Commentary on “Spread in model climate sensitivity traced to atmospheric convective mixing”


(02- 01-2014) By Fabrice Lambert,  Associate  Researcher (CR)2. 

A new research led by Prof. Steven C. Sherwood from the Australian Climate Change Research Center at the University of New South Wales indicates that Earth’s climate sensitivity may be higher than previously thought. A widely used metric for Earth’s climate sensitivity has been to simulate how much the global average temperature will rise if carbon dioxide (CO2) levels are doubled compared to pre-industrial levels (280 ppm, 2014 CO2 concentrations are ~400 ppm). The IPCC estimates for equilibrium climate sensitivity have been consistently in the range of 1.5 – 4.5°C, a spread that is due in large parts to the uncertainty in cloud radiative effect. This new study shows that the models which produced the lower estimates (i.e. a climate sensitivity below 3°C) are flawed and not consistent with observations of water vapor circulation. These models simplify the water cycle by assuming that evaporated water over the atmosphere always rises to the higher troposphere (10-15 km altitude) where it contributes to cloud formation. However, observations have shown that air masses containing newly evaporated water often only rise to half that height before falling back towards the surface. This shallower circulation pulls water vapor away from the part of the atmosphere where clouds form that cool the climate. According to the authors, the shallow water vapor circulation should increase in a warming climate, which will reduce the amount of clouds in the atmosphere that reflect incoming solar radiation. Because of this lack of clouds, the Earth’s surface will absorb more energy and surface temperatures will increase faster. Their results imply a most likely climate sensitivity of 4°C with a lower limit of ~3°C. The largest differences from previous climate sensitivities occur in tropical and subsidence zones.

Applied to Chile, this study suggest that northern and central Chile will be disproportionately affected by global warming. These regions may experience oppressive heat stress during summertime and increasingly difficult conditions for agriculture, although further studies are needed to estimate the detailed feedbacks on precipitation and wind currents along the western South American coast.

Paper:  Spread in model climate sensitivity traced to atmospheric convective mixing

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