As the world's top climate scientists released a report full of warnings this week, they kept insisting that the world still has a chance to avoid the worst effects of climate change.

"It is still possible to forestall most of the dire impacts, but it really requires unprecedented, transformational change," said Ko Barrett, vice chair of the Intergovernmental Panel on Climate Change. "The idea that there still is a pathway forward, I think, is a point that should give us some hope."

That hopeful pathway, in which dangerous changes to the world's climate eventually stop, is the product of giant computer simulations of the world economy. They're called integrated assessment models. There are half a dozen major versions of them: four developed in Europe, one in Japan, and one in the U.S., at Pacific Northwest National Laboratory.

"What we mostly are doing, is trying to explore what is needed to meet the Paris goals." says Detlef van Vuuren, at the Netherlands Environmental Assessment Agency, which developed one of the models.

How to cut greenhouse gas emissions to zero in 40 years

World leaders agreed in Paris to limit global warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit). The planet has already warmed about 1 degree Celsius, compared to pre-industrial levels.

Meeting that goal will mean cutting net greenhouse gas emissions to zero within about 40 years. It would require profound changes; so profound, it's not immediately clear that it's even possible.

That's why van Vuuren and his colleagues turned to their computer models for help. "How is it possible to go to zero emissions?" he says. "That's for transport, that's for housing, that's for electricity."

Each of these models starts with data about current sources of greenhouse emissions. They include cars and buses, auto rickshaws, airplanes, power plants, home furnaces and rice paddies. The models also include assumptions about international trade, prices, and the costs of new technologies.

Then the scientists force their virtual worlds to change course, by introducing limits on greenhouse emissions. The models then try to satisfy that requirement in the most cost-effective way, as long as it's technologically feasible and doesn't run up against limits like the supply of land or other natural resources.

The good news is that the models found a way to meet that target, at least in scenarios where world governments were inclined to cooperate in meeting their Paris commitments. In fact, according to Keywan Riahi, at the International Institute for Applied Systems, in Austria, they found multiple paths to zero carbon.

"The models tell us that there are, first of all, alternative pathways possible; that there are choices available to the decision-maker," he says.

Different models, using different assumptions, arrive at contrasting visions of the future world. But they're all dramatically different from the situation today.

Some models show people responding to higher energy prices or government regulations by changing their lifestyle. They move to more energy-saving houses, and give up their cars in favor of a new and better kind of public transit. In addition to traditional bus lines, autonomous vehicles respond like Uber — taking people where they need to go.

Riahi likes this version best. "I'm convinced that a fundamental demand-side restructuring would also lead to a better quality of life," he says.

Other scenarios show people still using plenty of energy, which in turn requires a huge boost in production of clean electricity. It would mean 10 or 20 times more land covered with solar and wind farms, compared to now, plus more power plants burning wood or other biofuels, outfitted with equipment to capture and store the carbon dioxide that's released.

Politics and individuals' preferences could foil the models

Riahi is quick to point out that what happens in the models may not be feasible in real life. They don't account for political obstruction, for instance, or human preferences. People may just want to drive an expensive car, rather than take public transit, even when the models says that choice isn't economically rational.

But the models also can be far too pessimistic, in particular about technological innovation. Ten years ago, van Vuuren says, they never anticipated the rise of cheap solar power. "We have been in the extremely fortunate situation that the cost of renewables has declined rapidly in the past decade." This has made the task of reducing carbon emissions much easier.

For all their shortcomings, though, these models remain the primary way that scientists and policymakers figure out options for the future. They quantify tradeoffs and consequences that may not be clearly apparent. If countries want to turn trees or crops into fuel, for instance, it means less land for growing food or for natural forests. Also, the models make it clear that international cooperation is essential, with rich countries helping poorer countries to cut their emissions.

The results of the computer modeling are like fuzzy maps, pointing out routes that could help the world avoid disaster.