temperature. Precipitation changes are calculated by multiplying predicted changes in precipitation by current rainfall values.

Using the district level changes for each climate scenario, we then predict how AEZs will change in each district using the weighted regression in Table 3. The total amount of land in Africa remains the same, but the amount of land in each AEZ shifts. As shown in Figure 2, there are three large land changes in the PCM scenario: a reduction of 195 million hectares of lowland humid forest, a reduction of 94 million ha of desert, and an increase of 217 million hectares of lowland sub-humid forest. There are much larger land shifts across AEZs in the CCC scenario. The model predicts a loss of 451 million ha of lowland humid forest, a loss of 335 million ha of desert, a gain of 390 million ha of lowland sub-humid forest, and a gain of 356 million ha of medium elevation sub-humid forest.

We map the AEZ changes across the landscape in Figure 3. By comparing the original predicted values in Figure 1b with the distribution of AEZs in Figure 3a, and Figure 3b, one can see how AEZs have changed in the PCM and CCC scenarios. There are large changes in AEZs between the baseline and the PCM maps in North, South, and East Africa. In particular, high elevation humid forests increase in Eastern Africa and midelevation moist savannahs increase in southern Africa. In the CCC scenario, one can see changes in East, Central, West and South Africa. The most notable change in the CCC scenario is the increase in all elevations of sub-humid forests and the reduction of lowland humid forests and deserts. The fact that the model predicts deserts in the southern Sahara shrink may seem counterintuitive but the African deserts are not too hot, they are simply too dry. According to the GCM predictions, there is enough increase in rainfall in these desert regions to change their status. Whether this translates into a change in crop production is a more complicated issue since desert crops depend on surface flows. Although future studies could analyze how surface flows change with warming, this analysis does not predict potential changes in surface flows.

The changes in the area of each AEZ have implications for the amount of cropland as shown in Table 5. The percent of cropland in each AEZ is assumed to remain constant so that as the size of an AEZ changes, cropland changes. Although the sizes of the AEZs change considerably, the effect on cropland is offsetting, so that the net changes in cropland for Africa are small in both scenarios. In the PCM scenario, the large decrease of 47 million ha of cropland in lowland humid forest is partially offset by the increases in cropland in high elevation humid forest and all elevation sub-humid forest, lowland moist savannah, and mid elevation dry savannah. The net effect in the PCM scenario is a loss of 27 million hectares (5%) of cropland. With the CCC scenario, the cropland in lowland humid forest is almost completely lost but it is offset by increases in cropland in lowland dry savannah and sub-humid forests at all elevations. The net impact is a gain of 20 million hectares or 4%.

With the PCM scenario, in addition to the 5% loss of cropland, there is a slight shift to lower valued cropland as shown in Table 6. With the PCM scenario, there is a $36 billion loss of annual crop revenue from lowland humid forests and another $10 billion loss from