Unless significant, steady reductions in the emissions of carbon dioxide from burning fossil fuels begin extremely soon, the Earth might be much closer to potentially catastrophic warming than is widely believed. So argues climatologist James Hansen of the Columbia University Earth Institute and an international team of colleagues in a new analysis published today in the journal PLOS One. Their paper further underscores other recent studies showing that even small delays in shrinking the industrial output of carbon dioxide (CO 2 ) could steeply complicate not only attempts to temper climate change but also any attempts by future generations to adapt to it.

Without abrupt action to restrict higher emissions "it will become exceedingly difficult to keep warming below a target smaller than 2° C" they write. Furthermore, they say, the supposedly safe “limit” for warming of 2 degrees Celsius—which has driven global climate negotiations for years—is too high. Anything more than 1 degree C could imperil the Earth's ecosystems and societies. Hansen in particular is concerned about inaction imposing a crushing burden on today's children, as reflected in the paper's title: "Assessing Dangerous Climate Change: Required Reductions of Carbon Emissions to Protect Young People, Future Generations and Nature."

Hansen and his co-authors note that these findings sharpen the need to implement carbon taxes and the expanded use of nuclear power, and also show the likely futility of geoengineering. "What's crucial is that we not put stuff in the atmosphere which is going to stay in the climate system forever," Hansen says. "For all practical purposes, that's what it does."

Whether the paper can have the galvanizing effect on climate policy its authors desire remains to be seen, however. At a minimum, it will encounter skepticism that the authors' 1 degree C target for CO 2 reduction is simply unrealistic, given the growing global demand for inexpensive energy now provided by fossil fuels.

For much of the past decade, the Intergovernmental Panel on Climate Change (IPCC) and other groups have recommended that policymakers try to keep the maximum amount of global warming to no more than 2 degrees C above the pre-industrial average. Temperatures have already risen 0.8 degree C since that time. Even warming of just 2 degrees C would raise sea levels, change agricultural productivity, destroy sensitive species and ecosystems, and alter patterns of disease in challenging ways, the groups acknowledge. Yet because further fossil-fuel use seems politically and economically unavoidable, particularly for developing countries fighting to raise their standard of living, scientists and policymakers have often jointly settled on 2 degrees C as an acceptable compromise between the consequences of warming and human ingenuity in adapting to them.

The PLOS One paper calls the safety of that compromise into question, however. Not only might the long-term effects of a 2 degrees C rise be worse than is commonly recognized, the authors say, but any circumstances that bring the world close to that limit by 2100 will most likely commit the world to further increases that might exceed 3 degrees C within another century. Such temperatures would make large segments of the globe virtually unlivable for humans.

Drawing on reconstructions of the prehistoric climate, or paleoclimate, from ice-core data, Hansen and his colleagues point out that Earth was last stably two degrees warmer than today during the Eemian interglacial period, between 130,000 and 114,000 years ago. Sea levels were typically four to six meters higher then, with more melting of ice in glaciers and at the poles. The authors note that a similar sea level rise in the future would swamp what are now densely populated coastal regions around the world.

The Eemian data also suggest that temperate climate zones crucial to agriculture would shift dramatically northward, leaving many of today's farmlands arid. Numerous species in imperiled ecosystems would become extinct, which could have unpredictable consequences.

Global warming of 2 degrees C would be “far into the dangerous range," they write.

The authors argue that this combination of sea-level rise, species loss and altered growing conditions, with consequences for poverty, disease and malnutrition, would almost certainly deal a blow to people around the world.

To be sure, not all these consequences would be imminent. Current estimates suggest that unrestrained warming could raise sea levels on the order of roughly a meter by 2100, for example. It might take centuries for the oceans to rise to levels seen in the Eemian.

Then again, it might not. Paleoclimate data suggest that extremely rapid changes have sometimes occurred during the span of decades, and sea-level increases of three to four meters within a century have been seen. "The paleo data doesn't allow you to see a discernible lag between the global temperature changes and the sea-level changes, so it doesn't give you any confidence that there's going to be a big lag," Hansen says.

