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Climate Change Science

Causes of Climate Change

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Earthâ€™s temperature is a balancing act

The greenhouse effect causes the atmosphere to retain heat

Changes in the sunâ€™s energy affect how much energy reaches Earthâ€™s system

Changes in reflectivity affect how much energy enters Earthâ€™s system

Earth's temperature is a balancing act

Models that account only for the effects of natural processes are not able to explain the warming observed over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming.

Click the image to view a larger version.Earth's temperature depends on the balance betweenÂ energy entering and leaving the planetâ€™s system. When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sunâ€™s energy is reflected back into space, Earth avoids warming. When absorbed energy is released back into space, Earth cools. Many factors, both natural and human, can cause changes in Earthâ€™sÂ energy balance, including:

Variations inÂ the sun's energyÂ reaching Earth

Changes in theÂ reflectivityÂ of Earthâ€™s atmosphere and surface

Changes in theÂ greenhouse effect, which affects the amount of heat retained by Earthâ€™s atmosphere

These factors have caused Earthâ€™s climate to change many times.

Scientists have pieced together a record of Earthâ€™s climate, dating back hundreds of thousands of years (and, in some cases, millions or hundreds of millions of years), by analyzing a number of indirect measures of climate such as ice cores, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in Earthâ€™s orbit around the sun.[2]

This record shows that the climate system varies naturally over a wide range of time scales. In general, climate changes prior to the Industrial Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations.[2]

Recent climate changes, however, cannot be explained by natural causes alone. Research indicates that natural causes do not explain most observed warming, especially warming since the mid-20th century. Rather, it is extremely likely that human activities have been the dominant cause of that warming.[2]

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Radiative Forcing

Radiative forcing is a measure of the influence of a particular factor (e.g. GHGs, aerosols, or land use changes) on the net change in Earthâ€™s energy balance. On average, a positive radiative forcing tends to warm the surface of the planet, while a negative forcing tends to cool the surface.

GHGs have a positive forcing because they absorb energy radiating from Earthâ€™s surface, rather than allowing it to be directly transmitted into space. This warms the atmosphere like a blanket. Aerosols, or small particles, can have a positive or negative radiative forcing, depending on how they absorb and emit heat or reflect light. For example, black carbon aerosols have a positive forcing since they absorb sunlight. Sulfate aerosols have a negative forcing since they reflect sunlight back into space.

NOAAâ€™s Annual GHGÂ Index, which tracks changes in radiative forcing from GHGs over time, shows that such forcing from human-added GHGs has increased 27.5 percent between 1990 and 2009. Increases in CO2Â in the atmosphere are responsible for 80% of the increase. The contribution to radiative forcing by CH4Â and CFCsÂ has been nearly constant or declining, respectively, in recent years.

The greenhouse effect causes the atmosphere to retain heat

When sunlight reaches Earthâ€™s surface, it can either be reflected back into space or absorbed by Earth. Once absorbed, the planet releases some of the energy back into the atmosphere as heat (also called infrared radiation). Greenhouse gasesÂ like water vapor (H2O), carbon dioxide (CO2), andÂ methane (CH4) absorb energy, slowing or preventing the loss of heat to space. In this way, GHGs act like a blanket, making Earth warmer than it would otherwise be. This process is commonly known as the â€œgreenhouse effect.â€?

The role of the greenhouse effect in the past

Over the last several hundred thousand years, CO2Â levels varied in tandem with the glacial cycles. During warm "interglacial" periods, CO2Â levels were higher. During cool "glacial" periods, CO2Â levels were lower.[2]Â The heating or cooling of Earthâ€™s surface and oceans can cause changes in the natural sources and sinks of these gases, and thus change greenhouse gas concentrations in the atmosphere.[2]Â These changing concentrations are thought to have acted as a positiveÂ feedback, amplifying the temperature changes caused by long-term shifts in Earthâ€™s orbit.[2]

Estimates of the Earth’s changing CO2 concentration (top) and Antarctic temperature (bottom), based on analysis of ice core data extending back 800,000 years. Until the past century, natural factors caused atmospheric CO2 concentrations to vary within a range of about 180 to 300 parts per million by volume (ppmv). Warmer periods coincide with periods of relatively high CO2 concentrations. Note: The past century’s temperature changes and rapid CO2 rise (to 400 ppmv in 2015) are not shown here. Increases over the past half century are shown in the Recent Role section.

Click the image to view a larger version.

Source: Based on data appearing in NRC (2010).

Feedbacks Can Amplify or Reduce Changes

Climate feedbacks amplify or reduce direct warming and cooling effects. They do not change the planetâ€™s temperature directly. Feedbacks that amplify changes are called positive feedbacks. Feedbacks that counteract changes are called negative feedbacks. Feedbacks are associated with changes in surface reflectivity, clouds, water vapor, and the carbon cycle.

Water vapor appears to cause the most important positive feedback. As Earth warms, the rate of evaporation and the ability of air to hold water vapor both rise, increasing the amount of water vapor in the air. Because water vapor is a greenhouse gas, this leads to further warming.

The melting of Arctic sea ice is another example of a positive climate feedback. As temperatures rise, sea ice retreats. The loss of ice exposes the underlying sea surface, which is darker and absorbs more sunlight than ice, increasing the total amount of warming.

