Dusting the climate for fingerprints; has the greenhouse warming arrived? Will we ever know? - searching for the cause of global warming; includes related article

Science News, June 10, 1995 by Richard Monastersky

After lurking in the back pages amid the ads for the past few years, the topic of global warming has once again clambered onto the front pages of newspapers around the world.

In recent months, an iceberg nearly as large as Rhode Island broke off an Antarctic ice shelf, apparently because of rising temperatures there. A statistician declared that the seasons have slipped out of sync with the calendar, perhaps because of greenhouse gas pollution. And just in time for a climate summit in Berlin 2 months ago, a German research team reported finding an abnormal pattern of change in climate records that does not correspond to any known natural causes.

Although the annual average global temperature has risen by about 0.5oC since the late 19th century, investigators have had difficulty determining whether natural forces or human actions deserve the blame. But in late February, Klaus Hasselmann, director of the prestigious Max-Planck Institute for Meteorology in Hamburg, Germany, stepped forward to point a finger.

The Max-Planck researchers find it highly improbable--only 1 chance in 20--that natural forces caused the temperature rise during the last century.

Environmental groups attending the Berlin climate summit rallied around the recent findings. Proof of greenhouse warming has arrived, many trumpeted.

Not quite, say Hasselmann and other researchers intimately involved in the hunt for human influences on climate. Recent developments do indeed bolster the theory that greenhouse gases have reset Earth's climate dial. But scientists acknowledge that uncertainties continue to plague studies aimed at detecting the human fingerprint in climate change--a distinctive change attributable only to human activities. Proof remains elusive.

What's more, researchers warn that the public should not hold its breath waiting for the unambiguous detection of human-caused greenhouse warming. Studies aimed at such a detection, while illuminating, will never give a definitive answer. "From the scientific point of view, I think it's just a sport, frankly. I really think it's wrong to pin all one's hopes on this proof," says Hasselmann.

Sport or not, the search for a greenhouse fingerprint has become the rage among climate scientists. As the name suggests, such efforts resemble the methods employed by police trying to crack a crime--especially the new technique of DNA fingerprinting, which has figured prominently in the current trial of O.J. Simpson.

In the forensic process of DNA fingerprinting, investigators compare a suspect's complex DNA pattern with samples found at the crime scene. In the case of global warming, the purported crime scene is the environment, and greenhouse gases are the prime suspects. Clues such as out-of-sync seasons (SN: 4/8/95, p.214) or giant icebergs (SN: 4/29/95, p. 271) do not, by themselves, implicate any one culprit. Researchers must sift through climate records for specific patterns that could only arise from the effects of greenhouse pollution.

What does the fingerprint of greenhouse warming look like? Hasselmann's group and others use computer climate models to calculate the kind of abnormal signal that human tinkering with the climate might create. The German team generated an anthropogenic, or human-caused, climate change by slowly boosting the amount of greenhouse gases in the model's atmosphere, starting from 1935 values. As the simulation progressed through the decades, a greenhouse fingerprint emerged: a specific pattern of global changes with temperatures increasing most dramatically in the interior of continents.

The team then compared this pattern with available temperature records for the period 1854 through 1993. Using statistical analyses, they tested how closely their computer-generated fingerprint matched the observations.

To solidify their case, Hasselmann's team had to address the same concerns that confront experts who use DNA fingerprinting in a criminal trial. Just as prosecutors must convince juries that the forensic technique will not implicate the wrong person, climate researchers must argue that their fingerprinting methods do not yield false matches.

Because natural factors can alter climate, Hasselmann and his colleagues needed to estimate the extent to which conditions can vary on their own. Ideally, this information would come out of the available climate data. But useful regional records of surface temperatures reach back only 140 years, not nearly long enough to give a full picture of the kind of changes that nature alone can produce. Moreover, the records are probably compromised because they contain changes wrought by greenhouse gases and other pollutants.

So the researchers again turned to computer climate models. To estimate Earth's own variability, they let the climate model run for 1,000 years, with greenhouse gas concentrations locked in at modern values. They also removed the estimated contribution of greenhouse gases from the climate observations to obtain another gauge of natural variability.