IPCC Climate Change 2007 Report: Physical Science Basis

views updated

IPCC Climate Change 2007 Report: Physical Science Basis


The Intergovernmental Panel on Climate Change (IPCC) is an international committee of scientists working under the auspices of the United Nations. Its job is to report periodically on the state of scientific knowledge on climate change. As of 2007, it had produced four major reports on the subject, the latest titled Climate Change 2007. This report was divided into three sections, each prepared by a different group of scientific experts.

The purpose of the report is to give citizens and government policymakers the most accurate, authoritative, and sweeping possible view of the state of our scientific knowledge about climate change so as to make whatever decisions are made about greenhouse-gas emissions and other climate-affecting activities as informed as possible. The first part to be released was Climate Change 2007: The Physical Science Basis, which provides a comprehensive assessment of the physical science of climate change. The IPCC does not recommend policies, but is restricted to describing scientific knowledge and its limits.

Historical Background and Scientific Foundations

Climate Change 2007: The Physical Science Basis was issued in stages beginning in January 2007 as part of the IPCC's Fourth Assessment Report (the other three were issued in 1990, 1995, and 2001). It describes the state of scientific understanding of how human activity and natural factors drive changes in climate. It also discusses observations of ongoing and prehistoric climate change, our knowledge of how climate works, and how climate is likely to change in the future. Because of increased urgency since 2001 regarding climate change, many new data have become available, new and better ways of analyzing the data have been developed, computer models of past, present, and future climate are more accurate, and there is better understanding of the ranges of uncertainty.

The Physical Science Basis report runs almost 1,000 pages. It offers both summary and in-depth views of the present state of climate science.

Structure of the Report

Since most politicians, citizens, and other interested persons will not have the time to read the bulk of the 2007 Assessment Report, each of its three parts offers a “Summary for Policymakers” that highlights the main conclusions of the report. A “Technical Summary” is also given that goes deeper into the scientific basis for the conclusions described in the “Summary for Policymakers.” The meat of the Physical Science Basis report is a series of eleven topic-specific chapters that review specific science areas in detail.

Chapter 1 reviews the history of climate change science. Climate is defined as average weather over some number of years. Many natural factors, including energy from the sun, Earth's orbit, volcanoes, and absorption and release of gases by living things and the oceans, affect climate. Over the last 40 years, scientists have found increasing evidence of a human influence on climate. Humans influence climate primarily by replacing natural landscapes with farmlands, deserts, and cities and by releasing smoke and gases such as carbon dioxide (CO2) and methane (CH4) into the atmosphere. Also, scientists have found convincing evidence that Earth's climate is indeed getting warmer (a fact that was not clear until the 1990s). Today, although many voices on the Internet and a relatively small number of scientists doubt a significant human role in global warming, the great majority of climate scientists worldwide are convinced that global warming is not only real but is caused primarily by human activity.

Chapter 2 describes observed changes over recent decades in the composition of Earth's atmosphere. Chapters 3, 4, and 5 delineate direct scientific observations of Earth's climate, including atmospheric composition, weather, snow and ice, and the oceans. Chapter 6 explores what is known about paleoclimate, the prehistoric climate record. Our detailed knowledge of global temperature and atmospheric composition now go back about 800,000 years, thanks to tubes of ice drilled from ancient snow deposits in Antarctica. Chapter 7 discusses links between climate and biogeochemistry, that is, the mutual effects of ecosystems and the chemistry of soils, seas, and air. Chapter 8 presents information on how computer models are used to simulate climate and compares their predictions to the observations described in Chapters 3, 4, and 5. Chapter 9 reveals how much observed climate change is human-caused and how much is natural. Chapter 10 covers the use of computer climate models to predict future climate change. Finally, Chapter 11 delves into computer-predicted climate change for different regions of the world.

Main Conclusions

The following points are highlighted in the “Summary for Policymakers” section of Climate Change 2007: The Physical Science Basis. Segments in italics are paraphrased from the original report to increase clarity.

Concentrations in the atmosphere of the greenhouse gases carbon dioxide, methane, and nitrous oxide (N2O have increased markedly as a result of human activities since 1750 (about the beginning of the Industrial Revolution) and are now far in excess of any values seen for many thousands of years. Increased carbon dioxide has been caused mostly by fossil-fuel burning and changes in land use, especially deforestation (destruction of forests). The increases in methane and nitrous oxide have been caused mostly by agriculture. Carbon dioxide is the most important greenhouse gas: it is responsible for most anthropogenic (human-caused) global warming. The pre-industrial concentration of CO2 in Earth's atmosphere was about 280 parts per million; as of 2005, it was about 379 parts per million, a 35% increase over pre-industrial values. Today's value far exceeds any value observed over at least the last 650,000 years. Recorded history, by comparison, spans only about 6,000 years.


AEROSOLS: Particles of matter, solid or liquid, larger than a molecule but small enough to remain suspended in the atmosphere. Natural sources include salt particles from sea spray and clay particles as a result of the weathering of rocks, both of which are carried upward by the wind. Aerosols can also originate as a result of human activities and in this case are often considered pollutants.

