academic institutions, and other organizations to compile and communicate key indicators related to the causes and effects of climate change. The indicators consist of peer-reviewed, publicly-available data from a number of government agencies, academic institutions, and other organizations. In addition to being published here, these data sets have been published in the scientific literature and in other government or academic reports. Details on each underlying data set as well as where to find it are provided in the technical documentation.

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What geographic areas and time periods do the indicators cover?

Trends relevant to climate change are best viewed at broad geographic scales and over long time periods rather than at localized scales or over a few years or a season. EPA's indicators are based on historical records that go back in time as far as possible without sacrificing data quality, so each indicator's time scale varies.

Most of EPA's indicators focus on the United States. However, some include global trends to provide context or a basis for comparison, or because they are intrinsically global in nature, such as Atmospheric Concentrations of Greenhouse Gases. EPA attempts to present multiple scales (national, regional, or location-specific) in cases where the underlying data allow such scaling. The geographic extent and timeframe that each indicator represents largely depend on data availability and the nature of what is being measured.

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Why do some indicator trends fluctuate over time?

Although the climate is continually changing, not every climate change indicator shows a constant pattern of steady change. The Earth is a complex system, and there are always natural variations from one year to the next—for example, a very warm year followed by a colder year. The Earth's climate also goes through other natural cycles that play out over a period of several years or even decades. Individual years or even individual decades can deviate from the long-term trend. Human factors may also influence a trend—for example, human activities and land management practices can affect wildfire activity from year to year.

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How are EPA's indicators chosen and developed?

EPA chooses its indicators using a set of general assessment factors and a standard set of criteria that considers data quality, transparency of analytical methods, and relevance to climate change. Based on the availability of this data, some indicators present a single measure or variable; others have multiple measures, reflecting different data sources or different ways to group, characterize, or zoom in on the data. See the technical documentation overview for a full description of these general assessment factors and criteria and for a description of EPA's process for evaluating, reviewing, and publishing indicators.

EPA's climate change indicators and the accompanying technical documentation are designed to ensure that the science and underlying peer-reviewed data are presented and documented transparently. EPA also receives feedback from scientists, researchers, and communication experts to help inform the content and new features of EPA's indicators. The entire set of indicators, including the technical support documentation, is then peer-reviewed by independent technical experts.

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Which greenhouse gases do these indicators track?

EPA's greenhouse gas-related indicators focus on most of the major, well-mixed greenhouse gases that contribute to the vast majority of warming of the climate when they are emitted into the atmosphere. These major gases are carbon dioxide, methane, nitrous oxide, and fluorinated gases. Some of these gases are produced almost entirely by human activities, while others come from a combination of natural sources and human activities.

Many of these major greenhouse gases remain in the atmosphere for tens to thousands of years after being released. They become globally mixed in the lower part of the atmosphere, called the troposphere (the first several miles above the Earth's surface), reflecting the combined emissions worldwide from the past and present. Due to this global mixing, concentrations of these gases are fairly similar no matter where in the world they are measured.

EPA's report also notes some other substances that have much shorter atmospheric lifetimes (i.e., less than a year) but are still relevant to climate change. Important short-lived substances that affect the climate include water vapor, ozone in the troposphere, pollutants that lead to ozone formation, and aerosols (atmospheric particles) such as black carbon and sulfates. Water vapor, tropospheric ozone, and black carbon contribute to warming, while other aerosols produce a cooling effect. In addition to several long-lived greenhouse gases, EPA's Atmospheric Concentrations of Greenhouse Gases indicator tracks concentrations of ozone in the layers of the Earth's atmosphere, while Figure 2 of the Climate Forcing indicator shows the influence of a variety of short-lived substances.

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How do indicators relate to human health?

Indicators provide key information on changes that are occurring in our environment, like increasing temperatures or more severe extreme weather events, which can threaten people's health. Tracking changes in climate impacts and exposures improves understanding of changes in health risk, even if the actual health outcome is difficult to quantify. For example, while only some indicators exist for human health outcomes related to climate change, like West Nile Virus, Lyme Disease, and Heat-Related Deaths, other indicators—like Ragweed Pollen Season or Wildfires—give insight into the changing risks to human health. There are many factors other than climate that affect human health; these are discussed in more detail in EPA's full section on the connections between climate change and human health.

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Climate Change Indicators

Climate Change Indicators: Heating and Cooling Degree Days

This indicator examines changing temperatures from the perspective of heating and cooling needs for buildings.

Figure 1. Heating and Cooling Degree Days in the Contiguous 48 States, 1895â€“2015

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This figure shows the average number of heating and cooling degree days per year across the contiguous 48 states.

Data source: NOAA, 20163
Web update: AugustÂ 2016

Interactive Map

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This map shows how the average number of heating degree days per year has changed in each state over time. Click each state to reveal the data. The map was created by comparing the first 60 years of available data (1895â€“1954) with the most recent 61 years (1955â€“2015). â€œWarmerâ€? colors indicate an increase in temperatures between the two periods, leading to less of a need to turn on the heatâ€”that is, fewer heating degree days. â€œCoolerâ€? colors indicate a decrease in temperatures, leading to more of a need to turn on the heatâ€”that is, more heating degree days.