Some types of clouds cause a negative feedback. Warming temperatures can increase the amount or reflectivity of these clouds, reflecting more sunlight back into space, cooling the surface of the planet. Other types of clouds, however, contribute a positive feedback.

There are also several positive feedbacks that increaseÂ GHG concentrations. For example, as temperatures warm:

Natural processes that are affected by warming, such as permafrost thawing, tend to release more CO2.

The ocean releases CO2 into the atmosphere and absorbs atmospheric CO2 at a slower rate.

Several types of land surfaces may release more methane (CH4).

These changes lead to higher concentrations of atmospheric GHGs and contribute to increased warming.

This graph shows the increase in greenhouse gas (GHG) concentrations in the atmosphere over the last 2,000 years. Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air.

Click the image to view a larger version.

Source: U.S. National Climate Assessment (2014).

The recent role of the greenhouse effect

Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO2Â and other heat-trapping gases to the atmosphere. These greenhouse gas emissions have increased the greenhouse effect and caused Earthâ€™s surface temperature to rise. The primary human activity affecting the amount and rate of climate change is greenhouse gas emissions from the burning of fossil fuels.

The main greenhouse gases

The most important GHGs directly emitted by humans include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several others. The sources and recent trends of these gases are detailed below.

Carbon dioxideÂ

Carbon dioxide is the primary greenhouse gas that is contributing to recent climate change. CO2Â is absorbed and emitted naturally as part of the carbon cycle, through plant and animal respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of CO2, causing concentrations in the atmosphere to rise.

Atmospheric CO2Â concentrations have increased by more than 40% since pre-industrial times, from approximately 280 parts per million by volume (ppmv) in the 18th century to over 400 ppmv in 2015. The monthly average concentration at Mauna Loa now exceeds 400 ppmv for the first time in human history. The current CO2Â level is higher than it has been in at least 800,000 years.[2]

Some volcanic eruptions released large quantities of CO2Â in the distant past. However, theÂ U.S. Geological Survey (USGS)Â reports that human activities now emit more than 135 times as much CO2Â as volcanoes each year.

Human activities currently release over 30 billion tons of CO2Â into the atmosphere every year.[2]Â The resultant build-up of CO2Â in the atmosphere is like a tub filling with water, where more water flows from the faucet than the drain can take away.

Atmospheric carbon dioxide concentration has risen from pre-industrial levels of 280 parts per million by volume (ppmv) to over 401 ppmv in 2016. Since 1959 alone (shown here), concentrations have risen by more than 85 ppmv. The yearly rise and fall in the chart reflects the growth and decay or northern hemisphere vegetation.

Click the image to view a larger version.

Source: NOAA

If the amount of water flowing into a bathrub is greater than the amount of water leaving through the drain, the water level will rise. CO2 emissions are like the flow of water into the world's carbon bathtub. "Sources" of CO2 emissions such as fossil fuel burning, cement manufacture, and land use are like the bathtub's faucet. "Sinks" of CO2 in the ocean and on land (such as plants) that take up CO2 are like the drain. Today, human activities have turned up the flow from the CO2 "faucet," which is much larger than the "drain" can cope with, and the level of CO2 in the atmosphere (like the level of water in a bathtub) is rising.

For more information on the human and natural sources and sinks of CO2Â emissions, and actions that can reduce emissions, see theÂ Carbon Dioxide pageÂ in the Greenhouse Gas EmissionsÂ website.

MethaneÂ

Methane is produced through both natural and human activities. For example, natural wetlands, agricultural activities, and fossil fuel extraction and transport all emit CH4.

Methane is more abundant in Earthâ€™s atmosphere now than at any time in at least the past 800,000 years.[2]Â Due to human activities, CH4Â concentrations increased sharply during most of the 20th century and are now more than two-and-a-half times pre-industrial levels. In recent decades, the rate of increase has slowed considerably.[2]

For more information on CH4Â emissions and sources, and actions that can reduce emissions, see EPAâ€™sÂ Methane pageÂ in the Greenhouse Gas Emissions website. For information on how methane is impacting the Arctic, see theÂ EPA reportÂ Methane and Black Carbon Impacts on the Arctic.

Nitrous oxideÂ

Nitrous oxide is produced through natural and human activities, mainly through agricultural activities and natural biological processes. Fuel burning and some other processes also create N2O. Concentrations of N2O have risen approximately 20% since the start of the Industrial Revolution, with a relatively rapid increase toward the end of the 20th century.[2]

Overall, N2O concentrations have increased more rapidly during the past century than at any time in the past 22,000 years.[2]Â For more information on N2O emissions and sources, and actions that can reduce emissions, see EPAâ€™sÂ Nitrous Oxide pageÂ in the Greenhouse GasÂ Emissions website.

Other greenhouse gases

Water vapor is the most abundant greenhouse gas and also the most important in terms of its contribution to the natural greenhouse effect, despite having a short atmospheric lifetime. Some human activities can influence local water vapor levels. However, on a global scale, the concentration of water vapor is controlled by temperature, which influences overall rates of evaporation and precipitation.[2]Â Therefore, the global concentration of water vapor is not substantially affected by direct human emissions.

