Climate change and crops
Changes in the climate are already having an effect on crop yields—but not yet a very big one
May 5th 2011 | from the print edition
THE problems climate change looks likely to bring in the future may increasingly be visible in the records of the past. Not just in the far-off ages of surging sea levels following ice-age thaws, spikes in prehistoric temperatures correlated with natural releases of greenhouse gas and ancient civilisations brought low by drought, but in records from living memory—which are based on reliable measurements made at the time. Using such data researchers have now compiled an estimate of global changes in crop yields which can be put down to recent increases in temperature and decreases in rainfall (the world as a whole is getting wetter, but the rain has stayed away from some agricultural plains). The bad news is that they find that climate change has lowered the amount of maize (or corn, if you prefer) and wheat produced in a given area. The good news is that the effect is so far reasonably small.
David Lobell and Justin Costa-Roberts, of Stanford University, and Wolfram Schlenker, of Columbia University, first put together temperature and precipitation figures for the parts of the world where four staple crops—wheat, maize, rice and soya—are grown. Those four crops, between them, account for about 75% of the calories people end up eating, although a lot of the soya is fed to animals first.
It turns out that during the seasons in which crops grow, these arable areas had on average become significantly warmer in the 29 years after 1979. Some bits of Europe that grow wheat, for example, have heated up by a couple of degrees since 1980. The researchers then assembled models of how the yields changed from year to year, and against the longer trend, to find changes linked to temperature and rainfall that are independent of improvements through better farming. Finally they compared today’s yields with what their models say yields would have been with today’s farming but in the 1980’s climate.
For both wheat and maize, the results, published this week by Science, were negative. Globally, wheat yields are down 5.5% compared with what they would have been with no climate change, and maize yields are down 3.8%. For soya, some places saw improvements, some saw damage, with no real net effect on the global scale. For rice, warming brought a clear benefit for crops at higher latitudes and some losses in warmer places. Temperature played a bigger role than precipitation. The results seem to fit with previous studies into such effects in individual countries.
But there are caveats. For example, the analysis does not track changes over time in the areas being farmed, using instead a crop map from around 2000. And many agronomists hold that relying on year-to-year yield changes for modelling exaggerates the damage due to longer climate shifts. Farmers will tend to adapt. That said, subtler effects of climate change such as more sudden rains and particularly hot days with disproportionate effects on yield are left out, which might mean the study underestimates the effects.
Some people will be surprised, even dismayed, that comparatively modest climate changes are already doing measurable damage. But in context, it is quite small. Yields have been going up around the world despite the warming climate—and over 29 years those increases swamp the estimated global reductions due to climate. The sort of loss that climate change inflicts in a decade is often the sort of gain that better farming brings in a year. What is more, for wheat the increasing level of carbon dioxide that is changing the climate also makes photosynthesis easier, which should have increased yields enough to wipe out perhaps half of the climate-related loss (maize, which photosynthesises in a different way, is thus the bigger loser of the two in net terms).
Then there is a point made by Richard Tol of VU University Amsterdam: farm yields show the worst of the situation. Easily achievable improvements in roads, markets and other things can increase the availability of food a lot even if farm yields stop rising. And people can adapt, at least to lowish levels of change; indeed the study provides evidence to help them do so as it shows which crops in a given country are the most affected by global warming. So Malthus looks beatable even when he sits astride the apocalyptic horse of climate change.
Moreover, there is a simple way more or less to abolish the effect of climate change on yield to date. According to William Cline of the Peterson Institute for International Economics in Washington, DC, at least 4% of the world’s grain is used to make ethanol for fuel. Most of this is doing little good for the environment, and stopping subsidies for such fuels would boost the supply of grain for feeding people on a scale similar to the hit that the past three decades of warming have provided.
Still, in a world where more than a billion people are undernourished and the population is still growing, every little hurts. The net effects of climate and carbon dioxide, Dr Lobell and his colleagues calculate, is to make grain prices about 5% higher than they would otherwise be. Dr Lobell reckons that means about $50 billion a year. In some countries the effects are particularly large: climate change means Russian wheat yields are more than 10% down. As Mr Cline points out, growth in yields has slowed recently. And climate effects could speed up. Earlier work by Dr Schlenker has showed that plants have skewed preferences: they dislike temperatures beyond the optimum more than they dislike temperatures below the optimum, so increasing warmth can have worsening effects beyond a certain point.
That might be keenly felt if patterns of warming shift. One of the reasons that the climate effects Dr Lobell and his colleagues have dug out of the data are not worse is that, although the planet as a whole has warmed up during the past 30 years, growing seasons in the parts of America which produce 40% of the world’s maize and soyabeans have failed to follow suit. No one is quite sure why this might be and no one knows if it will last. That climate change has not yet done very much harm may be cheering, but the past offers no firm guarantees for the future.