Geoscience Reference
In-Depth Information
the climate sensitivity of his model—the amount of temperature rise for doubled
CO
2
—was slightly higher than today's best estimate. Correcting for those two,
Hansen's 1988 projection was close to being right on the money.
Though scientists used the global climate models to look forward, it was also
possible to look back in time. Over the last quarter of the twentieth century, the
science of paleoclimatology made great advances, in part as the result of new data
from deep-sea sediments and polar ice cores. For two periods in particular, pa-
leoclimate specialists had a good estimate of both temperature and atmospheric
CO
2
levels. One was a cold period: the time of the last maximum Northern Hemi-
sphere ice advance 21,500 years ago. The other was a time of high temperatures
during the mid-Cretaceous, about 100 million years ago. A pair of paleoclimato-
logists used the temperature and CO
2
data from the two periods to calculate a cli-
mate sensitivity of 2.3 ± 0.9°C. Since these calculations incorporated the effects
of whatever had actually happened, they implicitly included changes in cloud cov-
er. Here was another piece of evidence that climate models work and that climate
sensitivity is not far from 3°C.
3
During the first decade of the twenty-first century, scientists confirmed two of
the most venerable predictions of global warming. As Arrhenius had predicted and
the Manabe-Wetherald model had projected, “from 1840 to the mid-20th century,
the Arctic warmed to the highest temperatures in four centuries.” The warming
“ended the Little Ice Age in the Arctic and caused retreats of glaciers, melting of
researchers corroborated the second prediction, finding that the “stratosphere has
cooled substantially over the past few decades.”
5
Adverse to the Well-Being of Humanity
Starting in the early 1970s, as the models had forecasted, global temperatures
began to rise. The models consistently showed that by the end of the twenty-first
century, business-as-usual CO
2
emissions would lead to a dangerous temperature
rise of several degrees. Growing concern led the World Meteorological Organiza-
tion (WMO) and the United Nations Environmental Program (UNEP) to sponsor a
1979 World Climate Conference in Geneva, Switzerland.
Theconferencereporturgednations“toforeseeandpreventpotentialman-made
to provide the required scientific advice, WMO and UNEP established a new body
they named the Intergovernmental Panel on Climate Change (IPCC).
