1238. Earth’s Sensitivity to Climate Change Could Be 'Double' Previous Estimates, Say Geologists

By Science Daily, December 10, 2013

The sensitivity of Earth's climate to CO₂ could be double what has been previously estimated, according to a statement issued by the Geological Society of London.

In an addendum to 2010's 'Climate change: Evidence from the Geological Record', the statement notes that many climate models typically look at short term, rapid factors when calculating Earth's climate sensitivity -- defined as the mean global temperature increase brought about by a doubling of atmospheric CO₂.

It is well known that a doubling of atmospheric CO₂ levels could result in temperature increases of between 1.5 and 4.5°C, due to fast changes such as snow and ice melt, and the behavior of clouds and water vapor.

Geological evidence from studies of past climate change now suggests that if longer term factors are taken into account, such as the decay of large ice sheets and the operation of the full carbon cycle, the sensitivity of Earth to a doubling of CO₂ could be double that predicted by most climate models.

Dr Colin Summerhayes, who led the statement's working group, says 'Geological studies of past climate change are throwing new light on how Earth may respond to growing emissions of CO₂. The climate sensitivity suggested by modern climate models may be fine for the short term, but does not encompass the full range of change expected in the long term as Earth's climate moves slowly towards equilibrium.’

The statement also highlights new data showing that temperature and CO₂ levels recorded in Antarctic ice cores increase at the same time. This, says Summerhayes, 'makes the role of CO₂ in changing Ice Age climate highly significant.’

Atmospheric carbon levels are current at just below 400 parts per million -- a figure last seen during the Pliocene, between 5.3 and 2.6 million years ago. At that time, global temperatures were 2-3°C higher than today, and sea levels were several meters higher, due to partial melting of the Antarctic ice sheet.

If the current rate of increase (2 ppm per year) continues, CO₂ levels could reach 600 ppm by the end of this century; levels which, says Summerhayes, 'have not been seen for 24 million years.'

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The above story is based on materials provided by Geological Society of London, The.