How overfed are we? refers, in which I express some doubt about the problems of food production. But Battisti and Naylor (Science 9 January 2009: Vol. 323. no. 5911, pp. 240 – 244; DOI: 10.1126/science.1164363) Historical Warnings of Future Food Insecurity with Unprecedented Seasonal Heat re-raise these problems, and they do it in Science, so lets have another look. You can also read it direct.
Higher growing season temperatures can have dramatic impacts on agricultural productivity, farm incomes, and food security. We used observational data and output from 23 global climate models to show a high probability (>90%) that growing season temperatures in the tropics and subtropics by the end of the 21st century will exceed the most extreme seasonal temperatures recorded from 1900 to 2006. In temperate regions, the hottest seasons on record will represent the future norm in many locations. We used historical examples to illustrate the magnitude of damage to food systems caused by extreme seasonal heat and show that these short-run events could become long-term trends without sufficient investments in adaptation.
As far as I’m concerned, the predictions of temperature increases are non-controversial, so I won’t discuss them other than to note that the pix in their article are not temperature increases, but increases in exceedance, which is why the tropics looks more affected by the mid-latitudes. Its not clear to me whether crops care more about absolute temperature change, or about change relative to current variance.
B+N don’t make any predictions at all about future food supply. They note that in the past, exceptional heat has been linked to decreases in a years crops, and they note that in the future the temperature is going to go up, but thats it. So there is no way to know where they stand related to the 25% crop reduction as an extreme scenario. Perhaps, as physical scientists, they would rather stay out of that debate.
There are two obvious problems with / counters to their analysis. First, that crops are more sensitive to precipitation than temperatures; and second, that we will adapt crop varieties. There are other issues, such as the expected advances in tech, that JA mentions in the comments to my first post; but thats not too clocely connected to their analysis. And they rather quickly skate over the expected improvements in yields in areas that are currently too cold.
Taking the second first, their historical analysis is based around individual warm years leading to problems. Since no-one predicts these warm years in advance, the farmers then couldn’t adapt their crops, much less their techniques, to an expected warmer year. Thats obviosly not true not, when we expect a fairly smooth increase in temperature. Most parts of the world, in 100 years time, will remain cooler than the warmest cropped regions now; and its not clear to me that we won’t be able to adapt, by switching crops, over that timeframe.
As to the first… B+N note the european summer 2003 as an example of high temperatures (a) killing people and (b) reducing crops. But I recall (wrongly? could be. Anyone know?) that it was also a dry year; and indeed the dryness contributed substanitally to the heat (less water to evap so less cooling). Future warming will not necessarily be associated with dryness in this way. B+N make some effort to get around this. In their analysis of historical heat waves, they note that 1972 was hot and dry and lead to poor crops in the USSR; but that previous dry summers hadn’t. The Sahel is less convincing. Famine there was originally from drought; B+N note that in recent years the rains have returned but crops haven’t fully recovered; I wouldn’t be too surprised if politics was part of that.
I think this bit of their analysis needs more work. The obvious naive question is, why didn’t they run their GCMs scenarios for precip as well as temperature? One obvious answer might be that they don’t really trust the precip from the GCMs. Another, that they aren’t feeding the GCMs into any kind of mechanistic crop model (but why not?). So they do their best to work their analysis in terms of temperature, and to claim that precip is less important. I’m not convinced.
Summary: there isn’t much new in this paper. The dependancy of crops on ppn and/or temp needs to be clarified.
[Update: ICE isn’t impressed -W]