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Year

FIGURE 14.56 Temperature anomalies in the Northern Hemisphere compared to the 1881-1960 mean, calculated based on measured changes in tree ring densities. The 95% confidence limit is ~ ±0.3°C. The line shows bidecadal smoothed levels. Arrows on the lower axis mark some of the major volcanic eruptions (adapted from Briffa et al, 1998).

Year

FIGURE 14.56 Temperature anomalies in the Northern Hemisphere compared to the 1881-1960 mean, calculated based on measured changes in tree ring densities. The 95% confidence limit is ~ ±0.3°C. The line shows bidecadal smoothed levels. Arrows on the lower axis mark some of the major volcanic eruptions (adapted from Briffa et al, 1998).

large-scale impacts on climate (e.g., see Enfield and Mayer, 1997; and Hurrell and Van Loon, 1997).

While the atmosphere and oceans are closely linked, how changes in one impact changes in the other is not as clear (e.g., Broecker and Denton, 1990). The oceans have well-documented circulation systems that play a major role in determining climate. For example, the Atlantic thermohaline circulation, often referred to as the "conveyor," consists of a complex combination of ocean currents that result in the transport of warmer surface waters from the North Pacific into the Indian Ocean, around the African continent, and into the northern Atlantic (Broecker, 1997). This provides a source of heat to air masses moving east in the winter, resulting in much warmer winters in Europe than would otherwise be the case. As discussed by Broecker (1997), this conveyor appears in the past to have jumped from one mode of operation to another, initiating substantial and rapid global climate changes. Furthermore, he suggests that it is possible that substantial increases in greenhouse gases such as C02 may also initiate such changes in the thermohaline circulation, with associated, and perhaps surprising, effects on global climate.

For example, increased water vapor at high latitudes and the associated increased precipitation, combined with melting glaciers, due to global warming could provide a layer of less dense surface water in the northern Atlantic. Since the conveyor is driven by high-density salt water, this could shut down this global ocean circulation system. Such a shutdown is expected to lead to cooling in the Northern Hemisphere but warming in the Southern Hemisphere since the heat transport associated with the conveyor no longer occurs (Kerr, 1998). That is, anthropogenic emissions that one normally associates with greenhouse warming may trip the ocean-atmosphere system in such a way that cooling could result in the Northern Hemisphere and warming in the Southern Hemisphere. Such feedbacks, hypothesized to have been triggered by closing of the Panamanian Isthmus, have been postulated to explain the Northern Hemisphere glaciation that occurred about 3 million years ago (Driscoll and Haug, 1998). Clearly, this is an area that needs to be explored further.

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