But most of the positive energy at this gathering came from people closer to the ground, like the 300,000 activists who marched last Sunday. They included mayors like New York’s Michael Bloomberg and his successor, Bill de Blasio, who both spoke of the critical role that cities can play in reducing emissions. They included governors like California’s Jerry Brown, who is justly proud of his state’s pathbreaking efforts to control automobile and power plant pollution. And they included institutions like Bank of America, which said it would invest in renewable energy, and companies like Kellogg and Nestle, which pledged to help stem the destruction of tropical forests by changing the way they buy commodities like soybeans and palm oil.

Underlying all these declarations was a palpable conviction that tackling climate change could be an opportunity and not a burden, that the way to approach the task of harnessing greenhouse gas emissions was not to ask how much it would cost but how much nations stood to gain by investing in new technologies and energy efficiency.

This burst of activity comes at a crucial time. A tracking initiative called the Global Carbon Project recently reported that greenhouse gas emissions jumped 2.3 percent in 2013, mainly because of big increases in China and India. This means it is becoming increasingly difficult to limit global warming to an upper boundary of 3.6 degrees Fahrenheit above preindustrial levels. Beyond that point, scientists say, a world already suffering from disappearing glaciers, rising seas and persistent droughts could face even more alarming consequences.

Avoiding such a fate is going to require a revolution in the way the world produces and consumes energy, which clearly has to involve national governments, no matter how much commitment there is on the streets and in the boardrooms. The odds are long that a legally binding treaty will emerge from Paris. Congress is unlikely to ratify one anyway. The smart money now is on a softer agreement that brings all the big polluters on board with national emissions caps, and there are reasons for hope that this can be done.

Mr. Obama is in a much stronger leadership position than he was at Copenhagen, having engineered a huge increase in automobile fuel efficiency and proposed rules that will greatly reduce the United States’ reliance on dirty coal. The Chinese, in part because their own air is so dirty, have been investing heavily in alternative energy sources like wind and solar, and they are giving serious consideration to a national cap on coal consumption. The cooperation of these two countries could by itself create the conditions for a breakthrough agreement. But what might really do the trick — if Climate Week is any guide — is the emergence of a growing bottom-up movement for change.