Declining Crop Yields

Why is food production slowing down? This is from Michael Roberts:

Is this because of changes at the intensive or extensive margins? That is, is the slowdown due to declining productivity on existing land, e.g. from bad luck with the weather for several years in a row, or a more permanent change like global warming? Or is it because world growth is bringing marginal, less productive land into production? Whatever the cause, Michael Roberts thinks it's likely a permanent rather than a temporary problem:

There are many reasons for high commodity prices. But recent data from FAO shows a pretty rapid slowdown in productivity growth. The price spike in 2008 occurred in a particularly bad year in which yields declined on a worldwide basis for three of the four largest food commodities. In 2009 all four of the majors saw yield declines, something that hasn't happened since 1974. 2010 couldn't have been much better and was probably worse, given how bad things were in the U.S, the world's largest producer and exporter (worldwide data for 2010 isn't available yet).

The yield slowdown comes at a particularly unfortunate time, with accelerating demand from emerging economies like China and subsidy-driven expansion of ethanol. Keep in mind: we need productivity growth to accelerate considerably to keep up with projected demand growth. FAO says we need 70 percent higher yields by 2050. (Although I'd like to do my own projections, and will one of these days...)

Maybe it's just bad luck with the weather. But I think it just may be a longer run phenomenon.

Yeah, resource scarcity will be in the news for awhile yet.

Thoughts on this?