Researchers Find Direct Evidence That Humans Cause Global Warming

While most climatologists agree that the earth's average surface temperature has increased by about 0.6 degrees Celsius over the past century, they have been uncertain about the cause--natural forces or human activity. Kaufmann and Stern examined the northern and southern hemispheres' historical temperature record from 1865 to 1994. Using a statistical technique known as the Granger causality test, they found that there was a "causal order" from the southern hemisphere to the northern hemisphere.

"What causal order means is that past values for temperature in the southern hemisphere help us predict temperature in the northern hemisphere better than just looking at past values for temperature in the northern hemisphere," says Kaufmann.

Temperature in the northern hemisphere, he adds, where most human activity takes place, is in a statistical--but not physical--sense, dependent on temperature in the southern hemisphere. The reverse is not true. Kaufmann and Stern then tried to account for the causal order with variables that represent natural forces and variables that represent the effects of human activity. Their results showed that changes in solar and volcanic activity alone could not account for the pattern of temperature change. The best explanation for this causal order pattern, says Kaufmann, was human activity--burning coal, oil and natural gas; cutting down forests; and emitting chloro-flouro carbons (gases used in air conditioners and refrigerators).

"The south-north causal order is generated by human activities that increase the warming effect of greenhouse gases globally, but also increase the cooling effects of sulfate aerosols mainly in the northern hemisphere," says Kaufmann. Sulfate aerosols, formed from particles when coal and oil is burned, reflect sunlight and produce a cooling effect on the earth's temperature. "Southern hemisphere temperature, therefore, reflects the increase in greenhouse gases," says Kaufmann.

In the northern hemisphere, this "greenhouse signal" is partly obscured by the sulfate aerosols in the lower atmosphere. The differential effect of the greenhouse gases and sulfates on temperature in the northern and southern hemispheres has increased over time.

"Eventually, the differential effects became strong enough to generate the causal order," says Kaufmann. "Only since the early 1970s has the greenhouse signal that we uncovered in our study emerged from the background noise of natural climate fluctuations."

To further validate their hypothesis, Kaufmann and Stern analyzed temperature data generated by a global climate model run on a supercomputer. They attempted to find the same south-to-north causality in simulations carried out by researchers at Britain's Hadley Centre, a UK Meteorological Office division.

"These researchers could not have foreseen our approach and could not have built into their model the kind of relationship we looked for," says Kaufmann. "And yet, the same results showed up in the simulation data."

The same south-to-north order appears when the model is run with an atmosphere that replicates the pattern of greenhouse gases and sulfates generated by human activity, says Kaufmann. When they ran the model excluding gases emitted by human activity--or with atmospheres that simulate future rates of human activity--the order was absent or ran in the opposite direction.

"The fact that the pattern of causal order changes with the concentration of greenhouse gases and sulfates in the atmosphere implies that the order of temperature change in the northern and southern hemisphere can be used as a fingerprint to identify the effects of human activity on global temperature."

For more information on Kaufmann and Stern's report, call Boston University's Office of Public Relations at (617) 353-3666.

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