Monday, February 18, 2008

The Day After Tomorrow

The movie "The Day After Tomorrow" is about the end of the world, and, unexpectedly, the cinematography is fairly good. I was surprised by how much I enjoyed seeing Los Angeles destroyed. New York is another matter: NY is destroyed every other weekend at the box office, and I've never lived there anyway, but I have lived near LA and I now believe that every good movie should destroy something important in your life.

Plus, the scientific side of the story was thought-provoking. Catastrophic (saltational) climate change can't be ruled out based on our present state of knowledge. The fossil record isn't precise enough to determine if past climate changes happened in 5, 50, or 500 hundred years. Also, the current level of Co2 is unprecedented for at least the last 400,000 years (the limit of our ice core data) and the rate of increase is likely unique in the entire history of the planet. We simply don't know enough from past experience to accurately anticipate how this is all going to play out.

The IPCC has issued a series of scenarios with likely average temperature increases, but a thorough understanding of the problems inherent in such models calls into question the precision of these averages. The most commonly cited average gives increases of of 2-6C over the next 100 years. But all of the models they currently use are inherently gradualist models (more on this below). One could be a skeptic whether these models are accurate at all, but their (retrograde) prediction of the cooling associated with the Mt. Pinatubo eruption convinced many of their validity. It is edifying to note the maturation of models that originally didn't even take into account atmospheric dust; this is certainly a new science and many problems remain to be solved, particularly the response of vegetation to increased CO2. (CF. McNaughton & Jarvis. Effects of spatial scale on stomatal control of transpiration. Agricultural and Forest Meteorology, 54, 279-301). There is a definite possibility that other, as yet undetermined, variables could play a significant role in future climate change.

But the problem with relying on IPCC averages as outer limits of possible climate changes goes deeper than possible 'out of left field' variables. A recent effort using contemporary day-to-day "matrix" models of weather (e.g. the kind that are used to predict the path of hurricanes) to predict long term climate change yielded slightly higher averages (up to 11C) than the IPCC,. But the most important result was buried in the calculation of these averages. It turns out that the researchers, like many other modelers, ran their simulation thousands of times and only averaged the results that seemed reasonable. A significant proportion of the models "crashed" to negative or positive infinity temperature. Obviously, these results could not be averaged with the simulations that "worked", but they instead point to the instability inherent in these equations.

The climate change models and the way they are interpreted are inherently biased toward gradualist results, but the real danger of climate change is unpredictability. A perfect storm probably won't jump-start a renewed ice age in a manner of days, as in The Day After Tomorrow, but more humble respect for the mysterious potentialities of nature may be wise. The hubris of those who calculate the economic cost of the IPCC predicted change are one example of what can go wrong in a cost-benefit analysis that doesn't factor in the possibility of catastrophes. Attaching specific temperatures to the future climate of the earth is an important undertaking, but it is not a perfect science. Nothing is.

No comments: