Global warming's surprising fallout

As we go on pumping carbon dioxide into the air, we might borrow a line from financial planners. Past performance is no guide to future results.

The buildup of carbon dioxide (CO2) is forcing scientists to rethink their expectations - not only about the buildup of heat on Earth but also about the implications for the natural world far beyond warming.

Take those powerful Alaskan earthquakes. We expect land to rise as the weight of glaciers melts away. Should we also adjust our assessment of earthquake risk?

Two geophysicists say "yes." Glaciers hold down earthquake action even in a seismically active region like Alaska, argue Jeanne Sauber with NASA's Goddard Space Flight Center in Greenbelt, Md., and Bruce Molnia with the US Geological Survey in Reston, Va. They use history and current data to make their case.

For example, earthquake action picked up in places where the ice masses retreated some 10,000 years ago, Dr. Sauber notes. Scandinavia had major quakes back then. Canada also had many moderate quakes as its glaciers melted.

Melting glaciers do not cause earthquakes: Quakes are created when forces within the crust build up strain in rock until something slips. Alaska is seismically active because a North Pacific crustal plate is ramming into southern Alaska, creating pressures that must be relieved at some point.

However, these pressures do push up high mountains where glaciers form - and the weight of the glaciers pushing down can stabilize the situation, if not eliminate the risk altogether. Remove that weight, and the likelihood of a quake goes up as the strain accumulates.

That's what happened with the 7.2 magnitude quake in Alaska's St. Elias region in 1979, Sauber and Dr. Molnia believe. Photographs show how glaciers in the fault area had thinned substantially during the 80 years since the previous earthquake activity.

Sauber says it now is clear that "in areas like Alaska where earthquakes occur and glaciers are changing, their relationship must be considered to better assess earthquake hazard." She adds that satellites are helping seismologists do this "by tracking the changes in extent and volume of the ice and movement of the Earth."

Another nonwarming implication of global warming is plant growth. Because plants use the carbon in CO2 to make their food and structures, they should grow faster as concentrations of the greenhouse gas go up. Many experts hope this will take some of the excess CO2 out of the air. They count on increased nitrogen fixation to supply the extra nitrogen to fertilize the plants.

Not so fast, warn Bruce Hungate at Northern Arizona University in Flagstaff. The experiments of Dr. Hungate and his collaborators show that this expected boon soon turns sour.

After burgeoning for a couple of years, the nitrogen fixers begin to lose their fixing ability. It looks as though molybdenum - a key nutrient - becomes less available as elevated CO2 levels change soil chemistry.

"Our results ... caution against expecting increased biological [nitrogen] fixation to fuel terrestrial [carbon] accumulation," the team warned in reporting its results in the journal Science last spring. The results also show the need for scientists to broaden their perspective when trying to foresee how Earth's ecosystems will respond to global change, the team added.

To adapt the financial guru's mantra, the way things worked in the past is an unreliable guide for expectations of how our planet will respond as humans force more unnatural change upon it.

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