No-one can be sure, but it is likely that crop production in sub Saharan Africa will become even more difficult than it is now. Evergreen (coniferous) forests will slowly move north, leading to a decrease in forest productivity and desiccation (drying out) of trees. This will cause wilting, defoliation (leaf loss) and an increased likelihood of fires.

Such changes will cause lack of water or temperature stress to animals, reducing their resistance to pests and a consequent increase in disease. Already in the UK we are seeing
foreign pests invading.

At the poles, glaciers will melt faster leading to a dramatic change in food webs there, and a reduction in biodiversity.

It's not all doom and gloom. In his recent book "Global Warming: the complete briefing", Sir John Houghton wrote

At northern latitudes in Siberia or northern Canada, a longer growing season is likely to provide for more and possibly different crops. Increased carbon dioxide also acts as a fertiliser for some plants and again, if other conditions are also right, could lead to higher agricultural production. This higher production is likely to occur in developed countries at mid-latitudes. By contrast, many developing countries may not be able to produce as much for their increasing populations.