The Intergovernmental Panel on Climate Change (IPCC) is an international organization that includes scientists and representatives of governments around the world. The IPCC was established in 1988 by the World Meteorological Organization (WMO) and by the United Nations Environment Programme (UNEP). The IPCC's stated role is to "assess on a comprehensive, objective, open and transparent basis the latest scientific, technical and socio-economic literature produced worldwide relevant to the understanding of the risk of human-induced climate change, its observed and projected impacts and options for adaptation and mitigation". Reports produced by the IPCC are intended to be neutral with respect to policy, to reflect a broad range of views and perspectives, and to include the most up-to-date scientific information.

IPCC Reports and Working Groups

The IPCC has issued (as of this writing in 2008) four Assessment Reports describing the current understanding of climate change science at the time each report was issued. The First Assessment Report was completed in 1990, and is often referred to by the acronym "FAR" (First Assessment Report). The Second Assessment Report (SAR) was completed in 1996. The Third Assessment Report (TAR) was released in 2001. Most recently, the Fourth Assessment Report (dubbed "AR4" for "Assessment Report 4", since "FAR" had already been used for the First Assessment Report) was completed in 2007.

The IPCC reports are issued as three volumes, corresponding to the three "Working Groups" of the IPCC. The Working Group I volume, titled "The Physical Science Basis" in AR4, deals with the science of climate change. The Working Group II volume, titled "Impacts, Adaptation and Vulnerability" in AR4, anlyzes possible or likely implications of climate change for human societies around the world. Finally, the Working Group III volume, called "Mitigation of Climate Change" in AR4, considers the possible steps humans might take to influence and alter climate change and the possible outcomes produced by various behaviors.

Each of volumes of the AR4 includes a rather lengthy full report as well as a briefer "Summary for Policymakers". These summaries are written in language that is much more accessible to non-scientists and are much briefer than the full reports. For example, the full Working Group I report in AR4 weighs in at 996 pages; the Summary for Policymakers is just 18 pages long. Here are links to the Summary for Policymakers documents for AR4 for the three Working Groups:

Frequently Asked Questions

The Working Group I report includes a new feature in AR4 that is especially valuable for educators. The report includes a 35-page document composed of 19 frequently asked questions about climate science and climate change along with concise (typically 1 to 2 page) responses to those questions. This document is a gold mine for teachers wishing to understand and explain to students the major issues in the field of climate science! The questions addressed are:

  1. IPCC Global temperature 1906-2005

    This figure from the FAQ document illustrates the response to question #15 in this list. The black line shows actual average global temperatures; the blue shaded region shows predictions from models that only take natural events into consideration; and the pink shading shows predictions from models that include natural and human-caused "forcings".

    Credit: Image courtesy of the IPCC (AR4 WG1 Summary for Policymakers Figure SPM.4).

    What Factors Determine Earth’s Climate?
  2. What is the Relationship between Climate Change and Weather?
  3. What is the Greenhouse Effect?
  4. How do Human Activities Contribute to Climate Change and How do They Compare with Natural Influences?
  5. How are Temperatures on Earth Changing?
  6. How is Precipitation Changing?
  7. Has there been a Change in Extreme Events like Heat Waves, Droughts, Floods and Hurricanes?
  8. Is the Amount of Snow and Ice on the Earth Decreasing?
  9. Is Sea Level Rising?
  10. What Caused the Ice Ages and Other Important Climate Changes Before the Industrial Era?
  11. Is the Current Climate Change Unusual Compared to Earlier Changes in Earth’s History?
  12. Are the Increases in Atmospheric Carbon Dioxide and Other Greenhouse Gases During the Industrial Era Caused by Human Activities?
  13. How Reliable Are the Models Used to Make Projections of Future Climate Change?
  14. Can Individual Extreme Events be Explained by Greenhouse Warming?
  15. Can the Warming of the 20th Century be Explained by Natural Variability?
  16. Are Extreme Events, Like Heat Waves, Droughts or Floods, Expected to Change as the Earth’s Climate Changes?
  17. How Likely are Major or Abrupt Climate Changes, such as Loss of Ice Sheets or Changes in Global Ocean Circulation?
  18. If Emissions of Greenhouse Gases are Reduced, How Quickly do Their Concentrations in the Atmosphere Decrease?
  19. Do Projected Changes in Climate Vary from Region to Region?


