CLIMATE CHANGE: A SHORT PRIMER.

THE FIRST CALCULATION 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 by 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, and looked forward to enjoying "ages with more equable and better climates, especially as regards the colder regions of the Earth, ages when the Earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind".

Since then, and particularly over the last twenty-five 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. It was a major driver in the development of remote-sensing Earth satellites and high performance computers.

Nevertheless this massive effort has yet to provide us with reliable forecasts of climate change. While part of the problem is that the earth-atmosphere system is extremely complicated, there are more fundamental reasons.

The atmosphere and the ocean are turbulent. This means that very small fluctuations can grow in size and alter the large-scale processes within the system. No matter how detailed are our measurements of atmosphere and ocean, there can be undetected fluctuations smaller than the distance between the measurements. They can grow and create large-scale changes which are inherently unpredictable. As a consequence there is a theoretical limit of about ten days to forecasting individual weather systems and the local temperature, rainfall and wind associated with them. It will never be possible to have skilful weather forecasts beyond a week or two.

Climate is concerned with the "average weather" of a region, and presents a slightly different forecasting problem. Average weather is controlled among other things by processes on land and in the ocean which change relatively slowly or not at all. To the extent that these processes are predictable, then so too may be those aspects of average weather which depend on them. "Relatively slowly" can mean the months and years associated with changes in the upper ocean and on the land surface. It can mean the centuries and millennia associated with changes in the deep ocean and the polar ice caps. Slow as these changes may be, many of them are turbulent fluctuations which, like those of the atmosphere, are inherently unpredictable.

Thus the science of climate change is plagued by uncertainty. At least part of that uncertainty, and perhaps most of it, will never be resolved.

Another of the problems is that modern instrumental observations of atmosphere and ocean have not been made for sufficient time to test forecasting techniques or to establish which type of change may be predictable. 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 when dealing with turbulent media.