With momentum building for U.S. action on global climate change, nearly 200 countries will take part in a United Nations summit on the issue next month in Copenhagen.

[IMGCAP(1)]A crisis more than a century in the making, climate change won’t be turned around over the course of a few days in Denmark. That much has long been clear.

The summit, though, has already focused the international community on what I believe to be the central environmental issue of our time.

We must use this moment of urgency and hope to consolidate the gains already made and map out the way ahead.

What’s striking to me, after working on this issue for more than two decades, is the extraordinary progress that’s been made — around the world and in America, too — in the months leading up to the summit.

President Barack Obama has used the opening year of his presidency to put climate change near the top of his agenda.

With Obama’s backing, the House passed historic clean energy legislation in June that would help put Americans back to work in fast-growing green technologies, reduce our reliance on foreign oil and strike a blow against climate change. Companion legislation is being considered in the Senate.

At Obama’s direction, the Environmental Protection Agency is issuing the first-ever rules to cut back the carbon emissions that are warming our planet.

And in a speech at the United Nations in September, Obama singled out climate change as one of four overarching foreign policy priorities that he intends to pursue, a position cited by the Nobel committee in tapping him for this year’s Peace Prize.

When he travels to China later this week, Obama will discuss climate change with President Hu Jintao in an effort to bring the world’s two largest carbon emitters nearer to an action plan that others around the world might quickly rally around.

Would all this have happened without Copenhagen? Hard to say. The summit, though, has provided a focal point for essential discussions that have helped diverse countries find common ground and set the stage in Copenhagen for a serious plan of action.

As Obama told reporters at the White House on Nov. 3, what he expects from Copenhagen is “an outcome that can start moving us down the path of a sustainable economy that is not accelerating the potential catastrophe of climate change.—

Seated in the Oval Office beside Swedish Prime Minister Fredrik Reinfeldt, the president reaffirmed the stakes in the upcoming U.N. summit.

“We are confident,— Obama said, “that if all countries involved recognize this is a unique opportunity, that we can get an important deal done, not that solves every problem on this issue but takes an important step forward and lays the groundwork for further progress in the future.—

What exactly must that deal include?

It must begin with a political commitment to make the carbon emissions cuts needed to keep the average global temperature — 57 degrees Fahrenheit — from rising by more than 3.6 degrees above preindustrial levels.

That, scientists worldwide largely agree, is a kind of tipping point.

Global warming above that level would trigger environmental catastrophe, accelerating the melting of glaciers and Arctic sea ice, the widening of deserts, the worsening of storms and related climate shifts that would further imperil our planet and people.

We’re already flirting with disaster. Average global temperatures are on track to warm by somewhere from 6.3 to 8.1 degrees Fahrenheit by the end of this century, the U.N. Environment Program warned in September. We simply have to take action to reverse those trends. A global accord to hold the line at 3.6 degrees is an essential first step.

We must also commit to help poorer countries pay the costs of reducing carbon emissions and adapting to the climate change effects that we can no longer avoid.

It is a tragic irony that the people who contribute least of all to climate change are already suffering its most dire effects. The Global Humanitarian Forum, a Geneva-based organization founded two years ago by former U.N. Secretary-General Kofi Annan, estimated last May that more than 300,000 people in developing countries are dying each year from climate change effects, including malnutrition, flooding, drought and the spread of disease.

Nor are the problems confined to far-away lands. Here in the United States, wildfires exacerbated by longer, drier, hotter summers are consuming 8 million acres of land every year — twice as much as just a decade ago — according to the U.S. Forest Service and other agencies. In the Rocky Mountains, ancient white bark pines are dying by the thousands. Throughout much of the West, water is becoming increasingly scarce.

We can address this problem in ways that make our economy stronger and our country more secure. We know we can put 1.9 million Americans back to work by investing in a new generation of energy-efficient cars, homes and workplaces. And we know that developing alternative fuels can reduce our reliance on foreign oil in ways that help us build energy independence.

The full promise of Copenhagen can only be fulfilled, though, by a third accomplishment: agreement on a schedule to finalize the details of a global climate change accord that calls for meaningful action by all major carbon emitters.

The United States is positioned to lead. For, as clean energy legislation moves forward in the Senate, it is becoming more apparent every day that it is no longer a matter of if, but of when, we will decide as a nation to take action against this widening scourge that threatens us all.

The Senate will move at its own pace. Clearly, though, the time for hesitation is over. The time has come to act. With American leadership at the fore and Copenhagen a milestone along the way, we can arrive, together, at the global accord on which our planet’s future depends.

Frances Beinecke is president of the Natural Resources Defense Council and author of the newly released book “Clean Energy Common Sense: An American Call to Action on Global Climate Change.—