Using the statistical technique of cointegration, the scientists compared these factors over time with global surface temperature in both the northern and southern hemispheres. Cointegration techniques are not confused by variables that tend to increase or decrease over time or contain some poorly measured observations. As such this is the first study to make a statistically meaningful link between human activity and temperature, independent of climate models, Kaufmann notes.
They found that eliminating any one variable - greenhouse gases, human sulfur emissions, or solar activity - made the errors larger; that is, all of those factors taken together are needed to explain changes in Earth's surface temperature.
They found also that the impact of human activity has been different in the two hemispheres. In the north, the warming effect of greenhouse gases was almost exactly offset by the cooling effect of sulfur emissions, making the temperature effects difficult to observe. In the southern hemisphere, where human sulfur emissions are lower, the effects are easier to see, they write.
Kaufmann says, "the countervailing effects of greenhouse gases and sulfur emissions undercut comments by climate change skeptics, who argue 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 says, "the warming effect of greenhouse gases was hidden by a simultaneous increase in sulfur emissions. But, since then, sulfur emissions have slowed, due to laws aimed at reducing acid rain, and this has allowed the warming effects of greenhouse gases to become more apparent."
Analysis of the data indicates that doubling the atmospheric concentration of carbon dioxide from its preindustrial level will increase will increase northern hemispheric temperature by 2.3 to 3.5 degrees Celsius [4.1 to 6.3 degrees Fahrenheit]. In the southern hemisphere, the increase will be between 1.7 and 2.2 degrees Celsius [3.1 and 4 degrees Fahrenheit], the scientists say, noting that this doubling is expected to be achieved over the next century.
Kaufmann observes that while to some, these projected changes may seem small, during the last ice age, more than 15,000 years ago, Earth's global temperature was only 3 to 5 degrees Celsius [5 to 9 degrees Fahrenheit] cooler than it is now.