May 4, 2006

Winds of Change?

Big news is coming out of Nature magazine that there has been a weakening of the atmospheric circulation over the tropical Pacific Ocean—and it is caused by anthropogenic changes to the earth’s greenhouse effect (of course). What effect might this have on the climate? According to an AP story, “It’s not clear what climate changes might arise in the area or possibly beyond, but the long-term effect might resemble some aspects of an El Nino event, a study author said.”

Hmmm. This sounds like an open door for anything. To paraphrase “We’re not sure what might happen, so anything bad that does happen anywhere might be related to this. So go for it!”

The AP story foreshadows this by writing “El Ninos boost rainfall in the southern United States and western South America and bring dry weather or even drought to Indonesia, Malaysia and else where in the western Pacific…As for the Pacific food chain near the equator, the slowdown might reduce population of tiny plants and animals up through the fish that eat them…”

It funny how the AP story left out the fact that El Ninos also act to reduce the number of hurricanes in the Atlantic Ocean. Detail, details…

But, despite all the hoopla, the changes in tropical Pacific circulation documented in the May 4th Nature by the research team led by NOAA/GFDL’s Gabriel Vecchi are pretty small—a weakening of about 3% since the mid-to-late 1800s, and a projected further decline of about 10% by the year 2100. In toto, this size change is barely more than natural variation.

This is demonstrated in Figure 1a (taken from Vecchi et al.) which shows the observed change in sea level pressure difference (the measured used to assess circulation changes) across the tropical Pacific since the mid-1800s. Figure 1b (also from Vecchi et al) shows how climate models project that it should have evolved.

Indo-Pacific SLP gradiant

Figure 1a (top). Time series of observations of the sea level pressure difference across the Indo-Pacific Ocean as recorded in different datasets (the dotted line contains the most up-to-date data). Figure 1b (bottom). Time series of the sea level pressure difference across the Indo-Pacific Ocean as modeled by different runs of a climate model. (source: Vecchi et al., 2006).

Now, we are not wanting to sound too skeptical here, but to us, it hardly seems as if these two records are of the same thing. The observations basically show large interdecadal variations, but no long term trend at all from the mid-1800s to the mid-1970s; a sharp drop in the late 1970s (can anyone say “Great Pacific Climate Shift?); and then a rise from the early 1980s to present levels that are characteristic of the period prior to the mid-1970s. The model projections, on the other hand, do capture the large degree of interdecadal variation, but indicate a slow, gradual, slight decline over the past 120+ years.

If the match between these observations and these model runs is strong enough to prove a human influence then…well, then anything can be proven to be related to global warming…which, is probably the main point here. If some aspect of the weather irks you, just blame it on pernicious industrial activity…you can’t be proven wrong.

Reference:

Vecchi, G.A., et al., 2006. Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature, 44, 73-76.




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