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

BIOGEOCHEMISTRY: The study of how substances and energy are exchanged between living things and the nonliving environment.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

MERIDIONAL: Relating to the meridians, imaginary north-south lines that define longitudes. Meridional circulations of air or water are those that are predominantly north-south in character with one direction of flow located above the other.

PALEOCLIMATE: The climate of a given period of time in the geologic past.

PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F (0°C) for several years.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).

Scientific understanding of anthropogenic influences on climate has improved since 2001. There is now very high

confidence that the global, average, net effect of human activity since 1750 has been to warm the Earth. “Very high confidence” is defined in the report as meaning that a statement is over 90% likely to be true. Scientists measure warming or cooling effects in terms of how much Earth's radiation of energy is increased or decreased. Decreased radiation causes warming, just as adding a blanket keeps a person warmer by preventing heat loss. Any change in average global energy radiation is described in units of watts per square meter. Such a change is called a radiative forcing term. The net radiative forcing from human-added carbon dioxide, methane, and nitrous oxide is now at þ2.30 watts per square meter, which is very likely (at least 80% likely) to be greater than any change in radiative forcing seen for at least 10,000 years.

Warming of the global climate system is unequivocal— that is, definitely real. This is shown by observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising sea levels. In other words, the world is getting warmer, and this is not a theory but an observed fact. Eleven of the 12 years from 1995 to 2006 were among the 12 warmest years since instrumental records of weather began to be kept in the 1800s. From the late nineteenth century to 2001–2005, the global temperature increased about 1.37°F (.76°C). Furthermore, the world is getting warmer faster: warming has happened about twice as fast over the last 50 years as it has over the last 100 years. The oceans are also getting warmer. Observations made since 1961 show that the oceans have warmed down to a depth of at least 1.9 mi (3 km). This ocean warming has absorbed at least 80% of the heat added to the climate system by global warming. Since water expands when it heats, the oceans have swelled; this, along with water added to the seas by melting ice, has caused sea levels to rise. The Greenland and Antarctic ice sheets, which contain enough water to raise sea levels by hundreds of feet (though they are unlikely to melt completely), are melting more rapidly and are at least 90% likely to have contributed to observed sea level rise. Sea level rose at about .07 in (1.8 mm) per year from 1961 to 2003, faster than during the preceding century. Sea level rose 6.7 in (17 cm) in the twentieth century.

Besides the global changes described earlier, many climate changes have been observed at the scale of continents, regions, and ocean basins. Climate changes at smaller-than-global scales include a rise in Arctic (North Polar and nearby) temperatures that is twice the global average, with 2.7% per-year loss of Arctic sea ice since 1978; about 10% loss of permafrost area in the Arctic since the nineteenth century; reduced precipitation in parts of the Americas, northern Europe, northern and central Asia, and parts of Africa; more intense and longer droughts since the 1970s, especially in the tropics and subtropics; more frequent heavy precipitation events; fewer cold spells and more hot spells; and increased tropical cyclone activity in the North Atlantic.

Not all aspects of climate have changed. Daily average global temperature range has not changed at least from 1950 to 2003. Antarctic sea ice coverage has not changed in any steady trend. There is not enough evidence to say whether there have been changes in the meridional overturning circulation of the ocean or small-scale weather events such as tornadoes and dust storms.

Paleoclimate (prehistoric climate) data show that the last 50 years have been unusually warm compared to the last 1,300 years. About 125,000 years ago, the last time the poles were significantly warmer than they are now for an extended period, polar ice melting caused sea level rises of 13 to 20 ft (4 to 6 m). Average temperatures in the northern hemisphere over the last 50 years were very likely higher than during any other 50-year period in the last 500 years, and were likely the highest in 1,300 years.

Most of the global warming since the middle of the twentieth century is at least 90% likely to have been caused by anthropogenic increases in greenhouse gases (mostly CO2, methane, and nitrous oxide). In 2001, the IPCC thought it only 80% or more likely that global warming was mostly human-caused; this uncertainty had decreased by 2007. Climate changes traced to human activity include ocean warming, continental-scale warming, more frequent temperature extremes, and changed wind patterns. Moreover, human greenhouse-gas emissions would probably have caused even more global warming than has been observed were it not for the cooling effects of aerosols—tiny solid particles floating in the air—released by volcanic eruptions and human activity. It is at least 95% certain that human beings have at least contributed to global warming. Without human activity, climate calculations show that the world would have probably cooled slightly, rather than warming significantly, from 1950 to the present. Every continent but Antarctica has been warmed by human activity. In Antarctica, significant warming is seen over the western peninsula, while little warming (or slight cooling) is seen over the rest of the continent.

In 2007, for the first time, computer climate models allow the statement of a likely (more than 80% probable) range of climate sensitivity to radiative forcing. Climate sensitivity is the degree to which global climate responds to a given amount of radiative forcing. In other words, climatologists are now able to predict, with reasonable confidence, the results of changes in radiative forcing produced by various human and natural factors, especially atmospheric CO2 concentration. Doubling atmospheric CO2 would, with 80% probability, produce global warming of 3.6-8.1°F (2-4.5°C). Clouds remain the largest source of uncertainty in climate modeling. They are an uncertain term because they tend to warm Earth by reflecting infrared radiation back down to the surface, like a reflective blanket, but tend to cool Earth by reflecting sunlight back into space.

