AR4 Synthesis Report

Released in 2007, the AR4 Synthesis Report combines the efforts of the three working groups of the Intergovernmental Panel on Climate Change into one document that assesses the state of research on climate change and makes possible recommendations for future action and research.

The IPCC and AR4 Synthesis Report

The AR4 Synthesis Report(AR4) is the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), an international group originally founded by two United Nations agencies, whose mandate is to review and assess the latest research findings on Earth’s climate. The AR4 is a synthesis of reports published by three separate working groups of the IPCC in 2007. Previous reports had been published in 1990, 1995, and 2001. The three working groups are tasked with evaluating, respectively, the scientific basis for climate change, the impact of climate change, and appropriate responses to mitigate the impact.

The AR4, like the three individual reports from the working groups, does not comprise independent research; rather, it comprises an evaluation of existing research for a large-picture evaluation of climate change. The report evaluates the events of climate change and the drivers and impacts of and responses to climate change. Finally, the report, which evaluates the uncertainties of predictions of climate change, is divided into six sections.

In the 2001 Third Assessment Report (commonly called TAR), links were established between changes in the climate and their impact. One of the goals established for the AR4 report was to determine ways to reverse climate trends, proposing solutions that would reduce emissions and, therefore, mitigate future impacts.

The AR4 also includes specific definitions (provided in the Principal Terms) for terms encompassing the uncertainty of statements in the report. As with any other area of science, the discussions in the report have some level of uncertainty, so the IPCC felt it was important to clearly define terms for scientists across different disciplines to explain the level of confidence related to conclusions in the report.

The levels of uncertainty include a qualitative and a quantitative scale. The qualitative scale is simple terminology explaining the level of agreement and the quantity of evidence (using terms such as “high agreement” and “medium evidence”).

The quantitative terms are more specific, referring to a numerical range of certainty based on statistics and the expertise in the field. When expressing confidence in a data set, a model, or analysis, confidence is expressed on a scale of 1 to 10, with descriptors of confidence: very high (9), high (8), medium (5), low (2), and very low (1). When evaluating a full body of evidence for a prediction or statement of causality, the report uses the scale listed in its glossary.

Changes in Conditions

The AR4 begins by stating that the warming of the global climate is “unequivocal.” The authors attribute this to three primary observations: temperature changes on land and water, a rise in sea level, and the melting of snow and ice cover.

Many of the observations on which current assessments are based began in 1970, when the availability of powerful remote-sensing satellites made such data gathering and analysis possible. While this relatively small data set yields insufficient data for some conclusions, it also illustrates that the years between TAR and AR4 have seen a significant increase in the availability of climate data. However, when it comes to observations on a local scale, there is still a profound difference in data gathering and analysis, as developing countries tend not to have the resources to monitor environmental changes in the same way.

Among the strongest effects found for global warming, the authors cite the number and size of lakes formed from melted glaciers, the number of predators at the top of a food chain in ocean ecosystems, and avalanches or other instances of instability in mountains or other regions covered by heavy snow. The authors state with high confidence that there have been increased water levels in lakes and rivers that have glaciers as their source earlier in a given year, a factor that has had an impact on global water temperature and quality. Additionally, plants and animals have begun sprouting and migrating earlier in the season. Similarly, oceanic plant life has increased because of higher temperatures.

The authors state with medium confidence that the temperature changes have affected crop planting times and have led to increases in the rates of certain human ailments, such as infectious diseases and heat-related health problems (in Europe, for example). Ultimately, the conclusion is that of the seventy-five studies encompassing twenty-nine thousand data sets, 89 percent are consistent with the idea that the earth is warming.

The authors note that some events and features of the environment have not appeared to change substantially or consistently in the period studied, although this could be a result of a lack of sufficient data. The ice coverage of Antarctica on the whole, and the surface temperatures on that continent, do not seem to have changed significantly. The authors also cite a lack of evidence to determine if the number of extreme weather events such as tornadoes and tropical cyclones has changed.

Causes of Change

In the second section of the AR4 report, the researchers explore potential causes of global temperature increases. The major observation is that the atmosphere is filled with historically high levels of the greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). These greenhouse gases lead to radiative forcing and, therefore, higher temperatures on Earth.

In reviewing the causes of climate change, the authors conclude that global warming is “very likely” because of greenhouse gas increases and that these gases have “likely” impacts on the warming of all six inhabited continents. Furthermore, they note, the warming “likely” has had an impact on many of the changes seen in local systems. In TAR, the human connection to global warming had been regarded as “likely.”

Despite their strong statements on the findings, the authors also note that there are some limits to the data, including a lack of data from developing parts of the world, limits on how much is available over time in some areas where averages can be thrown off by extreme weather events, and means to attribute changes in smaller geographic areas, where adaptation may reduce the effects observed.

