"The really big picture shows change in the ice and those changes look like what we get is a world that is a little warmer," says Dr. Richard B. Alley, the Evan Pugh Professor of Geosciences. "We currently do not include all these processes in the models that predict the global future."
Summarizing published literature, Alley notes that spring snows in the Arctic have decreased without a decrease in overall precipitation. A shift in the snowy season is an indication of warming.
As seen by satellites, Arctic sea ice is smaller and sea ice thickness measured by submarines is thinner.
"For the time period with especially good records, northern sea ice clearly shows a downward trend," says the Penn State researcher. "There may also be a downward trend in the south."
One explanation for the changes in the Arctic sea ice is a change in the circulation of the oceans and atmosphere. But the circulation change, which itself may be brought on by warming, does not seem to fully explain the changes in sea ice. Atmospheric warming may simply be causing melting.
"Mountain glaciers are typically more sensitive to temperature than other controls," says Alley. "Most mountain glaciers are getting smaller. Some are disappearing, others just shrinking."
Recent reports indicate that the large ice sheets in Antarctica and on Greenland are growing fatter in the middle due to slightly increased snow. Increased snow may be a sign of warming because warmer air holds more moisture for precipitation. In a few places on the margins of the ice, the ice sheets are getting smaller. The water from the shrinking ice sheets slightly raises ocean levels. "Both of these changes look like some sort of response to environmental warming," says Alley.
For floating ice, warm water beneath the ice sheet and warm air both may contribute to melting. But the real problem is that changes in the ice sheets are complicated, it is not simply melting. The centers of the ice sheets change on a scale of tens of thousands of years, while the edges change in years.
"We have been modeling ice sheets using ice flow models based on local information at the center of the ice sheets," says Alley. "But the ice sheets act globally as well."
The Penn State researcher explains that the models assume that if you push on one side of the ice sheet, the far side does not move. No transfer of energy takes place through the sheet. However, this approach does not work on a floating section of the ice sheet, the ice shelves. If you push on one side, the entire sheet moves.
"We have found that if the ice shelf calved off or a section melted, that the actual ice flow movement was much faster than our current models suggest," says Alley.
Obviously, some physical processes are not in the current models and these are important to the overall picture of the ice. Even little ice shelves can be important.
"However, if we include everything, everywhere, it will take a great deal of computer power and we are not quite ready for that yet," says the Penn State researcher.
What is clear is that ice around the globe is changing and the models are not adequate to fully understand what this means for the future.
"The ice changes are best explained by warming. Now we have to decide how to make better models and how to deal with the effects of warming," says Alley.