TroposphericÂ ozone (O3), which also has a short atmospheric lifetime, is a potent greenhouse gas. Chemical reactions create ozone from emissions of nitrogen oxides and volatile organic compounds from automobiles, power plants, and other industrial and commercial sources in the presence of sunlight. In addition to trapping heat, ground-level ozone is a pollutant that can cause respiratory health problems and damage crops and ecosystems.

Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), together calledÂ F-gases, are often used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. Unlike water vapor and ozone, these F-gases have a long atmospheric lifetime, and some of these emissions will affect the climate for many decades or centuries.

For more information on greenhouse gas emissions, see theÂ Greenhouse Gas EmissionsÂ website, including an expanded discussion of global warming potentials and how they are used to measure the relative strengths of greenhouse gases. To learn more about actions that can reduce these emissions, see What You Can Do.

Other climate forcers

Particles and aerosols in the atmosphere can also affect climate. Human activities such as burning fossil fuels and biomass contribute to emissions of these substances, although some aerosols also come from natural sources such as volcanoes and marine plankton.

Black carbon (BC)Â is a solid particle or aerosol, not a gas, but it also contributes to warming of the atmosphere. Unlike GHGs, BC can directly absorb incoming and reflected sunlight in addition to absorbing infrared radiation. BC can also be deposited on snow and ice, darkening the surface and thereby increasing the snow's absorption of sunlight and accelerating melt.Â For information on how BCÂ is impacting the Arctic, see EPA assessmentÂ Methane and Black Carbon Impacts on the Arctic.

Sulfates, organic carbon, and other aerosols can cause cooling by reflecting sunlight.

Warming and cooling aerosols can interact with clouds, changing a number of cloud attributes such as their formation, dissipation, reflectivity, and precipitation rates. Clouds can contribute both to cooling, by reflecting sunlight, and warming, by trapping outgoing heat.

For more information on greenhouse gas emissions, see theÂ Greenhouse Gas EmissionsÂ website. To learn more about actions that can reduce these emissions, seeÂ What EPA is DoingÂ andÂ What You Can Do.

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Changes in the sunâ€™s energy affect how much energy reaches Earthâ€™s system

The sun’s energy received at the top of Earth’s atmosphere has been measured by satellites since 1978. It has followed its natural 11-year cycle of small ups and downs, but with no net increase (bottom). Over the same period, global temperature has risen markedly (top).

Click the image to view a larger version.

Source: USGCRP (2009).Climate is influenced by natural changes that affect how much solar energy reaches Earth. These changes include changes within the sun and changes in Earthâ€™s orbit.

Changes occurring in the sun itself can affect the intensity of the sunlight that reaches Earthâ€™s surface. The intensity of the sunlight can cause either warming (during periods of stronger solar intensity) or cooling (during periods of weaker solar intensity). The sun follows a natural 11-year cycle of small ups and downs in intensity, but the effect on Earthâ€™s climate is small.[1]

Changes in the shape of Earthâ€™s orbit as well as the tilt and position of Earthâ€™s axis can also affect the amount of sunlight reaching Earthâ€™s surface.[1][2]

The role of the sunâ€™s energy in the past

Changes in the sunâ€™s intensity have influenced Earthâ€™s climate in the past. For example, the so-called â€œLittle Ice Ageâ€? between the 17th and 19th centuries may have been partially caused by a low solar activity phase from 1645 to 1715, which coincided with cooler temperatures. The â€œLittle Ice Ageâ€? refers to a slight cooling of North America, Europe, and probably other areas around the globe.[2]

Changes in Earthâ€™s orbit have had a big impact on climate over tens to hundreds of thousands of years. In fact, the amount of summer sunshine on the Northern Hemisphere, which is affected by changes in the planetâ€™s orbit, appears to drive the advance and retreat of ice sheets. These changes appear to be the primary cause of pastÂ cyclesÂ of ice ages, in which Earth has experienced long periods of cold temperatures (ice ages), as well as shorter interglacial periods (periods between ice ages) of relatively warmer temperatures.[1][2]Â

Rates of Climate Change Have Varied Over Time

Click to learnÂ aboutÂ how rates of climate change have varied over time.

The recent role of the sunâ€™s energy

Changes in solar energy continue to affect climate. However, over the last 11-year solar cycle, solar output has been lower than it has been since the mid-20thÂ century, and therefore does not explain the recent warming of the earth.[2]Â Similarly, changes in the shape of Earthâ€™s orbit as well as the tilt and position of Earthâ€™s axis affect temperature on very long timescales (tens to hundreds of thousands of years), and therefore cannot explain the recent warming.

Changes in reflectivity affect how much energy enters Earthâ€™s system

When sunlight reaches Earth, it can be reflected or absorbed. The amount that is reflected or absorbed depends on Earthâ€™s surface and atmosphere. Light-colored objects and surfaces, like snow and clouds, tend to reflect most sunlight, while darker objects and surfaces, like the ocean, forests, or soil, tend to absorb more sunlight.

