PRINCETON, N.J. — What we are doing to our planet, to our children and grandchildren, and to the poor, by our heedless production of greenhouse gases, is one of the great moral wrongs of our age. This Saturday is a day to stand up against this injustice.

Saturday is “350 Day.” The name comes from the number of parts per million of carbon dioxide in the atmosphere that, according to Jim Hansen — perhaps the world’s leading climate scientist — we should not exceed if we are to avoid potentially catastrophic climate change. CO₂ is already at 386 ppm, and is rising by two ppm a year.

We are approaching a point of no return, at which feedback loops will kick in and continue to warm the planet, no matter what we do. The melting of arctic ice is one example.

Four hundred years ago, explorers sought the legendary “Northeast Passage” across the north of Europe and Russia to China. They found the arctic ice impenetrable and gave up the quest. This year, commercial vessels successfully navigated the Northeast Passage.

That is one of many recent dramatic signs that our planet is warmer than it has been for a very long time. Ice-free arctic waters are more than a symptom of global warming. They are themselves a cause of further warming.

Ice and snow reflect the sun’s rays. An ice-free surface absorbs more warmth from the sun than one covered in snow or ice. In other words, our greenhouse-gas emissions have, by causing enough warming to melt the arctic ice, created a feedback loop that will generate more warming, and melt more ice, even if we were to stop emitting all greenhouse gases tomorrow.

Other feedback loops pose a similar danger. In Siberia, vast quantities of methane, an extremely potent greenhouse gas, are locked up in what used to be called “permafrost” — regions in which it was assumed that the ground was permanently frozen. But areas that used to be frozen are now thawing, releasing methane and thus contributing to further warming — and to further thawing, which releases more methane.

Developing nations are grasping how outrageous the distribution of greenhouse- gas emissions is. At the U.N. Summit on Climate Change in September, President Paul Kagame of Rwanda pointed out that, while developed nations outside Africa are almost entirely responsible for the problem, its greatest impact will probably be on Africa, which has few resources to cope with the challenge.

Kagame suggested giving every country an annual per capita quota for CO₂ emissions, and allowing developing countries that are below the quota to trade their excess quota with countries that are above theirs. The money that developing countries would receive for this would not be aid, but rather a recognition that the rich nations must pay for something that in the past they simply appropriated: far more than their fair share of our atmosphere’s capacity to absorb our waste gases.

Sri Lanka took a similar stance, using studies from the U.N. Intergovernmental Panel on Climate Change to calculate that in 2008, environmentally permissible carbon emissions totaled no more than 2,172 kilograms per person. In fact, the world’s per capita emissions were 4,700 kg, or more than double the permissible limit. While emissions in the rich nations were far above the permissible limit, Sri Lankan emissions were, at 660 kg, well below it.

As Sri Lanka’s government pointed out, “That means low-emitting countries like us could not emit more because our space has already been exploited by developed or globally heavy-polluting countries without our consent.”

This situation is an injustice of vast proportions, reminiscent of — and arguably much worse than — the now-repudiated colonialism of the Western powers in the 19th century. The task of remedying it must begin at the meeting on climate change in Copenhagen in December.

Many political leaders have expressed support for strong action on climate change, but what most of them regard as “strong action” will not be enough to get us back below 350 ppm. In some countries, including the U.S., there are major political obstacles to taking even modest steps.

On Saturday, people in nearly every country will be taking action to raise awareness of the need for an international treaty to bring our atmosphere back to 350 ppm of CO₂. There will be climbers hanging banners high in the Himalayas, where the glaciers are melting, and scuba divers at Australia’s Great Barrier Reef, which is threatened by climate change.

Churches will ring bells 350 times, 350 cyclists will circle towns, and in many places, 350 trees will be planted. At www.350.org you can find out what is happening near you and join in, or put your own idea online.

Don’t just sit back and hope that others make the impact. Your grandchildren will ask what you did to meet the greatest moral challenge of your time.

Peter Singer is professor of bioethics at Princeton University and Laureate Professor at the University of Melbourne. He is the author of “One World” and, most recently, “The Life You Can Save.” © 2009 Project Syndicate (www.project-syndicate.org)