The Case for Action: Creating a Clean Energy Future

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The Case for Action: Creating a Clean Energy Future
May 2010

The United States needs strong action now to reduce the risks of climate change, strengthen our energy independence, protect our national security, and create new jobs and economic opportunities. The Pew Center on Global Climate Change believes that the case for action has never been stronger. With a strong energy and climate policy the United States can lead the 21st century clean energy economy.


What is Black Carbon?

What Is Black Carbon?
April 2010

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Black Carbon (BC) has recently emerged as a major contributor to global climate change, possibly second only to CO2 as the main driver of change.[1] BC particles[2] strongly absorb sunlight and give soot its black color. BC is produced both naturally and by human activities as a result of the incomplete combustion of fossil fuels, biofuels, and biomass. Primary sources include emissions from diesel engines, cook stoves, wood burning and forest fires. Reducing CO2 emissions is essential to avert the worst impacts of future climate change, but CO2 has such a long atmospheric lifetime that it will take several decades for CO2 concentrations to begin to stabilize after emissions reductions begin. In contrast, BC remains in the atmosphere for only a few weeks, so cutting its emissions would immediately reduce the rate of warming, particularly in the rapidly changing Arctic. Moreover, reduced exposure to BC provides public health co-benefits, especially in developing countries. Technologies that can reduce global BC emissions are available today.

Black Carbon and Climate Change

BC warms the climate in two ways. When suspended in air, BC absorbs sunlight and generates heat in the atmosphere, which warms the air and can affect regional cloud formation and precipitation patterns. When deposited on snow and ice, it absorbs sunlight, again generating heat, which warms both the air above and the snow and ice below, thus accelerating melting. Because BC remains in the atmosphere for only one to four weeks, its climate effects are strongly regional. Its short lifetime also means that its climate effects would dissipate quickly if black carbon emissions were reduced, thus benefiting most directly the countries or communities that invest in policies to reduce BC emissions.

A recent study suggests that BC may be responsible for more than 30 percent of recent warming in the Arctic,[3] contributing to the acceleration of Arctic sea ice melting. Loss of Arctic sea ice would lead to more rapid warming and possibly irreversible climate change. BC is also driving increased melting of Himalayan glaciers, which are a major source of freshwater for millions of people in the region. BC may also be driving some of the observed reduction of the snowpack in the Pacific Northwest of the United States.

Different types of soot contain different amounts of BC—generally the blacker the soot, the more of a warming agent it is. Fossil fuel and biofuel soot are blacker than soot from biomass burning[4] (e.g., forest fires and wood fuel), which is generally more of a brownish color. Thus, controlling emissions of soot from fuel sources is an effective way of reducing atmospheric temperatures in the short term. Based on current information, the United States is responsible for about 6 percent of global BC emissions; while it has a history of making reductions to improve air quality, further improvements can be made. The majority of BC emissions come from the developing world: China and India together account for some 25–35 percent of emissions.

Control technologies that reduce BC include retrofitting diesel vehicles with filters to capture BC, fuel switching (e.g., from diesel to natural gas in buses), and replacement of inefficient cook stoves with cleaner alternatives. Adopting these alternatives would have positive co-benefits for public health, especially in the developing world. For example, retrofitting or replacing diesel buses and trucks would greatly improve urban air quality in densely populated cities. Replacement of dirty cook stoves with cleaner alternatives, such as solar cookers or newer models that burn fuel more completely, would improve indoor air quality, which is a major health concern in both urban and rural areas of the developing world.

Reducing BC emissions[5] represents a win-win scenario: it would have an immediate cooling effect on the Earth’s climate, potentially delaying temperature increases in the short run and helping reduce the risk of irreversible tipping points in the climate system, and it would reduce air pollution, resulting in fewer premature deaths and fewer missed work and school days.


1. Ramanathan, V. and G. Carmichael. 2008. Nature Geoscience, 1:221-227.

2. BC is a carbonaceous aerosol. An aerosol is a suspension of fine solid particles or liquid droplets within a gas. Examples include smoke, air pollution, smog, oceanic haze, and tear gas. Carbonaceous refers to a substance rich in carbon.

3. The Arctic warmed by 1.48 ± 0.28 °C during 1976–2007; BC is estimated to have caused 0.5–1.4 °C of that change (Shindell, D. et. al. 2009. Nature Geoscience, 2:294-300).

4. Soot from biomass burning generally tends to have a cooling effect on the climate.

5. The American Clean Energy and Security Act of 2009 reported out of the U.S. House Energy and Commerce Committee on May 21, 2009, has a significant section on BC emissions, directing the EPA Administrator to investigate BC sources, impacts, and mitigation technologies.


In Brief: Update on the 10-50 Solution: Progress Toward a Low-Carbon Future

January 2010

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Addressing the challenge of global climate change will require a significant reduction in annual greenhouse gas (GHG) emissions in the United States and throughout the world by 2050. This will necessitate a fundamental shift from an economy predominantly based on traditional fossil fuel use to one based on efficiently managed low-carbon energy sources, including technologies that capture and store carbon dioxide (CO2).

