Past the boiling point

ON JUNE 12, 1992, President George H.W. Bush, appearing at the Earth Summit in Rio de Janeiro, signed the U.N. Framework Convention on Climate Change. The convention set the goal of averting “dangerous” human interference with the climate system. After adding his name to it, the president called on world leaders to join him “in translating the words spoken here into concrete action.” When he subsequently submitted the treaty to the Senate, it was ratified by unanimous consent.

Tomorrow it will be 14 years since Bush pere signed the Framework Convention, and the U.S. remains committed, in theory at least, to avoiding dangerous climate change. Unfortunately, it’s hard to square this commitment with what has actually happened in the meantime.

Since 1992, American emissions of carbon dioxide -- the chief cause of climate change -- have continued to rise more or less at the same rate they were rising previously. Meanwhile, even as the Japanese and the Europeans have pledged to cut their carbon dioxide production, President George W. Bush has blocked all attempts to impose emissions limits in the U.S. In fact, the administration has gone so far as to oppose the efforts of those states, such as California, that are trying to reduce emissions on their own.

To the extent that the administration has offered any explanation for this contradiction -- promising to avert dangerous climate change on the one hand, blocking attempts to curb emissions on the other -- it’s to assert that the uncertainties about climate change make action premature. Thanks to the nature of global warming, this ostensibly cautious approach actually amounts to the worst sort of recklessness.

The climate system is highly inertial; it takes several decades for changes already set in motion to become apparent. Scientists probably won’t be able to determine just what level of greenhouse gases will trigger, say, the melting of the Greenland Ice Sheet until that level has been exceeded. But as anyone who has ever tried to push a stalled car can attest, systems that are hard to get moving also tend to be hard to stop. Although it sounds reasonable to argue that we ought to wait for certainty before taking action, if we do, effective action almost certainly will become impossible. Once we know for sure that the ice sheet is in danger of disappearing, it will be too late to reverse the process.

At least since the opening of the film “An Inconvenient Truth,” most Americans have probably heard enough about climate change to have a good idea of what “dangerous” change might be. Among the possibilities is a temperature increase sufficient to destroy entire ecosystems, cause mass extinction or disrupt the world’s food supply.

The disintegration of the Greenland Ice Sheet or the West Antarctic Ice Sheet often is held up as an exemplary catastrophe. Were either one to melt, sea levels around the world would rise by at least 15 feet. Were both to disintegrate, global sea levels would rise by 35 feet. A mere five-foot rise in sea levels would put much of South Florida underwater.

But 14 years after Rio, there still seems to be a good deal of confusion about how far away danger lies. The effect of adding greenhouse gases to the atmosphere is to throw the planet out of what scientists call “energy balance.” For balance to be restored, the entire planet has to warm up. This is a slow process, largely because the oceans have such an enormous capacity to absorb heat. That’s one of the reasons why, despite having already thrown billions of tons of carbon dioxide into the air over the last 50 years, we are just beginning to feel the effects now.

Meanwhile, it’s crucial to understand -- although the Bush administration would apparently prefer not to -- that uncertainty cuts both ways. As the administration likes to point out, the U.S. spends about $2 billion a year on climate-change research. It’s possible that as scientists learn more about how the climate works, they will discover that the threshold of dangerous change lies further away than is estimated, and Washington’s do-nothing policy will come to seem justified. But the reverse is just as likely. In fact, nearly everything that has been discovered about the climate system recently has tended to suggest that the threshold is closer than suspected.

In February, for instance, a team of scientists from the National Aeronautics and Space Administration and the University of Kansas announced that the flow of ice from glaciers in Greenland had more than doubled over the last decade. The following month, researchers from the University of Colorado, using the most sophisticated satellite data available, concluded that Antarctica was losing ice. The finding was particularly disturbing because climate modelers had predicted that as temperatures rose, the continent would actually gain ice because warmer air should produce more snowfall.

“If anything, the history of climate modeling has been one of conservatism and underestimating the impacts of climate change,” Ken Caldeira, a researcher at the Carnegie Institution Department of Global Ecology at Stanford University, observed recently.

Perhaps the most sobering data come from studies of climates past. Late last year, European researchers published an analysis of an Antarctic ice core containing climate information going back 650,000 years. (The ice contains tiny bubbles of trapped air, each a sample of an ancient atmosphere.) The core shows that carbon dioxide levels today are already higher than they have been at any point in that entire period.

The last time carbon dioxide levels were comparable to today’s, scientists believe, was 3 1/2 million years ago, during what is known as the mid-Pliocene warm period. It’s likely that they have not been substantially higher since the Eocene epoch, about 50 million years ago. In the Eocene, crocodiles roamed Colorado and sea levels were nearly 300 feet higher than they are now. A scientist with the National Oceanic and Atmospheric Administration put it to me -- only half-jokingly -- this way: “It’s true that we’ve had higher CO2 levels before. But then, of course, we also had dinosaurs.”

In 1992, when George H.W. Bush signed the Framework Convention on Climate Change, the dangers of global warming were already clear. They’re even clearer now. To wait for yet more clarity while carbon dioxide levels continue to rise is not a sign of prudence; it’s just the opposite. Dangerous climate change can still perhaps be averted. But not if we waste another 14 years.