“Human influence extremely likely to be the
dominant cause of observed warming since the middle of the last century,” was
the headline from Friday’s release of the first instalment of the
Intergovernmental Panel on Climate Change’s fifth assessment report. “Extremely
likely”—indicating a 95%-100% likelihood—was ratcheted up one notch from the
2007 fourth assessment report’s “very likely.” Yet compared to 2007, the IPCC
widened its estimate of the responsiveness of the climate system to carbon
dioxide by reducing the lower band to a 1.5°C increase from 2°C, qualifying the
new estimate as only “likely.”
This is a glaring discrepancy. How can the
IPCC be more confident that more than half the temperature rise since the
mid-20th century is caused by greenhouse-gas emissions when it is less sure of
the climatic impact of carbon dioxide? The explanation is that IPCC reports,
especially the summaries for policymakers, are primarily designed for political
consumption. And as if on cue, British Prime Minister David
Cameron commented on the IPCC report, “If someone said there is a 95%
chance that your house might burn down, even if you are in the 5% that doesn’t
agree with it, you still take out the insurance.”
But poke beneath the surface of the IPCC’s
latest offering and the confection is revealed for what it is. The IPCC’s
quantification of the separate components of the warming since 1951 (greenhouse
gases, cooling from aerosols, internal variability) is deemed only “likely”
(66%-100% likelihood). Only at the IPCC could the sum of these components be
given a greater likelihood than the individual building blocks. Perhaps the
most revealing aspect is that none of the climate scientists involved seems
embarrassed at this nonsense or protests at the manipulation of science for
political ends.