One interesting aspect of the process by which the IPCC develops its recommendations and reports is that its proclamations are arrived at by consensus. Thousands of people from around the globe either contributed to or reviewed portions of the IPCC's Fourth Assessment Report; somewhere between 300 and 350 representatives of governments and scientific organizations participated in the meeting at which details of the report were finalized. Essentially, all language included in the reports had to have the support of all representatives before it was included in the final release. As you can imagine, getting such a large and diverse group to agree on anything is a monumental challenge. There are strengths and weaknesses to this approach. Some participants feel that the reports are somewhat "watered down"; that any claim that was even remotely contentious in the eyes of any participant was vetoed, and hence some important and largely agreed-upon aspects of the science were left out. On the other hand, the consensus process does make it very difficult for naysayers to claim that the IPCC's reports are merely the ranting of a handful of extremists.

Uncertainty: Scientific and Social

Multiple IPCC Models

A typical graph from the IPCC's AR4 showing predicted global temperatures between 2000 and 2100. The graph displays the results from more than 20 different climate models (various colors) as well as the "ensemble average" of all the models (black dotted line).

Credit: Image courtesy of the IPCC (AR4 WG1 Report "The Physical Science Basis" Chapter 10 page 763 Figure 10.5).

A major goal of the IPCC reports is to offer reasonable predictions about the future climate of Earth. Lacking a perfect crystal ball, this endeavor must deal with two major sources of uncertainty in making such predictions. One source of uncertainty lies with the science of climate prediction. Though observations, theory, and climate models on supercomputers continue to improve, Earth's climate is an immensely complex system. Any attempt at predicting future climate is bound to have at least a few uncertainties about the science involved. A second major source of uncertainty is rooted in the behaviors of human beings and cultures. At what rate will world and regional populations continue to grow? What mix of renewable and fossil fuels will people use for energy in 10, 20, or 50 years time? Will people rely more on automobiles or public transportation to get around? Assumptions about the answers to such questions profoundly influence the possible scenarios (in terms of greenhouse gas emissions, etc.) climate scientists might use as the inputs for their climate models.

The IPCC reports address the scientific sources of uncertainty by reporting findings of numerous different groups of climate scientists running many similar, but different, climate models. Reports typically portray both an overall average result for a given climate variable (such as global average temperature in the year 2100, or precipitation forecasts for northern Africa for 2020) as well as a range of possible values representing the diverse predictions generated by the many different climate models. In the Fourth Assessment Report, results from 23 different climate models were reported in most cases.

The IPCC deals with uncertainty associated with the behaviors of people in the future by basing its projections on a series of different possible future scenarios. In 2000, the IPCC published the "Special Report on Emissions Scenarios" (SRES). The SRES outlines several alternative future scenarios, based on a range of possibilities for factors including: population growth rate, economic growth, per capita income, levels of energy use, mix of fossil fuel vs. other energy sources, and changes in land use. These scenarios are then used as drivers for the various climate models, which in turn produce predictions about future levels of atmospheric gases, temperatures (global and regional), precipitation patterns, and other aspects of climate. The different scenarios also vary in terms of the assumed levels of international cooperation involved, the extent to which disparities between "rich" and "poor" countries shrink or grow larger, the anticipated levels of technological change, and the extent to which "green" policies drive decision-making.

The scenarios are complex, so a brief summary is incapable of fully describing even the main ones employed by the IPCC. Instead, we'll try to explain some of the terminology you might encounter in IPCC reports and provide a few illustrative examples of key features in some scenarios. The IPCC groups these scenarios into four main families with accompanying descriptive "storylines". The main scenario groups are named A1, A2, B1, and B2. In the Fourth Assessment Report, three subdivisions of the A1 scenario are employed throughout; they are dubbed A1FI, A1B, and A1T. Are you lost in this alphabet soup yet? Here are some illustrative (but not comprehensive) descriptions of some aspects of the major scenarios:

