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Boston, MA — The average temperature at the earth’s surface is rising – but is this because of human activity, or is it only a phase in the earth’s natural cycle of change? Boston University researcher Robert K. Kaufmann and Australian National University researcher David I. Stern have found that for the 125-year period beginning in 1865, human activity directly contributed to changing the earth’s surface temperature. Their just-released study is the first to establish a statistically meaningful link between human activity and global temperature that isn’t based solely on computer models.
The study analyzes historical data for greenhouse gas concentrations, including carbon dioxide, methane, nitrous oxide, and chloroflurocarbons 11 and 12; industrial sulfur emissions; and variations in solar activity between 1865 and 1990. The researchers compared these factors to recorded global surface temperatures for this period. They found that when they eliminated any one variable – whether greenhouse gases, sulfur emissions, or solar activity – significant errors appeared in their statistical estimates of the recorded temperatures. This result means that all of the factors taken together are needed to explain the changes in the earth’s surface temperature that have been observed, including the products of human activity.
Kaufmann and Stern also found that the impact of human activity has been different in the earth’s northern and southern hemispheres. In the north, the warming effect of greenhouse gases has been almost exactly offset by the cooling effect of sulfur emissions. This phenomenon has made effects on temperature difficult to observe. In the southern hemisphere, where sulfur emissions from human activities have been lower, the effects are easier to see.
Boston University’s Kaufmann says, “The countervailing effects of greenhouse gases and sulfur emissions undercut the argument by skeptics of climate change that the rapid increase in atmospheric concentrations of greenhouse gases between the end of World War II and the early 1970s had little effect on temperature.” During this period, he explains, “what we’ve found is that the warming effect of greenhouse gases was hidden by the cooling caused by a simultaneous increase in sulfur emissions. But, since then, because of laws intended to reduce acid rain, sulfur emissions have slowed, and this has allowed the warming effects of greenhouse gases to become more apparent.”
The study also indicates that if the preindustrial atmospheric concentration of carbon dioxide were doubled, the temperature in the northern hemisphere would increase by as much as 6 degrees Fahrenheit. The southern hemisphere would experience a 4 degree Fahrenheit increase. Such a doubling is expected to take place over the next century.
Kaufmann observes: “These projected changes may seem small, a few degrees. But during the last ice age, when the climate was vastly different from today’s, the earth’s global temperature was as little as 5 degrees Fahrenheit different from what it is today.”
The two researchers used recently developed statistical “cointegration” techniques in their analysis. Cointegration allows researchers to avoid computational results that may show a relationship between different factors even when there is no actual relationship. Such spurious results are particularly likely when, as in the case of changes in temperature and human activity, the variables involved tend to increase or decrease over time or contain some poorly measured observations. Kaufmann and Stern’s study is the first analysis of global warming to make use of these corrective cointegration techniques.
Kaufmann is at Boston University’s Center for Energy and Environmental Studies, while Stern is at the Australian National University’s Centre for Resource and Environmental Studies. Their research appears in the just-released January issue of the “Journal of Geophysical Research” (“Atmospheres”). Note for journalists:
The paper by Robert K. Kaufmann and David I. Stern, “Cointegration Analysis of Hemispheric Temperature Relations,” appears in the Journal of Geophysical Research (Atmospheres) of January 2002. Its citation, which is to the online version, is Volume 107, no. 1, 10.1029JD000174, January 2002.
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