Achievement of this transition depends on both near-term and long-term actions that take advantage of current technologies and opportunities and that also make substantial investments in the technologies of the future. But most of all, the United States needs a clearly enunciated and sustained policy to guide those actions. Too often the debate over GHG emission reductions pits near-term actions against long-term investments in technology, when in fact both are necessary and more effective together.

In 2004, the Pew Center held a workshop (the “10-50” Workshop) to understand the technologies likely to enable a low-carbon future by mid-century (50 years) and identify policy options for the coming decade (10 years) to help “push” and “pull” these technologies into the market. This brief reviews some of the key policies and actions deemed important five years ago and reports on progress against those goals to date; it finds significant progress in pushing low-carbon technologies and underscores the critical remaining need for a policy, such as cap and trade, that puts a price on carbon and “pulls” those technologies into the marketplace.

Click here for more on the 10-50 Solution.




Key Scientific Developments Since the IPCC Fourth Assessment Report

Science Brief
June 2009

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The Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report in 2007, summarizing the scientific community’s current understanding of the science of climate change.  Since that time, a number of new scientific results have been published that expand our understanding of climate science.  This brief summarizes some of the key findings since the last IPCC assessment.





Global Warming Facts and Figures

These facts and figures are divided into three sections:

  1. U.S. Emissions
  2. International Emissions
  3. Land-Ocean Surface Temperatures
    and other information on our Basics page
  4. Main Greenhouse Gases

These sections explain the scientific evidence for human impacts on the climate system, specifically global warming.

Each section below contains several figures. Click on the section's heading or image to view all.


U.S. Emissions

International Emissions

Land-Ocean Surface Temperatures
and other information on our Basics page

Main Greenhouse Gases


Global warming facts and figures to explain the scientific evidence for human impacts on the climate system.

The Science of Climate Change: Global and U.S. Perspectives

The Science of Climate Change: Global and U.S. Perspectives

Tom M. L. Wigley, National Center For Atmospheric Research

Press Release

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This report is available for download only.

Basic Science on climate change:

  • Projections of future climate change suggest a global temperature increase of 1 to 6°C (2 to 10°F) from 1990 to 2100, with warming in most of the United States expected to be even higher.
  • Current scientific research shows that climate change will have major effects on precipitation, evapotranspiration, and runoff — and ultimately on the nation's water supply
  • While the net impacts of a doubling of atmospheric CO2 concentrations on U.S. agriculture as a whole are likely to be small, the impacts are likely to vary considerably from region to region.
  • Climate change will lead to substantial sea-level rise along much of the U.S. coastline, due mostly to thermal expansion of the oceans.
  • The very real possibility exists that warming over this century will jeopardize the integrity of many terrestrial ecosystems and will pose a threat to our nation's biodiversity.

The Wigley report provides more information on how climate is influenced by anthropogenic factors. You may download a pdf of the entire report by clicking on the report cover above, or read portions of the report in html by following the links in the "In This Section" box.



Eileen Claussen, Executive Director, Pew Center on Global Climate Change

This report on the science of climate change seeks to explain how climate is influenced by anthropogenic factors. Understanding the effect of greenhouse gas concentrations on the atmosphere is key to understanding the potential magnitude of the "greenhouse effect," evaluating possible environmental impacts, and considering policy responses.

A variety of factors determine the rate and magnitude of climate change, including the emissions of greenhouse and aerosol-producing gases, the carbon cycle, the oceans, biosphere, and clouds. As our understanding in each of these areas evolves, it is important that researchers, policy-makers, the press, and the public be kept informed since these developments affect our understanding of the seriousness and complexity of this issue.

As part of the Pew Center's series examining the potential impacts of higher atmospheric concentrations of greenhouse gases on the United States, this paper by the distinguished climate scientist Tom M.L. Wigley, senior scientist with the National Center for Atmospheric Research, addresses what is known and not known about the science of climate change. Its publication comes in an interim period between assessments of the science by the Intergovernmental Panel on Climate Change (which published its second assessment in 1996 and will publish its third assessment in 2001). The author uses preliminary estimates of greenhouse gas and sulfur dioxide emissions from the current IPCC review process as well as his own work to supplement previously published research.

The new research suggests the likelihood of slightly larger changes in temperature and sea level rise than projected in the most recent IPCC assessment. The temperature rise is expected to be greater in the U.S. than the average temperature increase across the globe. While changes in precipitation and extreme weather events such as hurricanes and other storms are more difficult to predict, it is possible that the intensity of rain and hurricane events could increase. Uncertainties in predicting the direction and magnitude of these changes make it difficult to predict the impacts of climate change. However, even small changes in climate can lead to effects that are far from trivial.

