Kaufmann and associate David Stern of the Australian National University's Centre for Resource and Environmental Study analyzed historical data for greenhouse gas concentrations, human sulfur emissions, and variations in solar activity between 1865 and 1990. They found that when they eliminated any one variable—whether greenhouse gases, sulfur emissions, or solar activity—significant errors appeared in their statistical estimates for recorded temperatures. This result means that all of the factors taken together are needed to explain the observed changes in the earth's surface temperature, including the products of human activity.

Additionally they found differences in the two hemispheres. In the north the cooling effect of sulfur emissions, produced primarily by coal-burning power plants, almost exactly offset the warming effect of greenhouse gases. In the south, where sulfur emissions are lower, the rise in temperature was more evident.

"Since sulfur emissions have slowed, due to laws aimed at reducing acid rain, the warming effects of greenhouse gases has become more apparent," says Kaufmann.

The scientists report that doubling the atmospheric concentration of carbon dioxide from its preindustrial level—expected to occur within the next century—will increase temperatures in the northern hemisphere by 2.3-3.5 degrees Celsius (4.1-6.3 degrees Fahrenheit). While this may seem like a minor change, Kaufmann observes that during the last ice age (15,000 years ago) the global temperature was only 3-5 degrees Celsius cooler than it is today.

This research was reported in the January 2002 issue of the Journal of Geophysical Research (Atmospheres).