Mass By Gas

by: Steven Carruthers

Carbon dioxide (C02) enrichment is one of the most interesting curiosities of modern horticulture, and is yielding valuable primary data as research continues into the effects of increased levels Of C02 in the atmosphere.

Unfortunately, the data collected is not generally published outside scientific journals, and seldom reaches the agricultural industry where the effects of increased levels Of C02 can be better understood.




During the past century, the level Of C02 in the atmosphere has been Dsteadily rising, largely from the combustion of fossil fuels. The atmospheric level Of C02 has climbed from an average of about 0.028% (280 ppm) in 1860 to 0.034% (340 ppm) by 1981, an increase of more than 1%. Current estimates predict that C02 levels will continue to rise, perhaps doubling within the next 70 years (Kellogg 1978).

While C02 in this context might be regarded as a pollutant by those concerned with the climatic implications of an increased "greenhouse effect", elevated levels Of C02 are likely to be beneficial to agriculture.

The genetic capability of plants to absorb higher levels Of C02 stems back to primordial times when plants adapted to C02 levels three to four times that which exists on earth today. In fact, for many years horticulturists have practised C02 enrichment in controlled environments to increase crop yields.

An important focus of current research is the exchange of carbon dioxide between the biosphere and the atmosphere, and the effects of elevated C02 levels upon plant species of economic value, in order to predict the likely outcome of future crop yields. On current projections, an increase in atmospheric C02 Will induce higher global crop yields. Kanemasu (1980) estimates that there will be a wheat yield increase in the United States of about 59% with C02 levels double that of today.

However, the implications for agricul-ture will depend strongly upon other weather-related factors, such as changes in rainfall patterns and the length of the growing season. Such predictions are also dependent upon:

1.The continued destruction of the great tropical and sub-tropical forests, since they act as the lungs of the planet for converting C02 into oxygen;
2.How much C02 is absorbed by the world's oceans, since they play a strong interactive role in both the global carbon cycle and the climate system; and
3.The continued burning of fossil fuels given likely alternative technologies of the future.

Tucker (1981) suggests that a doubling Of C02 could give an increase in photosynthesis of somewhere between 30% and 60%, but this may not necessarily be reflected in increased crop yields. He reasons that any increased yields are more likely to be attributed to increased precipitation, which in turn is attributed to global warming as a result of higher C02 levels.

Given a 30% increase in photosynthetic efficiency and a two-degree increase in average temperature, Baker and Lambert (1980) estimate a net increase in crop growth and development of 14% to 38%, depending upon the availability of water. Pimentel (1980) further notes that a decrease in rainfall of between 10% and 30% over the USA, together with a temperature variation of plus or minus two degrees, but with no change in ambient C02, will reduce expected wheat and corn yields by 10%-15%.

At present, these predictions are little more than conjecture owing to our inability to make accurate climate forecasts. However, there are a small number of scientists currently working to understand aspects of the biological consequences of elevated C02 in the future. Scientists can make much more progress in understanding the effects upon existing genotypes and the ensuing phenotypes. For example, yield responses can be studied for different crops, applying existing climate variations in order to estimate the future climate matrix, even if some of the more precise requirements concerning meteorological variables are the subject of conjecture.

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