Carbon Output Must Near Zero To Avert Danger, New Studies Say

A heavy haze could be seen in Beijing in August 2007. Two recent reports call for a heightened global effort to reduce carbon emissions. (By Greg Baker -- Associated Press)

"This is tremendous," Schmittner said. "I was struck by the fact that the warming continues much longer even after emissions have declined. . . . Our actions right now will have consequences for many, many generations. Not just for a hundred years, but thousands of years."

While natural cycles remove roughly half of human-emitted carbon dioxide from the atmosphere within a hundred years, a significant portion persists for thousands of years. Some of this carbon triggers deep-sea warming, which keeps raising the global average temperature even after emissions halt.

Researchers have predicted for a long time that warming will persist even after the world's carbon emissions start to fall and that countries will have to dramatically curb their carbon output in order to avert severe climate change. Last year's report of the U.N. Intergovernmental Panel on Climate Change said industrialized nations would have to cut emissions 80 to 95 percent by 2050 to limit CO2concentrations to the 450 ppm goal, and the world as a whole would have to reduce emissions by 50 to 80 percent.

European Union Environment Commissioner Stavros Dimas, in Washington last week for meetings with administration officials, said he and his colleagues are operating on the assumption that developed nations must cut emissions 60 to 80 percent by mid-century, with an overall global reduction of 50 percent. "If that is not enough, common sense is that we would not let the planet be destroyed," he said.

The two new studies outline the challenge in greater detail, and on a longer time scale, than many earlier studies. Schmittner's study, for example, projects how the Earth will warm for the next 2,000 years.

But some climate researchers who back major greenhouse gas reductions said it is unrealistic to expect policymakers to think in terms of such vast time scales.

"People aren't reducing emissions at all, let alone debating whether 88 percent or 99 percent is sufficient," said Gavin A. Schmidt, of NASA's Goddard Institute for Space Studies. "It's like you're starting off on a road trip from New York to California, and before you even start, you're arguing about where you're going to park at the end."

Brian O'Neill of the National Center for Atmospheric Research emphasized that some uncertainties surround the strength of the natural carbon cycle and the dynamics of ocean warming, which in turn would affect the accuracy of Caldeira's modeling. "Neither of these are known precisely," he said.

Although computer models used by scientists to project changes in the climate have become increasingly powerful, scientists acknowledge that no model is a perfect reflection of the complex dynamics involved and how they will evolve with time.

Still, O'Neill said the modeling "helps clarify thinking about long-term policy goals. If we want to reduce warming to a certain level, there's a fixed amount of carbon we can put into the atmosphere. After that, we can't emit any more, at all."

Caldeira and his colleague, H. Damon Matthews, a geography professor at Concordia University in Montreal, emphasized this point in their paper, concluding that "each unit of CO2emissions must be viewed as leading to quantifiable and essentially permanent climate change on centennial timescales."

Steve Gardiner, a philosophy professor at the University of Washington who studies climate change, said the studies highlight that the argument over global warming "is a classic inter-generational debate, where the short-term benefits of emitting carbon accrue mainly to us and where the dangers of them are largely put off until future generations."

When it comes to deciding how drastically to reduce greenhouse gas emissions, O'Neill said, "in the end, this is a value judgment, it's not a scientific question." The idea of shifting to a carbon-free society, he added, "appears to be technically feasible. The question is whether it's politically feasible or economically feasible."