Monday, October 25, 2010

Climate change and the Caribbean coral bleaching, again

There's an upswing of skepticism about climate change of late, thanks in part to the crazy season in the US (and I'm not talking about the baseball playoffs, although they have been pretty surprising too). While the pundits argue, the climate keeps on moving.

When I give talks about climate change and coral reefs, I almost always use the two slides on the right. The first slide is a map of "degree heating weeks" (DHW), a measure of accumulated heat stress experienced by corals, in the Caribbean in mid-October in 2005. Severe bleaching and coral morality is typically observed when the values of DHW exceed 8 deg C-week. Basically, the same long period of warm water temperatures that helps spawn the destructive 2005 Atlantic hurricane season caused unprecedented coral bleaching event in the eastern Caribbean.

In 2007, my colleagues and I published a study examining of the likelihood of the 2005 "hot spot" occurring with and without human influence on the climate system. The study contrasted model simulations of the Caribbean with historical data and then computed the statistics of extreme ocean temperature events. The second slide summarizes some of the key results of from study. In a nutshell, our best analysis concluded the 2005 Caribbean "heat wave" would likely be on the order of a once in a thousand year event, had there been no human-generated greenhouse gas or aerosol emissions since the Industrial Revolution ("natural forcing"). By the 1990s, the human forcings increased the odds to once in 10-50 years. And continued warming under "business as usual" would make such heat waves happen in three out of every four years.

Five years later, a Caribbean "heat wave" has happened again. I've been writing for months that there was a strong likelihood of extensive coral bleaching in the Caribbean this fall according to NOAA's advance forecast of sea surface temperatures (in fact, we had a good inkling of this last summer). Now we're getting reports of bleaching from observers in the Caribbean. Add this to the observations (following predictions, once again!) from Southeast Asia and the Equatorial Pacific, and we have what may be the most, or second most, extensive "global" coral bleaching event in recorded history.

For all those writing about this event, keep in mind the predictions. This is what the scientific community predicted was likely to happen. An event which we calculated would be a once in a millennium occurrence without human impact on the climate, happened again five years later.

There are caveats, for sure. There is uncertainty in the model simulations of interannual variability which can affect the specific calculations of odds (see the 2007 paper for some details). And once-in-a-thousand year events can in reality happen five years apart; we'd need to collect data for thousands and thousands of years to properly calculate the statistics. Obviously, that's not feasible, which is the very reason we have computers help us do the math on these problems.

The real climate doesn't care about the political climate.

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