Data source: NOAA, 20164
Web update: AugustÂ 2016

Interactive Map

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This map shows how the average number of cooling degree days per year has changed in each state over time. Click each state to reveal the data. The map was created by comparing the first 60 years of available data (1895â€“1954) with the most recent 61 years (1955â€“2015). â€œWarmerâ€? colors indicate an increase in temperatures between the two periods, leading to more demand for air conditioningâ€”that is, more cooling degree days. â€œCoolerâ€? colors indicate a decrease in temperatures, leading to less demand for air conditioningâ€”that is, fewer cooling degree days.

Data source: NOAA, 20165
Web update: AugustÂ 2016

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Figure 1. Heating and Cooling Degree Days in the Contiguous 48 States, 1895â€“2015

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Figure 2. Change in Annual Heating Degree Days by State, 1955â€“2015 Versus 1895â€“1954

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Figure 3. Change in Annual Cooling Degree Days by State, 1955â€“2015 Versus 1895â€“1954

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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

Heating degree days have declined in the contiguous United States, particularly in recent years, as the climate has warmed (see Figure 1). This change suggests that heating needs have decreased overall.

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Overall, cooling degree days have increased over the past 100 years. The increase is most noticeable over the past few decades, suggesting that air conditioning energy demand has also been increasing recently (see Figure 1).

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Heating degree days have generally decreased and cooling degree days have generally increased throughout the North and West. The Southeast, with the exception of Florida, has seen the opposite: more heating degree days and fewer cooling degree days (see Figures 2 and 3).Â

Background

Outdoor temperatures can affect daily life in many ways. In particular, temperature affects our comfort level and our demand for heating and air conditioning. Collectively, heating and cooling the spaces in which we live accounts for 48 percent of the energy that American households use every year.1Â As climate change contributes to an increase in average temperatures, an increase in unusually hot days, and a decrease in unusually cold days (see the U.S. and Global Temperature and High and Low Temperatures indicators), the overall demand for heating is expected to decline and the demand for cooling is expected to increase.

One way to measure the influence of temperature change on energy demand is using heating and cooling degree days, which measure the difference between outdoor temperatures and a temperature that people generally find comfortable indoors. These measurements suggest how much energy people might need to use to heat and cool their homes and workplaces, thus providing a sense of how climate change could affect peopleâ€™s daily lives and finances.

About the Indicator

This indicator uses daily temperature data from thousands of weather stations across the United States to calculate heating and cooling degree days. A â€œdegree dayâ€? is determined by comparing the daily average outdoor temperature with a defined baseline temperature for indoor comfort (in this case, 65Â°F). For example, if the average temperature on a particular day is 78Â°F, then that day counts as 13 cooling degree days, as a buildingâ€™s interior would need to be cooled by 13Â°F to reach 65Â°F. Conversely, if the average outdoor temperature is 34Â°F, then that day counts as 31 heating degree days, as a buildingâ€™s interior would need to be warmed by 31Â°F to reach 65Â°F. This does not mean that all people will actually heat or cool buildings to 65Â°F; it is just a number to allow for consistent comparisons over time and across the country. For reference, New York City experiences far more heating degree days than cooling degree days per yearâ€”a reflection of the relatively cool climate in the Northeastâ€”while Houston, Texas, has far more cooling degree days than heating degree daysâ€”a reflection of the much warmer climate in the South.2

Figure 1 shows each yearâ€™s average heating and cooling degree days across the contiguous 48 states. Figures 2 and 3 show how heating and cooling degree days have changed by state, based on a comparison of the first 60 years of available data (1895â€“1954) with the most recent 61 years (1955â€“2015). State and national averages were calculated by finding the total number of heating and cooling degree days per year at each weather station, averaging the results from all stations within regions called climate divisions (each state within the contiguous 48 has up to 10 climate divisions), then calculating state and national averages weighted by the population of each climate division. With this population-weighting approach, average state and national heating and cooling degree days more closely reflect the conditions that the average resident would experience.

Indicator Notes

Heating and cooling degree days suggest how temperature changes affect energy demand, but they do not necessarily reflect actual energy use. Many other factors have influenced energy demand over time, such as more energy-efficient heating systems, the introduction and increasingly widespread use of cooling technologies, larger but better-insulated homes, behavior change, and population shifts (such as more people moving to warmer regions). All of the population-weighting in this indicator is based on the population distribution according to the 2010 U.S. Census, so any changes in heating and cooling degree days over time in this indicator reflect actual changes in the climate, not the influence of shifting populations. A nationally applied baselineâ€”in this case, 65Â°Fâ€”has certain limitations considering the various climate regimes across the United States.

Data Sources

The data for this indicator were provided by 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.

Technical Documentation

Download related technical information PDF

References

1 EIA (Energy Information Administration). 2015. 2009 Residential energy consumption survey. Accessed May 2015. www.eia.gov/consumption/residential/index.cfm.

2 NOAA (National Oceanic and Atmospheric Administration). 2015. 1981-2010 U.S. climate normals. Accessed May 2015. www.ncdc.noaa.gov/data-access/land-based-station-data/land-based-datasets/climate-normals/1981-2010-normals-data.

