Probing Impact of Warming On the World’s Food Supply

One of the few potential advantages attributed to soaring carbon dioxide levels has been enhanced crop growth. But in an interview with Yale Environment 360, botanist Stephen Long talks about his research showing why rising temperatures and an increase in agricultural pests may offset any future productivity gains.

Stephen Long

The world’s demand for food is expected to rise 70 percent by 2050. Much of this demand will have to be met by boosting the supply of staple crops such as rice, maize and wheat, which provide about 60 percent of the calories consumed by humans. Yet yields of the most important food crops are stagnating, and new research spearheaded by botanist Stephen Long indicates that they are likely to fall further between now and 2050, as climate change worsens.

It has long been thought that climate change could enhance crop growth through the fertilizing effects of carbon dioxide. But research conducted by Long, a professor of crop sciences at the University of Illinois at Urbana-Champaign, shows that any gains from CO2 fertilization will be offset by damage to plants from higher air temperatures, increases in atmospheric ozone, and the greater efficiency of crop pests in a CO2-enriched world. Having just been awarded a $25 million grant from the Bill and Melinda Gates Foundation, Long’s research will now focus on finding a solution to the threat of diminished food supply.

In an interview with Yale Environment 360 contributor Olive Heffernan, Long discussed the impacts of climate change on some of the world’s most important crops, the challenge of boosting crop yields to meet the demands of a burgeoning human population, and how tinkering with the genes involved in plant photosynthesis may provide the solution scientists are seeking.

Yale Environment 360: It has previously been believed that an increase in atmospheric carbon dioxide would boost crop yields through fertilization. How does your research challenge this?

Stephen Long: The impact of CO2 on crops has been seen as the one benefit we would get from climate change. We do know that if we elevate CO2 in the laboratory, we see a boost in the yield of many crops. But what we’ve now begun to observe is that when we do this under open field conditions, without any enclosures, we do not see the large yield increases that were expected from laboratory experiments.

“One observation we made in experiments was that pest incidence goes up under elevated C02.”

The focus of our own work was on soybean and maize, but we also tracked work that had been done on wheat and rice. We found — for all of these crops — that in the field, the expectation that rising CO2 will boost yields by quite a significant amount was not realized to the extent expected. So, for example, at carbon dioxide levels predicted for 2050, in the laboratory you might get a 30 percent rise in yields of soybean, but we were seeing yield increases of under 15 percent in the field. And in the case of maize, we didn’t see any increase at all.

e360: Do you know why this is? What are the mechanisms at play here?

Long: Well, we know that temperature and ozone also affect crop yields, so those effects will also come into play with climate change — CO2 will not be acting alone. In field experiments, elevated ozone always results in a decrease in yield. One of the things that one of our research team picked up was that modern plant cultivars are actually more vulnerable to the effects of ozone than older cultivars, so there’s been no selection for improved tolerance of ozone. The possible explanation that we came up with for that was that modern cultivars have slightly higher photosynthetic rates and that appears to be one reason for their increased productivity. To allow more carbon dioxide to enter the leaf, they’re opening their stomata — the pores in the leaf — wider and this is allowing more ozone as well as CO2 to enter the leaf and so is causing damage.

In the case of temperature, it’s a little more complicated because if you’re growing a crop at its cold boundaries, for example, like wheat in northern Sweden or maize in Canada, increasing temperature may actually give you an increase in yield. But at the southern edge of crop growth in the northern hemisphere — where crops are at the upper limit of their thermal range — then any increase in temperature will cause a decline in yield. This is particularly problematic for many developing countries, which are in the tropics.

e360: Are some of the effects that we’re seeing due to the interaction of temperature, CO2, and ozone?

Long: Yes. At least in the laboratory, CO2 has been shown to provide some protection against ozone. At higher CO2 concentrations, stomata close a little more and so this decreases ozone uptake. But that doesn’t mean that that damage caused by ozone will go away as atmospheric CO2 increases, and in fact, the Intergovernmental Panel on Climate Change predicts that ozone levels globally will rise. This particularly affects the tropics and subtropics, which are the most vulnerable agricultural areas. In East Asia too, it’s predicted we’re going to see considerable rises in ozone. That’s due in part to more fossil fuel use, which generates nitrogen oxides, precursors of ozone. It’s also due to rising temperatures, as temperature accelerates the formation of ozone.

e360: How much do we know about what’s going on at the biological level? Are some of the effects you’ve observed due to the interactions of plants and animals and changes in those relationships in a climate-altered world?

Long: We understand why photosynthesis increases with higher CO2. We also know some of the mechanisms by which ozone directly affects crop productivity. But there is a lot that we don’t know. To give you one example, one of the observations we made in our field experiments was that pest incidence goes up under elevated CO2. For example, the Western corn rootworm is one of the biggest pests of corn crops and has actually adapted to the rotation that farmers use of growing soybean, corn, soybean, and so on.

“Rising ozone and rising temperature really take away the advantage to crops of rising CO2.”

The adults come to the soybean crop to lay their eggs, which will then hatch in time for the next year’s corn crop. What we found was that the Western corn rootworm was laying almost twice as many eggs under elevated CO2. Now the forecast is that the opposite should happen because under elevated CO2 there’d be more sucrose, less nitrogen and less protein, which the feeding animal needs. But we saw the reverse. In the end, this came down to the fact that the metabolic pathway that inhibits feeding of the insect was being inhibited under higher CO2. And colleagues have shown this with other pests. Performance of the Japanese beetle, another major pest of soybean —is certainly boosted under elevated CO2— as is that of the soybean aphid. So these are unexpected effects that impact our crops, and we are only really scratching the surface in terms of understanding them. There may be many other unforeseen problems that we haven’t uncovered.

e360: How significant are your findings in the context of global food production? Do you think that this could affect the world’s ability to feed itself in the future, given the projections are for a required 70 percent increase in food by 2050?

