Geoscience Reference
In-Depth Information
account: though the direct effect of increasing atmospheric CO
2
is relatively small,
about 1°C for doubled CO
2
, the various feedbacks kick the temperature increase
up to dangerous levels.
Rising Above the Noise Level
The models of the 1970s had many deficiencies. The best the Manabe and Wether-
ald model could do to simulate continents and oceans was to divide the Earth into
two regions, one wet and one dry, emulating a planet half-ocean and half-land. To
estimate the sensitivity of climate to increases in CO
2
, they had to run their mod-
el with the present concentration of CO
2
, double the input amount, run the model
again, and compare the results. But if atmospheric CO
2
were to double in reality,
it would not do so in one fell swoop but gradually, in small increments. Would
a gradual doubling of CO
2
lead to the same temperature rise as an instantaneous
doubling?
And what of the role of clouds and aerosols? Clouds reflect incoming sunlight,
tending to cool the Earth, but they also trap outgoing heat radiation, tending to
warm it. Aerosols include smoke, dust particles, and droplets of sulfuric acid that
volcanoes inject into the atmosphere. They too have the dual effect of reflecting
and absorbing radiation. The early modelers did not have enough information to
know whether the net effect of clouds and aerosols was to cool the Earth or warm
it. Nor did they know how the ocean transports and exchanges heat with the atmo-
sphere.
Another deficiency was fundamental. In the 1960s and 1970s, scientists did not
have enough global climate data to tell how well their models matched reality. Sa-
tellites had not yet begun to collect data on the temperature and composition of the
atmosphere at different elevations. Ocean temperature, salinity, and currents were
poorly sampled.
By the time these missing data had become available, the models had grown
enormously complex, stepping through millions of computations. As in the “but-
terfly effect,” a tiny error in input would multiply through successive calculations
and distort the results. To get around this problem, scientists “tuned” their models,
introducing arbitrary “flux adjustments”—aka “fudge factors”—until the models
gave a result that resembled observed climate. But would these same fudge factors
be reasonable if CO
2
concentration doubled? No one knew.
Gradually over the next three decades, the modelers tackled and largely over-
came each of these inadequacies, doing away with the tuning and building models
that generated climate from scratch without fudge factors. As Weart writes, “evid-
