THE EXISTENCE OF GLOBAL warming is now considered "unequivocal" by the Intergovernmental Panel on Climate Change, the United Nations body that has been evaluating the scientific evidence. And emissions of carbon dioxide and other greenhouse gases by humans and their machines are a big reason: "Most of the observed increase in globally-averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse-gas concentrations."

Now we have to move on to the softer science of economics. What are the costs and benefits if humanity is moved to change its emitting ways?

But first, here's news not so widely recognized: The report is surprisingly skeptical about some of the most science-fictional consequences of global warming often bruited in speeches and movies. It's worth quoting, even though the prose style is obfuscatory:

"Contraction of the Greenland ice sheet is projected to continue to contribute to sea-level rise after 2100. Current models suggest ice-mass losses increase with temperature more rapidly than gains due to precipitation, and that the surface-mass balance becomes negative at a global average warming in excess of 1.9 to 4.6 degrees Celsius. If a negative surface-mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea-level rise of about 7 meters."

As well, "Studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice-sheet mass balance."

Translation: Melting of the Greenland ice sheet may happen, but very slowly. It probably won't push sea levels up much for the next thousand years or so. And the Antarctic ice sheet may well get thicker.

A World of Words

The United Nations' climate-change corps will hold one of its periodic meetings for diplomats next month in Bali, Indonesia. It should be old-home week for thousands of members of the doomsday crowd, especially for those who buy carbon-emission credits to offset the exhaust from the jet aircraft that bring them to the exotic equatorial isle.

The meeting -- the United Nations Framework Convention on Climate Control -- has been called to draw a "road map" to negotiate a new world agreement on carbon-dioxide emissions. The pact would be a successor to the Kyoto Protocol that expires in 2012.

Participants will be negotiating about the negotiations they will hold in 2008 and 2009. They won't be negotiating about the dismal results of the Kyoto Protocol. It seems that most signatories have been more enthusiastic about maligning those countries that didn't sign than they've been about curbing their own emissions. The U.S., which didn't sign, has done more to limit carbon-dioxide emissions than many signatories.

To concentrate the minds of the delegates, the IPCC has just published a 22-page manifesto. A climate-official said he hoped everyone at the meeting would keep it close throughout the discussions and use it so much that many would wear them out and need new copies by the end.

Disproportionate Outcomes

Anyone who actually reads the document and its sources may be somewhat let down. A reader will learn, for example, that average greenhouse-gas emissions in the period from 1980 to 1999 were up more than a third over the average from 1850 to 1899. That's a big increase. But the let-down will come when he finds that the big increase in emissions led to average temperatures' increasing by a mere half-degree Celsius (about 1.3 degrees Fahrenheit) and to the global average sea level's increasing by about six inches.

That's considerably less than computer models suggest as consequences of future carbon-dioxide production. Is this really such a big threat? More warming could take place in the coming century, but how much?

Climate modelers tried six scenarios of greenhouse-gas emissions, from holding steady at the level of 2000 to a 250% increase by the end of the 21st century. An emission freeze still would produce another 0.6-degree-Celsius increase in average temperature over the half-degree already noted. A scenario with a low-emission increase offered a best estimate of 1.8 degrees C increased temperature and increased sea levels of seven to 15 inches. A scenario with a high-emission increase offered a best estimate of temperature increase of 4 degrees C and increased sea level of 10 to 23 inches. Best estimates are still quite uncertain: Plausible ranges were about 40% less than the best estimate on the low side to 60% more than the best estimate on the high side.

All this uncertainty ought to expose the limits of mathematical modeling as a font of wisdom -- something that Wall Street financiers and Main Street mortgage brokers at least dimly appreciate nowadays.

Global Respiration

The geologic record shows that temperatures have varied widely in the past couple of hundred thousand years, and sea levels have also gone up and down emphatically. The oceans certainly have been 80 feet higher -- and several hundred feet lower -- than they are now. There has been a mile of ice covering Cleveland, and there have been palm trees in Ontario. All this without the assistance of humankind.

The power of man to make and use tools has led us to try to measure the cost of making and using tools. But the benefits should be measured also. If humans hadn't advanced their control of their environment, there would already be too many people on Earth.

The IPCC recognizes this. It declares that stabilization of greenhouse gases in the atmosphere can be achieved, even within the next two or three decades, "by deployment of a portfolio of technologies that are either currently available or expected to be commercialized in coming decades, assuming appropriate and effective incentives are in place for their development, acquisition, deployment and diffusion and addressing related barriers." It's a big assumption.

The IPCC says that the world will spend $20 trillion or so on energy and infrastructure in the next 50 years. The delegates at Bali should be looking for ways to spend that capital wisely, without impoverishing ourselves along the way.

Editorial Page Editor THOMAS G. DONLAN receives e-mail at