Figure 6
Model Warming vs. Recorded Warming under 50 Percent
Greenhouse Gas Buildup Assumption
3.5
Model Predicted
2.9°F
2.9°F
3
2.7°F
2.5
2
1.8°F
Actual
1.5
1.1°F
Post-
1
1945
0.5
Pre-
1945
0
MPI
UKMO
NCAR
GISS
1890-1997
Source: Author's calculations.
Note: MPI = Max Planck Institute, UKMO = United Kingdom Meteorological Office, NCAR =
National Center for Atmospheric Research, GISS = Goddard Institute for Space Studies.
Change (IPCC) forecast for the year 2000 is
tends an acceleration of warming in the sec-
2°C, an almost 40 percent drop since the first
ond half of the greenhouse gas doubling
IPCC estimate in 1990.2 1 8
period.2 2 0 The "unrealized warming" has led
noted modeler James Hansen to predict
The downward revisions to date have
that the greenhouse "signal" of heat and
been primarily due to a lower assumed
drought will "increase notably" in the com
-
atmospheric buildup of greenhouse gases
ing years.2 2 1 Separating the recent El Niño
and the inclusion of the alleged warming
offset factor of sulfate aerosols. More realis-
effect from the "record" warming will be
tic model equations and calculations
the difficult first step in testing the Hansen
responsible for determining climate sensi-
prediction, at least in the short run.
tivity (discussed below) remain to be incor-
Lags or not, two recent developments may
porated. One-half of the predicted warming
portend a further reduction in model warm
-
in the revised models is still substantially
ing estimates. First, the rate of greenhouse
greater than the recorded warming, given a
gas buildup has been even slower than
50 percent increase in greenhouse gas
assumed because of flat methane emissions,
buildup to date (Figure 6). This suggests
a phaseout of chlorofluorocarbons, and
that further downward revisions may be
greater biomass intake of CO .2 2 2 Second, the
2
necessary.2 1 9
crucial water vapor climate feedback, which
in climate models doubles the warming esti-
Defenders of the models argue that a lag
mated from doubled CO alone, is likely to
effect produced by ocean absorption of
2
have been overestimated.
heat energy reconciles the past and por-
223
35