Four papers together give an insight into what global warming promises for agriculture and agriculturalists, and how to deal with it.
Some people will tell you that global warming is something we can cope with because it won’t actually create any new climates, just shift the old ones around a bit on the the surface of the Earth. They’re wrong.1 John Williams and his colleagues published an article in PNAS in the spring that shows conclusively that even the IPCC’s B1 scenario, in which modest reduction sees CO2 stabilized at 550 parts per million by 2100 AD, creates considerable risk of completely novel climates.
Williams and colleagues used the climate models to predict four variable within grid cells of about 2.8 degrees square. In each, they calculated an index that integrated four variables: mean rainfall and temperature for summer and winter. Then they asked whether the climate in that grid cell was novel, by measuring how different it was from the most similar modern climate, anywhere at all on earth. Given business as usual, “novel climates are likely to develop in lowland Amazonia, the southeastern US, the African Sahara and Sahel, the eastern Arabian Peninsula, southeast India and China, the IndoPacific, and northern Australia”. The same areas are affected under the B1 scenario, but at lower levels.
That’s the tropics and sub-tropics, where the poor people live, and where they depend on agriculture, which depends on climate. How much?
Andy Jarvis and Annie Lane, of Bioversity International and CIAT, used the same climate models to ask how the areas suitable for different crops will change. This one hasn’t been peer-reviewed yet, so I cannot really say too much about it. The idea was to plug the climates into FAO’s ECOCROP model of the growing conditions required by more than 1800 crop species. There are winners and losers. Geographically, northern temperate areas do OK, while the tropics suffer the biggest changes in areas suitable for agriculture. the greatest losses of suitable growing areas are predicted for sub-Saharan Africa and the Caribbean, regions least able to cope. Gains will be seen in Europe and North America, which perhaps need them least.
Among species, the biggest losers are cold-weather crops such as strawberry (-32% in areas suitable for cultivation), wheat (-18%), rye (-16%), apple (-12%) and oats (-12%). Among the winners are pearl millet (+31%), sunflower (+18%), common millet (+16%), chick pea (+15%) and soybean (+14%).
Quite often, however, the gains occur in places that have no cultural history to making use of those species. Land suitable for pearl millet is predicted to increase by more than 10% in Europe and the Caribbean, where it is an insignificant food, but not in Africa, where it is widely cultivated and an important element in food security. It is going to be small comfort to know that one can grow a crop that one has no idea how to cultivate, process or eat.
A2 (left) is business as usual, B1 (right) reflects moderate reduction in emissions. Above, risk of novel climates, below risk of disappearing climates, both constrained to 500 km. From Williams et al. (2007). Click to enlarge.
Ah, but people and their crops can move, can’t they? Back to Williams et al. As well as asking whether there is an analogous climate somewhere on Earth, they also asked whether there is an analogous climate less than 500 km away from the target grid cell. That makes things a whole lot worse, with “no-analog” climates right across the tropics and way into the polar regions. And 500 kilometres is quite a distance for agricultural systems to migrate in less than 100 years.
Will that make itself felt? You bet it will. John F. Morton, of the Natural Resources Institute in the United Kingdom, has a paper in PNAS entitled “The impact of climate change on smallholder and subsistence agriculture”.2 You might expect there to be a clear answer, but Morton’s main point is that we don’t really know enough about smallholder and subsistence agriculture to plug it into climate change models. Morton concludes:
Smallholder and subsistence farmers will suffer impacts of climate change that will be locally specific and hard to predict. The variety of crop and livestock species produced by any one household and their interactions, and the importance of nonmarket relations in production and marketing, will increase the complexity both of the impacts and of subsequent adaptations, relative to commercial farms with more restricted ranges of crops. Small farm sizes, low technology, low capitalization, and diverse nonclimate stressors will tend to increase vulnerability, but the resilience factorsâ€”family labor, existing patterns of diversification away from agriculture, and possession of a store of indigenous knowledgeâ€”should not be underestimated.
Which is a kind of we know what we don’t know plea for more research.
How, then, might one best help smallholder and subsistence farmers to weather the changes ahead? Perhaps predictably, the big boys are all in favour of setting their talented breeders to work to come up with varieties that are resistant to climate change. But there may be a better way. Give farmers access to a far greater range of diversity and improve their ability to select populations that will be adapted and adaptable.
This gets to the heart of one of the big questions of theoretical biology: which is faster, evolution based on new mutations arising, or evolution based on selection from existing variability. Rowan Barrett and Dolph Schluter, of the University of British Columbia, have a review in Trends in Ecology and Evolution that makes a convincing case for the superiority, in many cases, of adaptation from standing genetic variation.3 To cut a long (and very interesting) story short, there is every reason to believe that standing variation has played a role in the adaptation of some wild populations in the recent past. Would that be true in an agricultural setting, with artificial rather than natural selection? Probably even more so, given the very strong selection pressure that people can exercise over their crops and livestock.
To summarize: global warming will create entirely new climates; these will put enormous strains on agriculture, changing where crops grow best and what crops and animals people will be able to cultivate; and farmers may do better to find as much genetic variation as they can and select from that, rather than waiting for breeders to supply them with varieties that will be at best only narrowly adapted.Footnotes:
- Williams, J. W., Jackson, S. T., & Kutzbach, J. E. (2007). Projected distributions of novel and disappearing climates by 2100 AD. Proceedings of the National Academy of Sciences of the United States of America, 104(14), 5738-5742. [↩]
- Morton, J. F. (2007). The impact of climate change on smallholder and subsistence agriculture. Proceedings of the National Academy of Sciences of the United States of America, 104(50), 19680-19685. [↩]
- Barrett, R. D. & Schluter, D. (2007). Adaptation from standing genetic variation. Trends Ecol Evol. [↩]