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Numerous studies and laboratory experiments have shown that plants grown under higher carbon dioxide levels than at present do better — grow faster, bigger, use water more efficiently — than crops grown under atmospheric carbon dioxide levels. This is hardly surprising since most plants, including the progenitors of modern crop varieties, evolved at times when carbon dioxide levels on earth were much higher than today.
In a recent study, scientists examined a different question, how do crops fare under conditions of lower carbon dioxide, particularly carbon dioxide levels experienced during the most recent ice age.
The study in Global Change Biology examines the effect of lower carbon dioxide levels on plant growth. The researchers grew one type of wheat, wild barley, and two types of millet from seed to harvest in a controlled environment under two carbon dioxide levels, 180 parts per million (ppm), the level of carbon dioxide during the last glacial maximum, and 270 ppm, corresponding to the levels carbon dioxide reached after the most recent ice age but before the industrial revolution.
The research showed grain production for every crop was significantly lower under 180 ppm of carbon dioxide than under 270 ppm. Wheat yields were 32 percent lower, barley yields 34 percent lower, broomcorn millet yields 9 percent lower, and foxtail millet yields 23 percent lower. The researchers also found the crops had “depressed values of light-saturated rates of photosynthesis, reduced seed germination rates, and a decline in water use efficiencies at the lower, as opposed to the higher, CO2 concentration.”