Carbon Dioxide:

Is It Increasing Our Crop Yields?

Everyone knows that bad old carbon dioxide, mainly resulting from burning fossil fuels, is increasing in our atmosphere. All schoolchildren know that this contributes to dreaded global warming.

What's often overlooked is that carbon dioxide is known to make most plants grow better and yield more fruit. That shouldn't be too surprising, given that almost all the plants that we grow for food or fiber evolved millions of years ago, when the carbon dioxide content of the atmosphere was greater (Hint to the chicken vs. egg crowd: it was warmer then—but, which came first, the gas or the heat?).

The carbon dioxide-mediated yield increase results from an increase in the efficiency of photosynthesis that no one really understands completely. But, one of the known correlates is that plants become more efficient with regard to their consumption of water as carbon dioxide increases.

That's all speculation. But the numbers for Virginia get very interesting. Once upon a time, under Federal supervision, the State Climatology Office was involved in modeling of Virginia crop yields. Rather than simply subtracting the technological trend away (in order to see the effect of weather), we explicitly tried to figure out what was driving up yields.

We factored in the obvious (fertilizer) and the nonobvious to try and determine the cause of the upward trend. But, try as we might, there was still a rise that we could not explain. Had we known then about the growth-enhancing effects of CO2, we might have found the cause.

There are two very strong pieced of evidence arguing that we are making the planet greener. One is that changing the natural greenhouse effect mainly warms the driest airmasses, which happen to be the cold ones. Not much happens to the warm, soppy ones of summer, which is why climatological cognoscenti chuckle when someone blames a Richmond day with a temperature of 98° and a soaking dewpoint of 75° on global warming.

So greenhouse warming mainly congregates in the extremely cold airmasses that ring the North Pole in winter. Warming them a bit should hasten the time in which spring arrives and retard the first massive freeze in fall. According to Myneni, et al., writing in Nature in 1997, this turns out to be the case. He argues that satellite measurements of the greenness of the northern latitudes show the growing season is now a wee bit longer.


The other mechanism for planetary greening is the direct stimulation of vegetation by carbon dioxide. The evidence here is also pretty compelling. As can be seen in the figure above, there is an annual oscillation in the concentration of atmospherice carbon dioxide. In the spring, leaves emerge on the trees, and their photosynthetic factories draw it out of the air and turn it into more plant material during the ensuing summer. In the fall, the leaves fall off the trees, to ultimately decompose over the winter and spring.

This magnitude of this annual cycling of carbon dioxide, shown below, has been increasing as the concentration has risen. There are two possible reasons: either there's more plant material being formed each year (explaining the low points) or plants are respiring more. This last is related to growing season temperatures which haven't changed enough to result in much of an increase. Conclusion? Planetary greening.


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