  • All of the A1 scenarios presume rapid economic and technological growth, a low rate of population growth, and a very high level of energy use. Disparities between "rich" and "poor" countries narrow.
    • The A1FI scenario is based on high levels of fossil fuel use for energy.
    • The A1T scenario assumes non-fossil energy sources will predominate.
    • The A1B scenario presumes that a balance of fossil and non-fossil fuels will be used for energy.
  • The A2 scenario assumes that population growth will be high, that technological change and economic growth will be slower and more disparate (between countries and regions) than in other scenarios, and that energy use will be high.
  • The B1 scenario is based on a "high level of environmental and social consciousness combined with a globally coherent approach to a more sustainable development". Population growth is low, economic and technological advancement rate is high, while energy use is low. There is greater change in land use in this scenario than in others; area devoted to crops and grasslands decreases, while reforestation efforts expand the amount of area covered by forests.
  • The B2 scenario is similar to the B1 scenario in that presumes an emerging effort towards more sustainable and environmentally conscious world economy and society. More disparities between industrialized and developing nations persist in this scenario than is the case for the B1 scenario. Technological and economic growth is also slower than in B1, and population growth is greater (though still less than in A2). Energy use is also midway between B1 and A2. Changes in land use are also less dramatic than in B1.

We are not, by any means, suggesting that you need to be familiar with the details of these scenarios. We do feel, however, that some passing familiarity with them will help you understand the underlying assumptions that go into predictions of future climates based on these scenarios. If you do want to delve deeper into the IPCC scenarios, here are some useful links:

Select one of six IPCC scenarios for possible future climate to see carbon dioxide emissions, CO2 atmospheric concentration, and modeled temperatures through 2100 for each of the scenarios.

Credit: Image courtesy of the IPCC (AR4 WG1 Chapter 10 p. 803 Figure 10.26).

Main Findings

We'll dig into predictions of future climate change in the coming pages; for now, here are some of the main claims made by the IPCC in the Fourth Assessment Report:

  • Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land use change, while those of methane and nitrous oxide are primarily due to agriculture.
  • The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to very high confidence (defined as having a 90% chance of being correct) that the global average net effect of human activities since 1750 has been one of warming.
  • Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. Since the TAR, progress in understanding how climate is changing in space and in time has been gained through improvements and extensions of numerous datasets and data analyses, broader geographical coverage, better understanding of uncertainties, and a wider variety of measurements. Increasingly comprehensive observations are available for glaciers and snow cover since the 1960s, and for sea level and ice sheets since about the past decade. However, data coverage remains limited in some regions. Eleven of the last twelve years (1995–2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).
IPCC Sea Level 1870-2005

Global sea level rise from 1870 to 2005, as reported in the AR4 WG1 Summary for Policymakers. Gray dots are annual values; black line is the average smoothed over a decade. The red line (upper right) represents satellite-based measurements; the shaded blue region represents data from tide gages and includes known uncertainty ranges. Values are in comparison to the 1961-1990 average.

Credit: Image courtesy of the IPCC (AR4 WG1 Summary for Policymakers Figure SPM.3).

  • Palaeoclimatic information supports the interpretation that the warmth of the last half century
    is unusual in at least the previous 1,300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 meters of sea level rise.
  • Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.12 This is an advance since the TAR’s conclusion that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations". Discernible human infl uences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.
  • For the next two decades, a warming of about 0.2° C per decade is projected for a range of SRES
    emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1° C per decade would be expected.
  • Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.
  • Anthropogenic warming and sea level rise would continue for centuries due to the time scales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilised.

Accuracy of Future Climate Projections?

Models vs. actual temperature trendsThis graph shows how well the IPCC's projections of future temperatures have compared to actual global temperatures. The black dots show actual observed temperatures from 1990 to 2005 (as compared to the average from 1961 through 1990). The black line is a smoothed fit of those yearly observed temperatures. The colored lines and shaded regions show IPCC projections for the same timeframe from the First (1990), Second (1996), and Third (2001) Assessment Reports (FAR, SAR, and TAR, respectively). The shaded region for each Assessment Report shows the range of projections for different models. For the FAR and SAR, there is also a solid line for the 'best estimate' model projections (the TAR did not have a 'best estimate' model projection).

Credit: Image courtesy of the IPCC (AR4 WG1 Report "The Physical Science Basis" Chapter 1 page 98 Figure 1.1).

Last modified April 11, 2008 by Randy Russell.

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