Increased ocean heat transports and warmer climate

Abstract

We investigated the effect of increased ocean heat transports on climate in the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The increases used were sufficient to melt all sea ice at high latitudes, and amounted to 15% on the global average. The resulting global climate is 2°C warmer, with temperature increases of some 20°C at high latitudes, and 1°C near the equator. The warming is driven by the decreased sea ice/planetary albedo, a feedback which would appear to be instrumental for producing extreme high‐latitude amplification of temperature changes. Resulting hydrologic and wind stress changes suggest that qualitatively, for both the wind‐driven and thermohaline circulation, the increased transports might be self‐sustaining. As such, they would represent a possible mechanism to help account for the high‐latitude warmth of climates in the Mesozoic and Tertiary, and decadal‐scale climate fluctuations during the Holocene, as well as a powerful feedback to amplify other climate forcings. It is estimated that ocean transport increases of 50–70% would have been necessary to reproduce the warmth of various Mesozoic (65–230 m.y. ago) climates without changes in atmospheric composition, while the 15% increase used in these experiments would have been sufficient to reproduce the general climatic conditions of the Eocene (40–55 Ma). A companion experiment indicates that increased topography during the Cenozoic (0–65 Ma) might have altered the surface wind stress in a manner that led to reduced heat transports; this effect would then need to be considered in understanding the beginning of ice ages. Colder climates, or rapid climate perturbations, might have been generated with the aid of such altered ocean transports. The large high‐latitude amplification associated with ocean heat transport and sea ice changes differs significantly from that forecast for increased trace gases, for which water vapor increase is the primary feedback mechanism. The different signatures might allow for discrimination of these different forcings; e.g., the warming of the 1930s looks more like the altered ocean heat transport signal, while the warming of the 1980s is more like the trace gas effect. The actual change of ocean heat transport and deep water circulation both in the past and in the future represents a great uncertainty.