Over the next 20 years, given actual and projected increases in greenhouse-gas emissions, warming will probably continue at about .36°F (.2°C) per decade. Even if greenhouse gas and aerosol concentrations were held steady at 2000 levels, about half that much warming would occur anyway. The IPCC predicted in 1990 that global warming of .27-.54°F (.15-.3°C) per decade would occur from 1990 to 2005. The actual value was about .36°F (.2°C), which tends to support the methods used to make the predictions.

Continuing greenhouse-gas emissions at today's rates or higher will cause warming and other regional and global climate changes in the twenty-first century that are very likely to be greater than the changes that occurred in the 20th century.

Scientific confidence in predictions of regional-scale changes in temperature, wind patterns, precipitation, ice, and extreme weather is higher than in 2001. Increased confidence comes from the agreement between independently developed mathematical models that have a proven ability to predict past climate changes based on earlier data. Snow cover in the Northern Hemisphere will decrease; sea ice will shrink in the Arctic and Antarctic seas; heat waves and heavy precipitation events will become more common; typhoons and hurricanes will become more intense; increased precipitation is very likely at high latitudes (far from the equator) and decreased precipitation is likely in subtropical zones; and the meridional overturning circulation of the oceans is very likely to slow down during the twenty-first century. It is, however, very unlikely that the overturning circulation of the global ocean will stop or undergo other rapid, large changes during the twenty-first century.

Even if greenhouse gas concentrations were to be stabilized, instead of increasing steadily as they are today, anthropogenic warming of the climate and rise of sea level would continue for centuries because of greenhouse gases already added to the atmosphere. The amount of warming and sea-level rise will be affected, however, by how much greenhouse gas humans continue to add to the atmosphere.

Impacts and Issues

No other paper, book, or document reflects the combined knowledge of so many of the world's climate and weather scientists as the IPCC's Fourth Assessment Report. This document is therefore of unique importance in the global dialogue on climate change. It is consulted by hundreds of governments in considering possible responses to the problem of climate change; cited in thousands of essays, editorials, news reports, and scientific papers; read closely by activists and environmentalists seeking to change government policies and patterns of human behavior; and attacked by doubters of human-caused (anthropogenic) climate change.

The 2007 assessment is founded on significantly more and better data than the 2001 assessment, but has been subject to criticism from several quarters. For example, critics employed by or sympathetic to large fossil-fuel corporations have produced many Web sites and editorials calling the report panicky and biased. As of late 2007, however, few papers tending to show that climate change is less severe or human-caused than the report says had appeared in the peer-reviewed scientific literature.

Some scientists have accused the assessment of being too optimistic; for example, even before the Physical Science Basis report's release in early 2007, top U.S. climate scientists were saying that its predictions of sea-level rise were too low because, in the words of polar ice specialist Lonnie Thompson of Ohio State University, they “don't take into account the gorillas—Greenland and Antarctica.” Thompson added, “I think there are unpleasant surprises as we move into the twenty-first century.”


At the 10th Session of Working Group I of the Intergovern-mental Panel on Climate Change (IPCC) in Paris, France, during February 2007, the Working Group assigned levels of confidence to data and analysis. In their Summary for Policymakers, the following levels of confidence were used “to express expert judgments on the correctness of the underlying science: very high confidence at least a 9 out of 10 chance of being correct; high confidence about an 8 out of 10 chance of being correct.”

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

The IPCC has also been accused of reflecting a conservative or anti-global-warming bias, as opposed to exaggerating the prospects of global warming. According to some news sources, the roundabout process for releasing IPCC reports, which requires repeated approval by most of the world's governments before release, was supported in the 1980s by doubters of global warming who hoped to impede the release of reports favorable to the reality of global warming. In 2002, news outlets reported that the oil company ExxonMobil urged the administration of President George W. Bush, in a confidential memo that was eventually leaked to the press, to have U.S. climate scientist Robert Watson replaced as chair of the IPCC because he was too vocal about the reality of global warming. However, Watson's replacement, Indian engineer and environmentalist R. K. Pachauri, who was IPCC chair during the preparation of the 2007 Assessment Report, has also proved to be a supporter of the scientific consensus view of global climate change.

See Also Great Conveyor Belt; IPCC Climate Change 2007 Report; IPCC Climate Change 2007 Report: Criticism; IPCC Climate Change 2007 Report: Impacts, Adaptation and Vulnerability; IPCC Climate Change 2007 Report: Mitigation of Climate Change.



Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.


Borenstein, Seth. “New Climate Report Too Rosy, Experts Say.” The Washington Post (January 8, 2007).

Borger, Julian. “U.S. and Oil Lobby Oust Climate Change Scientist.” The Guardian (April 20, 2002).

Rosenthal, Elisabeth. “Science Panel Says Global Warming Is ‘Unequivocal.’” The New York Times (February 3, 2007).

Larry Gilman