Geoscience Reference
In-Depth Information
The IPCC was set up not to conduct research itself but to review the scientific
literature and advise world governments onthe state ofclimate science. Thousands
of scientists participate in the IPCC's work, almost all of them unpaid volunteers.
Diplomats and government representatives from well over one hundred nations
oversee their work and vet their reports, approving the IPCC's summaries for
policy makers line by line. The process guarantees that the IPCC's reports are con-
servative and more prone to understatement than exaggeration.
The First IPCC Assessment Report appeared in 1990, two years after the organ-
ization's founding. Already the panel had the confidence to say that “emissions
resulting from human activities are substantially increasing the atmospheric con-
centrations of the greenhouse gases, enhanc[ing] the greenhouse effect, resulting
on average in an additional warming of the Earth's surface.”
Global temperatures had “increased by 0.3 to 0.6°C over the last 100 years . . .
broadly consistent with predictions of climate models, but also of the same mag-
nitude as natural climate variability.” Under business-as-usual carbon emissions,
“global mean temperature during the [twenty-first] century [would increase] about
0.3°C per decade, greater than that seen over the past 10,000 years.”
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Five years later the Second Assessment Report concluded that “the balance of
language was qualified, the message was clear: the IPCC was on its way to con-
firming that human activity is changing the climate.
The 2001 Third Assessment Report (AR3), instead of giving a range, pinned
down the global temperature rise during the twentieth century to 0.6°C. Most of
the increase during the immediately preceding fifty years was attributable to hu-
man activities, the report stated. It included a chart showing the results of climate
model simulations of twentieth-century temperatures assuming three different in-
puts: first, only natural factors like volcanic eruptions and changes in the intens-
ity of the Sun; second, only greenhouse gas emissions; and third, both together.
Neither natural factors nor greenhouse gas emissions alone could explain the ob-
served temperature rise, but the two combined came close. The summary warned:
“Human influences will continue to change atmospheric composition throughout
the 21st century. Global average temperature and sea level are projected to rise un-
der all IPCC scenarios.”
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By the time of the Third Assessment Report, climate models still could not fully
account for the global cooling that had occurred from the 1940s through the 1970s.
Then in 2007, scientists discovered that the discrepancy was as much a function
of the data as of the models. Before World War II, most of the data on sea-sur-
face temperatures came from U.S. vessels, which measured the temperature of
