In the Lessons from Past Predictions series, we've examined some impressively accurate global warming projections made over 30 years ago, for example by James Hansen in 1981 and Wallace Broecker in 1975. We recently learned of an even earlier successful prediction, made by John Stanley (J.S.) Sawyer in a paper published in Nature in 1972.
J.S. Sawyer was a British meteorologist born in 1916. He was elected a Fellow of the Royal Society in 1962, and was also a Fellow of the Meteorological Society and the organization's president from 1963 to 1965.
His 1972 paper reveals how much we knew about the fundamental workings of the global climate over 40 years ago. For example, Sawyer addressed the myth and misunderstanding that as a trace gas in the atmosphere, it may seem natural to assume that rising levels of carbon dioxide don't have much impact on the climate. Sawyer wrote,
"Nevertheless, there are certain minor constituents of the atmosphere which have a particularly significant effect in determining the world climate. They do this by their influence on the transmission of heat through the atmosphere by radiation. Carbon dioxide, water vapour and ozone all play such a role, and the quantities of these substances are not so much greater than the products of human endeavour that the possibilities of man-made influences may be dismissed out of hand."
Sawyer referenced work by Guy Callendar in the late 1930s and early 1940s, in which Callendar estimated that the amount of carbon dioxide in the atmosphere had increased by about 10 percent over the prior 100 years (a remarkably accurate measurement, as current estimates put the increase during that time at about 9 percent). Sawyer also referenced the Keeling Curve, which included continuous reliable measurements of the amount of carbon dioxide in the atmosphere beginning in 1958. Compared to measurements of human carbon dioxide emissions from burning fossil fuels, Sawyer noted that only about half of those human emissions were remaining in the atmosphere. The other half, climate scientists had concluded, was being absorbed by the oceans and the biosphere. Sawyer wrote,
"Industrial development has recently been proceeding at an increasing rate so that the output of man-made carbon dioxide has been increasing more or less exponentially. So long as the carbon dioxide output continues to increase exponentially, it is reasonable to assume that about the same proportion as at present (about half) will remain in the atmosphere and about the same amount will go into the other reservoirs."
Lower curve: observed increase in atmospheric carbon dioxide from Mauna Loa. Upper horizontal lines: potential increase in atmospheric carbon dioxide from human fossil fuel burning, if all emissions remained in the atmosphere. From Sawyer (1972).
Indeed, over the past four decades, human carbon dioxide emissions have continued to increase more or less exponentially, and about half has continued to remain in the atmosphere with the other half accumulating in natural reservoirs. The carbon dioxide being absorbed by the oceans has contributed to the problem of ocean acidification, sometimes referred to as "global warming's evil twin."
Climate scientists also had a good idea how quickly carbon dioxide levels in the atmosphere would continue to rise as a result of human activities.
"Bolin has estimated that the concentration of carbon dioxide will be about 400 ppm by the year 2000. A recent conference put the figure somewhat lower (375 ppm)."
That prediction at the referenced 1971 conference on "the Study of Man's Impact on Climate" turned out to be quite accurate. In 2000, atmospheric carbon dioxide concentrations were measured at about 370 ppm.
In his paper Sawyer discussed the predicted impacts resulting from a continued rise in atmospheric carbon dioxide. He noted that directly "it might make some vegetation grow a little faster," which is generally true, although the situation is complicated. Sawyer noted that rising carbon dioxide levels would cause an increased greenhouse effect, and the associated warming would lead to more evaporation and more water vapor in the atmosphere. As a greenhouse gas itself, that rise in water vapor would act to further amplify human-caused global warming.
"...if world temperatures rise due to an increase in carbon dioxide, it is almost certain that there will be more evaporation of water–the water vapour content of the atmosphere will also increase and will have its own effect on the radiation balance."
Sawyer referenced a 1967 paper by Manabe and Wetherald, who had calculated that a doubling of atmospheric carbon dioxide would by itself cause approximately 1.3°C global surface warming, but that warming would be amplified by a further 1.1°C due to rising water vapor concentrations if the relative humidity were to remain constant. Observations have indeed unequivocally shown that water vapor strongly amplifies human caused global warming, for example as found by Dessler & Wong (2009).
Sawyer put all this information together to predict how much average global surface temperatures would warm between 1972 and 2000.
"The increase of 25% CO2 expected by the end of the century therefore corresponds to an increase of 0.6°C in the world temperature – an amount somewhat greater than the climatic variation of recent centuries."
Remarkably, between the years 1850 and 2000, atmospheric carbon dioxide levels did increase by very close to 25 percent, and global average surface temperatures also increased by just about 0.6°C during that time.
Sawyer also discussed that melting ice and snow in a warming world would act to amplify global warming, but suggested that increasing cloud cover might dampen global warming and act to regulate the global climate. However, recent research has shown that clouds may actually weakly amplify global warming as well. Sawyer discussed that climate scientists in the early 1970s understood that it would take on the order of 100 years (now understood to be even longer) for the planet to reach a new energy equilibrium due to the thermal inertia of the oceans, causing a lag in the global temperature response to an energy imbalance. Sawyer also understood that significant global warming would cause changes in weather and wind patterns around the world.
All in all, Sawyer's 1972 paper demonstrated a solid understanding of the fundamental workings of the global climate, and included a remarkably accurate prediction of global warming over the next 30 years. This goes to show that climate scientists have understood the main climate control knobs for over four decades.
Posted by dana1981 on Wednesday, 19 March, 2014
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