Many people think that our concern about carbon dioxide and global warming is a modern preoccupation driven by the attention of high-profile personalities, politicians and green activists. But Al Gore did not discover global warming. Nor did Tim Flannery, Peter Garrett, Greenpeace or Malcolm Turnbull. Scientific concern about global warming is not new. A single scientific paper, published more than three decades ago, can place the discussions about climate change into historical perspective.
Tomorrow it will be 35 years since the leading science journal Nature published a review paper entitled “Man-made carbon dioxide and the ‘greenhouse’ effect”, by the eminent atmospheric scientist J. S. Sawyer, director of research at the United Kingdom Meteorological Office. In four pages, Sawyer summarised what was known about the role of carbon dioxide in enhancing the natural greenhouse effect leading to warming at the earth’s surface, and made a remarkable 28-year prediction of the warming expected to the end of the 20th century.
His prediction can now be compared with what has been observed. We can also compare his review of the science in the early 1970s with that in the latest (2007) assessment from the Intergovernmental Panel on Climate Change. After summarising recent calculations of the likely impact of increasing carbon dioxide concentrations on global surface temperature, Sawyer concluded that the “increase of 25 per cent in carbon dioxide expected by the end of the century therefore corresponds to an increase of 0. degrees in world temperature – an amount somewhat greater than the climatic variations of recent centuries”.
Examination of the global surface temperature over the latter part of the 20th century shows that in fact the temperature rose about 0. 5 degrees between the early 1970s and 2000. Considering that global temperatures had, if anything, been falling in the decades leading up to the early 1970s, Sawyer’s accurate prediction of the reversal of this trend, and of the magnitude of the subsequent warming, is perhaps the most remarkable long-range forecast ever made.
Sawyer’s succinct summary of the climate change science understood at that time can be compared with the four volumes of the IPCC Fourth Assessment on Climate Change being released through 2007. The IPCC assessment involves more than 400 authors, about 2500 reviewers, and runs to several thousand pages with many thousands of references. Such a comparison shows that much has been done to address the concerns and uncertainties expressed by Sawyer at the time. He was concerned that the rudimentary understanding of cloud processes and other climate system feedback resulted in uncertainties regarding predictions of warming.
At the time, climate models were in their infancy, but Sawyer saw them as the best way to examine this feedback and reduce the uncertainties in climate change predictions. Since then, models have improved substantially and now include many more processes in more detail than was possible in the early 1970s, and the various climate processes that may enhance or offset the effects of carbon dioxide have been studied in detail. Despite these advances, our best estimate of the warming to be expected from a doubling of atmospheric carbon dioxide concentration has changed little from Sawyer’s time.
Our best estimate of the temperature increase that would result from a 25 per cent increase in atmospheric carbon dioxide concentrations is still around 0. 6 degrees. The scientific consensus of Sawyer’s time was very similar to the scientific consensus in 2007. Of course, better climate models and improved data and analyses have allowed the IPCC to discuss and even project possible changes in many other meteorological variables than could Sawyer, including extreme weather of various kinds as well as sea-level.
The IPCC now also looks in detail at regional aspects of climate change – a subject not even considered by Sawyer. Perhaps the greatest difference, however, is the emphasis on the impacts of climate change. While the IPCC assessment devotes a volume to this subject, Sawyer could only conclude, after conceding that climate variations of only a fraction of a degree can have “considerable economic importance” that “although there may be no immediate cause for alarm about the consequences of carbon dioxide in the atmosphere, there is certainly need for further study”.
Perusal of the IPCC volume devoted to the impacts of climate change on natural and human systems leaves one feeling far less sanguine than Sawyer was 35 years ago. The anniversary of Sawyer’s paper reminds us that the understanding of the effects of carbon dioxide on the global climate was sufficiently advanced 35 years ago to allow an accurate 28-year prediction of warming.
Despite claims to the contrary, our understanding of the greenhouse effect and global warming is not reliant on modern climate models and nor is it a modern preoccupation. Nor is it correct to claim that in the 1970s climate scientists were predicting global cooling – Sawyer’s paper accurately predicted exactly the opposite, based on the best science available. Other scientific papers around that time also drew attention to the warming expected from the anthropogenic increases in greenhouse gas emissions.