The Signature of Global Warming
by Ralph Kahn, December 1995
Is the everyday burning of coal and oil really warming Earth's climate? The Intergovernmental Panel on Climate Change, an international group of climate researchers, faces this daunting question. Its new report, the update of a volume published in 1990, was completed this month.
The cautiously-worded document does not provide a simple "yes" or "no" answer. This bespeaks the struggles of a community trapped between the orderly reasoning of scientists who must work with limited data, and the pragmatic demands of policy makers.
But in comparison to the assessment made 5 years ago, the new IPCC report shifts toward implicating human activity. It expresses heightened concern that the warming trend already observed "is unlikely to be entirely due to natural causes," and warns governments and citizens to begin taking steps to mitigate the likely consequences. For the first time since the Antarctic ozone hole, human activity is a leading suspect in a global-scale climate change.
The IPCC's increased concern is driven by recent improvements in computer models of Earth's climate. So many factors are involved in determining the climate that such models are needed to interpret global temperature measurements -- to pinpoint the causes of the observed trends. The pivotal advance of recent models is that they include the effects of tiny atmospheric particles called "aerosols," which originate from volcanoes, forest fires, windblown deserts, and industrial sources. Aerosols represent one more piece of the global warming picture that is fitting into place.
There is consensus among researchers about some aspects of the global warming issue. Scientists generally agree that the global average temperature rose about 1.5 degrees F over the past 100 years. If this increase were to continue, rainfall patterns and growing seasons would alter, polar ice would melt at a faster pace, and extreme weather could become commonplace. The disruption to water supplies, agriculture, coastal settlements, and natural habitats would be monumental.
Circumstantial evidence for human involvement in global warming is easy to find. Due to the burning of fossil fuels such as coal and oil, the amount of carbon dioxide in the atmosphere has steadily increased for at least 150 years. The greenhouse effect explains how, other factors remaining equal, an increase of certain gases such as carbon dioxide will lead to higher temperatures at the Earth's surface.
But no one wakes up in the morning, takes a deep breath, and says: 'Man, there's a lot of CO2 in the air today.' Connecting "potential causes" of climate change, such as the well-established increase in carbon dioxide, with "effects we care about," such as the actual trend in global temperature, is not so easy. The century-long upward trend in temperature contains numerous wiggles and bumps. The record is punctuated by a 10-year cooling episode in the 1950s, and a leveling off in the early 1980s. Before an indictment of our role in global warming can be issued, the "signature" of human activity must be separated from the natural fluctuations in temperature.
Observed temperatures represent the combined effects of human-induced and natural processes. In addition to greenhouse gases, clouds play a role in controlling temperatures at Earth's surface -- and the amount and location of clouds may change as the temperature rises. Ground cover, which determines the amount of sunlight the surface will absorb, also affects the temperature. Atmospheric aerosols are involved as well.
Computer-based climate models are used to sort out the contributions of these and other processes; they provide the link between potential causes and climate effects. Such models take into account as much as possible about the way the atmosphere, oceans, land surface, and biosphere function. Information about changing conditions, such as the varying amount of atmospheric carbon dioxide, is put into the model. The model then calculates the resulting temperature and other climatic consequences.
But even the most sophisticated models are only simplified descriptions of Earth's climate system. They leave out some processes that are not well understood, and abbreviate others to keep the size of models within the limits of existing computers. The models have had a tough time reproducing the past century's global average temperatures. Feeding in the measured change in greenhouse gases, most models predicted that the temperature would increase at about twice the observed rate.
However, in the last few years, aerosols have been added to the computer descriptions of the climate. Aerosols reflect sunlight, making the surface cooler. They account for the lack of warming in the early 1980s, when Mexico's El Chichon volcano erupted, creating a blanket of reflecting particles in the upper atmosphere that persisted for more than a year. Without aerosols, global average temperatures would be even warmer.
By including the effects of aerosols, the models show unprecedented agreement with the 100-year historical record. Increased confidence in the model results is the key element behind the IPCC's new findings.
Scientists are searching past and present climate data for other ways to improve our confidence in the greenhouse warming models. For example, some models anticipate a greater frequency of climate extremes, such as floods, droughts, and cold spells, and an increase in the severity of storms. Recent weather events seem to support this prediction, but firm conclusions await a longer record of precise measurements.
To fully address policy makers' questions about global warming, the models must take the additional step of predicting future climates. This is even more difficult than reproducing the climate of earlier times. Factors of little significance in the past may become important as conditions change. For instance, no one knows how cloud cover will be affected by variations in regional temperature and humidity. These issues are also areas of continuing study.
The IPCC report is understandably cautious. It offers not one, but a range of predictions. The report points to "substantial uncertainty" in some aspects of the way Earth's climate is determined, and in our knowledge of certain attributes of conditions today. It stresses that what the future brings will depend in part on how the economy performs, and on how people and governments react to unfolding environmental events.
So the IPCC's work is not over. There must be further assessments of the global warming threat as models improve and more data is taken. But even without a clear signature of human involvement, policy makers, as well as citizens, must consider those suggestive fingerprints as we make decisions that may affect the future of Earth's climate.
Ralph Kahn is a research scientist in Pasadena, California, who frequently writes about Earth and space science issues.
Copyright © 1995 by R. Kahn. All rights reserved.