As President Joe Biden convenes a virtual climate summit with 40 world leaders on Thursday, he faces a difficult task: how to propose a nonbinding yet symbolic target to curb greenhouse gas emissions that will have a meaningful effect on climate change initiatives not just in the United States, but globally.

The carbon target, which will be enthusiastically anticipated on both sides of the climate debate, will signal how vigorously Biden intends to act on climate change, a controversial and costly problem that has riled Republicans to warn about job-killing government overreach while those on the left fear Biden has not gone far enough to resolve a fundamental challenge to the world.

The climate crisis presents a more challenging political challenge for Biden because it is more difficult to see and measure than either the coronavirus pandemic relief package or the infrastructure bill.

According to Kate Larsen, a former White House strategist who helped shape President Barack Obama’s climate action plan, the goal Biden chooses “sets the tone for the degree of urgency and the speed of pollution reductions over the next decade.”

The figure must be realistic by 2030 but still being ambitious enough to please scientists and activists who see the coming decade as a critical, make-or-break moment for slowing climate change, according to Larsen and other experts.

Scientists, advocacy activists, and even industry leaders are urging Biden to set a goal of reducing greenhouse gas pollution in the United States by at least 50% below 2005 levels by 2030.

The 50 percent goal, which most economists believe is a possible result of vigorous White House negotiations, would virtually double the country’s previous pledge and would necessitate drastic improvements in the electricity and transportation industries, including major growth in renewable energy such as wind and solar power and steep cuts in emissions from fossil fuels such as coal and oil.

Anything less than that target, analysts warn, might jeopardise Biden’s pledge to keep temperatures from rising more than 1.5 degrees Celsius, while also provoking harsh opposition from foreign partners and Biden’s supporters.

The aim is important not only as a visible goal for the United States to meet after four years of climate inaction under President Donald Trump, but also for “leveraging other countries,” according to Larsen. “Which helps domestically in the war that follows, which is putting policies in place to accomplish that goal. We will present a stronger political argument at home if other countries behave with the same degree of ambition as the United States.′′

The 2030 goal, known as a Nationally Determined Contribution, or NDC, is an important component of the Paris climate agreement, which Biden entered on his first day in office. It’s also a significant step toward Biden’s ultimate target of net-zero carbon emissions by 2050.

“Clearly, the evidence requires at least 50 percent” reductions in greenhouse gas emissions by 2030, according to Jake Schmidt, a climate analyst at the Natural Resources Defense Council, a major environmental organisation.

In an interview, he said that the 50% goal is “ambitious, but realistic.” It’s also a positive climate appeal, he says: “People get what 50% entails — it’s half.”

Whatever goal Biden chooses, the climate summit itself “demonstrates the United States is back in rejoining the international effort′′ to combat climate change, according to Larsen, now a director at the Rhodium Group, an independent consulting company.

She described the summit as “the starting gun for climate diplomacy” after a four-year “hiatus” under Trump. Biden’s climate envoy, John Kerry, has been pushing world leaders in person and online in the run-up to the summit for promises and partnerships on climate initiatives.

According to Nathaniel Keohane, a former Obama White House strategist who is now a vice president at the Environmental Defense Fund, analysts have agreed on the need to cut pollution by at least 50% by 2030.

“The number has to begin with 5,” he said, adding, “We did the math.” We need at least 50%.′′

The 2030 target is only one of many, often contradictory, climate-related targets highlighted by Biden. He has also stated that he plans to introduce a renewable energy standard that will make power fossil-free by 2035, as part of a larger target of achieving net-zero carbon emissions across the economy by 2050.

Gina McCarthy, Biden’s environment advisor, admitted that the sheer amount of data can be perplexing. She and former New York Mayor Michael Bloomberg said last week at a conference that climate campaigners should rely on measures for the next decade.

“Let’s stop dreaming about 2050,” said McCarthy, who is overseeing White House efforts to create 2030 climate commitments for the United States.

Bloomberg, 79, went even further: “2050 is a decent number for people who give speeches, but I don’t know anybody who gives such speeches who will be alive in 2050.”

Some Republican politicians argue that focusing on lowering US emissions is counterproductive, claiming that Biden’s proposal will lift oil prices and destroy American jobs while encouraging Russia, China, and other countries to increase their greenhouse gas emissions.

“While our enemies maintain the status quo, the Biden administration would set punishing priorities for the United States. That will not address climate change,′′ said Wyoming Senator John Barrasso, the top Republican on the Senate Energy Committee. According to Barrasso, the United States currently leads the world in cutting carbon emissions, and Biden should continue to “make American electricity as renewable as we can, as soon as we can, without raising consumer costs.”

Those on the left believe Biden does not go far enough.

Rep. Alexandria Ocasio-Cortez, a New York Democrat who has advocated for a big Green New Deal, said Biden deserves “a tonne of credit” for the ambition and reach of his infrastructure strategy, but that it stops well short of what is needed to meaningfully tackle the climate crisis. She and her allies are urging the federal government to spend at least $10 trillion over the next decade to fix climate change and other issues.

McCarthy refuted the notion that Biden had reneged on campaign pledges to lead on climate change.

“We’re either doing too little or too much,” she said earlier this month to reporters. “However, be assured that the president placed a (spending) amount out here that he thought was not only defensible, but also necessary to reach this moment in time.”

Many of the increased climate change funding is found in Biden’s $2.3 trillion infrastructure package.

McCarthy said that if Republicans believe that less funds can be spent on renewable energy and utilities, “then we’ll have some talks.”

Marcella Burke, a former Trump administration official who is now an energy lawyer in Houston, gives Biden a “A-Plus” for passion on climate change but a “F” for specifics. “We’ve got a lot of targets released, just not a lot of plan to get there,” she said. “So the verdict is already out.”