The term albedo refers to the amount of solar radiation reflected from an object or surface, often expressed as a percentage. Earth as a whole has an albedo of about 30%, meaning that 70% of the sunlight that reaches the planet is absorbed.[3]Â Absorbed sunlight warms Earthâ€™s land, water, and atmosphere.

Reflectivity is also affected by aerosols. Aerosols are small particles or liquid droplets in the atmosphere that can absorb or reflect sunlight. Unlike greenhouse gases, the climate effects of aerosols vary depending on what they are made of and where they are emitted. Those aerosols that reflect sunlight, such as particles from volcanic eruptions or sulfur emissions from burning coal, have a cooling effect. Those that absorb sunlight, such as black carbon (a part of soot), have a warming effect.

The role of reflectivity in the past

Natural changes in reflectivity, like the melting of sea ice, have contributed to climate change in the past, often acting as feedbacksÂ to other processes.

Volcanoes have played a noticeable role in climate. Volcanic particles that reach the upper atmosphere can reflect enough sunlight back to space to cool the surface of the planet by a few tenths of a degree for several years.[2]Â These particles are an example of cooling aerosols. Volcanic particles from a single eruption do not produce long-term change because they remain in the atmosphere for a much shorter time than GHGs.[2]

The recent role of reflectivity

Human changes in land use and land cover have changed Earthâ€™s reflectivity. Processes such as deforestation, reforestation, desertification, and urbanization often contribute to changes in climate in the places they occur. These effects may be significant regionally, but are smaller when averaged over the entire globe.

In addition, human activities have generally increased the number of aerosol particles in the atmosphere. Overall, human-generated aerosols have a net cooling effect offsetting about one-third of the total warming effect associated with human greenhouse gas emissions. Reductions in overall aerosol emissions can therefore lead to more warming. However, targeted reductions in black carbon emissions can reduce warming.[1]

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References:

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

[2]Â IPCC (2013).Â Climate Change 2013: The Physical Science Basis.Â Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate ChangeÂ [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]Â NRC (2010).Â Advancing the Science of Climate ChangesÂ . National Research Council. The National Academies Press, Washington, DC, USA.

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Rates of climate changeÂ have varied over time

Studies of Earthâ€™s past climate suggest periods of relative stability as well as periods of rapid change.

Periods of relative stabilityÂ

			Interglacial climate periods such as the present tend to be more stable than cooler, glacial climates. For example, Earthâ€™s climate during the current interglacial period is more stable than the most recent glacial period. The glacial period was characterized by widespread, large, and abrupt climate changes. In contrast, the previous interglacial period was similarly stable.[1]

This image shows a glacier calving, when a mass of ice suddenly releases and breaks away. Source: USDA

Periods of abrupt climate changeÂ

			Abrupt climate change refers to sudden (on the order of decades), large changes in some major component of the climate system, with rapid, widespread effects. Abrupt or rapid climate changes tend to frequently accompany transitions between glacial and interglacial periods (and vice versa).[2]Â For example, a significant part of the Northern Hemisphere, particularly around Greenland, may have experienced very rapid warming of 14Â°F-28Â°F over several decades during and after the most recent ice age.[2]

Abrupt climate changes occur when a threshold or â€˜tipping pointâ€™ in the climate system is crossed, causing large changes or impacts to the climate. Scientific data show that abrupt changes in climate at the regional scale have occurred throughout history and are characteristic of Earthâ€™s climate system. Warming from greenhouse gas emissions, as well as other human changes to the Earth system may increase the possibility of large and abrupt regional or global climatic events.

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Climate Change Science

Understanding the Link Between Climate Change and Extreme Weather

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Changes in Extreme Weather and Climate Events

Trends in Specific Extreme Weather Events

Adaptation: Reducing the Threat of Climate Change and Preparing for Impacts

Changes in Extreme Weather and Climate Events

What is extreme weather?

Extreme weather and climate events are events that:

typically don't happen very frequently, such as droughts or floods that have historically occurred on average only once in 100 years.

vary from "the norm" in severity or duration, like heat waves.

have severe impacts, like hurricanes.

Scientists study many aspects of change in extreme weather and climate events. These include:

Frequency: Are events occurring more often than they did in the past?

Intensity: Are events getting more severe, with the potential for more damaging effects?

Duration: Are events lasting longer than "the norm"?

Timing: Are events occurring earlier or later in the season or the year than they used to?

Extreme weather is typically rare. But climate change is increasing the odds of more extreme weather events taking place.

Climate Change and Extreme Weather

 Watch the Extreme Weather Bell Curve Animation(1 pg, 342 K)
 to learn how this shift is happening. (requires Flash)

Establishing the most likely causes behind an extreme weather event can be challenging, since these events are due to combinations of multiple factors, including natural variability. Nevertheless, scientists have been able to draw a connection between some types of extreme climate patterns—an even some individual events—and climate change. A good way to think about this connection is to focus on whether an extreme weather event was made more likely by climate change.

Key Points

Extreme weather events are becoming more frequent and/or severe around the world. This is consistent with what we expect with a warming planet.

Increasingly frequent and/or severe weather events have serious consequences for society and ecosystems.

Between 2011 and 2013, the United States experienced 32 weather events that each caused at least one billion dollars in damages.

Changes in some weather events are more closely linked to climate change than others.

