Climate variations significantly affect agriculture through complicated mechanisms. Therefore, agriculture is expected to be especially vulnerable to climate changes. The impacts of climatic warming on agriculture productions are: (1) the direct effects of increased atmospheric CO2 concentration on crop and weed, (2) the indirect effects of climate change on crop, weed, and pests, and (3) the effects of sea level rise on farmland at a low altitude. Thus, in agriculture sector, there are many complex interactions among the related factors. Experimental results, detailed modeling of basic processes, and knowledge of physical and biological processes provide a basic understanding of the direct and indirect effects of climate warming on agricultural production. Experimental results in growth chamber and temperature gradient tunnel are (1) grain yield of rice is generally promoted by high CO2, (2) yields increase 30% or more with doubled CO2 in the absence of other limiting factors, and (3) rice spikelets have the highest sensitivity to high temperature at anthesis (flowering), and are liable to be sterile due to a failure in pollination, when the flowering temperature is approximately 35°C. Crop models with climate change scenarios projected by general circulation models predicted the yield changes of crops: (1) rice yield is expected to increase in northern Japan, but decrease in western Japan, (2) the yield variability would decreases in northern and north-central Japan, but increase in south-central and south-western Japan, (3) maize yield is expected to increase in Hokkaido, but change little in central Japan, and decrease in southwestern Japan, and (4) wheat yield would decrease at all sites.