Moreover, history may not be a good guide because, as the authors note, the greenhouse-effect forcing from fossil-fuel CO 2 is more intense than any other warming that scientists can find in the paleoclimate record. They also note that with current warming below 1 degree C, scientists have already begun to see effects on the environment that were not expected to arise until the advent of higher temperatures.

The authors also argue that 2 degrees C is a poor policy target for another, more disturbing reason. To climate modelers, CO 2 emissions qualify as a so-called fast feedback forcing, because they start to raise temperatures relatively rapidly. At currently projected rates of increase, the CO 2 alone would make the world more than 2 degrees C warmer within a century.

But at higher temperatures, the authors caution, other slow feedback forcings—warming from the loss of reflective sea ice, methane seeping out of thawing bogs previously under permafrost and other sources—also come into play more significantly. As a result, allowing temperatures to rise 2 degrees C by 2100 would almost certainly lead to further follow-on warming to 3 degrees C above the pre-industrial average within another century. When the Earth was 3 degrees C warmer during the Pliocene epoch more than 2.6 million years ago, sea levels stood 15 to 25 meters higher than today.

To the authors, a more prudent target would therefore be just 1 degree C warming—a goal that, even if it proved unachievable, would help to restrain the slow forcings associated with more runaway temperature rise.

The PLOS One article (reviewed further here) goes on to look at how rapidly CO 2 emissions would need to fall to check warming to the desired degree. The warming effect caused by industrial CO 2 depends on the cumulative amount of the gas released to the atmosphere. The authors note that industrial civilization has already released about 384 gigatons of carbon from fossil fuels into the air, and calculate that holding the total to roughly 500 gigatons, with an additional 100 gigatons stored in plants and soil, could keep warming to about 1 degree C. Continued use of fossil fuels at current and projected rates would push the total to 1,000 or more gigatons of carbon by the end of the century.

Once released, CO 2 persists. "A pulse of CO 2 injected into the air decays by half in about 25 years as CO 2 is taken up by the ocean, biosphere and soil, but nearly one-fifth is still in the air after 500 years," the authors write.

Consequently, even small delays in reducing CO 2 emissions can have a profound effect on the extent of eventual warming, sometimes called the warming "in the pipeline." Hansen and his team calculate that had the human race started cutting back on CO 2 emissions in 2005, warming could in theory have been kept to 1 degree C by 2100 with annual average reductions of just 2 percent. If those reductions started today, the decrease would have to be 6 percent a year. Putting them off until 2020 pushes that figure up to 15 percent. If reductions do not start within about a decade, temperature increases to 2 degrees C and higher are all but inevitable and would persist for hundreds of years.

To Hansen and his coauthors, cutting back on the use of fossil fuels, however difficult it might be, is the only way to head off dangerous warming. Reducing emissions of the greenhouse gas methane, restricting deforestation, and other measures that could lessen non- CO 2 influences on the climate might seem more immediately achievable, they write, but as long as CO 2 keeps going up, its effect will overwhelm any good they do.

Those messages echo the findings of a pair of recent reports by Myles R. Allen of the University of Oxford and his colleagues, published two weeks ago in Nature Climate Change. Using a methodology different than Hansen's, one of those papers looked at the impact of delaying CO 2 reductions, and the other at the role of non- CO 2 climate pollutants. Both papers treated 2 degrees C as a reasonable limit to warming. The researchers found that even under a variety of assumptions, delaying reductions by a decade could add about half a degree C to the eventual global temperature, and warned that "peak committed warming is rising substantially faster" than current observed warming would indicate.

They also concluded that early action on non- CO 2 factors would have little impact on peak warming "in the absence of simultaneous CO 2 mitigation."

The PLOS One paper argues for setting a price on carbon—a fee per ton of CO 2 emitted—as an immediately available way to discourage reliance on fossil fuels and to encourage energy conservation. Distributing the proceeds of such a carbon tax among the general population could make it more politically appealing, the authors say, pointing to studies that suggest most citizens would receive more from such a payout than they would pay in higher prices. The authors also suggest that China and the U.S. are in a strong position to lead the way in establishing a carbon tax, and that other nations would naturally follow their example even in the absence of international negotiations.

