I’ve written before about my view that we need to get away from food production that relies on seasonal weather patterns, and some new research has been published that reinforces that opinion. In particular, the capacity for irrigation to mitigate the effects of drought:
“Plants have to balance water supply and demand. Both are extremely critical, but people overlook the demand side of the equation, especially in the U.S. Corn Belt,” says Kaiyu Guan, principal investigator on two new studies, Blue Waters professor in the Department of Natural Resources and Environmental Sciences and the National Center for Supercomputing Applications at Illinois.
The demand Guan refers to is atmospheric dryness, often expressed as vapor pressure deficit (VPD). The drier the air, the more moisture is sucked out of pores, or stomata, in plant leaves. Plants have to open stomata to take in carbon dioxide as their food, but if they sense the atmosphere is too dry, they’ll close pores to avoid drying out. Keeping stomata closed too long leads to reductions in photosynthesis, plant growth, and grain yield.
The kicker? Plants shut down stomata due to atmospheric dryness even when there’s an adequate supply of moisture in the soil.
“If you only consider rainfall and soil moisture, which is how most people think about drought, that’s mostly describing the supply side. Of course if you have low soil moisture, plants will be stressed by how much water they get. But the supply is often pretty sufficient, especially here in the U.S. Corn Belt,” Guan says. “However, the demand side from the atmosphere can also severely stress plants. We need to pay more attention to that drought signal.”
Guan’s two recent studies used multiple technological approaches, including field measurements, various sources of satellite data, hydrological model simulations, and government crop yield statistics. The first study, published in Agricultural and Forest Meteorology, used data from seven sites across the Corn Belt to conclude VPD accounts for nearly 90% of the changes in crop stomatal conductance, a proxy for drought stress, and approximately 85% of changes in gross primary productivity, a measure of productivity.
“By comparison, soil moisture typically accounts for 6-13% of these measures for corn and soybean, and up to 35% when considering time lag effects,” says Hyungsuk Kimm, doctoral student in Guan’s group and the study’s lead author.
In the other study, published in the Journal of Hydrology, Guan’s team focused on grain yield. Yield depends on many factors related to water cycles, but the researchers found that VPD explains the biggest proportion of variability in crop yield and also provides the earliest warning for yield loss when comparing with other water cycle metrics and traditional drought indices.
“This led us to build a new drought index integrating VPD, soil moisture, and measures of evapotranspiration, which can account for more than 70% of yield variation. Our index outperforms all the existing drought indices,” says Wang Zhou, postdoctoral researcher in Guan’s group and the study’s lead author.
Guan adds, “In these two studies, we tried to understand the demand side of drought from two major angles, one using eddy covariance data which measures landscape water and carbon use very accurately — the gold standard — and the other leveraging satellite data and model-simulated hydrological variables correlated with regional yield,” Guan says. “In both, we demonstrate VPD is more important than soil moisture to explain the crop drought response in the U.S. Midwest.”
The researchers are continuing to look into things like how to breed more drought-resistant crops, but I honestly feel that that is too little, too late. There are limits to how drought-resistant you can make any given plant, and from what I can tell there are not predictable limits on how severe droughts and heat waves may be getting in the coming decades. If we want to avoid food shortages, we should be working on indoor food production. That could be the various techniques mentioned in my recent post on climate change and agriculture, or it could be something like seawater greenhouses, but in any case, there is zero question that the world’s food production is vulnerable to droughts, and there is zero question that droughts and heat waves are going to continue getting worse.
In my opinion, there is no greater threat to the United States in the coming decades than the degree to which our food production relies on the stability of a climate that is now so unstable that it’s warming at a rate that may be unprecedented in the history of life on this planet.
I continue to believe that if we take urgent action, we can survive what’s coming, and even build a better human society in the process, but we do need to act, and the longer we continue to delay, the higher the death toll will become.
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