It is amusing for an old atmospheric physicist to recollect that in the late 1970s, as a result of a one-degree cooling trend, many of the same activists who are now advocating all sorts of measures to avoid the global warming catastrophe were at the forefront of saving us from the next ice age.

We were told by Newsweek in 1975 that scientists "are almost unanimous in the view that the cooling trend will reduce agricultural productivity for the rest of the century".

And governments were vilified for not immediately adopting measures such as covering the Arctic with soot to produce ice melt.

The predicted catastrophe has remained - just the direction of the temperature change has reversed. It must be wonderful to be so certain about such complex issues that you can tell the world's governments how to save their citizens from catastrophe.

The fundamental laws of radiative transfer were discovered in the 1890s, and it was clear by the end of the century that what would come to be called the greenhouse effect (more than 95 per cent caused by clouds and water vapour) had elevated the earth's surface temperature by more than 25 degrees and continues that process.

Rather than being a problem, it keeps us out of a permanent ice age.

Worldwide attention was drawn to the catastrophic global warming hypothesis when the US Environmental Protection Agency claimed that as the atmosphere warmed, ocean levels would go up 8m. This caused great panic in low-lying countries, including Pacific atolls.

The school-level error in physics in their analysis was that much of the ice predicted to melt - including nearly all the Arctic - was already floating and that, according to Archimedes (250 BC), when floating ice melts there is no change in ocean level. The result was the predicted increase in ocean depth was nearer 25cm than 25 feet (7.62m).

A mantra of the 1980s was that the Amazon rainforest was the "lungs of the world", absorbing huge amounts of carbon dioxide, and that destruction of this forest would cause the promised catastrophe.

I was involved in a Nasa-sponsored experiment in the 1980s to measure gas fluxes into and out of the Amazon rainforest. Walking through the forest, it was apparent that there was no accumulated vegetable debris on the ground which prompted the question as to where it had all gone - until you sat down on a log and got bitten by a huge variety of insects whose main occupation was eating fallen vegetation.

The experiment went on to prove that the Amazon rainforest did, as expected, absorb a lot of carbon dioxide, but that it ejected an equal amount of carbon as carbon dioxide and methane.

Methane is a stronger greenhouse gas than carbon dioxide, so the net effect of the forest is to add to the greenhouse effect (New Zealand's cow flatulence is not even close).

The important unresolved problem is that as carbon dioxide produces a modest (maybe one degree) temperature rise, this will put more water vapour in the earth's atmosphere through increased evaporation.

The difficulty is that low clouds have a cooling effect whereas high clouds cause extra warming, so the relative amounts of high and low clouds produced will determine whether we have a problem.

The models are largely unhelpful in this regard since this ratio is often assumed or calculated crudely. What is needed is more modelling and field work, supported by careful analysis of the time evolution of cloud statistics and their heights, as observed by satellites. This painstaking work is under way, but results are preliminary.

My concern about the present situation is not that we may or may not reasonably expect catastrophic global warming. It is that anyone who has the temerity to try to discuss the issue will be the recipient of ad hominem attacks designed to shut down the debate - essentially because, even if the disaster is not imminent, it ought to be because it forces governments to move on a green agenda, some parts of which, in my view, are legitimate and some less so.

An example of a move to suppress debate was Greenpeace's attempt to prevent a visit to New Zealand by Professor Richard Lindzen, an eminent Massachusetts Institute of Technology meteorologist and one of the first to point out and analyse the cloud problem, because it would "undermine efforts to protect the earth's climate by promoting sceptical views on global warming".

This is nothing other than a modern version of medieval book-burning that in effect shuts down science.

We are running out of fossil fuels. In addition, burning fossil fuels have elevated air pollution levels in many cities, causing the premature death of at least 100,000 people a year.

These are excellent and entirely credible reasons for developing alternative renewable energy sources. It could also be sensible to divert the huge amount of money New Zealand is likely to have to pay under our Kyoto Protocol obligations to local development of renewable energy sources including biofuels, tide, wave and wind energy systems.

This might even result in exportable technologies.

* Geoff Austin is professor of geophysics at Auckland University.
Reader comment: What an excellent article, tho I'm not so sure about your comments re depletion of fossil fuels (apart from the fact ,that if we use some there is obviously less!).
- - - posted 9.47am July 28, 2006 by Alan