As environment ministers hashed out the details of a climate-change agreement, FERN correspondent Daniel Grossman sat down with two prominent experts in Paris to talk about the impact of climate change on agriculture. Chris Field, a professor of biology and environmental earth science at Stanford University, co-chaired the committee that wrote, “Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation” for the UN Intergovernmental Panel on Climate Change. Katherine Mach, a senior research associate in Stanford’s department of global ecology, co-directed the scientific activities of the committee.
Is it possible that agriculture might no longer be productive in large parts of the world in the future?
CF: Climate change is really a challenge in understanding, managing and reducing risks. There may be only a 1-percent probability that, for example, heat waves or drought would become so prominent across parts of sub-Saharan Africa that you could no longer practically do agriculture. But that 1 percent is important in how you think about the future. We don’t have a very mature science for evaluating these low-probability, high-consequence outcomes, but they should be part of the discussion.
KM: And it’s not just how much food you get from a given area of land but how that food matches with cultural preferences. If you were to map everywhere on the planet that you could grow crops, you could imagine a world where you rapidly adapt by shifting what’s grown where, but that’s actually a very substantial challenge in terms of understanding the feasibility of those types of movements.
How much will yields change with climate change?
CF: Think about what factors control whether or not agriculture is practical in a given area. One is whether you frequently get conditions that are so hot or so dry that the crops can’t survive. We know that many crops have a very sharp temperature threshold above which yield goes down very rapidly and that temperature threshold is around 80 degrees Fahrenheit for corn. Just a few days at temperatures of 100 degrees will cut corn productivity substantially. And these temperature thresholds have not really changed very much with crop breeding. We know there are lots of different varieties that are adjusted to different parts of the world but corn yields, for example, are much, much higher in Illinois than they are in Alabama and we haven’t been able to do anything about that. High temperature is simply a very stressful condition for most food crops. Crops also require substantial amounts of water, so a serious drought can heavily impact production, especially with commodity crops like corn, wheat and soy.
We have a reasonably clear picture of what we expect heat waves and high temperature conditions to look like in the future. We looked at 1,070 computer runs for crop yields. And by the end of the century 20 percent of them project decreases of 50 percent or more. And the simulations don’t generally include interactive factors that present some of the biggest challenges, like water availability and the exposure to strong storms.
One of the things that we expect in a changing climate is more variable conditions: extended drought, storms. All those things make it more and more difficult to get yields on a consistent basis. When farmers decide to grow a crop in a given location it is typically conditioned on the expectation that they’re going to have a reasonable probability of bringing the crop in. When people choose to abandon an area from agriculture it’s because the experience hasn’t been reliable enough to make it worth their investment.
So there’s a big risk of a substantial decline?
CF: It might be that conditions improve as a consequence of technological advances or it might be that they are more challenging as a result of more demands on water resources or, say, higher prices for fertilizer. One of the things we want to do is shift thinking to risk management so that we recognize that there’s a real possibility of outcomes that are unacceptable, especially for poor people who don’t have access to advanced technology, fancy cultivars and good predictive ability.
KM: It’s very clear that in some places it’s going to be harder to grow what we’re growing now.
What kinds of scenarios are you thinking about?
CF: For example, repeated extremes of very high temperature, heavy precipitation or drought. And the world’s grassland areas have historically had the most variable climates. California, where I’m from, is a good example. It’s in the midst of a historic drought, a drought that is more severe than we’re pretty sure we’ve had for at least 500 years. That’s a clear fingerprint of the influence we expect from a changing climate, whether climate change caused it or not.
California is a perfect example of an area where the big constraint on agriculture is the availability of water resources. We know that California’s water storage system, which is really a marvel of engineering, gets about 30 percent of its water from storage in the snow pack. In the winter of 2014, 2015 we had essentially zero snow pack and really lost all of that water from the water system. More than 95 percent of California’s crop land acreage is irrigated, and without irrigation water we don’t grow anything.
Is it possible that by the end of the century some areas that will no longer be able to feed their inhabitants?
CF: Yes. We can’t yet quantify it but there certainly is some risk. I think it’s important to recognize, though, that it’s not going to be because year-on-year crop yields dwindle to nothing. Rather, you might get a good yield one year, followed by three years where the crops fail, then an okay year followed by two more years of failure. It’s much like what happened in the Dust Bowl era in the U.S.