While the analysis presented is the work of one author, this report has been subject to extensive peer review. The Pew Center and the author are indebted to many scientists and organizations for their constructive comments on previous drafts of this paper or sections of this paper. Their comments have helped improve the text substantially, and so, while the opinions expressed in this report are solely those of the author, we gratefully acknowledge their input: E. Barron, B. Felzer, C. Hakkarinen, A. Henderson-Sellers, M. Hulme, M. MacCracken, M. McFarland, J. Mahlman, G. Meehl, N. Nakicenovic, B.D. Santer, M.E. Schlesinger, K.P. Shine, J.B. Smith, and S.J. Smith. The A1, A2, B1, and B2 scenarios developed in the current IPCC working group process have been used with the kind permission of their producers, represented by T. Morita, A. Sankovski, B. deVries, and N. Nakicenovic. D. Viner of the Climate Impacts LINK Project (UK Dept. of the Environment, Regions and Transport contract EPG1/1/68) supplied the HadCM2 data on behalf of the Hadley Centre and UK Meteorological Office. In addition, the Pew Center would like to acknowledge and thank Joel Smith and Brian Hurd of Stratus Consulting for their management of this Environmental Impacts series.

Executive Summary


The average surface temperature of the globe has warmed appreciably since the late 1800s, by about 0.6°C. Since this warming cannot be adequately explained by natural phenomena such as increased solar activity, human-induced increases in greenhouse-gas concentrations appear to be at least partly responsible. In addition to the warming effect of greenhouse-gas increases, however, changes in temperature over the past century are likely to have been significantly influenced by the cooling effect associated with changes in the sulfate aerosol loading of the atmosphere, arising from fossil-fuel-derived sulfur dioxide (SO2) emissions. When greenhouse-gas, sulfate aerosol, and solar influences are considered together, observed climate changes are consistent with model predictions.

Projections of future global-mean temperature and sea level change made by the Intergovernmental Panel on Climate Change (IPCC) in its 1996 Second Assessment Report used emissions scenarios developed in 1992. Preliminary versions of new emissions scenarios produced by the writing team for the IPCC Special Report on Emissions Scenarios (SRES) are now available. The most important difference between the old (1992) and new (SRES) scenarios is that the new scenarios have much lower emissions of sulfur dioxide. The reduction in sulfur dioxide emissions (and their attendant cooling effects through the production of sulfate aerosols) results in a slight increase in temperature and sea level rise projections from those previously given by the IPCC. If central estimates of model parameters are used, global-mean warming from 1990 to 2100 ranges from 1.9°C to 2.9°C. Sea-level rise estimates over the same period range from 46 to 58 cm. For temperature and sea level changes over the next few decades, projections are virtually independent of the emissions scenario.

Based on results from a number of climate models, the rate of future warming over the United States is expected to be noticeably faster than the global-mean rate. Future regional-scale precipitation changes are highly uncertain. The only result that is common to all climate models is an increase in winter precipitation in northern latitudes, from the northern Great Plains to the northeastern states. Even in the absence of large precipitation changes, there could still be significant changes in the availability of water for agriculture, human consumption, and industry because of the increased evaporation that should accompany warming. This factor alone would lead to drier summer soil conditions and reduced runoff. The effects of increased evaporation, however, may be partly offset by the direct plant-physiological effect that carbon dioxide (CO2) has in improving plant water-use efficiency and, hence, lowering evapotranspiration rates.

Changes in weather and climate extremes over the United States are certain to occur as the global climate changes. The frequency of extremely hot days is almost certain to increase, and the frequency of frosts should decrease. Changes in the frequency of daily precipitation extremes are highly uncertain, although there is evidence for an increase in the frequency of wet extremes. For hurricanes and tropical storms, the evidence suggests that there could be small increases in their windspeeds. It is also likely that future such storms will be accompanied by larger rainfall amounts. While there is no credible model-based information on changes in the number of hurricanes and tropical storms per year worldwide, there is empirical evidence that suggests that a small increase in frequency is possible in the North Atlantic region. For all extreme events, however, it is unlikely that the projected changes will become evident in a statistically convincing way for many decades, with the exception of temperature extremes, which should become evident sooner.

About the Author


Tom M.L. Wigley

Tom M.L. Wigley (B.Sc., Ph.D.), formerly Director of the Climatic Research Unit, University of East Anglia, Norwich, U.K., currently holds a Senior Scientist position with the National Center for Atmospheric Research, Boulder, CO. One of the world's foremost scientists in the area of climate change, he has published in diverse aspects of the broad field of climatology. His main interests are in carbon cycle modeling, projections of future climate and sea-level change, and interpretation of past climate change particularly with a view to detecting anthropogenic influences. Recently, he has concentrated on facets of the global warming problem, and has contributed on many occasions to Intergovernmental Panel on Climate Change (IPCC) reports and assessments.


Tom M. L. Wigley
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