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As countries have awakened to the hazards of climate change, they have slowly started curbing their greenhouse gas emissions in recent years. Some energy giants, like BP, now expect global demand for oil to plateau in the decades ahead.

But scientists warn that governments aren’t acting nearly fast enough to avoid severe global warming. And every year of delay makes the problem harder to solve.

The clock is ticking. Global temperatures have already risen 1 degree Celsius from preindustrial times. For that increase to stay well below 2 degrees Celsius, the Intergovernmental Panel on Climate Change says that global carbon dioxide emissions need to get down to zero by around mid-century. Blowing past that limit, the panel warned, could bring about a world of worsening food and water shortages, collapsing polar ice sheets, and a mass die-off of coral reefs.

Getting emissions all the way down to zero would entail a staggeringly rapid transformation of the world economy.

In the United States, climate activists have rallied around the Green New Deal, which envisions getting the United States to net zero emissions as quickly as possible in order to give less-wealthy nations more time to make their own transitions. Mr. Biden has called the Green New Deal a “crucial framework” but has distanced himself from its particulars, instead aiming to zero out America’s emissions by 2050.

Doing so would require doubling or tripling the pace at which clean electricity sources like wind or solar get installed, one recent study found, all while slashing pollution from cars, trucks, buildings, industry and agriculture.

Mr. Trump hasn’t committed to any climate targets. One analysis by Wood Mackenzie, an energy research firm, suggested that another four years of delay would “dramatically reduce the possibility” of the nation meeting the 2050 goal for eliminating carbon emissions.