"Under all the plausible scenarios for economic growth, population growth and technological change, what we come up with is that the atmospheric concentration of carbon dioxide will increase from 368 parts per million today to maybe, if we are lucky, as low as 540 ppm or if we are unlucky as much as 970 ppm by 2100."
That would mean an increase in global mean temperature of anything from 1.4 deg C to 5.8 deg over the course of this century.
If it is at the high end of the range it would mean as much warming this century as has occurred altogether since the last ice age.
"Some areas will become drier, some wetter," Dr Watson said. "We project more heatwaves, more floods, more droughts, more El Nino-type conditions."
Agricultural production would drop in the tropics and eventually, after an initial boost, in the middle and high latitudes as well. The challenge to water supplies would increase.
Sea level rise would threaten not only low-lying island states, but heavily populated river deltas, for example in Bangladesh and Egypt.
A degree of uncertainty surrounds these forecasts, Dr Watson readily acknowledges, arising both from the climate change model and the likely future track of greenhouse gas emissions, which in turn is a function of economic growth, population growth and technological change.
"But why assume that the uncertainties mean we are overstating the effects of human activities on climate change? We may be understating them."
It is a weak argument for doing nothing, Dr Watson says, especially as there is a lot of inertia in the climate system itself.
"Let's say, hypothetically, you could stop all carbon dioxide emission at once. The temperature would continue to rise for many decades and sea levels would continue to rise for centuries."
The IPCC is also looking at what can be done to mitigate climate change and what it is likely to cost.
The Kyoto Protocol set the goal of reducing greenhouse emissions by developed countries to 5 per cent below 1990 levels over the next 10 years.
The IPCC estimates the cost of doing that would reduce those countries' combined gross domestic product by anything from 0.2 to 2 per cent by 2010. It is not saying annual GDP growth rates would be that much lower, but that by 2010 economic output would be up to 0.2 to 2 per cent lower than it would otherwise have been.
But that is if countries have to rely on measures within their own borders to meet their target. If international emissions trading is allowed, as the Kyoto Protocol envisaged, the cost would be halved.
Allowing credits for forestry and other land use changes which reduce carbon would cut the costs further still, Dr Watson said.
Those costs had to be weighed against the cost of doing nothing - lost agricultural output, the challenge to water resources and so on.
"We also looked at the technologies to reduce emissions between now and 2020.
"The conclusions of the macro-economists were that half of the potential emission reductions could be achieved at negative cost - they would actually save money.
"The other half would cost up to $US100 a tonne of carbon avoided. In the US context, $100 a tonne would mean about 20c a gallon on gasoline." The New Zealand equivalent is 13c a litre. But to realise that technological potential would mean a major policy change."
The prospects of concerted international action have been dealt a blow by Washington's rejection of the Kyoto Protocol.
In part that reflects a belief on the part of the Bush Administration that the economic costs are too high.
"Some of their economists have advised them that it's $200 or $300 per tonne of carbon avoided," Dr Watson said. "Naturally I question that."
"But maybe what one could do, within the Kyoto Protocol, to make them see there will not be major economic dislocation is to cap the economic exposure of any country to say $30 or $40 or $50 a tonne of carbon."
Any part of a country's emission reduction obligations which it could not meet for less than that cap would go into an international pool, with the country paying the capped price. "You would then look around the world for the cheapest options ... it's another way of using a trading system."
Feature: Climate change
Intergovernmental Panel on Climate Change
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Summary: Climate Change 2001
United Nations Environment Program
World Meteorological Organisation
Framework Convention on Climate Change
