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Progress in Climate Change Sciences The Role of the Thermohaline Circulation in Climate Change Zhou Tianjun LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences 2. Response of thermohaline circulation to climate change Response of the thermohaline circulation to global warming has drawn great attention of the international climate research community, particularly in European-USA countries. The basic global change during global warming is the substantial warming of high latitudes, and with this condition the reduction of the equator-pole temperature gradient and the increase of poleward moisture flux. Such a re-arrangement of heat and freshwater fluxes tends to weaken the thermohaline circulation. If the meridional overturning conveyor collapsed due to the warming climate, the pole-ward oceanic heat transport would stop, and this undoubtedly would be a serious disaster for the European people. In the past 1. Impact of thermohaline circulation on climate decades, dozens of numerical experiments have been performed Satellite measurements of the Earth radiation budget reveal the to test the sensitivity of the thermohaline circulation to the increase surplus of incoming absorbed solar energy over outgoing longwave of atmospheric greenhouse gasses (e.g. CO2). Some results indicate radiation in low latitudes, while the reverse is true at high latitudes that in response to CO2 increasing, rainfall over high latitudes of on an annual mean bias. Thus, radiative processes are continually the North Atlantic would increase, the freshwater cap covering acting to cool the high latitudes and warm the low latitudes, and it the sea surface would prevent the down-welling of the surface is only the poleward heat transport that serves to offset the energy water, finally the thermohaline circulation would decrease in loss, in which, the oceanic poleward heat transport has great intensity and eventually shuts-down. This result is greatly not contributions. Under current climate system, the Atlantic Ocean convinced, since the climate system models used above are still plays a key role in transporting heat from tropics to high latitudes, far from the target of “state-of-the-art”, the ocean’s response is to which is mainly due to the existence of the meridional overturning great extent dependent on models (IPCC, 2001). circulation, namely thermohaline circulation (Broecker, 1991). As one example, response of the IAP LASG GOALS model to Circulation of the 90% of ocean water beneath the surface zone is CO2 doubling is shown in Figure 2. The initial concentration of driven by the force of gravity as dense water sinks and less dense CO2 is set to pre-industrialization level (345ppm), then it increases water rises. Since the ocean water density is largely a function of gradually with a rate of 1% per year and reaches the doubled level temperature and salinity, the movement of deep water drived from (i.e. 692ppm) after 70 years for integration. Following the increase the density gradient is called “Thermohaline Circulation”. In of atmospheric CO2, the global mean surface air temperature comparison with other regions of same latitudes, the wintertime increases gradually. When the concentration of CO2 is doubled, North Europe experienced a warmer climate (Fig. 1,see color page 4 ), the air becomes 1.65 warmer. Simultaneously, the North Atlantic partially due to the climate impact of the North Atlantic thermohaline circulation also decreases gradually. However, it does thermohaline circulation by transporting meridional heat (Bryden, not collapse, the mean annual intensity remains to be 19.8Sv 1993). Any kinds of potential changes of the thermohaline (1Sv=106m3s-1) when the CO2 is doubled. If the model integration circulation would cause serious impacts on the northern Europe continues, when the CO2 concentration reaches a threshold value, regional climate (Zhou, et al, 2000; Wang, et al, 2004). the thermohaline circulation might be shut-down. Previous studies 44 ClimateChangeNewsletter found that quadrupling the atmospheric CO2, the thermohaline of the thermohaline circulation would lead to a colder North circulation in the North Atlantic would eventually collapse. Atlantic climate, in reverse, the recovering of the thermohaline Signals in the Greenland ice core and the North Atlantic ocean circulation would result in a rapid warming. Some scientists bottom sediments indicated that the North Atlantic experienced suggest that variability of the thermohaline circulation might be rapid climate change in the past hundred thousands years, an closely linked to the ice-sheet covering North America and Europe example of which is the Younger Dryas event (1.1 ~ 1.0 kaBP) during the glacial and interglacial periods. Associated with the (Dansgaard,1989). However, in the recent 10 thousand years and freshwater input resulted from the breakdown of ice-sheet or ice- in particular 8.0kaBP, climate over this domain was relatively melting, sea surface salinity decreased and the deep-water stable. Also there are evidences indicating that the rapid climate formation reduced, which led to a weakened or even collapsed change events of the North Atlantic have affected the global climate thermohaline circulation, thereby colder climate over the domain system to some extent, relevant signals have probably been found and vice versa (Alley, et al., 1999). in other regions of the world. Generally, since the temperature Conclusively, the North Atlantic domain indeed has experienced changes happen within very short periods, they can not be rapid climate change in the past time, which might be closely linked Global mean temperature of sigma 0.991 to the mode transformation of the thermohaline circulation. Progress in paleo-climate studies provides abundant evidence for the people’s worrying about the potential response of the thermohaline circulation to global warming, thereby the suddenly rapid cooling of the western and northern Europe. E-mail: [email protected] Integration time (unit: a) North Atlantic THC Index in Sverdrup Integration time (unit: a) Fig. 2 Response of the global mean surface air temperature (a) and the North Atlantic thermohaline circulation index (b) to the increasing of atmospheric CO2 in IAP/LASG GOLAS model. (Figure (a) in unit of , Figure (b) in unit of Sv, solid square corresponds to the sensitivity experiment, thin line corresponds to the control integration. 1Sv=106 m3s -1) explained simply in terms of the traditional earth orbital parameters theory. The exact nature of the Younger Dryas event is still under intensive study. Scientists speculated that the abrupt climate change results from the abrupt shift of the thermohaline circulation from one mode of operation to another (Manabe and Stouffer, 1988; Muscheler et al, 2000). In general, the weakening and collapsing References Alley, R.B. P.U.Clark, L.D.Keigwin, R.S.Webb, 1999: Making sense of millennial-scale climate change. In: Mechanisms of global climate change at millennial time scales, eds. Clark, P.U., Webb, R.S., Keigwin, L.D. Washington, D.C.: American Geophysical Union, 394 p. Geophysical Monograph 112. Broecker, W. S. 1991: The great ocean conveyor. Oceanography, 4(2): 79-89. Bryden, H.L, 1993:Ocean heat transport across 24°N latitude. In: McBean G A, Hantel M, Eds. Interactions between global climate subsystems. Geophysical Monograph 75. American Geophysical Union. 65-75. Dansgaard, W., J. W. C. White, and S. J. Johnsen, 1989:The abrupt termination of the Younger Dryas climate event. Nature, 339: 532-533. Intergovernment Panel on Climate Change (IPCC), Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernment Panel on Climate Change. Houghton, J. T., Y. Ding, D. J. Griggs, M. Noguer, P. J. Van der Linden, X. Dai, K. Maskell, and C. A. Johnson (eds). 2001:Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 562-565pp. Manabe, S., and R. J. Stouffer, 1988: Two stable equilibria of a coupled ocean-atmosphere model. Journal of Climate, 1(9): 841-866. Muscheler, R, J.Beer , G.Wagner, R.C.Finkel , 2000: Changes in deepwater formation during the Younger Dryas event inferred from 10Be and 14C records. Nature. 408(6812): 567-570. Wang, S.W., T.J. Zhou, J.N. Cai, J.H. Zhu, Z.H. Xie, D.Y. Gong, 2004: Abrupt Climate Change at 4ka BP: Cause and Social Consequence. Advances in Atmospheric Sciences, 21(2): 298-302. Zhou, T.J., X.H.Zhang, S.W.Wang, 2000: The relationship between the thermohaline circulation and North Atlantic Oscillation. Chinese Science Bulletin, 45(11): 1052-1056. ClimateChangeNewsletter 45