Understanding the links between climate change and extreme events can help us plan for the future.

Related Links

EPA: Extreme Heat: What You Can Do to Prepare

EPA: Natural Disasters

Online course: Extreme Weather 101 Exit

Blog: Heat Waves

Blog: When It Rains, It Pours: Extreme Precipitation and Drought

Blog: Resiliency in the Face of Stronger Storms

There have been changes in some types of extreme weather events in the United States over the last several decades, including more intense and frequent heat waves, less frequent and intense cold waves, and regional changes in floods, droughts, and wildfires.[1] This rise in extreme weather events fits a pattern you can expect with a warming planet. Scientists project that climate change will make some of these extreme weather events more likely to occur and/or more likely to be severe.

Trends in Specific Extreme Weather Events

Learn more by clicking on each tab below:

Heatwaves

Droughts

Wildfires

Extreme Rainfall

Winter Precipitation

Hurricanes

Tornadoes

Floods

Heat Waves

Why does it matter? Heat waves can have serious health consequences, particularly for older adults, young children, the poor, and people with certain pre-existing health conditions, like asthma or heart disease.[1] Excessive heat can also kill or injure crops and livestock, and it can lead to power outages as heavy demand for air conditioning strains the power grid.

How does it relate to climate change? Even a small rise in average temperature brought on by climate change can boost the odds of extreme heat and heat waves.

What's happening? Climate change has increased the likelihood of more frequent and more severe heat waves. Heat waves have generally become more frequent and intense across the United States in recent decades, particularly in the western United States (including Alaska).[1] The impacts of heat waves are greatest in the Northeast and Midwest,[2] and in urban areas, where the urban heat island effect increases vulnerability to heat-related health impacts.

What's ahead? Heat waves are expected to become more frequent, longer, and more intense in the years ahead.[2] The number of extremely hot days is projected increase throughout the United States.[1]

How sure is the science? Scientists are highly confident[2] that heat waves and other extreme heat events have and will continue to become more frequent and intense due to climate change.

Droughts

Why does it matter? Droughts can mean crop and livestock failures for farmers, which in turn can cause higher food prices and possibly even food shortages. Droughts can also stress water supplies and contribute to wildfires.

How does it relate to climate change? As temperatures rise because of climate change, more water evaporates from land and water bodies. Along with changes in precipitation patterns, this can contribute to unusual dryness in some areas.

What's happening? In recent decades, some regions have experienced more intense and longer droughts, while other regions have seen less frequent, less intense, or shorter droughts.[3] There has been no general trend in the overall extent of drought across the contiguous United States since 1900.[1] However, large portions of the Southwest have experienced drought conditions since weekly Drought Monitor records began in 2000.[4] Droughts are expected to be a normal condition for the southern and central United States in the next century.[1]

What's ahead?  Summer droughts are expected to intensify almost everywhere in the continental United States due to longer periods of dry weather and more extreme heat. A number of studies project that widespread drought will become more common over much of the southern and central United States, with amplified drought severity. A reduction in soil moisture, which exacerbates heat waves, is projected for much of the western and central United States.[1]

How sure is the science? Scientists are highly confident[1] that droughts have become more frequent and intense in some regions of the United States, and that climate change increases the likelihood of these severe droughts in the future, particularly in the Southwest. Factors such as changes in a region's land use or high draws on water supplies can also play a role, especially at the local level.

Wildfires

Why does it matter? Wildfires and their associated air pollution can cause deaths, injuries, and eye, respiratory, and cardiovascular illnesses. Large wildfires can disrupt or displace communities and have significant economic costs related to fire suppression, property damage, and losses in recreation, tourism, forestry, and related industries, They can also threaten wildlife and degrade local ecosystems.

How does it relate to climate change? Climate change can contribute to dry conditions through higher temperatures, increased rates of evaporation, and earlier spring snowmelt (resulting in longer dry seasons), which all increase the risk of wildfires.

What's happening? In recent decades, the extent of wildfires appears to have increased, particularly in the western United States.[1][4] Increased warming, drought, and insect outbreaks, all caused by or linked to climate change, have increased wildfires and impacts to people and ecosystems in the Southwest.[1]

What's ahead? There is very high confidence[1] that under projected climate change, forests and communities in the western United States will be increasingly affected by more frequent, larger, and more intense wildfires.

How sure is the science? Scientists are highly confident[1] that wildfires have increased in size in the western United States and very highly confident[1] that dry conditions associated with climate change increase the likelihood of large wildfires in the future. Other factors, such as land management, land use changes, and impacts of pests, can also play a role in forest vulnerability.

Extreme Rainfall

Why does it matter? Extreme rainfall events can damage crops, erode soil, and increase flooding. In addition, runoff from precipitation can degrade or contaminate water quality as pollutants deposited on land wash into water bodies used by people for drinking, irrigation, and other activities.

How does it relate to climate change? Warmer temperatures cause more water to evaporate into the air. This moisture-laden air can produce more intense precipitation in the form of extreme rainfall events and storms.

