As an economist, Henry Jacoby was schooled in the hardheadedness of that numbers-driven profession. So in the ’70s, when first introduced to the climate change issue, with its century-long time horizons, he decided not to dive in.

“If you go out 100 years,” says the MIT management professor, in a drawl that reflects his Texas roots, “the uncertainties are very great. So I said, ‘I’m not going to work on that. It’s too hard.'”

He held to that stance until about 10 years ago, when he decided to help form a new interdisciplinary group dedicated to probing the climate-change issue from a variety of perspectives. “I said to myself, ‘I’m going to do it this time, because I think it’s an important problem,'” says Jacoby, “and it’s turned out to be the right thing to do.”

One reason he feels so is that the group — the MIT Joint Program on the Science and Policy of Global Change — may have found a way to move the global-warming debate beyond what Jacoby describes as “throwing things and yelling.”

Why is it so hard to figure out the long-term impacts resulting from the atmospheric buildup of the so-called “greenhouse gases?” Well, predicting anything on a century-long time-scale is very tough, notes Jacoby. “If you were at the end of the last century, and had tried to predict what the world would look like in 2001, how much would you have gotten right?” he asks.

But there are specific uncertainties linked with climate issues. For example, suppose the U.S., with 20 percent of its electricity from nuclear energy, over time reached France’s 70 percent level. The U.S.’s output of CO2 and other gases associated with coal, oil and natural gas would slow dramatically.

And the list of uncertainties can be expanded almost indefinitely. How fast will the world’s economy grow? Will the planet get cloudier as it gets warmer? How big will the world’s population get?

Jacoby and his associates have created a massive computer model to help them deal with such issues. Run on a battery of interconnected computers, the model, says Jacoby, “lets you run simulation after simulation using different values of the inputs.” Not only that, the model can give you the odds that any one of a kaleidoscopic range of scenarios will actually be what the world experiences.

The results may be seen in the accompanying chart. A warming below 1 degree F? Conceivable, but highly unlikely. Above 9 degrees F? Also very unlikely.

If the actual course of climate change stays near the lower bound, it appears impacts would probably be modest. “There’d be some rise in sea level, but it would happen slowly,” says Jacoby. “Weather patterns would change, and there would be some effect on ecosystems, though those are poorly understood.”

If near the upper bound, of course, the impacts could be dramatic. But the key point is that policymakers now have a rough idea of the odds on different temperature increases.

For his own part, Jacoby feels these odds point to a need for some action now. “What really matters is what we emit over the next 50 to 100 years, so we have time to get it right,” he says. “But I ‘m of the view that it’s inappropriate to sit and do nothing — even if that just means a lot more R&D on how to deal with carbon emissions, and going after the cheaper reductions that are available to us.”

Meanwhile, the economist has developed a way to think about those century-long time horizons that initially caused him to balk at climate-change studies. His wife’s father, he notes, is alive and well at 98. “I have a new granddaughter, who was born in December,” he notes. “If that little girl lives to her great granddad’s age, the year will be 2098.”