3 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental Information. Accessed January 2016. www.ncei.noaa.gov.

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

5 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental Information. Accessed January 2016. www.ncei.noaa.gov.

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Weather and Climate

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Health and Society

Heat-Related Deaths

Heat-Related Illnesses

Heating and Cooling Degree Days

Lyme Disease

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Length of Growing Season

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Climate Change Indicators: Ocean Heat

This indicator describes trends in the amount of heat stored in the worldâ€™s oceans.

Figure 1. Ocean Heat Content, 1955â€“2015

This figure shows changes in ocean heat content between 1955 and 2015. Ocean heat content is measured in joules, a unit of energy, and compared against the 1971â€“2000 average, which is set at zero for reference. Choosing a different baseline period would not change the shape of the data over time. The lines were independently calculated using different methods by government agencies in three countries: the National Oceanic and Atmospheric Administration (NOAA), Australiaâ€™s Commonwealth Scientific and Industrial Research Organisation (CSIRO), and Japan Meteorological Agencyâ€™s Meteorological Research Institute (MRI/JMA). For reference, an increase of 1 unit on this graph (1 x 1022 joules) is equal to approximately 18 times the total amount of energy used by all the people on Earth in a year.4

Data sources: CSIRO, 20165; MRI/JMA, 20166; NOAA, 20167
Web update: August 2016

Figure 1. Ocean Heat Content, 1955â€“2015

			Key Points

			Background

About the

			Indicator

			Indicator Notes

			Data Sources

Technical

			Documentation

Key Points

In three different data analyses, the long-term trend shows that the oceans have become warmer since 1955 (see Figure 1).

Although concentrations of greenhouse gases have risen at a relatively steady rate over the past few decades (see the Atmospheric Concentrations of Greenhouse Gases indicator), the rate of change in ocean heat content can vary from year to year (see Figure 1). Year-to-year changes are influenced by events such as volcanic eruptions and recurring ocean-atmosphere patterns such as El NiÃ±o.

Background

When sunlight reaches the Earthâ€™s surface, the worldâ€™s oceans absorb some of this energy and store it as heat. This heat is initially absorbed at the surface, but some of it eventually spreads to deeper waters. Currents also move this heat around the world. Water has a much higher heat capacity than air, meaning the oceans can absorb larger amounts of heat energy with only a slight increase in temperature.

The total amount of heat stored by the oceans is called â€œocean heat content,â€? and measurements of water temperature reflect the amount of heat in the water at a particular time and location. Ocean temperature plays an important role in the Earthâ€™s climate systemâ€”particularly sea surface temperature (see the Sea Surface Temperature indicator)â€”because heat from ocean surface waters provides energy for storms and thereby influences weather patterns.

Increasing greenhouse gas concentrations are trapping more energy from the sun. Because changes in ocean systems occur over centuries, the oceans have not yet warmed as much as the atmosphere, even though they have absorbed more than 90 percent of the Earthâ€™s extra heat since 1955.1,2 If not for the large heat-storage capacity provided by the oceans, the atmosphere would warm more rapidly.3 Increased heat absorption also changes ocean currents because many currents are driven by differences in temperature, which cause differences in density. These currents influence climate patterns and sustain ecosystems that depend on certain temperature ranges.

Because water expands slightly as it gets warmer, an increase in ocean heat content will also increase the volume of water in the ocean, which is one cause of the observed increases in sea level (see the Sea Level indicator).

About the Indicator

This indicator shows trends in global ocean heat content from 1955 to 2015. These data are available for the top 700 meters of the ocean (nearly 2,300 feet), which accounts for just under 20 percent of the total volume of water in the worldâ€™s oceans. The indicator measures ocean heat content in joules, which are units of energy.

The National Oceanic and Atmospheric Administration has calculated changes in ocean heat content based on measurements of ocean temperatures around the world at different depths. These measurements come from a variety of instruments deployed from ships and airplanes and, more recently, underwater robots. Thus, the data must be carefully adjusted to account for differences among measurement techniques and data collection programs. Figure 1 shows three independent interpretations of essentially the same underlying data.

Indicator Notes

Data must be carefully reconstructed and filtered for biases because of different data collection techniques and uneven sampling over time and space. Various methods of correcting the data have led to slightly different versions of the ocean heat trend line. Scientists continue to compare their results and improve their estimates over time. They also test their ocean heat estimates by looking at corresponding changes in other properties of the ocean. For example, they can check to see whether observed changes in sea level match the amount of sea level rise that would be expected based on the estimated change in ocean heat.

Data Sources

Data for this indicator were collected by the National Oceanic and Atmospheric Administration and other organizations around the world. The data were analyzed independently by researchers at the National Oceanic and Atmospheric Administration, Australiaâ€™s Commonwealth Scientific and Industrial Research Organisation, and the Japan Meteorological Agencyâ€™s Meteorological Research Institute.

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. Levitus, S., J.I. Antonov, T.P. Boyer, O.K. Baranova, H.E. Garcia, R.A. Locarnini, A.V. Mishonov, J.R. Reagan, D. Seidov, E.S. Yarosh, and M.M. Zweng. 2012. World ocean heat content and thermosteric sea level change (0â€“2000 m), 1955â€“2010. Geophys. Res. Lett. 39:L10603.

