The PETM stands for the Paleocene–Eocene Thermal Maximum. This was the most severe episode of global warming to strike the Earth in the geological record. It took place some 55 million years ago and the fossil record shows that there was a massive extinction event and a rise in the fraction of biota occupied by mammals. Temperature rose by about 6 °C (11 °F) over a period of approximately 20,000 years. This is believed to be due to a massive input of carbon dioxide to the atmosphere, although the mechanism of such an influx is not certain.
The PETM is often viewed as the best evidence for the resiliency of life on this planet and justification for outright rejection of my claim that global warming will result in extinction of all life on this planet.
My standard response to such skepticism has, until now, been to point out that the changes currently being experienced are happening so much more rapidly that life will not have time to adapt. This, too, is James Hansen’s main point in refuting the claim that the PETM is a comparable and reassuring example.
But I ran across a data point in my reading that makes the argument even stronger. In J.R. McNeill’s “Something New Under The Sun: an environmental history of the twentieth-century world,” the extent of deforestation is one of many trends discussed. While deforestation has been an issue for much longer than just the twentieth century, the practice became very sidnifican then.
The bottom line is that by 2000 the forests of the planet had been reduced by something between 15 and 45% (the estimates vary drastically) and the rate of decline was proceeding apace at the turn of the century. Vast swaths of forest, including in the equatorial rain forests we envision as impenetrable jungle, have fallen to the chainsaw and the plow.
Why is this so unsettling? Well the forests are the main means on land of removing carbon dioxide from the atmosphere. So, besides life not having geological time scales to adapt, we are presented with an additional handicap: life is going into a PETM-like period with at least one hand tied behind its back. One of Earth’s main weapons in fighting global warming is severely restricted. Going into this battle, which is coming at us with a pace at least 100 times as it came on in the PETM, we have perhaps as much as 50% fewer weapons with which to fight back.
McNeill, so far in my reading, hasn’t addressed the question of how plant microbes in the ocean fared in the twentieth century. However, if they suffered anything like the forests on land did, the use of the PETM is totally inappropriate as a comparison.