Climate Model Lapse

The "Great Debate" in climatology these days focuses on the differences between two temperature records: surface and satellite. The problem is this: Thermometer readings from across the planet's surface are warming at a greater rate than satellite temperature measurements of the lower atmosphere, or troposphere. That difference is probably real and not a result of errors in the data sets, since the satellite record is a measure of the temperature of the overlying atmosphere and not the surface. Meteorologists call the difference between the surface readings and those from the overlying atmosphere the "lapse rate," a term that refers to the rate at which temperature declines with height.

Figure 1 shows the details of the two records, which Gabriele Hegerl and John Wallace carefully examined in the latest issue of the Journal of Climate. The top panel is the satellite record (which began in 1979) of global temperatures (departures from the average) with the independent weather balloon record superimposed. Note the close correspondence between these two records during the overlapping period. The middle panel, which is a plot of global mean temperatures at the surface, depicts a larger temperature increase. To calculate the "lapse rate" (bottom panel), we subtract the middle panel from the top panel. The result? Declining lapse rates from 1964 to 1980 and increasing lapse rates thereafter. One of Hegerl and Wallace's goals was to determine the cause of that trend.

Figure 1. Global satellite-measured lower atmospheric temperature (top, with weather-balloon measurements superimposed), global mean surface temperature, and lapse rate, 1964–2000.

The latter period of increasing lapse rates during the era of satellite data is particularly interesting. When the spatial lapse rate patterns are computed (Figure 2), it's obvious that the biggest differences are present over the tropics. In other words, over the low latitudes, the surface is warming faster than the atmosphere.

Figure 2. Satellite-measured lower atmospheric temperature trend (top), surface temperature trend (middle), and the difference between the two (lapse rate) 1979–2000.

The authors attempt to account for this pattern by comparing it to El Niño/La Niña influences and other atmospheric stability factors that arise because warm air masses tend to reside over land and cold air masses are found preferentially over oceans. Yet even after accounting for these issues, the trends in lapse rate remained.

So the data were then run through a climate model, in both a control run and with changes in greenhouse gases and aerosols. Although the climate model could simulate the shorter-term, month-to-month changes in lapse rate well, it did not get the decadal scale changes right. The model had a much tighter coupling between the surface and the overlying atmosphere than is observed in nature.

The bottom line is that no one seems to know why these differences in temperature trends exist. Given that, it's unlikely a climate model would somehow magically figure it out. Indeed it didn't. There seems to be a lapse in our understanding of heat transfer between the surface and the atmosphere. And until we figure out that fundamental issue, climate models will continue to give us the wrong answers.

Reference:

Hegerl, G.C., and J.M. Wallace, 2002. Influence of patterns of climate variability on the difference between satellite and surface temperature trends, Journal of Climate, 17, 2412–2428.