The basic proposition behind the science of climate change is so firmly rooted in the laws of physics that no reasonable person can dispute it. All other things being equal, adding carbon dioxide (CO2) to the atmosphere—by, for example, burning millions of tons of oil, coal and natural gas—will make it warm up. That, as the Nobel Prize–winning chemist Svante Arrhenius first explained in 1896, is because CO2 is relatively transparent to visible light from the sun, which heats the planet during the day. But it is relatively opaque to infrared, which the earth tries to reradiate back into space at night. If the planet were a featureless, monochromatic billiard ball without mountains, oceans, vegetation and polar ice caps, a steadily rising concentration of CO2 would mean a steadily warming earth. Period.

But the earth is not a billiard ball. It is an extraordinarily complex, messy geophysical system with dozens of variables, most of which change in response to one another. Oceans absorb vast amounts of heat, slowing the warm-up of the atmosphere, yet they also absorb excess CO2. Vegetation soaks up CO2 as well but eventually re­releases the gas as plants rot or burn—or, in a much longer-term scenario—drift to the bottom of the ocean to form sedimentary rock such as limestone. Warmer temperatures drive more evaporation from the oceans; the water vapor itself is a heat-trapping gas, whereas the clouds it forms block some of the sun’s warming rays. Volcanoes belch CO2, but they also spew particulates that diffuse the sun’s rays. And that’s just a partial list.

Because including all these factors in calculations about the effects of CO2 increase is hugely difficult, it is no surprise that climate scientists are still struggling to understand how it all will likely turn out. It is also no surprise, given his track record as something of a climate change agitator, that James Hansen, director of the NASA Goddard Institute for Space Studies, has been circulating a preprint of a journal paper saying that the outcome is likely to turn out worse than most people think. The most recent major report from the Intergovernmental Panel on Climate Change in 2007 projects a temperature rise of three degrees Celsius, plus or minus 1.5 degrees—enough to trigger serious impacts on human life from rising sea level, widespread drought, changes in weather patterns, and the like.

But according to Hansen and his nine co-authors, who have submitted their paper to Open Atmospheric Science Journal, the correct figure is closer to six degrees C. “That’s the equilibrium level,” he says. “We won’t get there for a while. But that’s where we’re aiming.” And although the full impact of this temperature increase will not be felt until the end of this century or even later, Hansen says, the point at which major climate disruption is inevitable is already upon us. “If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted,” the paper states, “CO2 will need to be reduced from its current 385 ppm [parts per million] to at most 350 ppm.” The situation, he says, “is much more sensitive than we had implicitly been assuming.”

As with many of Hansen’s assertions, this one pushes the science further than some of his colleagues would be willing to go. Back in 1998, for example, Hansen was arguing that the human impact on climate was unquestionable, even as other leading climate scientists continued to question it. He was subsequently proved right, not only about the human influence but about the approximate pace of future temperature rise. But just as in 1998, the underlying motivation for his claims, if not all of his conclusions, is shared pretty much universally.

The problem is that conventional projections for how warm things will get come out of a calculation everyone knows is wrong. Called the Charney sensitivity, it estimates how much the global mean temperature will rise if atmospheric CO2 is doubled from its preindustrial levels, before people began burning coal and oil on a grand scale. In the mid-1800s carbon dioxide concentrations stood at about 280 ppm. Double that to 560 ppm, and the Charney sensitivity calculation tells you that temperatures should rise about three degrees C.

But the Charney sensitivity, though not quite as stripped down as the billiard ball model, is still an oversimplification. The calculation does take into account some feedback mechanisms that can modify the effects of increasing temperatures on short time­scales—changes in water vapor, clouds and sea ice, for example. But for the sake of simplicity, it assumes no change in other, longer-term factors, including changes in glaciation and vegetation; in particulates, such as dust; and in the ability of the ocean to absorb carbon dioxide, which diminishes as sea temperature rises.

Climate Models Struggle with Reality
“Many people, ourselves included, have tended to take [the Charney sensitivity] and apply it to the real world,” says Gavin Schmidt, who is also a climatologist at Goddard (though not a co-author on the new paper). “But the real world isn’t a model where a few things can change while others stay fixed.” At some point, Schmidt says, “we have to talk about the real climate.”

That’s what Hansen has attempted to do. He isn’t the first: other scientists, including Stephen H. Schneider of Stanford University, have talked for years about bringing additional real-world factors to standard climate models. The difficulty is that to add those factors, you have to come up with a reasonable way to weight them.

