The most interesting thing I did on a recent 10-day excursion to Alaska was charter a plane to fly around the Alaska Range. It is quite stunning to see how the vast majority of glaciers have retreated and thinned substantially. For me this was a much more dramatic show of the effects of climate warming than the retreat of the tidewater glaciers in southeast Alaska, which I had seen on several earlier trips.
Even though the magnitude of retreat of the tidewater glaciers is huge, there are internal dynamic instabilities that cause a small retreat to grow into a large one. But for the land glaciers in the Alaska Range there is nothing but persistent warming and increased melting that can explain their pervasive "dying glacier" profiles.
The bush pilots who do mountain flying in Alaska know exactly what is going on. They report that many of the landing sites they formerly used on glaciers in the 1970s are now completely unusable because the ice is gone.
On Tuesday, a four-year study of warming in the Arctic, conducted by more than 300 scientists and sponsored by the United States and seven other nations, is scheduled to be released. It will document the serious environmental changes climate warming is causing.
On my trip, I couldn't help thinking about times in the past when our planet has been warmer. The most recent period was the last interglacial 125, 000 years ago. The summertime sunlight was stronger than at anytime in our present interglacial. This likely was the root cause of the warmer climate back then.
For the warming of the past 30 years, in contrast, there is only one proposed root cause that has a strong basis in both data and theory, and that has survived thoughtful challenges: The warming is caused by atmospheric pollution, primarily carbon dioxide and methane.
Root causes are only one part of the issue, and the massively complex interactions within the environment provide much more to ponder with concern. Whether thinking globally or focusing on the great, dying glaciers of the Alaska Range, the actual magnitude of climate warming reflects a chain reaction of natural events.
One doesn't need to invoke anything particularly exotic -- there are several such events that are thought to operate with particular efficacy in the Arctic:
-- Melting of glaciers and ice caps exposes more dark ground, which absorbs more sunlight.
-- Earlier melting of seasonal snow cover exposes more dark ground, which absorbs more sunlight.
-- Retreat of sea ice exposes dark, absorptive ocean water and allows heat stored in the ocean to be released to the atmosphere.
-- Conifer forest replaces tundra and, because it's darker, absorbs more sunlight.
-- Tundra soils dry out, reducing the capacity of the land surface to buffer temperature changes.
All of these have occurred on the planet. And these alone are enough to convert a 6-degree Fahrenheit global warming to an 18- or 20-Fahrenheit warming in the Arctic, according to trends in past and present climates and climate models.
But what about the unexpected? Should we be worried about some unexpected and powerful event -- as some have suggested -- such as climate warming causing huge releases of naturally stored methane? A very good question and tough to answer.
I am of two minds: On one hand, it is unlikely that such an unexpected effect exists to any significant degree, because looking back through the Ice Age climate history there is a remarkable consistency in the warmth of the interglacial periods.
On the other hand, records of past climates have proven beyond a doubt that very rapid climate changes do occur during Ice Age-like climates. These are not fully explained (though they certainly involve changes of oceanic circulation) and, perhaps more importantly, are not predicted by climate models in any true sense.
A very important question is whether rapid climate changes can only happen during colder climates or whether they might happen in warmer climates too. If it can happen when things start to get hot, then global warming might trigger events that would exceed our worst expectations.
Kurt M. Cuffey is a professor of geography at UC Berkeley.
http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2004/11/07/ING739LK2T1.DTL