3. Levitus, S., J.I. Antonov, T.P. Boyer, O.K. Baranova, H.E. Garcia, R.A. Locarnini, A.V. Mishonov, J.R. Reagan, D. Seidov, E.S. Yarosh, and M.M. Zweng. 2012. World ocean heat content and thermosteric sea level change (0â€“2000 m), 1955â€“2010. Geophys. Res. Lett. 39:L10603.

4. Based on a total global energy supply of 13,541 million tons of oil equivalents in the year 2013, which equates to 5.7 x 1020 joules. Source: IEA (International Energy Agency). 2015. Key world energy statistics. www.iea.org/publications/freepublications/publication/KeyWorld_Statistics_2015.pdf.

5. CSIRO (Commonwealth Scientific and Industrial Research Organisation). 2016 update to data originally published in: Domingues, C.M., J.A. Church, N.J. White, P.J. Gleckler, S.E. Wijffels, P.M. Barker, and J.R. Dunn. 2008. Improved estimates of upper-ocean warming and multi-decadal sea-level rise. Nature 453:1090â€“1094. www.cmar.csiro.au/sealevel/thermal_expansion_ocean_heat_timeseries.html.

6. MRI/JMA (Meteorological Research Institute/Japan Meteorological Agency). 2016 update to data originally published in: Ishii, M., and M. Kimoto. 2009. Reevaluation of historical ocean heat content variations with time-varying XBT and MBT depth bias corrections. J. Oceanogr. 65:287â€“299.

7. NOAA (National Oceanic and Atmospheric Administration). 2016. Global ocean heat and salt content. Accessed May 2016. www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT.

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Climate Change Indicators: Great Lakes Water Levels and Temperatures

This indicator measures water levels and surface water temperatures in the Great Lakes.

Figure 1. Water Levels of the Great Lakes, 1860â€“2015

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This figure displays how water levels in each of the Great Lakes have changed since 1860. For each year, the shaded band shows the range of monthly average water levels, and the line in the middle shows the annual average. The graph uses the 1981 to 2010 average as a baseline for depicting change. Choosing a different baseline period would not change the shape of the data over time. Lakes Michigan and Huron are shown together because they are connected at the same water level.

Data source: NOAA, 20166
Web update: AugustÂ 2016

Figure 2. Surface Water Temperatures of the Great Lakes, 1995â€“2015

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This figure shows the average surface water temperatures in each of the Great Lakes, as measured by satellites. The graphs on the left show annual averages for the full time period, while the graphs on the right show how average daily temperatures have changed between two time periods for comparison: 2006â€“2015 versus 1995â€“2004.

Data source: NOAA, 20167
Web update: AugustÂ 2016

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Figure 1. Water Levels of the Great Lakes, 1860â€“2015

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Figure 2. Surface Water Temperatures of the Great Lakes, 1995â€“2015

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			Key Points

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			Background

About the

			Indicator

Â

			Indicator Notes

Â

			Data Sources

Technical

			Documentation

Key Points

Water levels in the Great Lakes have fluctuated since 1860. Over the last few decades, they appear to have declined for most of the Great Lakes (see Figure 1). The most recent levels, however, are all within the range of historical variation.

Since 1995, average surface water temperatures have increased slightly for each of the Great Lakes (see Figure 2).

Recent increases in water temperature have mostly been driven by warming during the spring and summer months (see Figure 2). These trends could relate in part to an earlier thawing of winter ice (see the Lake Ice indicator).

Background

The Great Lakes, which are Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario, form the largest group of freshwater lakes on Earth. These lakes support a variety of ecosystems and play a vital role in the economy of the eight neighboring states and the Canadian province of Ontario, providing drinking water, shipping lanes, fisheries, recreational opportunities, and more.

Water level and water temperature are two important and interrelated indicators of weather and climate change in the Great Lakes. Water level (the height of the lake surface above sea level) is influenced by many factors, including precipitation, snowmelt runoff, drought, evaporation rates, and people withdrawing water for multiple uses. Water temperature is influenced by many factors, too, but most directly by air temperature.

In recent years, warmer surface water temperatures in the Great Lakes have contributed to lower water levels by increasing rates of evaporation and causing lake ice to form later than usual (see the Lake Ice indicator), which extends the season for evaporation.1 Lower water levels in the Great Lakes forced ships to reduce their cargo tonnage by 5 to 8 percent between 1997 and 2000, which increased shipping costs. Lower water levels can also affect water supplies, the usability of infrastructure such as docks and piers, and shoreline ecosystems. These types of disruptions from low water levels are expected to continue as the climate changes.2

Another possible effect of warmer water, reduced ice cover, and increased evaporation is a corresponding increase in precipitation over nearby land, especially â€œlake effectâ€? snow (see the Snowfall indicator).3 Rising water temperatures are also expected to expand the ranges of and give new advantages to some invasive species such as the zebra mussel, and to encourage the growth of certain waterborne bacteria that can make people ill.4,5

About the Indicator

This indicator analyzes water levels and surface water temperatures in the Great Lakes. Water levels are recorded by gauges along the shore of each lake, some of which have been operated since the 1800s. Pre-1918 data came from one water level gauge per lake. Data since 1918 have come from a designated set of gauges in each lake. Figure 1 shows annual water level anomalies, or differences, in feet compared with the average water levels in each lake from 1860 to 2015. Lakes Michigan and Huron are combined because they are connected at the same water level.

