Wednesday, 25 January 2017

EPA climate change webpages 1


With climate change data under threat from the Trump administration, I am backing up some of the useful EPA climate change webpages here, before they are deleted or rewritten by climate change deniers.

Climate Change: Basic Information

How is the climate changing in the U.S.?

Observations across the United States and world provide multiple, independent lines of evidence that climate change is happening now.

Climate change is happening

Our Earth is warming. Earth's average temperature has risen by 1.5°F over the past century, and is projected to rise another 0.5 to 8.6°F over the next hundred years. Small changes in the average temperature of the planet can translate to large and potentially dangerous shifts in climate and weather.

The evidence is clear. Rising global temperatures have been accompanied by changes in weather and climate. Many places have seen changes in rainfall, resulting in more floods, droughts, or intense rain, as well as more frequent and severe heat waves.

The planet's oceans and glaciers have also experienced some big changes – oceans are warming and becoming more acidic, ice caps are melting, and sea levels are rising. As these and other changes become more pronounced in the coming decades, they will likely present challenges to our society and our environment.

What are climate change and global warming?

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 other effects, that occur over several decades or longer.

Humans are largely responsible for recent climate change

Over the past century, human activities have released large amounts of carbon dioxide and other greenhouse gases into the atmosphere. The majority of greenhouse gases come from burning fossil fuels to produce energy, although deforestation, industrial processes, and some agricultural practices also emit gases into the atmosphere.

Greenhouse gases act like a blanket around Earth, trapping energy in the atmosphere and causing it to warm. This phenomenon is called the greenhouse effect and is natural and necessary to support life on Earth. However, the buildup of greenhouse gases can change Earth's climate and result in dangerous effects to human health and welfare and to ecosystems. 

The choices we make today will affect the amount of greenhouse gases we put in the atmosphere in the near future and for years to come.

Climate change affects everyone

Our lives are connected to the climate. Human societies have adapted to the relatively stable climate we have enjoyed since the last ice age which ended several thousand years ago. A warming climate will bring changes that can affect our water supplies, agriculture, power and transportation systems, the natural environment, and even our own health and safety.

Some changes to the climate are unavoidable. Carbon dioxide can stay in the atmosphere for nearly a century, so Earth will continue to warm in the coming decades. The warmer it gets, the greater the risk for more severe changes to the climate and Earth's system. Although it's difficult to predict the exact impacts of climate change, what's clear is that the climate we are accustomed to is no longer a reliable guide for what to expect in the future.

We can reduce the risks we will face from climate change. By making choices that reduce greenhouse gas pollution, and preparing for the changes that are already underway, we can reduce risks from climate change. Our decisions today will shape the world our children and grandchildren will live in.

We can make a difference

You can take action. You can take steps at home, on the road, and in your office to reduce greenhouse gas emissions and the risks associated with climate change. Many of these steps can save you money; some, such as walking or biking to work, can even improve your health! You can also get involved on a local or state level to support energy efficiency, clean energy programs, or other climate programs.

EPA and other federal agencies are taking action. EPA is working to protect the health and welfare of Americans through common sense measures to reduce greenhouse gas pollution and to help communities prepare for climate change.

What EPA Is Doing about Climate Change

EPA is taking a number of common-sense steps to address the challenge of climate change.

Collecting Emissions Data

EPA collects various types of greenhouse gas emissions data. This data helps policy makers, businesses, and the Agency track greenhouse gas emissions trends and identify opportunities for reducing emissions and increasing efficiency.


Getting Reductions

EPA is reducing greenhouse gas emissions and promoting a clean energy economy through highly successful partnerships and common-sense regulatory initiatives.

  • Developing Common-sense Regulatory Initiatives: EPA is developing common-sense regulatory initiatives, to reduce GHG emissions and increase efficiency. For example, EPA's vehicle greenhouse gas rules will save consumers $1.7 trillion at the pump by 2025, and eliminate six billion metric tons of GHG pollution.
  • Partnering With the Private Sector: Through voluntary energy and climate programs, EPA's partners reduced over 345 million metric tons of greenhouse gases in 2010 alone - equivalent to the emissions from 81 million vehicles - and saving consumers and businesses of about $21 billion.
  • Reducing EPA's Carbon Footprint: EPA is monitoring emissions from its own energy use and fuel consumption and working to reduce greenhouse gas emissions by 25% by 2020. Learn more about how we're greening EPA.

Evaluating Policy Options, Costs and Benefits

EPA conducts economy-wide analyses to understand the economic impacts and effectiveness of proposed climate policies. Learn more about EPA's economic analyses on climate policies and the associated costs and benefits.

