Thursday, August 04, 2005

Global warming and crops

Rising temperatures are likely to reduce crop yields : Nature:
Rising temperatures are likely to reduce crop yields

John R. Porter

1. Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Hoejbakkegaard Avenue, 2630 Taastrup, Denmark


Your News story about the Royal Society meeting on climate change and food production ("Hikes in surface ozone could suffocate crops" Nature 435, 7; 2005) noted that rising CO2 levels will generally benefit crop growth, as this stimulates photosynthesis in most crop plants. However, the links between climate change and food production are even more complex than your story suggests.

Rising temperatures could extend the geographical distribution and growing season of some agricultural crops, such as pasture grasses, by allowing the threshold temperature for the start of growth to be reached sooner. This assumes that water and nutrients are supplied at a level that permits pasture crops to benefit from a longer growing season.

But in general, and contrary to common perceptions, most crop physiologists expect global warming to reduce crop yields. This is because higher temperatures shorten the life cycle of most cereals, hastening senescence and reducing the length of the growing season. Other effects such as an increase in tropospheric ozone level can exacerbate crop senescence, as noted in your News story. The staple cereal crops can only tolerate narrow temperature ranges, which, if exceeded during the flowering phase, can damage fertile seed production and thus reduce yield.

Global warming would also be expected to increase the frequency of exposure to extreme temperatures and thus damage crop fertility.

So far, efforts to predict climate change effects on food production and quality have been fragmented. For major crops, except wheat and soybean, we lack the agronomic-scale experiments needed to understand and robustly predict the direct effects of CO2 and ozone, and their interactions with temperature and water. With an extra three billion people to feed during the coming 40 to 50 years, closer cooperation among crop physiology, crop agronomy and climate science would be a positive outcome of the Royal Society meeting.
The costs of climate change are unclear, but Kansas farmers know the cost of drier summers, since several of the last few years have seen drought conditions in Kansas, letting South Dakota steal the wheat producing crown at least once.

I don't know if that's because of climate change. That's a statistical assessment that will require years of data. How many years of drought can Kansas tolerate in order to gather that data?

This is called the "precautionary principle." We can't fully estimate the effects of climate change because agriculture and climate are both incredibly complex systems, and their interaction is no less complex. But we can see serious dangers in one direction, and none in the other.

The precautionary principle says it's worth investing in a solution now, because by the time we have enough data for a full economic analysis it'll be too late to change and too expensive to help Kansas farmers.

A more moderate approach is to make an assessment of the probability of various outcomes (p_i) and the cost of each outcome (c_i) and spend the sum of p_i*c_i across all i. That'd be a lot more than anyone is spending on alternative energy as it is. I'm not an economist, so I don't have those numbers at hand, but I can eyeball it.