Surface water temperatures are measured by satellites. Figure 2 shows annual average temperatures over the entire surface of each lake, along with the pattern of daily temperatures over the course of the year. This figureâ€™s data begin in 1995, which was the first year with complete satellite data for all five lakes.

Indicator Notes

While climate change influences water levels, human activities such as dredging can also play a role. For example, the St. Clair river opening was enlarged in the 1910s, 1930s, and 1960s, contributing to greater outflows from Lakes Michigan and Huron.8 Similarly, natural year-to-year variability and other factors such as human use and wastewater discharges can influence water temperatures.

Data Sources

Water level data were provided by the Canadian Hydrographic Service and the National Oceanic and Atmospheric Administrationâ€™s Center for Operational Oceanographic Products and Services, and can be downloaded from: www.glerl.noaa.gov/data/dashboard/data. Surface water temperature data were provided by the National Oceanic and Atmospheric Administrationâ€™s Great Lakes Environmental Research Laboratory (satellite data at: https://coastwatch.glerl.noaa.gov).

Technical Documentation

Download related technical information PDF

References

1 Gronewold, A.D., V. Fortin, B. Lofgren, A. Clites, C.A. Stow, and F. Quinn. 2013. Coasts, water levels, and climate change: A Great Lakes perspective. Climatic Change 120:697â€“711.

2 Posey, J. 2012. Climate change impacts on transportation in the Midwest. U.S. National Climate Assessment, Midwest Technical Input Report.

3 Burnett, A.W., M.E. Kirby, H.T. Mullins, and W.P. Patterson. 2003. Increasing Great Lake-effect snowfall during the twentieth century: A regional response to global warming? J. Climate 16:3535â€“3542.

4 Rahel, F.J., and J.D. Olden. 2008. Assessing the effects of climate change on aquatic invasive species. Conserv. Biol. 22(3):521â€“533.

5 Kanoshima, I., L. Urmas, and J.-M. Leppanen. 2003. The influence of weather conditions (temperature and wind) on cyanobacterial bloom development in the Gulf of Finland (Baltic Sea). Harmful Algae 2:29â€“41.

6 NOAA (National Oceanic and Atmospheric Administration). 2016. Great Lakes water level observations. Accessed April 2016. www.glerl.noaa.gov/data/dashboard/data.

7 NOAA (National Oceanic and Atmospheric Administration). 2016. NOAA CoastWatch, Great Lakes node. Accessed February 2016. https://coastwatch.glerl.noaa.gov/.

8 Quinn, F.H. 1985. Temporal effects of St. Clair River dredging on Lakes St. Clair and Erie water levels and connecting channel flow. J. Great Lakes Res. 11(3):400â€“403.

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Stream Temperature

Tribal Connection: Trends in Stream Temperature in the Snake River

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Community Connection: Cherry Blossom Bloom Dates in Washington, D.C.

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Climate Change Indicators: Health and Society

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Changes in the Earthâ€™s climate can affect public health, agriculture, water supplies, energy production and use, land use and development, and recreation. The nature and extent of these effects, and whether they will be harmful or beneficial, will vary regionally and over time. This chapter looks at some of the ways that climate change is affecting human health and society, including changes in Lyme disease, West Nile virus, ragweed pollen season, heat-related deaths and hospitalizations, heating and cooling needs, and the agricultural growing season across the United States.

Because impacts on human health are complex, often indirect, and dependent on multiple societal and environmental factors (including how people choose to respond to these impacts), the development of appropriate health-related climate indicators is challenging and still emerging. It is important for health-related climate indicators to be clear, measurable, and timely to better understand the link between climate change and health effects.

Why does it matter?

Changes in climate affect the average weather conditions to which we are accustomed. These changes may result in multiple threats to human health and welfare. Warmer average temperatures will likely lead to hotter days and more frequent and longer heat waves, which could increase the number of heat-related illnesses and deaths. Increases in the frequency or severity of extreme weather events, such as storms, could increase the risk of dangerous flooding, high winds, and other direct threats to people and property. Warmer temperatures could also reduce air quality by increasing the chemical reactions that produce smog, and, along with changes in precipitation patterns and extreme events, could enhance the spread of some diseases.

In addition, climate change could require adaptation on larger and faster scales than in the past, presenting challenges to human well-being and the economy. The more extensively and more rapidly the climate changes, the larger the potential effects on society. The extent to which climate change will affect different regions and sectors of society will depend not only on the sensitivity of those systems to climate change, but also on their ability to adapt to or cope with climate change. Populations of particular concern include the poor, the elderly, those already in poor health, the disabled, and indigenous populations.

