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We emphasize that this scenario is meant to illustrate a low probability, "surprise" occurrence. Indeed, the rapidness of the changes in ocean circulation found by Mikolajewicz et al. are likely to be due to the nature of their modeling exercise, namely, that they omitted the effect of ocean feedbacks on the atmospheric energy balance. The IPCC notes that such rapid changes in ocean circulation are not computed by models that take ocean feedbacks into account (IPCC, 1992).
Other assumptions are the same as in the Conventional Wisdom scenario.
Cooling in the Northern Hemisphere has an important influence on the global build-up of greenhouse gases. For example, carbon uptake is especially reduced in northern boreal forests because of their extensive area, the cooling they are exposed to, and the assumed relationship between net primary productivity and temperature (Figure 11d). (See Klein Goldewijk et al., 1994 for a description of this relationship.) This effect is particularly pronounced in CIS where C uptake in year 2050 decreases from 2.6 Pg C a-1 to 1.0 Pg C a-1 between the Conventional Wisdom and Ocean Realignment scenarios (Figure 12b). The global biospheric uptake of the two scenarios differs by about 2.7 Pg C a-1 in year 2050 (Figure 12a). With reduced C uptake, atmospheric CO2 reaches 90 ppm higher than the Conventional Wisdom scenario in year 2100 (Figure 13a).
Another effect of the cooler temperatures in this scenario is a lower mixing ratio of water vapor in the atmosphere. This results in lower production of hydroxyl radical, which is the main atmospheric sink of CH4. Consequently, CH4 concentrations are higher in this scenario than in the Conventional Wisdom scenario (Figure 13b). Although higher levels of greenhouse gases increase radiative forcing, this is compensated by the reduced transport of heat from the tropics. Nevertheless, the trend of declining surface temperatures in the Northern Hemisphere is reversed after 2035 because of the increase in radiative forcing (Figures 14b and 15a). However, by the end of the century the temperature gain in the middle latitudes is only 1.5 0C as compared to 3 to 50C in the Conventional Wisdom scenario (Figures 14a, b).
Cooler temperatures also reduce potential crop productivity which leads to larger land requirements for the same amount of agricultural demand. This is especially important in the northern temperate regions such as the CIS where the area of agricultural land in year 2100 increases from 137 Mha in the Conventional Wisdom scenario to 164 Mha in this scenario. In Eastern Europe, agricultural area in 2100 increases from 61 to 70 Mha, and in OECD Europe from 111 to 146 Mha (Figure 7). The larger area of agricultural land comes partly at the expense of forest land; in year 2100 there is 60 Mha less global forest area in the Ocean Realignment scenario than in the Conventional Wisdom scenario (see Table 7).
Synopsis of the Ocean Realignment Scenario
A change in the circulation of the ocean can result in a temporary cooling rather than warming of the Northern Hemisphere. This cooling would reduce uptake of carbon in the northern boreal forests and other areas, and leads to a greater build-up of CO2 in the atmosphere than in the Conventional Wisdom scenario. The build-up of CO2 and other gases will eventually reverse the cooling trend, although temperatures will remain substantially cooler in the Northern Hemisphere as compared to the Conventional Wisdom scenario up to 2100 and beyond. One outcome of the cooler temperatures is the need for more land to produce the same amount of food in the North (assuming no change in trade patterns), and subsequently a lower rate of forestation of abandoned land. We repeat, however, that this is a low probability scenario and is most useful in illustrating the large differences between the Conventional Wisdom scenario, and an unexpected "surprise" scenario. These differences underscore the need to test the robustness of climate policies against different kinds of uncertainties and "surprises" (Clark, 1986).
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