When Ron Prinn spins one “Wheel of Fortune,” he arrives at a one in four chance of the Earth warming up at least 3 degrees centigrade, and the beginning of an irreversible melting of polar ice sheets. When he spins the other wheel, the odds of this level of dangerous warming fall to one in 40. The first wheel, Prinn suggests, represents the risks involved in doing nothing about climate change. The second wheel is attainable only by enacting a climate policy that stabilizes carbon dioxide levels in the near future.

Prinn arrives at this casino scenario by way of an enormously complex climate model, the Integrated Global System Model (IGSM), which takes into account man made and natural activities forcing climate change, to generate a “probability range of forecasts.” Data come from measuring variables in the atmosphere, ocean, and land ecosystems, as well as from human emissions. GDP, energy use, policy costs, agricultural and health impacts get factored in as well.

Research using 400-thousand-year-old ice samples shows that while temperatures and greenhouse gases have fluctuated, the temperatures today are the highest in the last 1200 years. 1998 and 2005 were the warmest years ever recorded. Given the current rise in carbon dioxide levels, polar regions are warming up at much faster rates than other parts of the world, which will exacerbate warming. As ocean ice melts, there’s less sunlight reflected back and more heat trapped at the poles; tundra thawing will release more gases as well. There are feedbacks in the system: small changes in gases such as methane can trigger very rapid changes in temperature.

Prinn admits to big uncertainties in the IGSM: clouds, which play a large role, are difficult to model. There are also uncertainties about emissions, and ocean-mixing, the churning of cooler and warmer waters, which can bring carbon buried on the ocean floor to the surface. Prinn’s caveat is “never seriously believe any single forecast of the climate going into the future.” However, by running the IGSM hundreds of thousands of times to estimate the probability of various amounts of climate change, Prinn and colleagues are, “in the Monte Carlo sense, building up a set of forecasts on which we can put a measure of the odds of being correct or incorrect.”

If we want better odds, we’ll need to prevent any major increase in carbon dioxide emissions from current levels (and no more than twice preindustrial levels). This is a tall order, given the growth of developing countries and the anemic response by the U.S. and other countries to the gathering crisis. Prinn adds to this dismal picture, noting that new energy solutions must permit scaling up on a global basis. “To get three terawatts out of windmills, you’d need 21 million of the current-style windmills.” Solutions that look good on a small scale “may be going in the wrong direction on a large scale.”