Wednesday, October 06, 2004

Global warming

Today's image from the shrinking polar ice caps. This September, scientists at the University of Colorado measured a 13.4% decline in the Arctic ice cap.

These images are destined to be the equivalent of the images of the ozone layer that got people together on banning CFCs. Of course the scientists who took these images are being cautious. They ought to get out front and scream like hell. Nothing else can account for this.

In their October 4, 2004, press release,University of Colorado scientists reported that the decline in Arctic Sea ice points to “acceleration of the downward trend.” They said one possible explanation for the continuing loss of sea ice in this region is that climate warming from human activities like the burning of fossil fuels is becoming more apparent. “Climate models are in general agreement that one of the strongest signals of greenhouse warming will be a loss of Arctic sea ice,” stated Mark Serreze, of the U.S. National Snow and Ice Data Center (NSIDC). “Some [models] indicate complete disappearance of the summer sea ice cover by 2070.”

In other Earth destroying news, a paper in Science will allege that climate change played a major role in Pleistocene megafaunal extinctions. They acknowledge a role for human hunting but argue that climate change played a role too. I'm not an expert on this, but I've followed it closely, and was always impressed by the data for the overkill hypothesis. What the climate change argument doesn't explain is: why are the major extinctions so staggered? Minor climate fluctuations ought to be manageable, and the effects should be harsher on smaller mammals, which are more restricted in their ranges and movements. Elephants and giant bears should be able to move around, so why, on every continent, did the biggest species die out preferentially? Why does it correlate so closely with the arrival of humans?

The paper's authors argue that some extinctions took place before humans arrived, but I wonder. The tail end of a species geological record is likely to be truncated, and we are unlikely to detect the very leading edge of an invasion. So the fact that the timelines for some species didn't overlap with humans isn't as compelling as it might seem at first.

The other thing that I've always been grabbed by is that the extinctions in Africa were much more moderate than those in the other continents. This makes a lot of sense in the overkill hypothesis. Humans evolved hunting African animals, so they evolved being hunted by humans. That co-evolution prevented overkills. When humans took the techniques they evolved - biologically and culturally - to Asia, Europe, North America, Madagascar and Australia, the animals had nothing to protect themselves.

There are other theories. That humans introduced diseases as they traveled, that they modified the landscape, that the landscape was changing on its own. But none of those give a satisfying, single explanation for the whole pattern. There was fairly constant biological exchange between Africa and Madagascar, the climate on the two is affected by the same processes, so why was the invasion of humans so devastating and prinicpally to large animals? Because humans hunted large animals preferentially.

It's obvious that those factors all would have played some role in all of this. Climate change stressed the animals, human hunting stressed them, anthropogenic and climate driven landscape change stressed the animals. But hunting is the common factor at the moment of extinction.