Dec. 6, 1999 For the first time, scientists placed space-based observations of Arctic sea ice retreats into a much longer-term context and have examined the likelihood that the sea ice decreases are in part because of human-caused climate change.
The team, led by Konstantin Vinnikov of the University of Maryland, used computer climate models to examine whether the decreases observed in the ice cover of the Arctic over the past few decades are the result primarily of natural climate changes or might also be influenced by human-induced global warming, says Dr. Claire Parkinson, a climatologist at NASA's Goddard Space Flight Center (Greenbelt, Md.) and one of the co-authors of the study.
The team used spacecraft observed and ground-based data to measure sea ice retreats. Scientists used a computer model to simulate how much ice there would be without human-added carbon dioxide in the atmosphere.
"Satellite data from November 1978 through March 1998 (19.4 years) reveal that the Arctic ice extent overall has shown a downward trend of 37,000 square kilometers per year, meaning a loss each year of an ice area well exceeding the combined areas of the states of Maryland and Delaware. The total loss over 19.4 years would be an area exceeding the size of Texas," said Parkinson. To put this into a longer-term context, the team used a 5,000-year run of a global climate model of NOAA's Geophysical Fluid Dynamics Laboratory at Princeton University. They found from this 5,000-year run that the probability of getting a negative trend over 19.4 years as large as that found from the satellite data was less than two percent, suggesting that the negative trend derives from more than just natural variability.
The team then examined outputs from computer simulations that include greenhouse gas increases, tending to warm the atmosphere, and aerosol increases, tending to cool the atmosphere. The model results with these human-induced changes included show atmospheric temperature increases and a much better match with the observed sea ice decreases than the model results simulating natural variability, suggesting that the Arctic sea ice decreases could partially be in response to increasing greenhouse gas levels during the second half of the twentieth century. Vinnikov says that the results suggest that melting Arctic sea ice is probably related to human-induced global warming.
"We only have satellite data for a relatively short period of time," says Parkinson. "It was interesting to be able to put the satellite data into a longer term context by using the model simulations."
The study, by a team of meteorologists, physicists, and climatologists from the University of Maryland, Rutgers University, NOAA, the University of Illinois, NASA, the Hadley Center in Great Britain, and the Arctic and Antarctic Research Institute in Russia, will appear in the Dec. 3 issue of Science.
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