The northern hemisphere contains more land area than the southern hemisphere, and conversely, a lower percentage of the world's oceans. Since oceans absorb more heat than land areas, it is not surprising that most climate models predict faster heating over the northern hemisphere than the global average. In addition, the models predict faster temperature increases at the higher latitudes.
North America has long been the breadbasket of the world, with a climate that made high yields of grains possible. The situation may soon change. If global warming trends continue, high temperatures everywhere in the US and reduced moisture in some parts of the country may combine to reduce US agricultural productivity to the point where the US can no longer export food. Besides the loss of a large export market for the US and Canada, countries now receiving grain shipments will surely suffer.
While most climate models have focused on the temperature question, S. Manabe and R. T. Wetharld, using a 3-dimensional atmosphere model coupled with a simple mixed-layer ocean model ran a simulation at the NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The model generated estimates of the percentage change in summer soil moisture levels for the entire world. The results are summarized for Senate testimony [Senate Comm: Energy & Nat. Resources,11/9/87], and have been reproduced in Dean Abramamson's book, 'The Challenge of Global Warming'.
The GFDL model predicts that few areas in the northern hemisphere will receive more moisture - only the Indian/Southeast Asian peninsulas, and a couple of areas in northern Siberia. The rest of the continental areas are projected to have drier summer soils, due in part to earlier snowmelts in the spring, and hotter, more cloudless summers, causing extra evaporation of ground moisture. Levels of soil moisture reduction are shown below for the areas of Pittsburgh, Omaha, Wichita, and Spokane. These moisture reductions will severely impact crop yields.
But this is only one computer model's prediction. Kellogg and Zhao compared the GFDL estimates with 4 global circulation model runs from various research groups and found all 5 agreed that moisture would drop in the Omaha area, and 3 or 4 models agreed reductions would occur in the other three cities.[Sensitivity of Soil Moisture...,Kellog et al., Journal of Climate 1:4,348-366] The models had the greatest disagreement in the California and Gulf Coast areas, with some predicting more moisture, and others less. This is understandable, for the current models do not accurately estimate the ocean's effects on heat transfer, and sea currents. Better models are needed to accurately predict moisture levels in coastal areas. ***
