Climate Change and Extinction
Its been a rough week on the climate change front. Work was published online in Science and Geophysical Research Letters that strongly indicates that the West Antarctic ice sheet has destabilized, meaning that it will likely retreat rapidly over the next century and end up contributing some 1.2m (4 feet) of sea level rise, which qualifies as catastrophic in human terms (especially if you add in other contributions from Greenland and other ice caps). On a related front, a study commissioned by the military has raised the threat posed by climate change to the security of the United States to being a primary threat capable of starting conflicts (as opposed to an earlier 2007 study that indicated that climate change could worsen conflicts).
In this vein, GG would like to offer a piece of information that some might find comforting. While human-caused climate change might well be a threat to civilization, it (at least on its own) might not be quite as horrifying in terms of extinguishing life on Earth as some might contend. One of the great unknowns in humanity’s ongoing experiment in global climate change is just how severely such change will alter the biosphere. Many presume rapid changes are sure to be catastrophic. Bill McKibben (for instance) wrote a book The End of Nature basically saying there is no way that most species can survive. Groups with a similar outlook make apocalyptic claims that most of the world will become a desert.
This book and others’ claims made the Grumpy Geophysicist grumpier. Why? Well, we have examples in the geologic past of some pretty extreme climatic shifts and we simply don’t see quite that extreme a response. Rapid changes in deglaciation near the end of the last Ice Age (the Younger Dryas event) led the climate to flip back from warm to glacial within a decade and somehow the flora and fauna of North America and Europe survived (we’ll argue the megafauna extinction another day; thank you very much for your patience).
This isn’t to poo-poo impacts of rapid climate changes too much. Major extinction events at the end of the Paleocene and end of the Eocene were both marked (and almost certainly caused by) dramatic shifts in the climate. It is just that in neither case was the land left barren and uninhabitable (and the Eocene was a lot warmer than today, too, so somebody out there can handle hot tropics–in fact there was diversification of flora in the tropics during the rapid warming at the end of the Paleocene).
Well, say climate catastrophists, those events were slower (other than the end of the Ice Age thing, which appears to be more regional to the northern hemisphere). And indeed it appears that the changes at the end of the Paleocene (going into the Paleocene Eocene Thermal Maximum or PETM), which is arguably the best natural experiment in rapid climate change, took about a thousand years. At least, that was the story until some recent work.
Our estimates of the rapidity of the change at the end of the Paleocene (or start of the PETM) has largely been based on deep sea records. The problem there is twofold. One is that sedimentation rates on the deep sea floor are low: a thousand years might be less than a centimeter of rock. The other is that many of the deep sea indicators are reflective of the ocean as a whole, but the ocean takes nearly a thousand years to reach equilibrium with the atmosphere. So two researchers, James Wright and Morgan Schaller at Rutgers looked at some shallow water sediments from this time preserved in southern New Jersey. They noticed that there are rhythmic layers that, they argued, were annual varves created as seasonal streamflows flushed sediment into the shallow ocean. They see the changes in temperature associated with the beginning of the PETM. And it happens within 13 years.
This is amazing on a few counts (GG is not easily amazed). First, to have a record with such fidelity that you could look not only at annual averages but look at seasonal variations [other such records are growth variations in organisms, but these are tougher to tie to a particular event]. Second, that this global change could occur in 13 years or less (it could be less because it still takes some time for even the shallow water to equilibrate).
If these authors are correct, the truly dramatic changes associated with the initiation of the PETM (estimated release of 3000 gigatons of carbon–or more–in just over a decade or less) outdo modern releases of carbon (~30 gigatons/year). We have lots of good reasons to suspect that extinction rates are tied to the rate of change in the environment; if the PETM was really this quick and only produced the level of extinction seen, we might expect that anthropogenic climate change, especially if we limit the rate to something like modern rates, might not (in and of itself) be a catastrophic “sixth extinction.”
Of course this study has invited skepticism; in some ways, the biggest surprise is that these criticisms don’t look particularly fatal. Arguably the largest potential concession might be that instead of 13 years, the record might be more like a couple hundred years if the varves are really from decadal (and not annual) variations. This is still pretty fast and comparable to modern fluxes of carbon. This will bear watching: the PETM is the best template we have in the geologic record for what massive infusions of carbon into the atmosphere will do.
Now before you happily go gassing up your Hummer before taking your private jet for a spin around the neighborhood, do keep in mind there were some notable extinctions of some mammals in the PETM and a big extinction of plankton in the oceans. Also, this time out there are a lot of other factors in play. Humans already caused an extinction of megafauna without needing climate change, and the current extinction event sweeping the amphibian world looks to be enabled by human trade and agriculture. So while anthropogenic climate change itself might not create a major mass extinction, toss in all the other things humans are doing and the prospect for a lot of species isn’t too bright.