About a year ago I decided that I should look hard at the issue of Anthropogenic Global Warming the notion that it is we human beings who are responsible for the warming of the earth.

I thought we had immediate environmental problems facing us, including our failure to manage water and our over-dependence on oil , and that we should be dealing with them. Global warming seemed a distraction.

Although it was plain that the learned academies, governments and the UN all seemed to believe AGW was true, what puzzled me was the stridency of the claims that we had to act now. If it was all so obvious, why weren't our governments acting to save us? What followed were months of discovery and learning.

I think the central AGW proposition can be put like this:

Human activity in burning coal and oil, and clearing forests has put an enormous amount of carbon dioxide into the atmosphere, where it has combined with water vapour to increase global temperatures in an unprecedented way. The evidence that this has occurred is clear-cut, and the increase in temperature will have, according to our computer models, dire effects on the planet. We must change our ways lest catastrophe strike us. It may already be too late.

That led me to find out what I could about the following:

(1) the extent to which the planet is warming;

(2) whether or not such warming is unprecedented;

(3) whether the warming is caused by burning fossil fuels;

(4) the likelihood of polar ice melting in a major way;

(5) the use of computer models in predicting future climates;

(6) the reluctance to admit uncertainty; and

(7) the extent to which we need to change to avoid catastrophe.

Twelve months later the outcome for me is, to say the least, uncertain. But this is what I think one can reasonably say.

Is the planet warming? Maybe. It depends on what measurements you think are relevant. It doesn't seem to have warmed for the last 10 years. The Intergovernmental Panel on Climate Change says 0.6 of a degree plus or minus 0.2 of a degree over the 20th century. I'm prepared to accept that.

Is the warming unprecedented? Probably not. There is abundant historical and proxy evidence for both hotter and cooler periods in human history.

Is it our fault? Again, maybe. The correlation of increasing warmth with increasing carbon dioxide concentrations is particularly weak; that with solar energy and with ocean movements is much stronger.

Are we likely to see rising sea-levels? Not in our lifetimes or those of our grandchildren. It is not even clear that sea-levels have risen at all. As so often in this domain, there is conflicting evidence. The melting of polar or sea ice has no direct effect.

How reliable are the computer models on which possible future climates are based? Not very. All will agree that the task of modelling climate is vast, because of the estimates that have to be made and the rubbery quality of much of the data.

Given all this, why is there such insistence that AGW has occurred and needs drastic solutions? This is a puzzle, but my short answer is that the IPCC has been built on the AGW proposition and of course keeps plugging it, whatever the data say. The IPCC has considerable clout. Most people shy off inspecting the evidence because it looks like science and must therefore be hard.

The media have been captured by AGW (it makes for great stories), the environmental movement and the Greens love it, and business is reluctant to get involved. Governments simply postpone making the draconian decisions they are urged to make.

What should we do? The current Garnaut inquiry is not of much help, because it too is based on the assumption that AGW is correct. A Royal Commission has been proposed, and I would be in favour of that course of action if the inquiry is run properly.

More generally, I would urge people to find out for themselves, because the issue is important. There are around four million Australians who have been to university. This domain is science, but the questions are straightforward and accessible. I warn that there is a lot of reading!

What we should not do is go down the path of carbon taxes, carbon trading and carbon caps before it is absolutely plain that there is no alternative. Why? At present such measures seem likely to be unnecessary and futile and to lead to rorts.

Professor Aitkin AO, a historian and political scientist, is a member of the Australian Science and Technology Council.

This is a much abridged summary of his address to the Australian Planning Institute yesterday.