Mitch McConnell, the Senate majority leader, has already said he will bring the plan to the floor, a move to force Democrats — particularly the six presidential candidates in the Senate who have endorsed the blueprint — to cast a vote that Republicans can use to brand them as socialists and extremists. Most 2020 hopefuls standing up for a Green New Deal have done little more than endorse it as a slogan and have rarely been pressed on its specifics.

But while the scope of the Green New Deal is enormous, experts believe that the economic trade-offs — saving trillions on potential catastrophe by spending trillions to prevent it — are worth serious consideration given the scale of the threat, and that a deep policy discussion would help voters and other Americans grapple with the environmental threats.

Technological challenges. And political ones.

The Green New Deal, which is a congressional resolution without the force of legislation, calls for a “10-year national mobilization” to make the United States carbon-neutral across the economy. That means, as much carbon would have to be absorbed as is released into the atmosphere. Mr. Holdren, who is now a professor of environmental policy at Harvard University, said the Green New Deal’s timeline of achieving that goal around 2030 is not feasible.

“As a technologist studying this problem for 50 years, I don’t think we can do it,” he said.

“There’s hope we could do it by 2045 or 2050 if we get going now,” he added.

Mr. Holdren said worldwide energy infrastructure — an investment of $25 to $30 trillion — turns over every three to four decades, and an aggressive transition to non-carbon energy begun today could achieve zero emissions by midcentury. That is the deadline urged by scientists from 40 countries in last year’s report from the U.N.’s Intergovernmental Panel on Climate Change.

The more ambitious Green New Deal was introduced by Representative Alexandria Ocasio-Cortez of New York and Senator Edward J. Markey of Massachusetts. Its sweeping targets also include supplying 100 percent of the country’s electricity from renewable and zero-emissions sources within a decade; digitizing the nation’s power grid; upgrading every building to be more energy efficient; and overhauling factories and transportation, including cars, trucks and trains “as much as is technologically feasible” to remove greenhouse emissions.