Like other climate scientists, Hansen and his co-authors use evidence from the deep past to sort out these feedback mechanisms. Over the past 800,000 years, for example, we know that the climate has oscillated between long ice ages and much shorter periods of interglacial warmth—much like the conditions we are in now. The relation between air temperature and CO2 is pretty well understood for that period, thanks largely to air bubbles trapped in ancient ice cores that have been drilled in Greenland and Antarctica (the CO2 concentration inside them can be measured directly; the global mean temperature can be calculated from the relative abundances of two different oxygen isotopes, which vary with how warm it gets).

But Hansen points out that the record contains other clues. “We also know how sea level changed over that period,” he says, from studies tracing the height of ancient shorelines. Because sea level rises and falls as continental ice sheets retreat and advance, you have a measure of what fraction of the earth was covered with a bright white, heat-reflecting coating. As ice retreats in a warming world, more dark surface is exposed to absorb solar radiation, which makes the world even warmer, melting even more of the ice. Conversely, a cooling world gets cold faster as ice sheets advance. This is one of the key feedback mechanisms left out of the Charney sensitivity calculation, partly because it is thought to happen only over hundreds of years, and, Hansen says, partly because “it just hadn’t sunk in that the paleoclimate record is a remarkable source of info on climate sensitivity.”

Using that record, for example, Hansen concludes that even if the human race could maintain today’s level of atmospheric CO2, which stands at 385 ppm—not even halfway to the atmospheric doubling we are headed for—sea level would rise several meters thanks to the disintegration of continental ice sheets. Moreover, he thinks disintegration may happen much faster than one might naively expect. “We didn’t have convincing data on this until we had the gravity satellites,” he says, referring to GRACE, a pair of orbiters that can detect tiny local changes in the earth’s gravitational field. “Greenland has gone from stable mass in 1990 to increasing ice loss. Another big surprise is West Antarctica, where despite little actual warming, the ice shelves are melting.” As those partially floating ice shelves melt, land-based glaciers are free to slide more rapidly to the sea. In Greenland, meltwater from the top of the glaciers is evidently pouring down through cracks to lubricate the underside of the ice sheets, easing their flow out to the ocean.

Warming temperatures, Hansen says, not only increase the amount of meltwater on the surface of the ice but also increase rainfall. “Ice-sheet growth,” he says, “is a dry process. Disintegration is a wet process, so it goes a lot faster.”

If today’s CO2 levels would lead to several meters of sea-level rise—putting many coastal areas, housing hundreds of millions of people, completely underwater—then letting CO2 rise to 560 ppm could lead to a disaster of unimaginable proportions. Even a rise to 450 ppm could be catastrophic, according to Hansen’s team’s analysis. Before about 35 million years ago, the planet was completely ice free, so warm-water alligators and lush redwood forests thrived above the Arctic Circle. The transition to large-scale glaciation in Antarctica began, the researchers estimate, when CO2 dropped to 425 ppm, plus or minus 75 ppm. Most of the ice should therefore disappear again if we reach that point—and if all of Antarctica’s and Greenland’s glaciers were to melt, sea level would rise many tens of meters. The only way to keep CO2 concentrations as low as that, Schneider says, is to have the entire world adopt California’s strictest-in-the-nation proposals for limiting carbon emissions—something that is hard to imagine even the other U.S. states agreeing to, let alone developing ­nations such as India and China.

That’s only taking the feedback from melting glaciers into account. “Changes in vegetation, in atmospheric and ocean chemistry, and in aerosols and dust in the atmosphere all appear to be positive feedbacks on temperature changes,” Schmidt says. “If global average temperatures change for any reason, those other elements will amplify the change.” Other positive feedbacks include the release of CO2 dissolved in the oceans, which will happen as they warm up, and the accelerated release of other greenhouse gases—methane, for example, from biomass that will begin rotting as permafrost melts in the Arctic.

Given Hansen’s eminence as a climate scientist, one might expect that his analysis would have triggered a general panic. And it has—but not among scientists. “This month may have been the most important yet in the two-decade history of the fight against global warming,” wrote journalist and author Bill McKibben in the Washington Post this past December, shortly after Hansen spoke about his new calculations at a conference: “[350 ppm is] the number that may define our future.” McKibben has even created an organization he calls to spread the word. Other activists and bloggers have reacted with similar alarm.