Summary of Key Pointsâ€‹

Heat-Related Deaths. Since 1979, more than 9,000 Americans were reported to have died as a direct result of heat-related illnesses such as heat stroke. The annual death rate is higher when accounting for deaths in which heat was reported as a contributing factor, including the interaction of heat and cardiovascular disease. People aged 65+ are a particular concern: a growing demographic group that is several times more likely to die from heat-related cardiovascular disease than the general population. Considerable year-to-year variability and certain limitations of the underlying data for this indicator make it difficult to determine whether the United States has experienced long-term trends in the number of deaths classified as â€œheat-related.â€?Â

Heat-Related Illness. From 2001 to 2010, a total of about 28,000 heat-related hospitalizations were recorded across 20 states. Annual heat-related hospitalization rates ranged from fewer than one case per 100,000 people in some states to nearly four cases per 100,000 people in others. People aged 65+ accounted for more heat-related hospitalizations than any other age group from 2001 to 2010, and males were hospitalized for heat-related illnesses more than twice as often as females.Â

Heating and Cooling Degree Days.Â Heating and cooling degree days measure the difference between outdoor temperatures and the temperatures that people find comfortable indoors. As the U.S. climate has warmed in recent years, heating degree days have decreased and cooling degree days have increased overall, suggesting that Americans need to use less energy for heating and more energy for air conditioning. This pattern stands out the most in the North and West, while much of the Southeast has experienced the opposite results.

Lyme Disease. Lyme disease is a bacterial illness spread by ticks that bite humans. Tick habitat and populations are influenced by many factors, including climate. Nationwide, the rate of reported cases of Lyme disease has approximately doubled since 1991. The number and distribution of reported cases of Lyme disease have increased in the Northeast and upper Midwest over time, where some states now report 50 to 100 more cases of Lyme disease per 100,000 people than they did in 1991.

West Nile Virus. West Nile virus is spread by mosquitoes, whose habitat and populations are influenced by temperature and water availability. The incidence of West Nile virus neuroinvasive disease in the United States has varied widely from year to year and among geographic regions since tracking began in 2002. Variation in disease incidence is affected by climate and many other factors, and no obvious long-term trend can be detected yet through this limited data set.

Length of Growing Season. The length of the growing season for crops has increased in almost every state. States in the Southwest (e.g., Arizona and California) have seen the most dramatic increase. In contrast, the growing season has actually become shorter in a few southeastern states. The observed changes reflect earlier spring warming as well as later arrival of fall frosts. The length of the growing season has increased more rapidly in the West than in the East.

Ragweed Pollen Season. Warmer temperatures and later fall frosts allow ragweed plants to produce pollen later into the year, potentially prolonging the allergy season for millions of people. The length of ragweed pollen season has increased at 10 out of 11 locations studied in the central United States and Canada since 1995. The change becomes more pronounced from south to north.Â

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Climate Change Indicators

Climate Change Indicators: Stream Temperature

This indicator shows changes in stream water temperature across the Chesapeake Bay region.

Figure 1. Changes in Stream Water Temperatures in the Chesapeake Bay Region, 1960–2014

This map shows the change in water temperature at 129 stream gauges across the Chesapeake Bay region from 1960 to 2014. Red circles show locations where temperatures have increased; blue circles show locations where temperatures have decreased. Filled circles represent sites where the change was statistically significant.

Data source: Jastram and Rice, 20153
Web update: August 2016

Figure 1. Changes in Stream Water Temperatures in the Chesapeake Bay Region, 1960–2014

			Key Points

			Background

About the

			Indicator

			Indicator Notes

			Data Sources

Technical

			Documentation

Key Points

From 1960 through 2014, water temperature increased at 79 percent of the stream sites measured in the Chesapeake Bay region (see Figure 1). More than half of these increases were statistically significant. Only 5 percent of stations had a significant temperature decrease over the same period.

Since 1960, the Chesapeake Bay region has experienced an overall increase in stream water temperature. Temperature has risen by an average of 1.2°F across all sites and 2.2°F at the sites where trends were statistically significant.

Stream temperatures have risen throughout the Chesapeake Bay region. The largest increases have occurred in the southern part of the region.

Background

Water temperature is an important physical property of every river and stream. Many plants, animals, and other organisms living in streams can flourish only in a specific range of water temperatures. Temperature can affect certain aspects of water quality. For example, higher temperatures reduce levels of dissolved oxygen in the water, which can negatively affect the growth and productivity of aquatic life. Persistently warmer temperatures in streams can accelerate natural chemical reactions and release excess nutrients into the water.1 A stream’s water temperature can also influence the circulation or mixing patterns in the water it flows into, like bays and estuaries, potentially affecting nutrient levels and salinity.

			Temperatures can vary naturally along the length of a stream, from cold temperatures near a source of meltwater to higher temperatures near its outlet to the sea. The temperature at any given point is a product of many different factors, including sources of water (for example, melted snow, a recent rainstorm, or groundwater), the amount of water in the stream (streamflow), air temperature, plants along the bank (for example, trees that provide shade), and the amount of development within the watershed. Over time, however, an area’s climate has the strongest natural influence on a stream’s temperature.

