If you’d asked Ronald Prinn a decade ago whether human activity played a significant part in global warming, he would have given you an “equivocal” answer. Today, he is no longer straddling the line, and indeed, has amassed forceful evidence that post-industrial society has brought about enormous change in earth systems, and may cause irreparable damage as this century progresses.

Prinn provides a short lesson on radiative forcing -- the process by which the earth absorbs solar radiation and gives off energy by emitting infrared radiation. These processes, which should be in balance, increasingly are not, due to manmade activities that trap the heat from the sun, and drive up the earth’s temperature. Prinn comes armed with MIT’s Integrated Global System Model, which helps show how human industry, agriculture and consumption feed into the delicate, interconnected physical and biological workings of atmosphere, ocean and earth. Forecasting the climate into the future, says Prinn, “is no longer a job for the natural sciences, but for a combination of natural and social sciences.”

Prinn’s illustrations depicting how human activity and earth systems interact are almost comically complex, and he acknowledges that his models must take into account major uncertainties. Clouds, ocean mixing and aerosols act as wild cards in terms of radiative forcing. However, observation of earth’s climate over millennia, and the running of computer simulations hundreds of times, have yielded some probabilities that Prinn believes policy makers must contend with.

Even assuming that civilization can limit its carbon dioxide emissions to twice preindustrial levels (550 parts per million), some very dramatic shifts will happen (or have already begun): the poles will heat up much faster than other parts of the world, melting ice and raising sea levels. Arctic tundra and soil will thaw and release methane, a much more potent greenhouse gas even than carbon dioxide. Another possibility: The ocean will reach its limit in absorbing atmospheric CO2, and, to put it bluntly, begin to die.

There are things we can and should do, says Prinn, if we want to avoid playing roulette with life on earth, and these actions are not priced beyond our means. We can make our transportation and building energy costs more efficient. We can continue to use coal if we figure out how to capture and store carbon underground. We’ll need to develop biofuels. Bills in Congress seeking to achieve 50-80% reductions in carbon emissions below 1990 levels won’t cripple our economy, Prinn’s models show. “Bottom line, we can afford this.”