Journal
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
Volume 373, Issue 2054, Pages -Publisher
ROYAL SOC
DOI: 10.1098/rsta.2014.0417
Keywords
climate sensitivity; ocean heat uptake; radiative forcing; climate change; climate modelling
Categories
Funding
- European Research Council under the European Community's Seventh Framework Programme FP7, ERC [247220]
- NCAS-Climate Programme
- Joint DECC/Defra Met Office Hadley Centre Climate Programme [GA01101]
- Natural Environment Research Council [ncas10009] Funding Source: researchfish
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In the Coupled Model Intercomparison Project Phase 5 (CMIP5), the model-mean increase in global mean surface air temperature T under the 1pctCO(2) scenario (atmospheric CO2 increasing at 1% yr(-1)) during the second doubling of CO2 is 40% larger than the transient climate response (TCR), i.e. the increase in T during the first doubling. We identify four possible contributory effects. First, the surface climate system loses heat less readily into the ocean beneath as the latter warms. The model spread in the thermal coupling between the upper and deep ocean largely explains the model spread in ocean heat uptake efficiency. Second, CO2 radiative forcing may rise more rapidly than logarithmically with CO2 concentration. Third, the climate feedback parameter may decline as the CO2 concentration rises. With CMIP5 data, we cannot distinguish the second and third possibilities. Fourth, the climate feedback parameter declines as time passes or T rises; in 1pctCO(2), this effect is less important than the others. We find that T projected for the end of the twenty-first century correlates more highly with T at the time of quadrupled CO2 in 1pctCO(2) than with the TCR, and we suggest that the TCR may be underestimated from observed climate change.
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