Increased CO2 caused big gains in US crop yields

Written by Science notes on July 2, 2024. Posted in Current News

From the US National Bureau of Economic Research: a new study by economists Charles Taylor and Wolfram Schlenker of Columbia University shows that rising atmospheric CO2 levels were better for crops than scientists had suspected up to now.

So more unsettled science… but for once it’s better than scientists thought, not worse. Experts have long known that something good was going on out in the fields. Starting around 1950 US agricultural output started soaring even while the number of workers was shrinking.

Introduction of improved seed and plant varieties helped, as did new equipment and techniques. But by the 1990s it was clear that agricultural productivity was growing much faster than the rest of the economy, and faster than could be accounted for by standard measures of technological improvement.

Using satellite-measurements of changing CO2 levels from 2015 to 2021 matched to county-level crop yields these economists found not only that extra CO2 makes crops grow better, which experts already knew, but the effect is much much better than previously believed.

The usual way of measuring how CO2 affects plants is to grow them in a greenhouse where the CO2 level can be artificially increased.

Taylor and Schlenker note that the effect is so good, so consistently, that commercial greenhouses typically raise the CO2 level to 900 ppm or more, at least double the average outdoor level.

But it’s hard to identify how plants would respond out in the field where other weather conditions can play a role.

One way to try to figure it out is to use little chimneys and pump CO2 into the air along a row of field crops, called the Free Air Carbon Enrichment or “FACE” method.

Those experiments have tended to show only small improvements in yield, but critics have argued the results aren’t very accurate since the CO2 gets blown away so the plants may not benefit from it.

There is another approach: look at the big picture. So Taylor and Schlenker made use of a satellite observatory that was put in space to measure the distribution of CO2 in the atmosphere.

While CO2 eventually mixes to a uniform average in the troposphere, closer to the ground it varies considerably over space because of the variation in sources (such as cars and factories) and sinks (like plants and forests).

It also varies seasonally, dropping in the spring and summer as plants grow then rising again as they die and decompose, and it trends up over time as CO2 emissions happen.

The satellite record yielded point-by-point estimates of the local CO2 level during the 2015-2021 period that the authors could then line up with local temperature, precipitation and air pollution records, then use to explain local variations in the output of corn, soybeans and wheat.

They found that every one-part-per-million increase in local CO2 yielded a gain of between 0.5% and 0.8% in output depending on the crop type. These benefits were far higher than estimates from FACE and other previous methods.

Looking back in time Taylor and Schlenker attribute 10% of the total increase in output of corn since 1940 to CO2, plus 30% of soybeans and 40% of wheat. Which is a remarkably good thing if you dislike hunger and hate deadly famines.

Indeed, if someone invented a machine that boosted crop productivity by that much it would be hailed as a miracle of modern technology. Instead we keep hearing how extra CO2 is going to kill us all. Well at least we’ll be well fed when the apocalypse arrives.

See more here Climate Discussion 

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