Streetwise Professor

May 24, 2007

Well Put

Filed under: Climate Change — The Professor @ 9:19 am

Roger Pielke Sr. has a very illuminating blog entry on climate modeling vs. weather modeling. A common objection to long range climate prediction is “If we can’t predict next week’s weather, how can we predict the next century’s climate?” The stock climate modeler response is that climate is stationary, so we can predict future averages (where averages are taken over relatively long periods) with considerable accuracy. The short run vicissitudes of weather around long run means are effectively averaged out when forecasting climate.

Pielke notes, however, that this is only part of the story, and arguably not a very important part. The “stationary” part of climate models is in the basic physics, but this represents only a fraction of the forces that affect climate. Climate also depends on numerous other processes and feedbacks. Climate models “parameterize” these processes. Unfortunately, some of these processes are very poorly understood (e.g., clouds, just to name one), so the parameterizations are crude. Moreover, these factors can interact in complex–and poorly understood–ways. Even if the climate system is indeed stationary, the relevant model may be badly misspecified due to incorrect parameterizations. This can lead to biased projections. Even if the vicissitudes in weather indeed average out, the models may be waaaaay off, because the estimate of the mean can be biased due to biased parameterizations. That is, in the presence of potentially large uncertainty in the parameterizations, it is a non sequitur to assert that stationarity implies that climate forecasts are more accurate than weather forecasts. Large model errors can swamp the effects of stationarity.

Dr. Pielke deserves plaudits for pointing this out. He does so in an evenhanded, thoughtful, and fairminded way. (In stark contrast to many in the climate sciences–at least many of those who have a high public profile.) He seems to be a serious scientist–one who acknowledges anthropomorphic effects on climate, but who skeptically evaluates competing claims and evidence. Would that more were like him.

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