Long: In my view, these results are very significant, because we’ve shown that rising ozone and rising temperature really take away the advantage to crops of rising CO2, which is perhaps the one benefit we can see on the horizon from global climate change. If we add in increased pest incidence, we’re probably looking at seeing crop losses, even in areas like the Corn Belt, where we expected that there might be some benefit from climate change. And the U.S. Corn Belt is probably the world’s largest exporter of grain, and has always being a major buffer of world basic primary foodstuff supplies. So if that area is affected, it has major repercussions on global grain supply. And of course the Corn Belt will be affected when many other areas in the world are affected by these changes as well. Given the projected increase in grain that we need, if we don’t start to do something about it, these effects could be quite profound.

e360: Do we know to what extent this trend is replicated elsewhere and for other crops? Could this be a much wider problem than is now appreciated?

Long: The expectation for soybean was that rising CO2 would give us about a 30 percent increase in yield. What we’ve seen in our experiments is below 15 percent. An earlier experiment in Arizona saw that type of shortfall with wheat, and experiments on wheat and rice in China have seen a similar shortfall. Also in Japan, with rice, experiments have shown, instead of 30 percent, an 11 percent increase in yield. Recently an experiment in Germany with maize failed to see any increase when rainfall was good, which matches what we saw with maize in the U.S. Corn Belt. So just from a very limited number of experiments, there is agreement.

e360: Is it realistic that we should expect the yields of the staple crops to continue to increase? Aren’t the benefits of the Green Revolution largely reaped at this stage?

Long: The major benefit of the Green Revolution was increasing the amount of biomass that ends up in the part of the crop we consume, the grain. With wheat and rice and soybean that’s now approaching 60 percent of the shoot weight. Assuming we’re still going to have some stems and leaves, there may not be much more to gain from those avenues. Of course, responsiveness to fertilizer was also a key part of the Green Revolution. These routes appear to be reaching their biological limits and although there is more to be gained there, and it’s important that it is gained, it’s the law of diminishing returns. In the 1970s and 80s, global yields of wheat were improving by 30 percent per decade. The last decade, that’s almost down to zero. Rice has shown a similar decline.

e360: If we can’t gain further yields from these staple crops, what about using other crops in their stead?

Long: We have systems in place for dealing with these crops, from farmers who know the crops right through to amazing research, both public and private. Society has a huge investment in dealing with these crops, and of course, these are the crops that society is used to.

“We are exploring avenues where we could boost photosynthesis in crops.”

So we really need to be looking at new routes for improving them — understanding why, for example, these crops in the field do not show the CO2 boosts we see in the laboratory. One area that the Green Revolution did very little to improve on is the process of photosynthesis, which converts sunlight energy into crop biomass. Over the last 50 years we have developed a very deep understanding of photosynthesis, and we now know that we that we can engineer genes to give us quite a boost in photosynthesis. So there are avenues that we could pursue to give us the next jump in crop yield to meet the increase in demand we will see by 2050.

e360: You recently received a grant of $25 million from the Bill and Melinda Gates Foundation. What will this research grant fund?

Long: This grant will fund us to explore avenues where we could boost photosynthesis in crops in general, although our targets are rice and cassava. But because the process of photosynthesis is very similar in rice, wheat, cassava, soybean, and so on, finding a way in which we can boost it in one of those crops is probably going to tell you how to boost it in the others as well. And so we are following four or five leads, where we think we could make a significant boost in photosynthesis and in crop yield.

e360: Given the trends of slowing yields of crops with climate change, by when would we need to see this boost from photosynthesis become a reality?

Long: We really need to start seeing results within five years of doing this, because if we can show an increase in photosynthesis in our research, it could take 15 or 20 years to get that into the farmers’ fields. We need to be able to test it in multiple locations, and it needs to meet regulatory requirements for different countries. So really, to be able to provide this food increase by 2050, it’s very urgent that we start work on these problems today.

e360: Do you think it’s realistic that the yield jump that you would see from photosynthesis would actually counter the decrease that we’re seeing as a result of climate change? Could it be a one-stop solution?

Long: I wouldn’t say it’s the only solution, but I think it’s a very important one in raising the potential yield of our major crops, the yield their genetic make-up will allow under optimal conditions. Obviously, improved agronomy has to come along with that as well. We shouldn’t underestimate the importance of that, particularly in developing countries, where yields of major staple crops are often far below their potential. This could be achieved using improved methods of managing the crop, funding farmers to be able to fertilize their crops adequately, and adequate soil testing to understand what needs to be done to improve the soil and crop yields.

At the same time, if we can improve the yield potential of the crop so that it’s genetically capable of getting a higher yield, that typically results in higher yields under suboptimal as well as optimal conditions. We need both of these approaches if we’re going to meet the yield increases we need.

e360: So what is the next step?

Long: The grant we’ve just received from the Bill and Melinda Gates Foundation will enable to us continue and greatly improve our work on improving photosynthesis and crop yields. We’ve been working for almost ten years now on trying to boost yields from photosynthesis. We have, for example, already found two genes, which we’ve manipulated, each giving us a significant increase in photosynthesis and yield under laboratory conditions, and in our initial field trials. One of these genes has given us a potential yield increase of 15 percent. Now if we can add those together, we may be able to see the big increases of the kind we’ll need in the future.