These results were checked using a basic crop growth simulation mod_l (LID), outlined by Penning de Vries et al. (1988) for the three planting seasons of 1983. It was found that while the yield of the June planting under the changed-climate scenario decreased slightly, the other two either equalled or exceeded the yield under the present climate. In Hokkaido, Japan, Yoshino et al. ( 1987) found increases of up to 25 per cent over the yield under the present climate. Thus, there is no reason to doubt the results of Irawati's model, and it may be concluded that climatic change alone would not alter rice yield seriously.

However, rice production could suffer serious set-backs from secondary causes. First, there would be heavier erosion in the upstream area, which may have to be abandoned and reforested. Secondly, some of the fertile coastal alluvial land would be inundated by the sea-level rise. The three coastal districts of the Citarum River basin would lose a total of more than 20 000 hectares of paddy fields. In the district of Subang alone, more than 25 000 hectares would be inundated, of which almost 12 000 hectares are irrigated farm lands which, with two plantings annually, produce about 110 000 tonnes of rice and almost 4 000 tonnes of maize and soya bean. To maintain the present level of the three districts' production, the yield would have to be increased by 37.5 per cent beyond the current yield.

Yoshino et al. (1987) present data indicating that by planting mid-to-late maturing varieties, the negative impact of the climatic change could be reversed. In fact, with this technology, rice yield in Hokkaido could be boosted under the 2 x CO2 climate, to surpass the present yield by 16 (lowest) to 47 per cent (highest). It might therefore be necessary to switch to late-maturing varieties to balance the loss in production in the inundated coastal plains and the abandoned upstream areas. Whether this would compensate for the total loss in production remains to be tested.

One important aspect of global climatic change remains to be clarified. This is the effect of the increased atmospheric carbon dioxide (CO2) content on the rate of photosynthesis. None of the models considers the increased CO2 concentration. By logic, however, the increased air temperature is expected to speed up the biochemical processes in the photosynthetic chain. In turn, the carbon intermediate sink is kept constantly large. Add to this the higher ambient CO' concentration, and the CO2 gradient would be large at all times. Even if the temperature range of CO2 assimilation, as reported by Uchijima (1975 6), is controlled by stomata! closure, the CO2 assimilation rate can be expected to be higher than at present.

Thus, rice yield could benefit from the atmospheric CO2 increase, assuming there is no reduction in the irradiance of the surface. This hypothesis needs to be tested in an environment of increased temperature and relative humidity.