The Chesapeake Bay is the largest estuary in the United States, an important habitat for countless aquatic species, and a driver of the regional economy. As rising air temperatures (see the U.S. and Global Temperature indicator) cause stream temperatures to rise, warmer stream water coming into the bay can stress plants and animals and worsen the effects of nutrient pollution that the bay is already facing.2

About the Indicator

The U.S. Geological Survey maintains thousands of stream gauges across the United States. Each gauge measures water levels several times a day, typically every 15 minutes. Field technicians visit each gauging station an average of eight times a year to measure various stream conditions, including water temperature. This indicator shows how average water temperatures throughout the year changed between 1960 and 2014 at 129 stream gauges located across the Chesapeake Bay region. These stations were selected because they had high-quality data for this entire time period.

Indicator Notes

In addition to climate, changes to a stream’s average water temperature can be influenced by other factors such as industrial discharges, changes to local hydrology (such as construction and operation of dams and channels), and changes to land cover in the watershed. This indicator uses measurements from all stream gauges with sufficient data. Thus, it includes some stream sites that are likely more disturbed by human activities than others. A more detailed analysis of this data set found that water temperature tends to increase more quickly than air temperature in agricultural areas without major dams, but more slowly at forested sites and in areas influenced by dams.4 Nonetheless, a comparison of 35 relatively undisturbed reference stations with the remaining 94 stations in this indicator showed no significant difference in trends between the two groups of stations.5

This indicator is based on measurements that are collected an average of eight times per year, but more in some years and less in others. The data were analyzed in a way that accounts for variations in timing and makes it possible to compare average temperatures across many years.

Data Sources

This indicator is based on an analysis developed by the U.S. Geological Survey (USGS).6 Periodic temperature data were collected by the USGS, and these data are stored in the USGS National Water Information System.

Technical Documentation

Download related technical information PDF

References

1 Duan, S.W., and S.S. Kaushal. 2013. Warming increases carbon and nutrient fluxes from sediments in streams across land use. Biogeosciences 10:1193–1207.

2 Duan, S.W., and S.S. Kaushal. 2013. Warming increases carbon and nutrient fluxes from sediments in streams across land use. Biogeosciences 10:1193–1207.

3 Jastram, J.D., and K.C. Rice. 2015. Air- and stream-water-temperature trends in the Chesapeake Bay region, 1960–2014. U.S. Geological Survey Open-File Report 2015-1207. https://pubs.er.usgs.gov/publication/ofr20151207.

4 Rice, K.C., and J.D. Jastram. 2015. Rising air and stream-water temperatures in Chesapeake Bay region, USA. Climatic Change 128(1):127–138.

5 Jastram, J.D., and K.C. Rice. 2015. Air- and stream-water-temperature trends in the Chesapeake Bay region, 1960–2014. U.S. Geological Survey Open-File Report 2015-1207. https://pubs.er.usgs.gov/publication/ofr20151207.

6 Rice, K.C., and J.D. Jastram. 2015. Rising air and stream-water temperatures in Chesapeake Bay region, USA. Climatic Change 128(1):127–138.

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Learn about other indicators in this section

Climate Indicators

Climate Change Indicators

Greenhouse Gases

Weather and Climate

Oceans

Snow and Ice

Health and Society

Ecosystems

Wildfires

Streamflow

Stream Temperature

Tribal Connection: Trends in Stream Temperature in the Snake River

Great Lakes Water Levels and Temperatures

Bird Wintering Ranges

Marine Species Distribution

Leaf and Bloom Dates

Community Connection: Cherry Blossom Bloom Dates in Washington, D.C.

Frequent Questions


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Climate Change Indicators

Climate Change Indicators: Bird Wintering Ranges

This indicator examines changes in the winter ranges of North American birds.

Figure 1. Change in Latitude of Bird Center of Abundance, 1966â€“2013

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This figure shows annual change in latitude of bird center of abundance for 305 widespread bird species in North America from 1966 to 2013. Each winter is represented by the year in which it began (for example, winter 2013â€“2014 is shown as 2013). The shaded band shows the likely range of values, based on the number of measurements collected and the precision of the methods used.

Data source: National Audubon Society, 20144
Web update: May 2014

Figure 2. Change in Distance to Coast of Bird Center of Abundance, 1966â€“2013

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This figure shows annual change in distance to the coast of bird center of abundance for 272 widespread bird species in North America from 1966 to 2013. This figure covers 272 species instead of the 305 species shown in Figure 1 because 33 of the species in Figure 1 need access to salt water, which means they cannot move inland. Each winter is represented by the year in which it began (for example, winter 2013â€“2014 is shown as 2013). The shaded band shows the likely range of values, based on the number of measurements collected and the precision of the methods used.

Data source: National Audubon Society, 20145
Web update: May 2014

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Figure 1. Change in Latitude of Bird Center of Abundance, 1966â€“2013

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Figure 2. Change in Distance to Coast of Bird Center of Abundance, 1966â€“2013

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			Key Points

Â

			Background

About the

			Indicator

Â

			Indicator Notes

Â

			Data Sources

Technical

			Documentation

Key Points

Among 305 widespread North American bird species, the average mid-December to early January center of abundance moved northward by more than 40 miles between 1966 and 2013 (see Figure 1). Trends in the center of abundance moving northward can be closely related to increasing winter temperatures.2

On average, bird species have also moved their wintering grounds farther from the coast since the 1960s (see Figure 2). A shift away from the coasts can also relate to changes in winter temperatures. Inland areas tend to experience more extreme cold than coastal areas, but those extremes are becoming less severe as the climate warms overall.3

Some species have moved farther than others. A total of 48 species have moved northward by more than 200 miles. Of the 305 species studied, 186 (61 percent) have shifted their wintering grounds northward since the 1960s, while 82 (27 percent) have shifted southward. Some others have not moved at all.

