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A short primer on climate change and the greenhouse issue

By Garth Paltridge - posted Thursday, 21 June 2001


The first calculations of the effect of burning fossil fuel on the world's climate were made more than a hundred years ago. They suggested that the average temperature of the Earth would rise a degree or more because extra carbon dioxide in the atmosphere would act rather like a blanket and reduce the normal cooling of the Earth's surface. At the time people viewed the change in a very positive way.

Since then, and particularly over the past 25 years, an extensive and highly planned international research effort has attempted to translate the broad prediction of global warming to a more detailed assessment of the likely change of climate of individual regions. The effort taught us a lot about the workings of the earth-atmosphere system, and returned us a lot in terms of new technology.

Nevertheless, this massive effort has yet to provide us with reliable forecasts of climate change. The science of climate change is plagued by uncertainty. At least part of that uncertainty, and perhaps most of it, will never be resolved.

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Research has concentrated on the complex numerical models fashioned to give the best possible simulation of today's climate. It is assumed that, if the simulation of today's climate is good, then the model may also be good at simulating the change to a future climate. The assumption is highly questionable.

The recent Inter-governmental Panel on Climate Change (IPCC) assessment, predicting a rise of average temperature over the next century somewhere in the range 1.2 to 5.8 degrees Celsius, presents the extreme values obtained from a set of numerical climate models, each of which was fed with various scenarios of future atmospheric carbon dioxide and dust pollution. The significant point is that there is no practical way to assess which (if any) of the models is more likely to give a correct answer, so that there is no way to assess the confidence which one might place in the results. At best they ‘provide support for’ the results of the simpler calculations of climate change.

The bottom lines of all this are as follows.

We have some confidence (not nearly 100% but at least some confidence) that, over the next century, and as a consequence of man's extra input to the atmosphere of carbon dioxide and other greenhouse gases, the Earth's average surface temperature will rise a degree or three above the temperature it would have been otherwise. With much lesser confidence we might accept that the temperature will rise a degree or three above that of today. The lesser confidence comes about because there is considerable evidence from palaeo-records that the Earth's temperature changed rapidly (i.e. over periods of the order of decades and centuries), considerably (i.e. by several degrees) and naturally (i.e. as a result of self-generated fluctuations) at apparently random times in the past.

There is reason to believe that such an increase of temperature would be greater in the polar regions than at the equator – perhaps by a factor of two. There is reason to believe that the overall global sea level might rise by a few tens of centimeters per century. There is reason to believe that the global average rainfall might increase by something of the order of 10 or 20 percent.

No confidence can be placed in forecasts of climate change at any greater level of detail .

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To put the projected change in a broader context, climate has been much the same as at present for the past 10,000 years. The average temperature over this period seems to have fluctuated within the range of perhaps a degree or so. Over the past several million years, there were repeated retreats into ice ages where the average temperature was perhaps five to ten degrees colder than it is now, and where great ice sheets covered much of North America and Europe. The last ice age was at its peak about 20,000 years ago. The average temperatures of the relatively short warm periods between the ice ages were much the same as today. There were large fluctuations of temperature during the ice ages, and possibly also during the intervening warm periods.

The instrumental record of the past hundred years suggests that the Earth has warmed by a few tenths of a degree over that time. This is consistent with calculations about global warming and the enhanced greenhouse effect, but is certainly far from proof of them. The warming is still well within the range of what seems to have been natural fluctuation over the last ten thousand years – or indeed over the last thousand years.

Extremes versus Averages.

If the average temperature of a place should rise by one or two degrees, one might reasonably expect that there will be more extremely hot days and less extremely cold days, more extremely hot seasons and less extremely cold seasons, and so on. People notice a change in extremes and in the frequency of rare events much more readily than a change in the average. In the jargon of statisticians, the main concern is with the 'tails' of the frequency distributions of rainfall, temperature and the other variables of climate. This again brings great uncertainty into the climate issue.

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This is an edited and abridged extract from an article that was first published in Quadrant, April 2001.



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About the Author

Emeritus Professor Garth Paltridge is an atmospheric physicist and was a Chief Research Scientist with the CSIRO Division of Atmospheric Research before taking up positions in Tasmania as Director of the Institute of Antarctic and Southern Ocean Studies and CEO of the Antarctic Cooperative Research Centre. He retired in 2002 and continues to live in Hobart. He is an Honorary Research Fellow at the University of Tasmania and a Visiting Fellow at the Australian National University. He is a Fellow of the Australian Academy of Science.

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