Acclimation and Heat Stress of Plants, and Future Crop Failures

A field of sweet corn, Flat Rock, Indiana. Photo: Jeffrey St. Clair.

One of the most popular ideas that springs into people’s minds when mulling over remedies for slowing the advance of climate change because of the ever increasing accumulation of carbon dioxide in the atmosphere, is to plant more trees, bushes and grasses. Let a greater quantity of plant photosynthesis filter our atmosphere of excess CO2.

This is not an entirely bad idea — especially in its more nuanced formulation as multi-crop regenerative agriculture coupled with wildland, wetland and forest conservation and reforestation, ending industrialized chemical pesticide monoculture farming and drastically reducing the entire meat industry, along with a popular shift to plant-based diets — though it is an entirely inadequate tactic for absorbing the ever increasing load of CO2 in the atmosphere being fed by gargantuan torrents of anthropogenic CO2 emissions exhausted as waste products from the fossil fueled engines powering today’s capitalism and militarism, which remain requirements by our capitalists and militarists for the continuation of our present civilizational paradigm.

So, planting trees is being done and will continue because it is something that many people can do to try to help, and because it poses no real threats to capitalism or militarism. But one of the cruelties of global warming is that high concentrations of CO2 combined with elevated global temperatures reduce the rate of photosynthesis and plant growth. These effects are called “acclimation” and “heat stress” of plants, respectively (https://www.bbc.co.uk/bitesize/guides/z8c6gdm/revision/3).

Acclimation is either an enhancing or inhibiting effect on photosynthesis by high CO2 concentrations. Generally, photosynthesis is enhanced as CO2 concentration is increased from a low level. Then above an elevated threshold concentration, the rate of photosynthesis saturates and can even be reduced. The mechanism of the effect is involved and has been the subject of research for many years by agricultural scientists interested in maximizing crop yields (for example in greenhouses).

Elevated temperatures can cause heat stress in growing plants by dehydrating them: as in their fatally drying out in a drought. However, a growth inhibiting (and even growth killing) heat stress can also occur to well-watered plants by the high temperature “denaturing” of the enzymes that control the reaction rates (the chemical reactions) of the photosynthesis process within plant leaves.

Current research on plant growth under the combined effects of elevated temperature and high CO2 concentration shows that “in heat-stressed plants at normal or warmer growth temperatures, high CO2 may often decrease, or not benefit as expected, tolerance of photosynthesis to acute heat stress. Therefore, interactive effects of elevated CO2 and warmer growth temperatures on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.” (https://pubmed.ncbi.nlm.nih.gov/19017125/)

There are now scientific projections of crop yield reductions for several agricultural regions, due to anticipated rises of CO2 concentrations and their related elevated regional temperatures. A report issued by Chatham House (https://www.chathamhouse.org/2021/09/climate-change-risk-assessment-2021) on 14 September 2021 describes the following:

“The planet could be struck by a wave of ‘unprecedented’ crop failures in the next 20 years if global greenhouse gas emissions continue as usual… researchers detailed a litany of risks that climate change could pose to [food security]… global agriculture will need to produce nearly 50 percent more food by 2050 to feed a growing population. But as global demand increases, crop yields could drop by 30 percent as farmers contend with a hotter and more volatile planet… By 2050, an anticipated 40 percent of the planet’s cropland will be exposed to severe drought for at least three months per year, and the breadbaskets of the United States and southern Russia could be among the regions most affected. Europe, the report said, is likely to experience the largest increase in agricultural drought, ‘with the central estimate indicating that nearly half the cropland area will experience severe periods of drought by 2050’… By the 2040s, the United States, China, Brazil and Argentina, which grow 87 percent of the world’s maize, could suffer a steep drop in their maize production — all at the same time. ‘The probability of a synchronous crop failure of this order during the decade of the 2040s is just less than 50 percent.’… Farmers will also have to contend with a decline in the length of crop seasons and long stretches of water scarcity… East and South Asia will be particularly hard hit, with 230 million people subjected to prolonged drought by 2040. Outside of Asia, Africa will likely have the greatest number of people facing drought, exceeding 180 million by 2050. Many regions also will have to manage coastal and river flooding. By 2100… 75 million people in East, South, and Southeast Asia will face coastal flooding every year. ‘Across these three regions around 11 times more people will be impacted by coastal flooding than under a scenario in which climate change is averted.’” (https://thefern.org/ag_insider/crop-yields-seen-falling-without-climate-action-report/)

This all leads to a bleak vision of our planet’s future, where lives are shorter, food is more scarce, and 3.9 billion people “are likely to experience major heat waves.”

My purpose in describing all this is not to feed into more self-indulgent wallowing in depression and flaccid fatalism over the anticipated ‘collapse of civilization’ and ‘human extinction,’ but to show how elevated CO2 concentrations along with elevated global-regional temperatures will physically reduce our food security — crop yields — and in that way very directly shorten human life globally. This is intended to prod the public mind to get on with the job of effectively responding to global warming climate change, by cutting through the many excuses for continuing to cling to the dysfunctional behaviors (fossil fueled capitalism and militarism) driving the planetary crisis, and to change those behaviors to ensure we all have sufficient good food and clean water in an enduring future.

[Thanks to Peter Carter for pointing me to the Chatham House report.]

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