Background

Changes in climate can affect ecosystems by influencing animal behavior and ranges. Birds are a particularly good indicator of environmental change for several reasons:

Each species of bird has adapted or evolved to favor certain habitat types, food sources, and temperature ranges. In addition, the timing of certain events in their life cyclesâ€”such as migration and reproductionâ€”is driven by cues from the environment. For example, many North American birds follow a regular seasonal migration pattern, moving north to feed and breed in the summer, then moving south to spend the winter in warmer areas. Changing conditions can influence the distribution of both migratory and non-migratory birds as well as the timing of important life cycle events.1

Birds are easy to identify and count, and thus there is a wealth of scientific knowledge about their distribution and abundance. People have kept detailed records of bird observations for more than a century.

There are many different species of birds living in a variety of habitats, including water birds, coastal birds, and land birds. If a change in behavior or range occurs across a range of bird types, it suggests that a common external factor might be the cause.

Temperature and precipitation patterns are changing across the United States (see the U.S. and Global Temperature indicator and the U.S. and Global Precipitation indicator). Some bird species can adapt to generally warmer temperatures by changing where they liveâ€”for example, by migrating farther north in the summer but not as far south in the winter, or by shifting inland as winter temperature extremes grow less severe. Non-migratory species might shift as well, expanding into newly suitable habitats while moving out of areas that become less suitable. Other types of birds might not adapt to changing conditions and could experience a population decline as a result. Climate change can also alter the timing of events that are based on temperature cues, such as migration and breeding (especially egg-laying).

About the Indicator

This indicator looks collectively at the â€œcenter of abundanceâ€? of hundreds of widespread North American bird species over a 48-year period. The center of abundance is a point on the map that represents the middle of each speciesâ€™ distribution. If a whole population of birds were to shift generally northward, one would see the center of abundance shift northward as well.

For year-to-year consistency, this indicator uses observations from the National Audubon Societyâ€™s Christmas Bird Count, which takes place every year in early winter. The Christmas Bird Count is a long-running citizen-science program in which individuals are organized by the National Audubon Society, Bird Studies Canada, local Audubon chapters, and other bird clubs to identify and count bird species. The data presented in this indicator were collected from more than 2,000 locations throughout the United States and parts of Canada. At each location, skilled observers follow a standard counting procedure to estimate the number of birds within a 15-mile diameter â€œcount circleâ€? over a 24-hour period. Study methods remain generally consistent from year to year. Data produced by the Christmas Bird Count go through several levels of review before Audubon scientists analyze the final data, which have been used to support a wide variety of peer-reviewed studies.

Indicator Notes

Many factors can influence bird ranges, including food availability, habitat alteration, and interactions with other species. As a result, some of the birds included in this indicator might have moved north for reasons other than changing temperatures. This indicator does not show how responses to climate change vary among different types of birds. For example, a more detailed National Audubon Society analysis found large differences among coastal birds, grassland birds, and birds adapted to feeders, which all have different abilities to adapt to temperature changes.6

Some data variations can be caused by differences among count circles, such as inconsistent level of effort by volunteer observers, but these differences are carefully corrected in Audubonâ€™s statistical analysis.

Data Sources

Bird center-of-abundance data were collected by the annual Christmas Bird Count organized by the National Audubon Society and Bird Studies Canada. Recent and historical Christmas Bird Count data are available at: www.audubon.org/conservation/science/christmas-bird-count. The National Audubon Society published a previous version of this analysis in 2009,7Â available at: http://web4.audubon.org/bird/bacc/index.html.

Technical Documentation

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References

1 La Sorte, F.A., and F.R. Thompson III. 2007. Poleward shifts in winter ranges of North American birds. Ecology 88(7):1803â€“1812.

2 National Audubon Society. 2009. Northward shifts in the abundance of North American birds in early winter: A response to warmer winter temperatures? http://web4.audubon.org/bird/bacc/techreport.html.

3 National Audubon Society. 2009. Northward shifts in the abundance of North American birds in early winter: A response to warmer winter temperatures? http://web4.audubon.org/bird/bacc/techreport.html.

4 National Audubon Society. 2014 update to data originally published in: National Audubon Society. 2009. Northward shifts in the abundance of North American birds in early winter: A response to warmer winter temperatures? http://web4.audubon.org/bird/bacc/techreport.html.

5 National Audubon Society. 2014 update to data originally published in: National Audubon Society. 2009. Northward shifts in the abundance of North American birds in early winter: A response to warmer winter temperatures? http://web4.audubon.org/bird/bacc/techreport.html.

6 National Audubon Society. 2009. Northward shifts in the abundance of North American birds in early winter: A response to warmer winter temperatures? http://web4.audubon.org/bird/bacc/techreport.html.

7 National Audubon Society. 2009. Northward shifts in the abundance of North American birds in early winter: A response to warmer winter temperatures? http://web4.audubon.org/bird/bacc/techreport.html.

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