What's happening? In recent years, a larger percentage of precipitation has come in the form of intense single-day events.[4] The amount of rain falling on the heaviest rain days has also increased over the past few decades.[1]

What's ahead? It is likely that the frequency and intensity of extreme precipitation events will increase over most of the United States and many other areas of the globe.[5] A trend towards increase heavy precipitation will continue to occur, even in areas where total precipitation is projected to decrease.[1]

How sure is the science? Scientists are highly confident[1] that across most of the United States, the heaviest rainfall events have become more intense and frequent, especially in the Midwest and Northeast, and that the frequency and intensity of extreme rainfall events will further increase in the future for most areas in the United States. Other factors, such as weather systems and ocean cycles like El Niño, can also play a role.

Extreme Winter Precipitation

Why does it matter? Heavy winter storms can disrupt transportation, the flow of goods, and emergency and medical services. A buildup of snow can collapse roofs, knock down trees and power lines, and cause flooding when it melts.

How does it relate to climate change? Warmer temperatures cause more water to evaporate into the air. If the temperature is still below freezing, this moisture-laden air can produce more intense precipitation in the form of unusually heavy snow, sleet, and freezing rain events, even in years when total snowfall is lower than average.

What's happening? Winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States.[1]

What's ahead? It is likely that the frequency and intensity of extreme winter precipitation events will increase in some areas of the United States, particularly in the northern states.[1]

How sure is the science? Scientists have medium confidence[1] that winter storms have increased slightly in frequency and intensity.

Hurricanes

Why does it matter? Powerful hurricanes that make landfall can cause significant numbers of deaths and injuries, and disrupt or displace communities. When combined with coastal waters made higher by sea level rise, strong winds can create damaging storm surges. Hurricanes are also among the most costly extreme weather events, with severe storms causing billions of dollars in economic losses.[6][7] Even hurricanes that don't make landfall can influence waves, currents, and storm tides, which can result in property damage, flooding, and coastal erosion.

How does it relate to climate change? Increases in hurricane activity are linked to warming ocean temperatures because hurricanes draw more energy from warmer water. But the link between ocean temperature and hurricanes is complex, and other factors can also play a role in the formation and intensity of these storms.[8]

What's happening? There has been an increase in the intensity, frequency, and duration of hurricanes and in the number of strong (Category 4 and 5) hurricanes in the North Atlantic Ocean since the early 1980s.[1] However, changes in observation methods over time make it difficult to know for certain whether tropical storm activity has shown a long-term increase.[8]

What's ahead? The intensity of the strongest hurricanes is projected to continue to increase as the oceans continue to warm, bringing stronger winds and heavier rains. Whether changes in hurricane frequency or landfall will occur is less certain.[1]

How sure is the science? Scientists have medium confidence[1] that hurricane intensity and associated heavy rainfall will continue to increase under a changing climate, but significant uncertainties remain.

Tornadoes

Why does it matter? Strong tornadoes can cause deaths and injuries, disrupt or displace communities, and inflict severe damage to crops, trees, buildings, and infrastructure.

How does it relate to climate change? A warming climate can lead to stronger and more frequent thunderstorms, and these storms can spur tornadoes.[8] But scientists do not yet fully understand all of the ways in which tornadoes may be linked to climate change.

What's happening? Although the number of tornado reports has increased with better observation practices, there has been little change in the frequency of the strongest tornadoes over the past 55 years in the United States.[9]

What's ahead? With a warming climate, the thunderstorms and weather conditions that give rise to tornadoes could increase in some areas. But challenges in observing and modeling these events result in many uncertainties about how the frequency and intensity of tornadoes will change.[1][10]

How sure is the science? Scientists have low confidence[1] in projections of trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds. This is in part due to a lack of long-term and the fact that such small and often remote storms are difficult to monitor and model.

Floods

Why does it matter? Flooding can cause disease, deaths, and injuries; damage property and critical infrastructure such as sewer systems and wastewater treatment facilities; and disrupt or displace communities.

How does it relate to climate change? Heavy rainfall events, more intense storms, and changes in the timing of snowmelt can lead to more frequent or intense flooding in some areas.[1]

What's happening? Many regions of the United States are experiencing significant changes in the magnitude of river flooding. When averaged over the entire nation, however, the increases and decreases cancel each other out and show no national level trend. For instance, there has been an increase in flooding events in the Midwest, Great Plains, and Northeast in the last several decades, where the largest increases in heavy rain amounts have occurred. But flooding has decreased in the Southwest.[1]

What's ahead? Heavy rainfall events and more intense storms in some regions could lead to more frequent or intense flooding in many United States regions, even in areas where total precipitation is projected to decline.[1] There is medium confidence[10] that projected increases in heavy rainfall would contribute to increases in local flooding in some regions.

How sure is the science? The link between floods and climate change is a relatively new area of research, and many other factors, like land use and management practices, can trigger these events or influence how damaging they become. Scientists have high confidence[1] that there have been regional trends in floods. However, scientists have low confidence[10][1] in projections of future changes in flood frequency and intensity, because the causes of regional changes are complex.

Adaptation: Reducing the Threat of Climate Change and Preparing for Impacts

Extreme weather and climate events pose a serious threat to the health and welfare of American families and businesses. For instance, between 2011 and 2013, the United States experienced 32 weather events that each caused at least one billion dollars in damages.[7] 2012 ranks as 2nd costliest year on record, with more than $110 billion in damages.

