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 2øC warmer, with temperature increases of some 20øC at high latitudes, and IøC 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 deepwater circulation both in the past and in the future represents a great uncertainty.

Climate change and trace gases

“…CO 2 amount ca 3 Myr ago has been estimated as 380-425 ppm (Raymo et al 1996), but the proxy measures of CO 2 make the uncertainty difficult to quantify. Change of meridional heat transport (Rind & Chandler 1991), perhaps associated with orographic and ocean bottom topography changes, could have contributed to mean temperature change.…”

Sensitivity of East Asian climate to the progressive uplift and expansion of the Tibetan Plateau under the mid-Pliocene boundary conditions

“…The important features of the PRISM2 data set, compared to the present, can be summarized as follows: (1) greatly reduced continental ice volume with a small Greenland ice cap being the only continental ice in the Northern Hemisphere; (2) greatly reduced sea ice extent with the Arctic being seasonally ice free; (3) a positive sea level anomaly of 25 m; (4) increased sea surface temperature at high latitudes; (5) expansion of evergreen forests to the margins of the Arctic Ocean, a reduction of desert area in equatorial Africa, essential elimination of polar desert and tundra regions in the Northern Hemisphere, and a small amount of deciduous vegetation being present at the edge of the Antarctic continent (Dowsett et al, 1999). Additionally, since atmospheric CO 2 concentration was not too far from 315 ppmv at the midPliocene (Rind and Chandler, 1991), this value was taken in all of the experiments. Except for the aforementioned changes, other boundary conditions were kept at the present configuration.…”

Precipitation rates and atmospheric heat transport during the Cenomanian greenhouse warming in North America: Estimates from a stable isotope mass-balance model

“…Increased global temperatures increase the saturation vapor pressure of the troposphere, and modify latent heat transfer from low to high latitudes . Furthermore, as water vapor is a greenhouse gas, the increased vapor content of the lower atmosphere enhances the "greenhouse effect" (Rind and Chandler, 1991). Elevated atmospheric pCO 2 levels from large igneous province eruptions (Larson, 1991;Tarduno et al, 1998), and the presence of continental seaways (Poulsen et al, 1999) may have reduced regional thermal gradients by as much as 15°C.…”