How fast must we act on global warming?

Stern also dismissed two other arguments for inaction: that humans will easily adapt to climate change and that its effects are too far in the future to address now. Putting the burden of dealing with climate change on future generations is "unethical," Stern said.

Prior to Stern's report, models for fighting climate change suggested that the best way to avoid economic disruption was to start modest efforts in the near term and ratchet up spending over both the medium and long term. The Stern review turns that thinking on its head and argues that strong immediate efforts are needed or a crucial window of opportunity will be lost.

Economists who hold to the slower start say Stern has misjudged the effect of climate change on future generations as more significant than in other economic models. Stern asks for greater immediate sacrifices (a tax on carbon emissions) on behalf of future generations, in essence taking more wealth away from society today and giving it to our grandchildren.

But by spending less on curbing CO2 now, another line of reasoning goes, we will be richer in the future and thereby better able to pay for mitigating climate change then. Spending on climate abatement must be weighed against other social costs, they say. For example, scientists expect malaria to spread more quickly as the climate warms. But would money put into CO2 abatement be better spent on developing a vaccine or other malaria-fighting steps? And if future generations are many times richer per capita than today, as some models predict, might they be less affected by the costs of global warming than Stern suggests?

If CO2 cannot be captured from fossil fuels and sequestered, are we going to leave oil, gas, and coal in the ground unused? If so, we'll pay a high cost, says Robert Mendelsohn, a Yale economist. And fossil-fuel alternatives have their own costs, he says. Are we ready to double the number of nuclear power plants? Scatter 2 million windmill generators across the landscape? Convert large areas of agricultural land to grow "energy crops" like corn for ethanol and risk higher food prices or shortages?

Delay also means more high-carbon-emitting sources – power plants and vehicles – will go into service, says Jeffrey Sachs, a Columbia University economist. The cost of retrofitting or decommissioning these sources, he says, is higher than building greener ones in the first place.

If carbon can be captured from power plants and stored underground, the picture brightens considerably, Professor Sachs says. Engineers have thought a lot about energy efficiency, and Sachs is optimistic they will find quick and economical solutions once they concentrate on carbon capture and sequestration.

Sachs suggests imposing a $25 per ton tax on carbon for the next 40 years. Schemes to cap-and-trade CO2 emissions between countries and industries may not send clear enough signals to markets about the real cost of carbon emissions, he says. Yet another approach would be to set standards for carbon emissions, industry by industry, akin to the fuel-economy standards now in place for cars.

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