For some prominent climate scientists, that is the upper limit for heat-trapping carbon dioxide in the atmosphere, measured in parts per million. If the gas concentration exceeds that for long, they warn, the world can expect decades of disrupted climate patterns, rising sea levels, drought and famine. The current concentration is 387 parts per million.
Organizers said their goal, in the prelude to global climate talks in Copenhagen in December, was to illustrate the urgent need to cut emissions by pointing out that the world passed the 350 mark two decades ago. Yet while agreeing that unabated emissions pose serious risks, some prominent scientists and economists focusing on climate policy said the 350 target was so unrealistic that the campaign risked not being taken seriously — or could convey the wrong message.
“Three-fifty is so impossible to achieve that to make it the goal risks the reaction that if we are already over the cliff, then let’s just enjoy the ride until it’s over,” said John M. Reilly, an economist at M.I.T.
“The message needs to be that there are risks at the current level, and those risks increase the further we push the system,” he said.
In a prominent recent study, scientists concluded that carbon dioxide levels were almost certainly headed beyond any levels experienced on the planet in the last 15 million years.
Michael Oppenheimer, a scientist previously at the Environmental Defense Fund who is now at Princeton, said it would be a herculean accomplishment to hold concentrations below 450 parts per million in decades.
But Bill McKibben, the author and activist who founded 350.org, the group coordinating the protests, defended its approach, saying that settling on a concrete goal articulated as a number was the only way to build a “global community” for climate action.
“We need to be thinking about reducing, not going up more slowly,” he said. “Three-fifty is the number that says wartime footing, let’s see how fast we can possibly move, and let’s hope against hope that it’s fast enough.”
Mr. McKibben spent Saturday morning in an office in downtown Manhattan with 20 volunteers coordinating an accelerating flow of videos and photographs from other time zones that captured demonstrations planned in 170 countries. Events focusing on the number were held on every continent and from pole to pole, with climbers unfurling a banner on a mountain peak in Antarctica and artists forming a 350 out of hunks of ice on a gravel beach in Greenland, in the Arctic.
The Cairo Cyclist Club posed with a banner and activists in front of one of EgyptÂ’s great pyramids, while more than 350 roller skaters swept through Tel Aviv.
Some participants had gotten a head start. Thousands of students assembled in a plaza in Addis Ababa, Ethiopia, waving “350” placards. Grinning American soldiers in eastern Afghanistan e-mailed photographs of a 350 fashioned from sandbags.
Robert J. Brulle, a sociologist at Drexel University who analyzes environmental campaigns, expressed a mixed view of the 350 campaign. It represents “a new wave of civic environmentalism” that has proved vital to solving problems, he said, but that approach was abandoned long ago by many big environmental groups that now focus mainly on legislation and litigation.
But Dr. Brulle questioned the core symbol and message of 350.org. He suggested that it might be too technical and that it focused on deeply cutting emissions without providing a clear path for accomplishing the task. Nonetheless, the effort has been endorsed by dozens of prominent figures, including James E. Hansen, a NASA scientist who has been most closely associated with defining the climate threshold as 350 parts per million. He has campaigned for halting emissions from coal burning altogether by 2030.
The demonstrations were also endorsed by Rajendra K. Pachauri, the chairman of the Intergovernmental Panel on Climate Change, and Desmond M. Tutu, the former leader of the Anglican Church in South Africa.
Mr. McKibben of 350.org said it was up to national leaders to assume a moral obligation to match their actions to the science.
As the pictures flowed in via e-mail and the Web, he noted that there were hardly any showing celebrities or officials. “Ordinary people are just haunted by this,” he said. “Now they’re saying one thing very loudly, and very beautifully.”
Gavin A. Schmidt, a climate scientist who works with Dr. Hansen and manages a popular blog on climate science, realclimate.org, said those promoting 350 or debating the number might be missing the point.
“The situation is analogous to people trying to embark on a cross-country road trip to California, but they’ve started off heading to Maine instead,” Dr. Schmidt said. “But instead of working out ways to turn around, they have decided to argue about where they are going to park when they get to L.A.
“If you ask a scientist how much more CO2 do you think we should add to the atmosphere, the answer is going to be none.”