Geoscience Reference
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example that low- and mid-elevation clouds cause cooling, whereas high-elevation
clouds cause warming. The net effect of aerosols is cooling.
As a test, the group used their simple model to try to replicate the known global
climate effects of the 1963 eruption of Mt. Agung in Indonesia. The model “hind-
casted” that after a lag of one to two years, Agung should have caused the average
global temperature to dropby0.1to 0.2°C, after which temperature would recover,
which was just what had happened.
The simple model showed that the most important factors in determining the
Earth's temperature are CO
2
, volcanic eruptions, and changes in the Sun's intens-
ity. Considering only those three, the Hansen model replicated observed temperat-
ures since 1885 quite well. Together the three factors explained more than 75 per-
cent of the variance, a statistician's way of saying the fit is quite good, though not
perfect.
The NASA scientists predicted that CO
2
warming should “rise out of the noise
level of natural climate variability in this century.” Another major volcanic erup-
tion would slow the warming, they said, but only temporarily. Using a fast eco-
nomic growth scenario, they projected that by the end of the twenty-first century,
global temperature could rise by as much as 4.5°C.
Hansen and colleagues noted some of the consequences of doubled CO
2
. Warm-
ing at high latitudes would be “two to five times the global mean warming,”
16
causing “melting of the world's ice sheets” (965) and opening “the Northwest and
Northeast passages along the borders of the American and Eurasian continents”
(957). The model projected that “all the [Arctic] sea ice may melt in summer”
(966) and that “much of the western two-thirds of the U.S. and Canada would suf-
fer hot, dry conditions” (965).
The article ended with a paraphrase of Revelle: “The climate change induced by
anthropogenic release of CO
2
is likely to be the most fascinating global geophysic-
al experiment that man will ever conduct” (966). As the years passed, Hansen has
offered increasingly dire predictions of the consequences of global warming. For
Hansen and a rising number of scientists, fascination has given way to fear for the
future of humanity.