Advancing the Science

EPA contributes to world-class climate research through the U.S. Global Change Research Program and the Intergovernmental Panel on Climate Change. EPA's Office of Research and Development conducts research to understand the environmental and health impacts of climate change and to provide sustainable solutions for adapting to and reducing the impact from a changing climate.

Partnering Internationally

EPA is engaged in a variety of international activities to advance climate change science, monitor our environment, and promote activities that reduce greenhouse gas emissions. EPA establishes partnerships, provides leadership, and shares technical expertise to support these activities. Learn more about EPA's International Climate Partnerships.

Partnering With States, Localities, and Tribes

EPA's State and Local Climate and Energy Program provides technical assistance, analytical tools, and outreach support on climate change issues to state, local, and tribal governments. See the progress made by our pilot communities.

Helping Communities Adapt

EPA's Climate Ready Estuaries and Climate Ready Water Utilities programs help coastal resource managers and water utility managers, respectively, plan and prepare for climate change. Learn more about EPA's efforts on adapting to climate change.

Causes of Climate Change

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 between energy entering and leaving the planet’s system. When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sun’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 Earth’s energy balance, including:

These factors have caused Earth’s climate to change many times.

Scientists have pieced together a record of Earth’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 Earth’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]

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 Earth’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 Earth’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.

NOAA’s Annual GHG 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 CO2 in the atmosphere are responsible for 80% of the increase. The contribution to radiative forcing by CH4 and CFCs has been nearly constant or declining, respectively, in recent years.

The greenhouse effect causes the atmosphere to retain heat

When sunlight reaches Earth’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 gases like water vapor (H2O), carbon dioxide (CO2), and 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 “greenhouse effect.”

The role of the greenhouse effect in the past

Over the last several hundred thousand years, CO2 levels varied in tandem with the glacial cycles. During warm "interglacial" periods, CO2 levels were higher. During cool "glacial" periods, CO2 levels were lower.[2] The heating or cooling of Earth’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] These changing concentrations are thought to have acted as a positive feedback, amplifying the temperature changes caused by long-term shifts in Earth’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.

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 planet’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 increase 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 CO2 and other heat-trapping gases to the atmosphere. These greenhouse gas emissions have increased the greenhouse effect and caused Earth’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 dioxide 

Carbon dioxide is the primary greenhouse gas that is contributing to recent climate change. CO2 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 CO2 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 CO2 level is higher than it has been in at least 800,000 years.[2]

Some volcanic eruptions released large quantities of CO2 in the distant past. However, the U.S. Geological Survey (USGS) reports that human activities now emit more than 135 times as much CO2 as volcanoes each year.

Human activities currently release over 30 billion tons of CO2 into the atmosphere every year.[2] The resultant build-up of CO2 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.
Source: 
NOAA


If the amount of water flowing into a bathtub 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 CO2 emissions, and actions that can reduce emissions, see the Carbon Dioxide page in the Greenhouse Gas Emissions website.

Methane 

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 Earth’s atmosphere now than at any time in at least the past 800,000 years.[2] Due to human activities, CHconcentrations 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 CH4 emissions and sources, and actions that can reduce emissions, see EPA’s Methane page in the Greenhouse Gas Emissions website. For information on how methane is impacting the Arctic, see the EPA report Methane and Black Carbon Impacts on the Arctic.

Nitrous oxide 

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] For more information on N2O emissions and sources, and actions that can reduce emissions, see EPA’s Nitrous Oxide page in the Greenhouse Gas 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] Therefore, the global concentration of water vapor is not substantially affected by direct human emissions.
  • Tropospheric 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 called 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 the Greenhouse Gas Emissions 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) 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. For information on how BC is impacting the Arctic, see EPA assessment 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 the Greenhouse Gas Emissions website. To learn more about actions that can reduce these emissions, see What EPA is Doing and What You Can Do.

Changes in the sun’s energy affect how much energy reaches Earth’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 Earth’s orbit.

Changes occurring in the sun itself can affect the intensity of the sunlight that reaches Earth’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 Earth’s climate is small.[1]

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

The role of the sun’s energy in the past

Changes in the sun’s intensity have influenced Earth’s climate in the past. For example, the so-called “Little Ice Age” 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 “Little Ice Age” refers to a slight cooling of North America, Europe, and probably other areas around the globe.[2]

Changes in Earth’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 planet’s orbit, appears to drive the advance and retreat of ice sheets. These changes appear to be the primary cause of past cycles 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] 

Rates of Climate Change Have Varied Over Time

The recent role of the sun’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-20th century, and therefore does not explain the recent warming of the earth.[2] Similarly, changes in the shape of Earth’s orbit as well as the tilt and position of Earth’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 Earth’s system

When sunlight reaches Earth, it can be reflected or absorbed. The amount that is reflected or absorbed depends on Earth’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] Absorbed sunlight warms Earth’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 feedbacks 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] 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 Earth’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]

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.] U.S. Global Change Research Program.

