BALTIMORE, June 25 (UPI) -- Climate scientists are getting close to proving a link between human activity and global warming, but translating that knowledge into long-term forecasts remains as tricky as ever as the data become more complex, researchers report.
Based on the latest research, global warming over the past 50 years now can be attributed to human-caused greenhouse gases "with a high degree of statistical confidence," said Gabriele Hegerl, a research professor at Duke University in Durham, N.C.
Hegerl, with Duke's Nicholas School of the Environment and Earth Sciences, said the connection between observed warming over the past half-century and greenhouse gases is "always large and always positive."
Hegerl and others spoke at CLIVAR 2004, a climate variability and prediction conference held this week in Baltimore.
When the effects of natural influences on climate are removed from the data, she continued, there remains a large effect from human-caused -- known technically as anthropogenic -- factors. Only when scientists examine scales smaller than global trends -- for instance, a hemisphere, continent or smaller area -- does the reliability of the analysis decline.
Even on a continental scale, however, Hegerl said, climate warming is detectable statistically beyond the range of sampling error.
"Anthropogenic influence on climate has been detected in large-scale surface and atmospheric temperature, and in ocean heat content," she said. "The signal can be distinguished from natural influences, such as volcanism and changes in solar irradiance, showing with a high degree of statistical confidence that human activity -- particularly the burning of fossil fuels -- does influence climate."
Signs of anthropogenic influence have emerged at sub-global scales as well, she said, "such as continental surface temperature, and in non-temperature variables, such as sea-level pressure."
Climate models -- computer simulations of global climate -- are what scientist use to try puzzle out what will happen in the future in a world that is heating up.
"The climate system is very complex," John Mitchell, chief scientist with the British Meteorological Office, told United Press International, "and there are a lot of processes we know little about."
Predictions of warming by 2100, for instance, vary from an increase of 2.5 degrees Fahrenheit to 9 degrees (1.5 degrees Celsius to 5 degrees), depending upon the model examined. For the next 40 years or so, however, the models are fairly consistent among one another, showing a gradually warming world.
Mitchell concurred. "Models run with only natural (influences) don't explain what's going on," he said.
Though it is undeniable that the sun, volcanoes, atmospheric dust and other natural processes influence climate, human-generated carbon dioxide emissions must be included in models to reproduce the kind of warming seen in observations.
One of the largest uncertainties in developing an accurate climate model is to reproduce the effect of clouds, whose effects can be very complex. In addition to contributing rain and snow, they reflect sunlight back into space and at the same time trap heat rising from the Earth's surface. Models do a very poor job of reproducing cloud formation accurately.
"It is a very difficult problem," Mitchell said, "and some of the more obvious approaches have not turned out to be particularly helpful."
Clouds are intangible objects -- it is difficult even to establish where one begins and ends.
Susan Solomon, senior scientist at the National Oceanic and Atmospheric Administration in Boulder, Colo., said if atmospheric carbon dioxide levels remain stable at today's levels -- about 376 parts per million -- there still would be a significant increase in average global temperatures over the next 50 years.
"We'll have to live with (policy) decisions for a very long time," Solomon said. CO2 particles in the atmosphere are very long-lived -- up to 150 years.
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Dan Whipple covers the environment for UPI Science News. E-mail [email protected]