Most climate experts turn out to be much less exercised, even though they take the danger of global warming very seriously. The reason: Hansen and his colleagues base their new estimate of climate sensitivity and of the various tipping points represented by different feedback mechanisms on a record of ancient conditions that are not really well understood. “The problem,” Schmidt says, “is that the further back you go, the less you really know. The error bars get very large.” The planet’s ice-free periods are, he admits, “very interesting—they’re like the periods we think we’re heading for, and in principle they can tell us a lot about the climate sensitivity to CO2 changes.”

And although you can infer atmospheric CO2 levels indirectly, from changes in the acidity of ocean sediments, for instance, Schmidt notes that this involves assumptions that might be wrong. “People generally think CO2 was higher then, but you can’t get a precise number or a precise time series. Jim would say that the true climate sensitivity is twice the Charney sensitivity, but it could be three times, it could be one.” Similar uncertainties surround the ebb and flow of continental ice sheets. “It’s quite possible,” Schmidt says, “that the ice sheets across North America might have been more sensitive than those in Greenland, which would explain why Greenland’s have persisted.”

That appears to be a widespread consensus. “Jim’s analysis is very shrewd,” says Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University. “It’s something we all should be thinking about, but the uncertainties are so large that it’s a weak point.” Schneider, too, offers praise overall but caution about specifics. “Jim’s doing great work,” he says, “but I wish he’d get off absolute numbers. It’s not as though the world is okay with 1.8 degrees of warming but turns into a pumpkin at 2.2 or something.”

Seeking a Workable Solution
Indeed, although few climate scientists are ready to buy Hansen’s argument in detail, they agree that the changes already observed are ominous. “Where I’ve come down on this,” says Fred Krupp, president of the Environmental Defense Fund, “not sparked by Jim Hansen but by watching coral reefs die and Antarctic ice sheets break off sooner than we ever expected, is that we need to stabilize CO2 at current levels or below.”

Lurking behind this general tone of caution is the sense that reducing the growth of CO2 emissions is a daunting enough prospect by itself, given that the world’s population continues to grow and that countries such as India and China are determined to catch up to the developed world economically. Halting that growth entirely would be even more difficult, and actually drawing down the amount of CO2 in the atmosphere seems largely inconceivable. Nevertheless, Hansen and his co-authors lay out a possible strategy. “The only way I can see of doing it,” Hansen says, “is, first of all, to cut off emissions from coal entirely by 2030.” Coal, he points out, is the single biggest fossil-fuel reservoir of carbon, and because it is only burned in power plants, not as transportation fuel, “it can be captured at just a few sources rather than millions of tailpipes.”

To push coal-based carbon emissions down to zero, he and his colleagues suggest, the world has to agree that, starting right away, no new plants will be built unless they have the capability to capture waste CO2 before it leaves the smokestack. At the same time, existing plants will either have to be retrofitted with capture technology or phased out by 2030.

A second major effort, the authors say, would involve massive reforestation of areas that have been denuded of trees. “Deforestation contributed a net emission of 60 ± 30 ppm over the past few hundred years,” they write, “of which ~20 ppm CO2 remains in the air today.” Regrowing forests, they argue, could absorb all of that excess and more. And finally, they favor the use of “biochar,” or charcoal made from agricultural waste and other biomass. If burned or left to decay, this biomass releases CO2. When converted to charcoal and tilled into the soil, it does two things. First, it is exceptionally stable, so it keeps carbon sequestered for centuries, at least. Second, it increases soil fertility, because it adsorbs nutrients and keeps them available for new crops. “Replacing slash-and-burn agriculture with slash-and-char,” they write, “could provide a CO2 drawdown of ~8 ppm in half a century.” More speculative technologies might also eventually be able to draw CO2 out of the atmosphere and lock it up in minerals, although their potential scale and expense are still guesswork.

But possibility and plausibility are two different things. Hansen and his colleagues have produced a road map; getting all the biggest carbon emitters to go along will be tough. The scheme would take decades to implement even if every nation agreed to it today. The task, the scientists admit, “is Herculean yet feasible when compared with the efforts that went into World War II.” Schneider, though just as concerned as Hansen about the dangers posed by increasing atmospheric carbon, has a less optimistic view. “It has no chance in hell,” he says. “None. Zero. The best thing we can do is to overshoot, reach 450 or 550 parts per million, then come back as quickly as possible on the back end.” And even that, given the political barriers to quick and effective action, will be difficult.

Note: This article was originally printed with the title, "Beyond the Tipping Point".