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.