Under President Donald Trump, the U.S. officially withdrew from the Paris Agreement on global warming on Nov. 4. Trump’s successor, Joe Biden, wants back in. But averting the worst effects of climate change will be an uphill battle even with the involvement of the U.S., the second-largest producer of carbon dioxide emissions, behind China. Global warming already is fueling wildfires, hurricanes and mass migrations, and in the U.S., Trump dismantled regulations meant to stifle greenhouse gas emissions from power plants, automobiles and oil wells.

1. What’s the Paris Agreement?

The landmark 2015 accord among almost 200 countries brought together the developed and developing worlds to pledge limits on the fossil-fuel pollution that causes climate change. Those pledges are voluntary and non-binding. The goal is to hold the rise in temperatures to below 2 degrees Celsius (compared with preindustrial levels), and preferably to 1.5 degree, at the end of this century, to avoid the rising seas and superstorms that climate models predict.

2. How can the U.S. rejoin?

That’s simple. The U.S. notifies the United Nations, with re-entry active 30 days later. The U.S. also would need to submit a specific pledge to reduce emissions, known as a nationally determined contribution. As a candidate, Biden said he would he apply to rejoin on his first day as president and then “lead an effort to get every major country to ramp up the ambition of their domestic climate targets.”

3. What is Biden’s plan to reduce U.S. emissions?

His $2 trillion clean-energy and infrastructure proposal calls for an emissions-free electric grid in 15 years and a target of net-zero emissions across the entire economy by 2050. It’s one of the most ambitious climate proposals in the world, surpassing China’s recently announced goal of being carbon neutral by 2060.

4. What had the U.S. pledged under the Paris agreement?

Under President Barack Obama, who signed the pact in 2015, the U.S. pledged to cut its carbon dioxide emissions 26 to 28 percent from 2005 levels by 2025. In addition, the U.S. was being counted on to contribute heavily to a Green Climate Fund that helps poorer nations invest in renewable energy. The fund is falling short of its target, with the U.S. halting payments in 2017 and Australia forswearing new contributions in 2019.

5. What will the U.S. pledge under Biden?

Environmentalists say they expect Biden to ultimately develop a more stringent carbon-cutting pledge for the U.S., as the nation seeks to restore trust with other signatories after the Trump-led exodus from the pact.

6. What was Trump’s gripe with the agreement?

He called the pact “a total disaster for our country” that would hurt American competitiveness by enabling “a giant transfer of American wealth to foreign nations that are responsible for most of the word’s pollution.” He said rules and directives put in place by Obama to meet the U.S. targets for emissions hurt the U.S. economy by killing jobs related to fossil fuels, especially coal mining.

7. Has the Paris agreement helped slow global warming?

Not enough so far. Human activities are estimated to have already caused about 1 degree Celsius of warming and are increasing at a rate of about 0.2 degrees Celsius per decade. The UN World Meteorological Organization has said that global temperatures are on track to rise 3 to 5 degrees by the end of this century, well beyond the targeted cap of 2 degrees. The years 2015 to 2019 were the warmest five years on record, and 2010 to 2019 was the warmest decade on record, according to the UN agency. Climate Action Tracker, a research project, agrees that current policies and pledges will leave the planet “well above” the Paris accord’s “long-term temperature goal.” Even with the U.S. involved, academics were concerned that the world was headed for “extensive” species extinctions, serious crop damage and irreversible increases in sea levels.

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