The levers of extinction

Carl Zimmer's survey of research and discussion on the matter of the sixth extinction and the role of climate change (Multitude of Species Face Climate Threat) is useful:

scientists who study the impact of global warming on biodiversity are pushing back against the pressure for detailed forecasts. While it’s clear that global warming’s impact could potentially be huge, scientists are warning that it’s still impossible to provide fine-grained predictions.

At a talk on the future of life in Oxford last Sunday Kathy Willis said that while climate change is a matter of huge concern, the direct destruction of natural habitat by human activity on the ground is a much greater threat to biodiversity in the 21st century.

Prof. Willis pointed out that during the PETM, when CO2 concentrations were over 1000ppm (?), tropical forests thrived to a far greater extent than they do today. The implicit corollary (as I understand it): reduce degradation/deforestation and tropical forests will do OK. [For a different view see the note added on 2 May below]

I asked her whether a (presumably) much greater rate of change in atmospheric concentrations during the 21st century (a hundred years or so as against 10 to 20,000 years 55.8mya) could make a difference? Her brief, informal answer was: the rise in CO2 at the time of the PETM was also rapid (constrained, she said, to less than 10,000 years).  She did, however, think that the current rapid rate of change in ocean pH was a significant cause of concern for the marine biota.

P.S. 30 April: Michael LePage has good overview of some instances of human forcing of evolution, and the question of extinction

P.S.  2 May: William Laurance is less optimistic. In an overview of the impact of climate change on tropical forests he concludes:

we know that climates have changed in the Earth’s past, and that species have shifted their latitudinal and elevational ranges in response to this. However, even in periods of rapid warming, such as the interglacial periods of the Pleistocene or the Paleocene-Eocene boundary, temperatures rose only at a few percent of the rate happening today. On top of this, the impacts of future climate change will be worsened by massive habitat loss and fragmentation. The synergistic impacts of these combined body blows could well be the greatest of all threats to tropical biodiversity.