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Is the Increasing CO2 Better for Plants?

Mon ,10/01/2011

The concentration of CO2 in the air is increasing rapidly due to our use of fossil fuels. Since plants require CO2, it would seem that the CO2 increase would be good for plants. However, that is a great simplification as the increased CO2 levels may not be good for most plants or for mankind.

CO2: In 1900, Arvid Hgbom found that factories were emitting about the same amount of CO2 as all the world’s volcanoes. This worried some people – but they were told not to worry, the plants and oceans would soon take up any excess. However, that has not been the case. We have destroyed many of the forests that take up CO2. Our factories and other activities now are emitting thirty billion tons of CO2 each year, around 200 times as much CO2 as all the world’s volcanoes. The CO2 dissolves in water to form carbonic acid, which has made the oceans 20% more acidic in the last century. This has damaged the coral and phytoplankton that take up CO2. Also, since CO2 is less soluble in the more acidic waters, the oceans are losing their ability to take up the excess CO2. Measurements by Charles Keeling of atmospheric CO2 levels have shown that the plants and the oceans are not taking up the CO2 nearly as fast as we are producing it. The concentration of CO2 in the air has increased rapidly over the last century, from 280 parts per million (ppm) to 385 ppm, a 38% increase. It would seem that one bright spot might be that the higher CO2 level would be good for plant growth. However, that may not be the case.

Historical Data shows that the concentration of CO2 in the air has varied widely over the Earth’s history, from over 7000 ppm in the Paleozoic era to a low of 180 ppm during ice ages. Ice core studies show that during the last four ice ages, the CO2 concentration was about 180 ppm during the cold periods and rose to about 280 ppm during the warmer interglacial periods. Warmer temperatures occur at the same times as the higher CO2 levels, so it is difficult to sort out the effect on plants caused by CO2 changes from those caused by temperature changes. Ancient plants that grew at higher temperatures and CO2 levels during the Paleozoic Era adapted to the drop in CO2 level and temperatures near the end of that era by developing leaves. Ancient leaved species, such as Ginko and Magnolia, when exposed to higher levels of CO2, change their leaf structures. Other plants have adapted by changing the altitude at which they grow. For the last 6000 years, we have been in an interglacial period. The Earth’s temperature has been relatively stable and the CO2 concentration has been about 280 ppm, except for the last century.

Plant Growth: According to Dr. Surendra Singh, a biologist with a background in botany, modern plants have adapted to the 280 million ppm CO2 concentration that has existed over the last thousands of years. There is no reason to believe that the increasing CO2 concentration would be better for plants as CO2 is seldom the limiting agent in plant growth and seed production. Plants also require water, nitrogen, phosphorous, potassium, sunlight, and trace elements. Over-fertilizing, over-watering, or putting a shade plant in bright sun will harm the plant. More is not always better, and the increasing the CO2 levels might harm plants that are not adapted to higher CO2 levels.

Greenhouses: Commercial greenhouses have found that adding CO2 helps plants grow better. Some people claim that that proves CO2 stimulates plant growth, but that is not necessarily true. In an enclosed greenhouse, the plants quickly deplete the supply of CO2 if more is not made available. That may also be the case with hydroponic gardeners, who claim better production at higher CO2 levels. Specific crops may do better at the higher levels of CO2 – if all the other nutrients are supplied at an optimum level and plenty of water is available. That is not possible for plants in the outdoors. We do fertilize many crops, but doing so depletes the soil of other nutrients and there is an energy cost in producing and spreading the fertilizers.

Experiments: It is difficult to do outdoor experiments on the effect of CO2 levels on crop growth, but a few have been done. One experiment found that wheat grown at higher CO2 levels has more leaf mass and more kernels; however, the kernels are smaller and have less nitrogen, making them less valuable as a food source. In another experiment, higher CO2 levels in wheat used for grazing correlated with lower nitrogen in the leaves, making the crop less suitable for grazing. Agriculture experts are saying that the result of increasing CO2 levels coupled with increasing temperatures will lower crop yields or quality. That has been found to be true in rice production, and rice is a staple for half the world. In an inadvertent experiment, we have found that some invasive species, such as Kudzu, are well adapted to the increasing temperatures and CO2 levels. They have prospered and are expanding their range northward.

Certainly, plants need CO2, but to say more CO2 will make all plants grow better is a great supposition and oversimplification. The Earth’s atmospheric CO2 level is now 385 ppm, possibly higher than it has been in the last 20 million years, and it is increasing every year. By pouring more CO2 into the air each year, we are conducting a great experiment with unknown results. What will happen if the production from agricultural plants decreases significantly?

Note added on 12/07/ 2012:   This graph from the National Research Council projects the effect of rising temperatures on crop production. It includes both the effect of temperature and CO2 increases.

(C) 2011  J.C. Moore

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