Is drought becoming more or less important for agriculture?
As the world continues to warm, many agricultural regions are seeing an increased frequency and severity of conditions that lead to drought (see, e.g. here). An ongoing question has been just how damaging these climate changes will be, and how much we can lessen their impact by adapting. We know from decades of work that the ongoing climate trends are overwhelmingly negative for crops in many (but not all) systems. But there are lots of potential ways the system could adjust to become less sensitive. If we can identify ways that are really effective, and invest in accelerating those adjustments, it could make the difference between impacts that are catastrophic and those that are merely inconvenient.
In a recent study published in Nature Food, we look at the last two decades in the United States, and whether there is any evidence that new technologies have helped adapt to climate trends. The good news is that we find evidence for sizable productivity gains under drought conditions, presumably driven by factors such as hardier seeds and improved management. The bad news is that, even with these gains, the negative impact of drought appears to be growing.
This question of whether drought impacts are changing is a really tricky one to study. Most systems get hit with a strong drought only once or twice in a decade, so the sample sizes are quite small. Even when looking over the past 40 years, when most agricultural areas have been warming, it can be hard to detect a difference in the response to drought over time. Many people have tried, but it hasn’t worked very well.
In our study, we decided to take a new approach. Instead of trying to look over time, we first look across space. Specifically, we compare maize (corn) yields for farms that have soils with low vs. high capacity to store soil water. In this way, we can look at yields along gradients of drought stress, and can do this each year to see if things are changing over time.
To do this well, it’s important to avoid comparing farms that have different soils but also differ in lots of other characteristics. To that end, we exploit a new dataset of high-resolution maize yields that we’ve developed based on satellite data, which is shown below. These estimates are for every year and for each 30x30m pixel that is growing maize within a 9-state region that is the core of the “Corn Belt.” That’s more than 4 billion estimates of maize yields!
Because we have so much data, we can restrict ourselves to comparing fields with different soils within the same county. For example, the figure below shows the yields and soil plant-available water storage (PAWS) for Wabash County, Indiana in 2018. You can see a couple of things that are typical of most counties. First, there is a pretty sizable range of PAWS within a county, with some soils able to hold twice as much water as others. Second, there’s a pretty noticeable difference between yields on the different soils, with generally higher yields for soils that hold more water.
Then we can do this comparison for each county and each year. The figure below shows the average effect of being on a soil with a given PAWS, compared to a PAWS of 250 mm (roughly the average PAWS for the region). The different lines correspond to different modeling choices, and show that the result is not really sensitive to these choices
Now to the interesting part, we can look at if this sensitivity to PAWS (the slope of the line in the figure above) has changed at all over the last 20 years. The figure in panel (a) below shows estimates for each year, along with a trend line. You can see that, on average, the sensitivity to PAWS has gone up a lot, by about 55% over 20 years. In panel (b) we show the results for both average and change in sensitivity for each state. The drier states like Missouri and South Dakota tend to have both higher sensitivity and bigger increases.
So what’s going on — why would the sensitivity be growing over time? We describe various explanations in the paper, but the most likely one is that the technologies that help farmers to continue raising yields — things like new seeds that can withstand very dense sowing — are more beneficial when the weather is good and the soils can hold plenty of water. These technologies still help when conditions are dry, but not as much as when they are wet.
This brings us to a point of frequent confusion. The question “how is the impact of drought changing?” is not the same as the question of “is agriculture getting better at coping with drought?” There’s no doubt that agriculture does better now in dry years or poor soils than it used to. A common comparison is the U.S. drought of 1988 vs. 2012. Even though the latter was arguably more severe, yields in 2012 were 50% higher than in 1988.
So we are getting quite good at producing grain under drought conditions, but we are getting even better at producing grain under non-drought conditions. As a result, the “cost” of drought seems to be rising. Another way to say this is that the potential benefits of avoiding further increases in drought, by reducing the degree of climate change, are increasing.
By all means, we should continue to invest in drought tolerant varieties and other technologies. But we shouldn’t expect it to reduce the costs of climate change. This is counter-intuitive to some people. But you can think of it as the consequence of breeders and farmers being really clever about squeezing the most out of new technologies in good conditions.
A related conclusion is that the benefits of having deep soil are rising. One would expect, for example, that the soil rating will play an even greater factor in determining land rents and prices over time.
You might ask “who cares about a growing effect of drought, as long as we continue to push up average yields?” That’s a version of a common refrain that “we are drowning in corn, do we really need more of it?” While it’s true that corn supply is now quite strong compared to demand (as reflected in low prices), demand around the world continues to grow. Or if agriculture is not your thing, it’s also true that the better we get at feeding people, the more land we can retire from agriculture and put to other uses. Either way, things that make it harder and more costly to produce grain are generally a net negative for society.
Finally, it’s worth mentioning the usual caveat about how the past doesn’t necessarily predict the future. Maybe some new technology will emerge that only helps farmers in bad conditions, and therefore lowers the impact of bad weather. It’s possible, but again farmers are typically really good at pushing their crops to the limits. If it does happen, at least now we’ll be able to spot it with the types of satellite measurements and approaches used here.
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