Another of the authors' recommendations that will assuredly be controversial is for a speedy expansion of nuclear power worldwide. (To the ire of many environmentalists, Hansen has previously gone on the record as favoring more nuclear power.) They favor using new third- and fourth-generation reactors designed not to suffer disastrous meltdowns and that would yield far less nuclear waste.

Such new reactors are still controversial, however, because critics see them as making it easier for terrorists and unfriendly states to acquire nuclear weapons. Hansen and his coauthors acknowledge those risks but maintain that appropriate policies could contain them.

Although many environmental scientists and activists also disapprove of geoengineering—intentionally altering the atmosphere to mitigate climate problems—it is often cited as a potential technological fix that is more affordable or practical than reducing CO 2 emissions. The PLOS One paper's authors are highly skeptical, however. They describe schemes that would add acidic aerosols to the stratosphere to reflect away more sunlight as "a temporary bandage for a problem that will last millennia," and fault it for the potential unintended consequences.

Plans aimed at extracting CO 2 from the atmosphere do no better: the authors point out that there are "no proven technologies capable of large-scale capture of CO 2 ," and that even the most favorable estimates peg the cost at $500 per ton of carbon. Because of the dynamics of atmospheric CO 2 levels, the net reduction of CO 2 from the atmosphere would be only about half of what geoengineers captured. Thus, according to the authors' model, reducing atmospheric concentrations by 52 parts per million—only a little more than the amount by which CO 2 levels already exceed the concentration of 360 ppm preferred by many climatologists—could cost between $100 trillion and $400 trillion dollars.

And even if the costs of carbon capture fell precipitously, the authors point out that disposal of the captured carbon would be a challenge: geoengineers would need to grab hundreds of gigatons of carbon to be effective, but even a single gigaton turned into carbonate bricks would occupy the volume of 3,000 Empire State Buildings.

Despite the gloomy picture, Hansen and his colleagues write hopefully about the possibility that phasing in measures such as carbon taxes that pay dividends to the pubic might help propel climate activism past "tipping points" that would win over the support of industry and governments. Indeed, according to Hansen, the paper is intended as a document that might help judges understand the climate issues when they deliberate in lawsuits such as Alec L. v. Jackson now in the U.S. District Court in Washington, D.C., concerning the U.S. government's obligation to restrict greenhouse gases.

However persuasive some may find the PLOS One paper, critics are very likely to be outspoken. Even many environmentalists sympathetic to the goal of sharply curtailing climate change are guaranteed to balk at a program of aggressive nuclear-power expansion, though the alternatives may be hard put to accomplish equal CO 2 reductions. To most of the energy industry and many politicians and economists, a carbon tax is a nonstarter.

Although Hansen warns against complacency that sea-level increases and other effects of warming will be gradual, skeptics in the past have mocked suggestions that the worst of those consequences might take less than centuries to materialize. (Witness criticisms of Al Gore's presentation about sea-level rise in An Inconvenient Truth.) Critics may also question the calculations that Hansen and his coauthors used to estimate the sensitivity of future warming to CO 2 emissions, although the Allen papers and other research provide some independent confirmation and seem to rule out more optimistic scenarios.

The most common skeptical reaction, however, is very likely to be that notwithstanding the papers' assertions, a goal of anything less than 2 degrees C is simply unrealistic given the economic sacrifices that would be required, the difficulty of transforming the energy infrastructure, the determination of the developing world to improve its lot, the inertia built into the political system and the risks of overzealous climate reform.

"But I would say, from a policy point of view, whether it's one degree or two degrees, they both imply the same thing right now, which is a need for a dramatic change in the global energy system," says Jeffrey Sachs, director of the Earth Institute and a coauthor on the paper. He adds that "this is a 30-year process. But it has to not only begin now; it has to envision a very deep change at the end."

John Rennie, a former editor in chief of Scientific American, is an adjunct instructor at NYU and writes The Gleaming Retort blog