Smog might have some advantages

Press, 4 August 1983, Page 18

Smog might have some advantages

The steady build-up of carbon dioxide in the atmosphere has led to fears of the “greenhouse effect.” That need not be bad, because crops grow well in greenhouses, as Dr John Gribbin, author of the book, “Future Weather,” points out in this article reprinted from the “Guardian.”

Carbon dioxide is building up in the atmosphere as a result of mankind’s activity—burning fossil fuel, especially coal, and the destruction of the tropical rainforests. Between 1860 and 1979 the proportion of carbon dioxide in the atmosphere increased from about 290 parts per million (ppm) to 335 ppm (0.00335 per cent) that is by more than 15 per cent. Although carbon dioxide is present only in small quantities in the atmosphere, such a dramatic proportional change is likely to have some impact on the environment, and because carbon dioxide gas traps heat that could otherwise escape into space climatologists have become concerned about the prospect of a “carbon dioxide greenhouse effect,” warming our planet in the decades ahead and altering climate unpredictably. The experts disagree among themselves on how big an effect there might be on climate, and how soon. The most extreme possibility seems to be an increase in global mean temperatures of some 2 per cent, by about the middle of the 21st century, when the “natural” concentration of carbon dioxide might have doubled. Would this be such a bad thing? All the gloomy prognostications have come from climatologists, physical scientists who naturally worry first about things like temperature changes. But most of their glooomy scenarios concern food, and they are not experts in that department. Few people would disagree with the sentiment that a warmer world might be a better place to live, other things being equal, and the only problem seems to be if the climatic changes do affect agriculture adversely. Did you ever hear a gardener complain that his produce does worse inside the greenhouse than outside? In the last couple of years some agriculturalists have begun to respond to the gloomy prognostications of the climate experts. “Don’t you worry about the

greenhouse effect,” they say, “a little more carbon dioxide can only be good for our crops.”

One of the agriculture experts now speaking up in praise of the greenhouse effect is Professor Norman Rosenberg, of the University of Nebraska-Lincoln, who has looked at the way common crop plants respond to enhanced concentrations of carbon dioxide.

Green plants are classified into three main groups according to the way they carry out photosynthesis, and two of these are especially important to agriculture. The so-called C 4 plants include corn, sorghum, millet and sugarcane, while the C 3 plants include wheat, barley, alfalfa, soyabean and many others. The essential difference between the two families is that although both of them can carry out respiration by a process independent of the presence of light, C 3 plants have a second respiration system, called photorespiration, which operates only in daytime. C 4 plants are more efficient at photosynthesis — and therefore at making food — under optimum conditions, but they need strong sunlight and high temperatures. C 3 plants are less productive in the tropics, but more rugged, so that they continue to do reasonably well in colder latitudes (above 7 degrees C) where C 4 crops do

Controlled experiments in greenhouses show that both types of plant are affected by increased carbon dioxide. At very high concentrations — ten or more times the present atmospheric concentration — there are detrimental effects. But at the levels likely to be encountered over the next 50 to 100 years the effects are all beneficial.

Both types of plant are more efficient at photosynthesis if provided with more carbon dioxide and this effect is most pronounced in the C 3 varieties which thrive at high latitudes; both use water more efficiently if provided with

more carbon dioxide, because the leaf pores through which they exchange gases and water vapour with the air (the stomata) close up. This effect is more pronounced in the C 4 types that thrive in the tropics. Both effects are just what the farmer would order, if he could.

Indeed, one of the things puzzling Sylvan Wittwer, Director of the Agriculture Experiment Station at Michigan State University, is why more farmers don’t order more carbon dioxide for their crops. He says that the “currently low level of atmospheric carbon dioxide may well be the most limiting factor in over-all agricultural productivity,” and describes the experiments which have shown the value of enriching the air inside real greenhouses with carbon dioxide as “an abandoned goldmine” of agricultural productivity. Almost all crops respond favourably to increased carbon dioxide, with yields increased by about 20 per cent for carbon dioxide concentrations around 400 ppm. “The potential for increased photosynthesis is about 0.5 per cent for each one per cent increase in the concentration of atmospheric C 02,” according to his studies, in the range up to 500 ppm. At the same time, for crops including soyabean and maize, water requirements per unit of biomass are halved at carbon dioxide concentrations twice those of the present atmosphere. Could these benefits be counterbalanced by adverse changes in climate in a warmer world? Far from it.

• The all important corn belt of North America has already extended its range 500 miles further north in the last 50 years thanks to improved techniques, and a slight warming of the globe could only help to consolidate that position, says Wittwer. “Making the Minnesota climate that of Texas would not eliminate many crops.” As for the Third World, where rice remains the staple food for most of the global population, any warming can only be beneficial.

Professor Suresh Kumar Sinha, of the Agricultural Research Institute in New Delhi, has pointed out that even if rainfall decreased a temperature increase of 1 to 2C

would improve rice yields, while if rainfall also increases (more likely in a warmer world, because more water evaporates from the sea and gets into circulation) yields could soar by 10 per cent or more, quite apart from the gains due to improved photosynthesis. If carbon dioxide is so good for crops, why haven’t we noticed these effects already? Perhaps we have.

Wittwer points out that there has been a great increase in crop productivity of our planet over the last three decades ... the “green revolution” has coincided with the period of recorded rapid increase in concentration of atmospheric carbon dioxide ... are we already “hooked” on high concentrations of carbon dioxide and their associ-

ated high levels of crop productivity? The tragedy will be if the perceived “threat” of the greenhouse effect leads the rich nations into an ill-conceived and expensive search for solutions, gobbling up resources that could be better used to improve world agriculture, reduce our susceptibility to all weather variations, and prepare the way to take advantage of any benefits that might accrue as the carbon dioxide concentration increases.

Efforts devoted to minimising the concentration of carbon dioxide in our atmosphere may be minimising the yields from our farms — and that prospect of misguided effort is the real carbon dioxide “problem.”