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Saturday, June 9th, 2007
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Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us about Our Future

by Peter D. Ward

This Is How the World Ends

A review by Doug Brown

Many books on global warming are based upon crude computer models (crude compared to our planet's actual climate) and hypothetical what-ifs. Thus they are easily dismissed by skeptics as alarmist litanies of, "Here are some really bad things that could maybe possibly happen if the worst-case outcomes of this model which is built on untested assumptions turn out to be right." Peter Ward, a paleontology professor at the University of Washington (and astrobiologist for NASA), takes a different and much scarier approach. Rather than hypothetical speculations into the future, he starts with actual data from the past. Can we examine the fossil and climate record to identify past instances of greenhouse global warming, and see what happened then? The answer, very disturbingly, is yes.

The first section of Under a Green Sky covers how scientists have examined mass extinctions over time, and how causes are determined. After the Cretaceous-Tertiary event (a.k.a. the extinction of the non-avian dinosaurs) was shown to have been largely caused by a meteor slamming into the earth, extraterrestrial impacts became the assumed cause of all mass extinctions. Everyone ran around looking for craters of the approximate correct age to have caused other events. Ward espoused a more systematic approach, where the fossil record itself was first examined in detail to see if extinctions happened slowly, in phases, or all at once (only the latter favoring an impact). The granddaddy of all mass extinctions, the Permian extinction, was a study target for both Ward and the impact crowd. In the Permian event, almost 90% of species died. To find the cause of this event would garner much fame. Thus, when the impact folks thought they found their crater, they promptly reported to the press the extinction had been solved. The fossil data said otherwise. Ward's wonderfully written book Gorgon discusses this particular debate in more depth, but the short story is the crater turned out to be the wrong age by several million years, and the fossil record indicated waves of extinctions over a short period of time.

If not an impact, what could have made so many things die so quickly? Here's where global warming enters the picture. When carbon dioxide and other greenhouse gas levels were indirectly measured (via isotope ratios in rocks and counting stomata in fossil leaves), it was found a greenhouse event did take place at the end of the Permian, and also at the end of the Triassic (the first part of the "age of the dinosaurs"). Okay, so it got warm and stuffy, but so what? Don't reptiles like the heat? Heat, yes, noxious gases like hydrogen sulfide, no. It was the examination of ocean floor extinctions that finally completed the picture. In impacts like the Cretaceous-Tertiary event, things in the upper half of the ocean die, but not so much in the lower half. In the Permian and Triassic events, the opposite trend was seen; the extinctions started on the ocean floor. Also, dark bands in the rocks signaled the presence of anoxic bacteria in deep water.

Ordinarily, there is a conveyor belt running through all the oceans, both at the surface and at deep levels. The Gulf Stream is a famous part of this conveyor. Warm water moves toward the poles, then sinks down to the ocean floor and heads back towards the equator. This deep water, having come from the surface in polar regions, is well oxygenated. In previous global warming events such as the Permian and Triassic, changes in atmospheric gases were enough to stop the conveyor. With no oxygenated water on the ocean floor, everything there died and anoxic bacteria took over. Ward posits these bacteria produced large amounts of hydrogen sulfide (the gas made by rotten eggs), which then burped up to the surface in large bubbles. Ward and his colleagues calculated there was plenty enough of this nasty gas to account for the extinctions. The scary thing is how fast the conveyor stops. In a matter of decades, the climate can significantly alter, and within a hundred years extinction is the order of the day.

Which brings us to the present. Thanks to us tool-pushing primates, carbon dioxide levels are rising precipitously, setting up circumstances very similar to those seen before. And when those circumstances arose, really bad things happened. Ward closes Under a Green Sky with three hypothetical scenarios for the future, based in part on past occurrences. In the first, we get our act together and cut emissions drastically. If we can keep atmospheric CO2 below 450 ppm (parts per million) come the year 2100, things will get a bit warmer and some ice will melt, but otherwise we should largely be okay. However, this is unlikely, as the current level is 360 ppm (and rising at 2 ppm per year), and much of the world is industrializing as fast as it can, which may push the rate of increase to 4 ppm per year. In scenario two, Ward assumes we hit CO2 levels of 700 ppm by the year 2100. Sea level will have risen several feet, the ocean conveyor will have recently shut down triggering climatic changes, resulting in massive numbers of refugees. In scenario three, Ward assumes year 2100 CO2 levels of 1,100 ppm. Earth would be 10 degrees Celsius warmer. All of the world's ice would be melting, and much of the world's population displaced by rising waters. The conveyor would have shut down decades earlier, and signs of deep ocean anoxic bacteria beginning to show. Due to changes in the atmosphere, the sky would be turning a sickly shade of green. The sixth great mass extinction would be underway.

What makes this such a terrifying book is it isn't based on theoretical mathematics. Rapid increases in greenhouse gases have shut down the ocean conveyor several times before, resulting in severe climate change and mass extinction. If Ward's analysis is correct, we know what caused it and we know how to make it not happen again. The question is: can we save us from ourselves? Perhaps if people read Under a Green Sky and tell their friends about it, we might have a chance. Many people are apathetic about global warming because the press concentrates on superficial metrics like mean temperature and sea levels rising a few feet. So we grow oranges in Alaska, who cares? Peter Ward offers a reason why we should all care, and right now.

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