This map summarizes the number of times each state has been affected by weather and climate events over the past 30 years that have resulted in more than a billion dollars in damages. The Southeast has been affected by more billion-dollar disasters than any other region. The primary disaster type for coastal states such as Florida is hurricanes, while interior and northern states in the region also experience sizeable numbers of tornadoes and winter storms. For a list of events and the affected states,

see: http://www.ncdc.noaa.gov/billions/events

Source: USGCRP (2014) Billion Dollar Weather/Climate Disasters.

EPA is taking a number of common-sense actions to reduce greenhouse gas emissions and help cities and towns build more resilient communities to prepare for the impacts of a changing climate, including the weather extremes described above.

For more information about climate adaptation and things you can do to prepare for changes in extreme weather events, see Climate Change Adaptation.

For more information on how you can reduce greenhouse gas emissions at home, on the road, and in your workplace, see What You Can Do.

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References

[1] USGCRP (2014). Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds. Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp.

[2] US EPA (2006). Excessive Heat Events Guidebook.

[3] CCSP (2008). Weather and climate extremes in a changing climate. Synthesis and Assessment Product 3.3.

[4] US EPA (2014). Climate Change Indicators in the United States.

[5] AMS (2013). Kunkel, K.E., T.R. Karl, H. Brooks, J. Kossin, J. Lawrimore, D. Arndt, L. Bosart, D. Changnon, S.L. Cutter, N. Doesken, K. Emanuel, P.Ya. Groisman, R.W. Katz, T. Knutson, J. O'Brien, C. J. Paciorek, T. Peterson, K. Redmond, D. Robinson, J. Trapp, R. Vose, S. Weaver, M. Wehner, K. Wolter, and D. Wuebbles. 2013. Monitoring and understanding changes in extreme storm statistics: State of knowledge. Bulletin of the American Meteorological Society 94:499-514.

[6] NOAA (2013). Smith, A.B., and R.W. Katz. 2013. U.S. billion-dollar weather and climate disasters: Data sources, trends, accuracy and biases. Natural Hazards 67:387-410.

[7] NOAA (2013). Billion-Dollar Weather and Climate Disasters. National Oceanic and Atmospheric Administration, National Climatic Data Center.

[8] IPCC (2013). Climate Change 2013: The Physical Science Basis. 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, 1535 p.

[9] NOAA (2013). Historical records and trends. National Oceanic and Atmospheric Administration, National Climatic Data Center.

[10] IPCC (2012). Managing the risks of extreme events and disasters to advance climate change adaptation. Cambridge, UK: Cambridge University Press.

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Methane and Black Carbon Impacts on the Arctic: Communicating the Science

This document describes the scientific understanding of the impacts of methane and black carbon emissions on the Arctic climate. This information is provided to help facilitate communication of the evolving science, evaluate mitigation strategies, and inform decision-making.

      Methane and Black Carbon Impacts on the Arctic: Communicating the Science (PDF)(19 pp, 7 MB)

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Overview of Climate Change Science

On This Page:

Overview

Earth's climate is changing

Natural causes alone cannot explain recent changes

Human causes can explain these changes

The extent of future climate change depends on us

Climate change impacts our health, environment, and economy

Overview

Earth's climate is changing. Multiple lines of evidence show changes in our weather, oceans, ecosystems, and more.

Natural causes alone cannot explain all of these changes. Human activities are contributing to climate change, primarily by releasing billions of tons of carbon dioxide (CO2) and other heat-trapping gases, known as greenhouse gases, into the atmosphere every year.[1]

Climate changes will continue into the future. The more greenhouse gases we emit, the larger future climate changes will be.

Human activities, like driving, manufacturing, electricity generation, and the clearing of forests contribute to greenhouse gas emissions and warm the planet.

Changes in the climate system affect our health, environment, and economy. We can prepare for some of the impacts of climate change to reduce their effects on our well-being.

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Earth's climate is changing

Related Links

USGCRP: National Climate Assessment, 2014

USGCRP: Global Climate Change Impacts in the United States

NRC: America's Climate Choices Reports Exit

IPCC: Fifth Assessment Report Exit

IPCC: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation Exit

The global average temperature has increased by more than 1.5°F since the late 1800s.[2] Some regions of the world have warmed by more than twice this amount. The buildup of greenhouse gases in our atmosphere and the warming of the planet are responsible for other changes, such as:

Changing temperature and precipitation patterns [1][2]

Increases in ocean temperatures, sea level, and acidity

Melting of glaciers and sea ice [1]

Changes in the frequency, intensity, and duration of extreme weather events

Shifts in ecosystem characteristics, like the length of the growing season, timing of flower blooms, and migration of birds

Increasing effects on human health and well-being

Learn more about the indicators of climate change.

These maps show temperatures across the world in the 1880s (left) and the 1980s (right). Blue colors represent cooler temperatures compared with the average temperatures from 1951 to 1980, and red colors represent warmer temperatures compared to this average. The map on the left shows that it was colder in the 1880s in most places. The map on the right shows it was warmer in the 1980s in most places. Earth's average surface temperature has increased by more than 1.5°F since the 1880s. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.3°F – 0.4°F per decade.