Effects of Climate Change

In discussing the effects of climate change, the authors state there is “high agreement and much evidence” that higher temperatures will continue as long as greenhouse gas emissions-levels remain high. While some aerosols (for example, sulfur dioxide) have the effect of cooling the temperature, their levels are expected to drop.

Under current projections, temperatures are expected to rise 0.4 degree Celsius (0.7 degree Fahrenheit) through about 2030. It is very likely, researchers conclude, that the environmental changes observed through the twentieth century will continue to a greater degree during the twenty-first century, even without any significant growth in greenhouse gas emissions.

Some climate models are predicting that those changes will continue beyond the twenty-first century, even without an increase in emissions, because of radiative forcing’s impact on temperatures under current conditions. One major concern is water supply, as glaciers are often a source of freshwater for many parts of the world. The disappearance of some glaciers, combined with a growing world population, could have severe effects.

Responses to Climate Change

In the second half of AR4, the authors discuss sustainable development. In light of the need for development in much of the world, of primary concern is developing areas without exacerbating negative climate change. Humans are making changes to ameliorate climate change and its effects, but more changes are needed. Without these changes, humans will not be able to adapt to new conditions on Earth.

Although the impact of global warming is known, policy solutions to avoid global warming have proved challenging. Along with concerns about greenhouse gas emissions, there are concerns about development in many parts of the world that would result in increases in greenhouse gas emissions.

A primary concern is mitigation. Because of the higher emissions related to development, the strategy is to immediately attack the problem of reducing carbon emissions in developed parts of the world. By mitigating now, the hope is that emissions levels will be constant. Without this, global warming will worsen and more adverse effects from climate change will arise in the coming years.

Technologies available now or expected to be available by 2030 can help in this regard, with the hope that greenhouse gas levels will remain at their present levels or become lower. The expectation is that some of these steps will lower economic costs, with the highest cost being $100 (U.S. dollars) per ton of carbon dioxide emitted (or the equivalent).

The authors note that economic policies not directly tied to climate change may still have significant effects, and those effects remain poorly understood. There exist a number of other uncertainties about climate change. Solutions will depend on the specific challenges of a given region and on the resources available there, whether these resources are financial or governmental and institutional. Future patterns of use, and especially the technologies that may develop, will be key to whether the challenge of development can be met without harming Earth’s climate.

Principal Terms

about as likely as not: something that is 33 to 66 percent likely to be true, as used in the context of the AR4 Synthesis Report

climate change: as defined by the Intergovernmental Panel on Climate Change, a change in the climate that persists for roughly ten years or longer.

exceptionally unlikely: less than 1 percent likely to be true

extremely likely: greater than 95 percent likely to be true

extremely unlikely: less than 5 percent likely to be true

glacial lakes: lakes formed by the melting of a glacier; an indicator used in estimating average annual temperatures

greenhouse gas: any of the gases in the earth’s atmosphere, including carbon dioxide and methane, that absorb heat, thereby affecting atmospheric temperatures

ice core record: use of samples from ice sheets to ascertain temperature, atmospheric and volcanic activity, and precipitation in past years

likely: greater than 66 percent likely to be true

mitigation: with respect to greenhouse gases, strategies to prevent emissions from passing current baseline levels

more likely than not: greater than 50 percent likely to be true

radiative forcing: a measurement of how much the greenhouse gas in a given area will impact the solar radiation

tree-ring data: data harvested from tree rings that indicate historical climates; thicker rings indicate the presence of light and nutrients, which allow more growth

unlikely: less than 33 percent likely to be true

very likely: greater than 90 percent likely to be true

very unlikely: less than 10 percent likely to be true

virtually certain: greater than 99 percent likely to be true

Bibliography

Bernstein, Lenny, et al. Climate Change 2007: Synthesis Report. Geneva: IPCC, 2008. The complete AR4 Synthesis Report, which summarizes the findings of the three IPCC working groups.

Curry, Judith A., and Peter J. Webster. “Climate Science and the Uncertainty Monster.” Bulletin of the American Meteorological Society 92, no. 12 (2011): 1667-1682. In this article, which led to some controversy, the authors discuss how uncertainty is handled in the IPCC report.

Intergovernmental Panel on Climate Change. Climate Change 2001: IPCC Third Assessment Report. Geneva: IPCC Secretariat, 2001. A number of changes took place since the last report from the IPCC, both in methods and the strength of conclusions. This early report provides some insight into how climate change science and research has evolved.

‗‗‗‗‗‗‗. Working Group I. Climate Change 2007: The Physical Science Basis. New York: Cambridge University Press, 2007.

‗‗‗‗‗‗‗. Working Group II. Climate Change 2007: Impacts, Adaptation, and Vulnerability. New York: Cambridge University Press, 2007.

‗‗‗‗‗‗‗. Working Group III. Climate Change 2007: Mitigation of Climate Change. New York: Cambridge University Press, 2007. These three reports make up the most recent IPCC review of available information on climate change and subsequent recommendations.