A paper published today in the Journal of Climate finds that increases in ocean heat transport from the warmer tropics to the poles are unlikely to cause alarmist 'positive feedback' 'tipping point' scenarios as predicted by James Hansen and the IPCC. The paper finds that increases in ocean heat transport above the current are likely to be offset by increased cloud cover over the tropics, resulting in cooling of the tropics, without inducing significantly warmer climates than today. The paper shows yet another means by which negative feedbacks - not positive - dominate the climate.
Climate sensitivity to changes in ocean heat transport
Unidad de Ciencias de la Atmosfera, Facultad de Ciencias, Universidad de la Republica, Uruguay
Centro Euro-Mediterraneo per i Cambiamenti Climatici, and Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy
Abstract: Using an atmospheric general circulation model coupled to a slab ocean we study the effect of ocean heat transport (OHT) on climate prescribing OHT from zero to two times the present-day values. In agreement with previous studies an increase in OHT from zero to present-day conditions warms the climate by decreasing the albedo due to reduced sea-ice extent and marine stratus cloud cover and by increasing the greenhouse effect through a moistening of the atmosphere. However, when the OHT is further increased the solution becomes highly dependent on a positive radiative feedback between tropical low clouds and sea surface temperature. We found that the strength of the low clouds-SST feedback combined with the model design may produce solutions that are globally colder than Control mainly due to an unrealistically strong equatorial cooling. Excluding those cases, results indicate that the climate warms only if the OHT increase does not exceed more than 10% of the present-day value in the case of a strong cloud-SST feedback and more than 25% when this feedback is weak. Larger OHT increases lead to a cold state where low clouds cover most of the deep tropics increasing the tropical albedo and drying the atmosphere. This suggests that the present-day climate is close to a state where the OHT maximizes its warming effect on climate and pose doubts about the possibility that greater OHT in the past may have induced significantly warmer climates than that of today.