Click the image to view a larger version. Source: NASA.

Climate Change v. Global Warming

Click on the image for a pop-up on the difference between climate change and global warming.

Weather v. Climate

Click on the image to open a pop-up that explains the differences between weather and climate.

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Natural causes alone cannot explain recent changes

Natural processes such as changes in the sun's energy, shifts in ocean currents, and others affect Earth's climate. However, they do not explain the warming that we have observed over the last half-century.[1]

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Human causes can explain these changes

Models that account only for the effects of natural processes are not able to explain the warming observed over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming.

	Source: U.S. National Climate Assessment (2014).

Click the image to view a larger version.Most of the warming of the past half century has been caused by human emissions of greenhouse gases.[1] Greenhouse gases come from a variety of human activities, including: burning fossil fuels for heat and energy, clearing forests, fertilizing crops, storing waste in landfills, raising livestock, and producing some kinds of industrial products.

Greenhouse gas emissions are not the only way that people can change the climate. Activities such as agriculture or road construction can change the reflectivity of Earth's surface, leading to local warming or cooling. This effect is observed in urban centers, which are often warmer than surrounding, less populated areas. Emissions of small particles, known as aerosols, into the air can also lead to reflection or absorption of the sun's energy.

Learn more about past and present climate trends and their causes.

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This graph shows the increase in greenhouse gas (GHG) concentrations in the atmosphere over the last 2,000 years. Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air.

	Source: USGCRP 2009.

Click the image to view a larger version.

The extent of future climate change depends on us

The extent of the change will depend on how much, and how quickly, we can reduce greenhouse gas emissions. During the 21st century, global warming is projected to continue and climate changes are likely to intensify. Scientists have used climate models to project different aspects of future climate, including temperature, precipitation, snow and ice, ocean level, and ocean acidity. Depending on future emissions of greenhouse gases and how the climate responds, average global temperatures are projected to increase worldwide by 0.5°F to 8.6°F by 2100, with a likely increase of at least 2.0°F for all scenarios except the one representing the most aggressive mitigation of greenhouse gas emissions.[2] Learn more about the projections of future climate change.

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Climate change impacts our health, environment, and economy

Climate change affects our environment and natural resources, and impacts our way of life in many ways. For example:

Warmer temperatures increase the frequency, intensity, and duration of heat waves, which can pose health risks, particularly for young children and the elderly.

Climate change can also impact human health by worsening air and water quality, increasing the spread of certain diseases, and altering the frequency or intensity of extreme weather events.

Rising sea levels threaten coastal communities and ecosystems.

Changes in the patterns and amount of rainfall, as well as changes in the timing and amount of stream flow, can affect water supplies and water quality and the production of hydroelectricity.

Changing ecosystems influence geographic ranges of many plant and animal species and the timing of their lifecycle events, such as migration and reproduction.

Increases in the frequency and intensity of extreme weather events, such as heat waves, droughts, and floods, can increase losses to property, cause costly disruptions to society, and reduce the availability and affordability of insurance.

We can prepare for some of the likely climate change impacts to reduce their effect on ecosystems and human well-being. Making such preparations is known as adaptation. Examples of adaptation include strengthening water conservation programs, upgrading stormwater systems, developing early warning systems for extreme heat events, and preparing for stronger storms through better emergency preparation and response strategies.

Learn more about how climate change impacts are expected to affect different U.S. regions and sectors and how we can prepare.

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References

1. USGCRP (2014). Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds. 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. ExitContribution of Working Groups I, II and III to the Fourth 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.

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Climate Change vs. Global Warming

The term climate change is sometimes used interchangeably with the term global warming. However, the terms do not refer entirely to the same thing.

Global warming refers to the recent and ongoing rise in global average temperature near Earth's surface. It is caused mostly by increasing concentrations of greenhouse gases in the atmosphere. Global warming is causing climate patterns to change. However, global warming itself represents only one aspect of climate change.

Climate change refers to any significant change in the measures of climate lasting for an extended period of time. In other words, climate change includes major changes in temperature, precipitation, or wind patterns, among others, that occur over several decades or longer. Climate change can occur at the global, continental, regional, and local levels. Climate change may refer to natural changes in climate, or changes caused by human activities.

Weather vs. Climate

The difference between weather and climate is primarily a matter of time and geography. Weather refers to the conditions of the atmosphere over a short period of time, such as hours or days, and typically for a local area. Climate refers to the behavior of the atmosphere over a longer period of time, and usually for a large area.

Familiar examples of weather characteristics include the daily temperature, humidity, or the amount of precipitation produced by a storm. Weather also includes severe weather conditions such as hurricanes, tornadoes, and blizzards. Because of the dynamic nature of the atmosphere, it is not possible to predict weather conditions in a specific location months or years in advance.

Climate is typically defined based on 30-year averages of weather. Climate represents our expectations for the weather. For example, climate tells us how warm we expect a typical summer to be, how much rainfall would correspond to a wetter-than-average spring, or how frequently we expect a snowy winter to occur. Scientists can compare recent and long-term observations of the climate to detect the influence of greenhouse gases on climate conditions.

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