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Carbon Dioxide and the Climate

A 1956 American Scientist article explores climate change; two contemporary commentaries illuminate its relevance to the present

Gilbert N. Plass, James Rodger Fleming, Gavin Schmidt

Unfortunately it is difficult to obtain any direct evidence for the carbon dioxide content of the atmosphere during past geological epochs. In fact it is not even certain from direct measurements whether or not the carbon dioxide content has increased in the last 50 years. A plot of such measurements can be fitted nicely with a linear curve that increases by 10 per cent in that time interval. However, the probable error for most of the measurements is so large that this result is not very firmly established. Because of its importance to the climate, regular measurements of the atmospheric carbon dioxide content should be started at several different country locations and continued for a number of decades. Since the atmospheric carbon dioxide content varies somewhat with the past history of the air mass and the time of year, a number of measurements are necessary in order to obtain a reliable average. The present predicted rise of 3 per cent a decade could be easily observed with the present techniques of analysis. As to the carbon dioxide content of the atmosphere at earlier periods, only general discussions of the various factors that affect the carbon dioxide balance can be given at the present time. It is possible though that we will be able to calculate the carbon dioxide amount of a past epoch from measurements of the ocean temperature and the rate of carbonate deposition during that epoch together with further studies of the atmosphere-ocean equilibrium.

There is some interesting evidence which suggests that the carbon dioxide content of the atmosphere was once much larger than at present. It is known that plants grow more luxuriantly and rapidly in an atmosphere that has from five to ten times the normal carbon dioxide amount. In fact carbon dioxide is sometimes released in greenhouses in order to promote growth. Since plants are perfectly adapted to make maximum use of the spectral range and intensity of the light that reaches them from the sun for photosynthesis, it seems strange that they are not better adapted to the present carbon dioxide concentration in the atmosphere. The simplest explanation of this fact is that the plants evolved at a time when the carbon dioxide concentration was considerably higher than it is today and that it has been at a higher level during most of the ensuing time. Higher temperatures than today during most of the Earth’s history would have resulted from this higher carbon dioxide content. In fact the geological evidence shows that warmer climates than today have existed for at least nine-tenths of the time since the Cambrian period.

Further evidence as to the carbon dioxide amounts in the past is provided by the pH of sea water. There is a definite pH value associated with a given atmospheric carbon dioxide amount when the atmosphere-ocean system is in equilibrium. Further, many marine animals are very sensitive to the pH value, the higher marine animals being more sensitive in general than the lower. For example, herring are killed if the pH changes by more than one-half unit; lower marine animals such as sea urchins, diatoms, and algae cannot tolerate pH changes of more than one unit.

This suggests that the pH of the oceans has not varied by more than these amounts since the time when these animals evolved or at most that the pH has changed extremely slowly so that these animals could evolve to live in the changed environment. However, even with the stringent requirement that the pH of sea water should not change by more than one-half unit, the atmospheric carbon dioxide amount can still vary by a factor of fifty and maintain equilibrium between the atmosphere and the oceans. Thus very large changes in the atmospheric carbon dioxide amount can occur without influencing either marine or land animals; still larger variations would even be possible over time intervals sufficiently long to allow the animals to adapt to their new environment.

All calculations of radiocarbon dates have been made on the assumption that the amount of atmospheric carbon dioxide has remained constant. If the theory presented here of carbon dioxide variations in the atmosphere is correct, then the reduced carbon dioxide amount at the time of the last glaciation means that all radiocarbon dates for events before the recession of the glaciers are in question.

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