Data from dozens of meteorological stations show that the surface temperature across Arctic land and water keeps getting warmer. However, researchers at the University of Wisconsin-Madison now show that Arctic clouds and the climate conditions with which the clouds interact produce a cooling effect, possibly offsetting to some degree the effects of global warming in this region.
Xuanji Wang, UW-Madison graduate student and lead author of the paper, and Jeff Key, a scientist with the National Oceanic and Atmospheric Administration (NOAA) at UW-Madison's Cooperative Institute for Meteorological Satellite Studies (CIMSS), studied a number of climate changes across the Arctic region during the period of 1982 to 1999. Specifically, they noted changes in the surface temperature of the land and ocean, cloud coverage, and surface albedo - the amount of light reflected off surfaces, such as snow or ice.
While a number of researchers have monitored Arctic surface temperature and sea ice extent over the years, the Wisconsin scientists say few have studied other conditions, such as cloud cover, and none have examined how changes in these conditions interact.
"To understand how and why the climate is changing, you have to think about the feedback systems," says Wang. One of the most important feedback systems, he notes, is "cloud forcing." This system involves the interplay among clouds, surface temperature and surface reflectivity (albedo).
Clouds play an important role in climate: not only do they reflect energy from the sun to the ground, but they also can trap heat emitted by the earth and re-emit some of that energy back to the surface. Depending on other climate conditions, such as surface albedo, clouds can either enhance or inhibit surface warming, says Wang.
For instance, when the ground is covered by snow, as is the case for much of the Arctic, solar energy reflects off the snow and is absorbed by clouds. The result: the surface stays cool. But once the covering melts, the ground absorbs the solar energy and surface temperatures rise.
Because cloud coverage, albedo and surface temperature all contribute to the outcome, small changes in one of the factors can produce big changes overall: as the surface warms, ice begins to melt, the ground absorbs solar energy and the surface temperature rises even more.
Recognizing the interplay among climate factors, Wang and Key set out to understand how aspects of the Arctic climate respond to changes in surface temperature.
"Surface temperature is the most important variable of the energy budget," says Key. "But to understand why it is changing, we need to measure other characteristics of the climate."
To do this, the researchers used satellite data collected across the Arctic region to compute surface temperature, albedo and cloud properties. This information helped the Wisconsin team determine cloud forcing - a measurement of the warming or cooling effect of clouds that depends on the interactions among the various climate conditions. They averaged data for each season, as well as for each year.
The researchers found that Arctic surface temperature during the spring, summer and autumn has warmed at decadal rates of 1.1, 0.7 and 0.7 degrees Celsius, respectively. This data confirms similar trends noted in previous studies.
Adding to this information, the researchers also found that the amount of light reflected off the ground or water during these three seasons has decreased. A lower albedo in autumn, they say, indicates a longer melt season and a later onset of freezing or snowfall.
The researchers also found that spring and summer cloud coverage has increased by 2 to 4 percent per decade, but that winter cloud coverage has decreased over the years. When data for cloud coverage was averaged over the year, no changes were noticed.
"The average annual change doesn't tell the whole story," says Key. "Opposite trends in different seasons can cancel on an annual scale, but their seasonal effects are important."
Some of the seasonal changes the researchers found may seem inconsequential, but Key says they are significant: "The Arctic is a place where small changes can have big effects. These effects can signal climate changes elsewhere." He adds, "That's why it's so important to monitor the Arctic."
To understand the cumulative effects of these small changes on the Arctic, the researchers calculated cloud forcing. They found no trend during the spring, but they did find trends toward increasing cloud cooling during the winter, summer and fall seasons. Cloud cooling during the summer, the researchers say, was due in large part to the increased cloud coverage.
"It appears that if clouds conditions weren't changing," says Key, "the Arctic would be getting even warmer," which means even more ice would be melting. More clouds in spring and summer and fewer in winter, he says, appear to have dampened the consequences of global warming on this region.
Because of the height at which the clouds formed, the researchers say the trends they report are the result of not local processes, such as water evaporation, but large-scale circulation patterns. More research on this possible link, they add, needs to be conducted.
Wang and Key say the findings they present confirm and, more importantly, augment the existing data on Arctic climate change with information related to changes in albedo, cloud cover and cloud forcing. "We have added new information on how the climate responds to warming by looking at parameters not previously examined," adds Wang.
This information, the two atmospheric scientists say, will help researchers understand the effects of global warming on the Arctic and, ultimately, the rest of the globe.
-- Emily Carlson, (608) 262-9772, emilycarlson@facstaff.wisc.edu
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Science