Published Monday with its grim warning that climate change is likely to hit the global economy as hard as the Great Depression, the report from Sir Nicholas Stern comes hard on the heels of former Vice President Al Gore's film on the same theme, "An Inconvenient Truth." But whereas Gore was speaking to a global public (and from the Cannes film festival to the U.S. box office and the next Oscars and across the world it is clear that people are listening), Stern speaks to statesman and officials, policy-makers and this with the power to do something about it.

"The scientific evidence of global warming is overwhelming and it is clear that the consequences will literally be disastrous," Prime Minister Tony Blair commented Monday. "We cannot afford to wait. This disaster is not set to happen in some science fiction future many years ahead, but in our lifetime."

There is not much hard environmental science in the report, but a great deal of economic and political science about the real-world effects, bringing great intellectual rigor to the uncertainties of forecasting. This helps explain why Blair's government has already accepted its findings, and is now pledged to carry the message to its partners in the European Union and its allies like the United States. That is why Blair has hired Gore as a consultant to help promote the report's message in the United States.

"Our actions over the coming few decades could create risks of major disruption to economic and social activity, later in this century and in the next, on a scale similar to those associated with the great wars and the economic depression of the first half of the 20th century," Stern concludes.

"We have the time and knowledge to act. But only if we act internationally, strongly and urgently," he added.

But this former chief economist at the World Bank and senior British civil servant also looks on the bright side. Global warming can be tackled, and by investing now, its challenge can be met at a reasonable price, perhaps as little as one percent of global GDP over the next 45 years. The alternative would be an environmental crisis several orders of magnitude more costly, in both human and economic terms.

It is already very late in the game, Stern stresses, and some grim consequences are already inevitable, notably in Africa and other parts of the developing world that have few resources to cope with the threat of droughts, storms, floods and crop-destroying climate change. The best we can hope for is not to reverse the build-up of greenhouse gases (measured as equivalent of units of carbon dioxide) in the atmosphere, but to stabilize it at a far higher level than today. Before the industrial revolution, that level was around 250 parts per million. Today it stands at 430 parts per million. Stern's target is to level it off at 500-550 parts per million by 2050.

The goal is to cut the emissions of greenhouse gases per unit of GDP by 75 percent or more; the fuel systems and power trains of the global economy have to be completely re-invented, even as we try and keep the overall system functioning. What that means, Stern says, starts with the need to cut the use of carbon in power generation by two-thirds -- which almost certainly means a large investment in nuclear power.

Stern has studied other economic models on global warming that suggest that a rise of 2-3 degrees in temperature would mean a fall in global GDP of 3 percent or more. Stern goes further, suggesting that a temperature rise in the coming century of 5-6 degrees is "a real possibility." Looking at other impacts, including health costs and famines and epidemics, Stern says that without real action to moderate climate change, we could see a 20 percent drop in living standards across the globe.

Given that choice, between a world that is 20 percent poorer and with climate change still continuing, and a world that has invested one percent of its wealth but avoided the worst, Stern says the argument for massive investment now is unanswerable, and that governments, officials and policy-makers across the world will have to act.

His paper is written essentially for them, in arguments and language that are likely to convince and to resonate. But this does not mean Stern's arguments will win, or will escape controversy. It will not be easy to get politicians to accept that the rich countries should be responsible for the lions' share -- as much as 80 percent -- of the cost of cutting emissions.

The battle against climate change will have to be funded, either through general taxation, or preferably for Stern, through a carbon tax, along with government regulations limiting the amount of carbon any enterprise may produce. If a company must produce more emissions, it will have to buy the right to do so in a global emissions market, in which efficient producers may sell their unused emission rights (one such market already exists in Europe). One way or another, Stern says, governments will have to force "behavioral change" on their economies.

The British government is already responding, with new plans for taxes on air travel, new "pay as your drive" road taxes combined with incentives to curb car use and ownership, and more incentives for energy-efficient appliances and the use of renewable energy.

Some of the implications of this kind of massive shift in the global fuel system are just beginning to be understood. Countries rich in fossil fuels like Saudi Arabia, Russia and Iran, could be facing sharp falls in income if Stern's recommendations go into force. Car manufacturers, expecting a new global bonanza as the millions of newly prosperous Indians and Chinese start buying personal transport, may see the real growth come instead in buses and public transport.

Of course, the Stern report may be a one-week wonder, with big headlines for a few days, and then filed away and forgotten, rather in the way that Kyoto was virtually forgotten in the United States and the way that Europe set Kyoto targets to cut emissions but does not look on track to meet them.

"We can't wait the five years it took to negotiate Kyoto -- we simply don't have the time," Blair concluded Monday. "We accept we have to go further (than Kyoto)."

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