Watts Up With That?

Guest Post by Willis Eschenbach

There is a new paper in Nature magazine that claims that the tropics are expanding. This would be worrisome because it could push the dry zones further north and south, moving the Saharan aridity into Southern Europe. The paper is called “Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone”, by Robert Allen et al. (paywalled here , supplementary information here  , hereinafter A2012). Their abstract says:

Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed. This expansion is important because it is associated with shifts in large-scale atmospheric circulation and major climate zones. Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models. Here we use a climate model with detailed aerosol physics to show that increases in heterogeneous warming agents—including black carbon aerosols and tropospheric ozone—are noticeably better than greenhouse gases at driving expansion, and can account for the observed summertime maximum in tropical expansion.

Setting aside the question of their use of a “climate model with detailed aerosol physics“, they use several metrics to measure the width of the tropics—the location of the jet stream (JET), the mean meridional circulation (MMC), the minimum precipitation (PMIN), the cloud cover minimum (CMIN), and the precipitation-evaporation (P-E) balance. Figure 1 shows their observations and model results for how much the tropics have expanded, in degrees of latitude per decade.

FIGURE 1. ORIGINAL CAPTION FROM A2012: Figure 2 | Observed and modelled 1979–1999 Northern Hemisphere tropical expansion based on five metrics. a, Annual mean poleward displacement of each metric, as well as the combined ALL metric. … CMIP3 models are grouped into nine that included time-varying black carbon and ozone (red); three that included time-varying ozone only (green); and six that included neither time-varying black carbon nor ozone (blue). Boxes show the mean response within each group (centre line) and its 2σ uncertainty. Observations are in black. In the case of one observational data set, trend uncertainty (whiskers) is estimated as the 95% confidence level according to a standard t-test.

I note in passing that the error bars of the observations are very wide. In fact, they barely establish the change as being different from zero, and in a couple cases are not statistically significant.

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