The Reality of Climate Change

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It took a long time for the reality of climate change to be recognized, in particular the global rise in temperature at sea level. The story began with the pioneering work of the Swedish scientist Svente Arrhenius, who realized that industrial processes leading to the emission of carbon dioxide could warm the earth. This conclusion was supported by the independent research of Thomas Chamberlain. Their results attracted little attention because it seemed that human activities must be insignificant compared with many other effects on the climate such as the sunspot cycle and the changes of the earth's orbit around the sun. Furthermore, there is about fifty times as much carbon dioxide in the oceans than in the atmosphere and it was thought that they would easily absorb any extra pollution due to human activities (Maslin 2004).

During the nineteen fifties improved scientific studies of the atmosphere and the oceans showed that additional carbon dioxide in the atmosphere would warm the earth and that only about a third of this would be absorbed by the oceans. By about 1960 there was general agreement that the temperature of the earth should increase due to human activities. However this seemed to be contradicted by the overall fall in temperature from 1940 to 1975. Scientists then predicted continual cooling, leading to widespread famine due to floods and drought.

Subsequently, however, the global temperature began to increase and by 1990 the growth was unmistakable, although there were still grounds for doubt. In particular, there had been changes in the methods of measurement of temperature, the previous cooling was not understood and the satellite measurements of the upper atmosphere did not show the warming trend. Further work showed that for various reasons these indications are unreliable, and the steady increase of temperature was confirmed.

During the nineteen eighties people became more conscious of the importance of environment. This was due to many events, from the publication of Rachel Carson's book The Silent Spring in 1962, the Report of the Club of Rome on the limits to growth to the nuclear accidents at Three Mile Island and Chernobyl. The global nature of climate change was shown by the discovery in 1955 of the ozone hole in the atmosphere above Antarctica that was discussed in Section 6.3. In this case prompt international action in banning the use of chlorofluorocar-bons succeeded in averting the danger.

In addition to these global changes there is impressive evidence for the reality of climate change during the last few decades. Some of this has been described in a book by Sir Ghillean Prance, former Director of the Royal Botanic Gardens in Kew in London. He recalls that there were devastating floods in Mozambique and Venezuela and quite serious ones in England. In other countries there has been drought, that in the Midwest of the United States in 1988-1989 caused losses estimated at $39 billion. The hurricane Mitch killed ten thousand people in Central America. The average temperatures are rising in many countries: of the five warmest years ever recorded in the United Kingdom, four have been in the last decade. One result is that in some regions the growing season for plants is increasing, with earlier development in spring, and autumn events being delayed. Birds and animals are also affected, and some species, unable to cope with the climate change, have become extinct. Already British birds nest about twelve days earlier than they did a few decades ago.

For the last thirty years the isotherms have been moving steadily towards the North and South Poles at a rate of about thirty-five miles per decade. Some animals and plants are able to move and keep up with this, but others may find it difficult or impossible due to biological factors and man-made and natural barriers. Since they are in many ways interdependent this could disrupt the ecosystem, causing the extinction of some species. In the past, rises in temperature have caused mass extinctions of 50% to 90% of all species, and the same could happen again.

Such evidence raises many questions. Do these changes show that world climate is changing? If so, will it continue to change in the same way? Are these changes due to human actions? If so, what can we do about it?

Climate is determined by many natural causes, and in addition there is evidence that it is affected by human actions, as shown in Figure 7.1. We cannot do anything about the natural causes, but if there is a causal link between human actions and climate change we may have reason to expect the present changes to continue, and furthermore we will have a strong incentive to take action to mitigate the harmful effects of climate change.

1750 1800 1850 1900 1950 2000

Source: Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center,

Figure 7.1. The Effect of Anthropogenic Emissions on Atmospheric Concentrations of Carbon Dioxide.

1750 1800 1850 1900 1950 2000

Source: Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center,

Figure 7.1. The Effect of Anthropogenic Emissions on Atmospheric Concentrations of Carbon Dioxide.

A causal link between carbon dioxide emissions and global warming has been proposed; it is the greenhouse effect that is discussed in the next section.

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