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.
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.
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
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.
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.