President Joe Biden pledged to cut U.S. greenhouse gas pollution in half by 2030 at a virtual climate summit Thursday, outlining an aggressive target that would require sweeping changes to America’s energy and transportation sectors.

“These steps will set America on a path of a net-zero emissions economy by no later than 2050,” Biden said as the White House opened the two-day summit, attended by 40 leaders from around the world.

“We see multiple paths to reaching this goal,” the official said, noting, for example, that Biden has pledged to make the U.S. power sector 100% carbon-pollution free by 2035.

“We are all so delighted to have the United States back,” South Africa’s president, Matamela Cyril Ramaphosa, declared in his remarks at the summit. Boris Johnson, the United Kingdom’s prime minister, called Biden’s pledge “game changing” and offered a blunt retort to those who question the need to address climate change.

While other world leaders offered similarly dire assessments of the threat posed by climate change, only a few outlined aggressive new steps to address the crisis. Canada’s prime minister Justin Trudeau said his country would reduce carbon pollution by 40% to 45% by 2030, an increase from Canada’s previous 30% target. Japan’s Prime Minister Yoshihide Suga vowed to reduce emissions 46% below 2013 levels.

Sen. John Barrasso, a Wyoming Republican and fierce defender of the fossil fuel industry, slammed Biden’s 50% emissions-reduction plan, saying it would put the U.S. at a competitive disadvantage globally.

“President Biden is unilaterally committing America to a drastic and damaging emissions pledge,” Barrasso said in a statement Thursday. “As the president sets punishing targets for the country, America’s adversaries like China and Russia continue to increase emissions at will. The last thing the economy needs is higher energy prices and fewer jobs, but that’s exactly what we’re going to get.”

“The Biden administration inherited four years of dust and denialism on climate action, but that era is over,” Sen. Edward Markey, D-Mass, said Thursday. He called Biden’s 50% emissions-reduction target “technologically feasible and well within our reach.” With additional funding and political will, Markey said, “we can do even more to meet the moment on this existential crisis.”

The United States is the second-largest emitter of carbon dioxide (CO2), producing about 5.41 billion metric tons in 2018. China emits nearly twice that amount.

Experts say the world’s major economies need to dramatically scale back their carbon emissions to limit the rise of average global temperatures to 1.5 degrees Celsius, compared to pre-industrial levels. Although the U.S. and other countries are already seeing the impacts of global warming through more extreme weather events, scientists say the effects will become catastrophic and irreversible if the planet’s temperatures exceed the 1.5-degree threshold.

Biden has pledged to be the most aggressive president on climate change, setting a goal of de-carbonizing the U.S. power sector by 2035 and reaching net-zero greenhouse gas emissions by 2050. He has already taken several steps to address climate change, such as pausing oil drilling on public lands and halting construction of the Keystone XL Pipeline.

“The Biden-Harris administration will do more than any in history to meet our climate crisis,” Secretary of State Antony Blinken said Monday in a speech previewing this week’s summit. “This is already an all-hands-on-deck effort across our government and across our nation. Our future depends on the choices we make today.”

Blinken and other Biden advisers have countered the GOP criticism by pointing to the economic opportunities in transitioning to a clean-energy economy.

In his speech on Monday, Blinken said that climate change should be viewed not only as a threat but also an opportunity – to create new clean-energy jobs, to build a healthier society and to regain a competitive edge against China.

“It’s difficult to imagine the United States winning the long-term strategic competition with China if we cannot lead the renewable energy revolution,” he said. “Right now, we’re falling behind.”

He noted that China holds nearly a third of the world’s renewable energy patents and is the largest producer and exporter of solar panels, wind turbines, electric vehicles and other innovations.

“If we don’t catch up, America will miss the chance to shape the world’s climate future in a way that reflects our interests and values, and we’ll lose out on countless jobs for the American people,” Blinken said.