Many experts are discussing whether they should continue pressing for ambitious carbon cuts in the near term or adjust their goals given the prospect of a much warmer world.

“We have to begin the conversation about cruising past” the 2009 pledge on limiting global temperature increases, said John D. Po­desta, who chairs the Center for American Progress, a liberal think tank, and is a member of a United Nations advisory panel addressing climate change and other issues. “It’s hard to contemplate and scary to contemplate, but it has to be addressed at this point.”

In 2004, Princeton University professors Robert Socolow and Stephen Pacala wrote an influential paper outlining how the world could stabilize its greenhouse-gas emissions by mid-
century through a series of ­“wedges,” using current technology, such as sharply increasing nuclear power worldwide, eliminating deforestation and converting conventional plowing to no-tillage farming.

Socolow has more recently published an article in the Vanderbilt Law Review that he describes as his “let’s get real here” lecture. He says environmentalists will have to accept that fossil fuels will not disappear in the next few decades.

Several activists who track international climate talks insist, however, that the next three years are critical, saying negotiators need to forge a new pact by 2015 to lock in needed carbon cuts. Alden Meyer, who directs strategy and policy for the Union of Concerned Scientists, said major emitters will not agree to meaningful cuts until they view it as in “their national self-interest.”