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

[3] NRC (2010). Advancing the Science of Climate Changes . National Research Council. The National Academies Press, Washington, DC, USA.




Future of Climate Change

Increasing greenhouse gas concentrations will have many effects

Key Points

  • Continued emissions of greenhouse gases will lead to further climate changes. Future changes are expected to include a warmer atmosphere, a warmer and more acidic ocean, higher sea levels, and larger changes in precipitation patterns.
  • The extent of future climate change depends on what we do now to reduce greenhouse gas emissions. The more we emit, the larger future changes will be.

  Related Links


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


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

Future changes will depend on many factors

  • NRC Climate Stabilization Targets increase in greenhouse gas concentrations
  • Natural influences on climate (e.g., from volcanic activity and changes in the sun's intensity) and natural processes within the climate system (e.g., changes in ocean circulation patterns)

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

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

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

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

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


This figure shows projected greenhouse gas concentrations for four different emissions pathways. The top pathway assumes that greenhouse gas emissions will continue to rise throughout the current century. The bottom pathway assumes that emissions reach a peak between 2010 and 2020, declining thereafter.
Source: Graph created from data in the Representative Concentration Pathways Database (Version 2.0.5) 
http://www.iiasa.ac.at/web-apps/tnt/RcpDb

Future temperature changes

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

Key global projections

  • Increases in average global temperatures are expected to be within the range of 0.5°F to 8.6°F by 2100, with a likely increase of at least 2.7°F for all scenarios except the one representing the most aggressive mitigation of greenhouse gas emissions.[2]
  • Except under the most aggressive mitigation scenario studied, global average temperature is expected to warm at least twice as much in the next 100 years as it has during the last 100 years.[2]
  • Ground-level air temperatures are expected to continue to warm more rapidly over land than oceans.[2]
  • Some parts of the world are projected to see larger temperature increases than the global average.[2]

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

Click the image to view a larger version.

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

Click the image to view a larger version. 

Key U.S. projections

  • By 2100, the average U.S. temperature is projected to increase by about 3°F to 12°F, depending on emissions scenario and climate model.[1]
  • An increase in average temperatures worldwide implies more frequent and intense extreme heat events, or heat waves. The number of days with high temperatures above 90°F is expected to increase throughout the United States, especially toward the end of the century.[1] Climate models project that if global emissions of greenhouse gases continue to grow, summertime temperatures in the United States that ranked among the hottest 5% in 1950-1979 will occur at least 70% of the time by 2035-2064.[1]

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

Future precipitation and storm events

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

Projected changes in global annual mean precipitation for a low emissions scenario (left) and high emissions scenario (right). Blue and green areas are projected to experience increases in precipitation by the end of the century, while yellow and brown areas are projected to experience decreases.
Source: 
IPCC, 2013 

Click the image to view a larger version.

Key global projections

  • Global average annual precipitation through the end of the century is expected to increase, although changes in the amount and intensity of precipitation will vary significantly by region.[2]
  • The intensity of precipitation events will likely increase on average. This will be particularly pronounced in tropical and high-latitude regions, which are also expected to experience overall increases in precipitation.[2]
  • The strength of the winds associated with tropical storms is likely to increase. The amount of precipitation falling in tropical storms is also likely to increase.[2]
  • Annual average precipitation is projected to increase in some areas and decrease in others. The figure to the right shows projected regional differences in precipitation under two emission scenarios.[2]

Key U.S. projections

  • Northern areas are projected to become wetter, especially in the winter and spring. Southern areas, especially the Southwest, are projected to become drier.[1]
  • Heavy precipitation events will likely be more frequent, even in areas where total precipitation is projected to decrease. Heavy downpours that currently occur about once every 20 years are projected to occur between twice and five times as frequently by 2100, depending on location.[1]
  • The proportion of precipitation falling as rain rather than snow is expected to increase, except in far northern areas.[1]
  • The intensity of Atlantic hurricanes is likely to increase as the ocean warms. Climate models project an increase in the number of the strongest (Category 4 and 5) hurricanes, as well as greater rainfall rates in hurricanes.[1] There is less confidence in projections of the frequency of hurricanes.[1]
  • Cold-season storm tracks are expected to continue to shift northward. The strongest cold-season storms are projected to become stronger and more frequent.[1]

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