I spent a few minutes Wednesday with F. Sherwood Rowland, the atmospheric chemist from the University of California, Irvine, who shared a Nobel Prize for his work revealing the threat to the ozone layer from CFC’s and similar synthetic chemicals. He has a very sobering forecast for levels of carbon dioxide in the atmosphere.

He’s in New York for the World Science Festival, along with a large flock of Nobel laureates and other luminaries. We discussed the body of science pointing to troubles ahead from rising carbon dioxide levels during a break after Mayor Michael R. Bloomberg’s speech on the “tragic lag between what we know and what we do.”

The seasonal ozone “hole.” (NASA)The response to the threat from CFC’s — a treaty and phaseout that is working — has been held up as a model for what humanity can do with carbon dioxide, the main emission linked to warming and the biggest threat, because of its long lifetime in the air, to drive temperatures dangerously higher. But Dr. Rowland, along with others at the meeting, including Steven Chu, a Nobel laureate and director of the Lawrence Berkeley National Laboratory, said there are big differences.

The main one is that CFC’s were produced by a handful of companies and affordable substitutes were within reach. (And nature provided a stark warning with the unpredicted “hole” in the ozone layer over Antarctica.) Carbon dioxide emissions result from almost every activity in lives both prosperous and poor — from burning forests to microwaving popcorn or flying to Tokyo.

That linkage between emissions and lifestyles is one reason the concentration of the gas has been rising relentlessly, to around 385 parts per million now after never topping 280 parts per million for at least 650,000 years. (United States emissions from energy use dropped slightly in 2006 but yesterday the Energy Department reported they rose 1.6 percent in 2007.)

Avoiding a lot of warming and climate change while heading toward 9 billion people seeking a decent life will require an utter transformation of the multi-trillion-dollar energy system, Dr. Chu said. An audience member wondered whether spiking gas prices would propel the change. Dr. Chu said higher energy prices would not be enough on their own, adding that the necessary energy transformation will also require decades of sustained research, development, and deployment of new technologies.

During a break, I asked Dr. Rowland two quick questions. The first: Given the nature of the climate and energy challenges, what is his best guess for the peak concentration of carbon dioxide?

(Keep in mind that various experts and groups have said risks of centuries of ecological and economic disruption rise with every step toward and beyond 450 parts per million, with some scientists, most notably James Hansen of NASA, saying the long-term goal should be returning the atmospheric concentration to 350 parts per million, a level passed in 1988.)

His answer? “1,000 parts per million,” he said.

My second question was, what will that look like?

“I have no idea,” Dr. Rowland said. He was not smiling.

By Andrew C. Revkin