Paleoclimate Evidences for Instability of the Ocean Circulation
The issue of the stability of ocean circulation attracted a large attention after discovery of abrupt climate changes in the Greenland ice cores in the early 1990s. These records revealed that during the last glacial age, climate was rather unstable and was characterized by numerous abrupt shifts between cold and relatively warm states (Figure 3). The most prominent abrupt climate changes, known as Dansgaard-Oeschger events, correspond to abrupt warmings in Greenland by 10-15 °C over just several years or decades. This finding was corroborated later by numerous marine and terrestrial paleoclimate records from different locations which revealed abrupt climate changes apparently synchronous with that observed in the Greenland ice cores.
The initial idea of W. Broecker that these abrupt climate changes are related to the reorganizations of the Atlantic thermohaline circulations received in recent years a strong support from the analysis of different paleoclimate records and modeling studies. It has been shown that during the warm phases of the glacial age corresponding to Dansgaard-Oeschger events, the Atlantic thermohaline circulation was alike its present state and warm surface currents penetrated far into the high-latitude North Atlantic. During the cold periods, known also as 'stadials', although the Atlantic thermohaline circulation was still active, it was less extended to the north, and a much smaller amount of energy was transported toward the Nordic Seas. This caused a substantial southward expansion of the sea ice area and a strong cooling over the North Atlantic realm. At last, during periods of massive iceberg discharge into the North Atlantic from the North American and other Northern Hemisphere ice sheets, the Atlantic thermohaline circulation was completely shut down over centuries or even millennium causing the extreme cold climate conditions.
There is also a growing body of paleoclimate evidences suggesting that climate impact of Dansgaard-Oeschger events was not restricted to the North Atlantic
Figure 3 Paleoclimate records of Greenland temperature anomalies compared to present-day climate reconstructed from 18O isotope concentration (blue), atmospheric methane concentration (red), and relative abundance of woody pollen in sediments core in the southern Italy (green). Vertical dashed lines show a probable temporal correlation between different records. DO4, DO8, and DO12 marks the Dansgaard-Oeschger events number 4, 8, and 12, respectively; BA refers to Bolling-Allerod warm event; and YD refers to Younger Dryas cold event. Greenland and Antarctic data are from BlunierT and Brook EJ (2001) Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period. Science 291: 109-112. Pollen data are from Watts WA, Allen JRM, and Huntley B (1996) Vegetation history and palaeoclimate of the last glacial period at Lago grande di Monticchio, southern Italy. Quaternary Science Reviews 15: 133-153.
realm, and abrupt climate changes synchronous with Dansgaard-Oeschger events, recorded in Greenland, have been found in many paleoclimate records in Eurasia, tropics, and the Pacific Ocean. In the tropics, for example, abrupt climate changes are most pronounced in the paleoclimate proxies reflecting changes in hydro-logical conditions (precipitation) and the strength of summer and winter monsoons. This is fully consistent with results of model simulations showing a southward shift of ITCZ and weaker summer Asian monsoon for a weaker state of the Atlantic thermohaline circulation.
One of the most convincing arguments for the global-scale extent of abrupt glacial climate changes is coeval variations in methane concentration with the temperature changes in Greenland (Figure 3). Since the major sources of methane is the boreal and tropical wetlands, strong excursions in methane concentration comparable in the magnitude with the difference between glacial and modern conditions indicate large changes in temperature and precipitation over a large part of the globe.
It is generally recognized that an increase of atmospheric concentration of carbon dioxide and other greenhouse gases due to anthropogenic activity is the primary cause of observed global warming. While the temperature rise is the most known and well-established aspect of anthropogenic climate change, the rising concentration of greenhouse gases leads to a number of other changes, such as an intensification of hydrological cycle, changes in probability of extreme weather events, gradual melting and retreat of the ice sheets and glaciers, shrinking of sea ice area, which are already supported by the analysis of observational data. It is believed that the ocean circulation, as a rather sensitive and a strongly nonlinear component of the climate system, will also undergo considerable changes in the course of anthropogenic global warming. Based on results of modeling experiments, the Atlantic thermohaline circulation is considered as the most vulnerable component of the global ocean circulation and is expected to weaken considerably in the future. Two major factors affect the ocean circulation under global warming conditions: surface warming and freshening. Both factors affect the local sea water density and meridional density gradient, the primary factor controlling the strength of the Atlantic thermohaline circulation. Surface freshening in the high latitude of North Atlantic caused by increased precipitation, enhanced river runoff, and melting ofthe Greenland ice sheet will lead to a substantial decrease of surface density that can hinder the formation of the North Atlantic deep water masses, the key component of the Atlantic thermohaline circulation.
In a number of numerical experiments with coupled climate models, it was shown that continuous growth of atmospheric CO2 concentration will lead to a weakening, and, in some models, to a complete shutdown of the Atlantic thermohaline circulation. Some models, however, show only a very modest reduction of the Atlantic thermohaline circulation during the twenty-first century and do not show a complete shutdown even under a very high CO2 concentration. The models also disagree concerning the relative role of temperature and salinity changes for the thermohaline circulation change. When assessing the results obtained with different climate models, it is important to realize that ocean models are still relatively coarse resolution, and observational data provide no constrain on sensitivity of the oceanic circulation to temperature or salinity changes, because these changes are still relatively small to be detected with confidence. Another important uncertainty in the prediction of the future of the Atlantic thermohaline circulation is related to the changes in the mass balance of the Greenland ice sheet. Model experiments indicate that under global warming conditions, an increased melting of the ice sheet will overwhelm an increase in precipitation, which implies that Greenland may become an important additional freshwater source for the North Atlantic. The latter will additionally contribute to a freshening of the area where deep water masses are formed and to a slowdown of the thermohaline circulation. However, it is still unclear whether melting of Greenland will be fast enough to cause a complete shutdown of the Atlantic thermohaline circulation.
In spite of all these uncertainties, a general consensus is that in the course of the twenty-first century, the Atlantic thermohaline circulation will weaken, but it is unlikely that abrupt (on the timescale of several years or decade) shutdown will occur. However, if the concentration of greenhouse gases will continue to rise beyond the twenty-first century, a complete shutdown of the Atalntic thermohaline circulation will become more likely. It is important to note that although weakening of the Atlantic thermohaline circulation under global warming is a common feature of many climate models, even a complete shutdown of the thermohaline circulation does not imply immediate cooling or, moreover, entering of a new ice age. Modeling results suggest that greenhouse warming will overwhelm the effect of reduced oceanic heat transport and the warming in the North Atlantic will continue even in the case of substantially reduced thermohaline circulation. This warming, however, is expected to be smaller than in other regions of the planet. At the same time, it is possible that if the thermohaline circulation weakens considerably, it will take centuries for its complete recovering.
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