Over the course of the conference, he called fighting climate change 'a moral imperative'
President Joe Biden wrapped up a two-day virtual climate summit on Friday meant to signal the United States' return to global leadership on climate change.
Over the course of the conference, he called fighting climate change "a moral imperative," committed the United States to halve its emissions by 2030 and touted the job-creating potential of climate projects, as the more than 40 world leaders in attendance issued their own pledges.
Many foreign leaders heralded resurgent US interest in climate change after four years of backpedaling by former president Donald Trump.
Intended as a prelude to a major United Nations climate conference in Scotland this November, the summit saw countries put forward fresh climate pledges as part of a push to reach global carbon neutrality by mid-century.
This isn't the first time the United States - the world's second-largest greenhouse gas emitter - has tried to catalyze global efforts to fight climate change.
"Without the US I think it's hard to imagine a climate regime succeeding," said Daniel Bodansky, an international law professor and climate change expert at Arizona State University.
But Biden had to contend with an awkward backdrop: While the United States has often played a key role in driving international action on the issue, its record on implementing climate agreements is lackluster, and subject to the shifting impulses of a divided electorate.
Here's a look at America's past involvement in major climate agreements.
The Framework Convention and the Kyoto Protocol
In 1992, the United States and more than 150 other countries signed the UN Framework Convention on Climate Change in Rio de Janeiro, promising to return emissions to 1990 levels by 2000. That goal remains unmet.
Five years later, diplomats and dignitaries convened in Kyoto, Japan, to hash out the details of what would become the Kyoto Protocol, an international treaty based on the UN framework that committed countries to reducing greenhouse gas emissions.
Addressing world leaders gathered in Kyoto, then-Vice President Al Gore urged them to "take responsibility" and promised the US "remains firmly committed to a strong, binding target" for reducing emissions.
Recognizing that developed countries bore the brunt of responsibility for global warming, the agreement committed three dozen industrialized countries and economies in transition - plus the European Union - to legally binding emissions-reduction pledges.
These countries agreed to either cut back their own greenhouse gas emissions or reduce the global carbon footprint, by funding green development projects elsewhere, for example.
Altogether, the targets added up to an average 5% emissions cut from 2008 to 2012 compared to 1990 levels, according to UN the accord also established an adaptation fund for developing countries to mitigate the effects of climate change.
But the final agreement didn't oblige developing countries to reduce emissions. The US signed the accord, but the Senate - which had to ratify it - signalled it would be dead on arrival, so the Clinton administration never submitted it.
During his first presidential campaign, George W Bush told voters the Kyoto Protocol would "affect our economy in a negative way"; shortly after his inauguration, he formally withdrew the United States.
The Copenhagen Accord
In December 2009, world leaders convened in Copenhagen, to renew efforts to tackle climate change. The Obama administration hoped the gathering would lead to an ambitious new agreement on binding emissions pledges.
But participating countries were only able to agree on a political declaration drafted by the US, China and several other countries stipulating that nations would "take action" to reach peak emissions as soon as possible.
While Obama labelled the accord an "unprecedented breakthrough," it dashed the hopes of environmental advocates and many diplomats.
In the wake of the conference's perceived shortfalls, an influential paper jointly published by Oxford University and the London School of Economics concluded that climate policy following the Kyoto Protocol approach had "failed to produce any discernible real world reductions in emissions."
Documents leaked by whistleblower Edward Snowden later showed the US spied on other countries during the Copenhagen conference, prompting angry reactions from developing countries.
The Paris agreement
Current global efforts are guided by the Paris Climate Agreement, which was adopted in December 2015 and took effect in November 2016. Nearly 200 countries signed onto the accord, which aims to limit global warming to "well below" 2°C above preindustrial levels and ideally closer to 1.5°C. Scientists predict warming beyond that temperature would cause irreversible damage.
The agreement works differently than the Kyoto Protocol: It invited developing countries to make pledges alongside developed countries - and under its bottom-up approach, countries set their own emissions reduction targets and implementation plans, but aren't legally bound to carry them out.
Then-Secretary of State John F. Kerry helped forge the landmark accord, and Obama championed it alongside Chinese President Xi Jinping during an era of warmer bilateral relations.
Promising to cut emissions by 26% to 28% compared with 2005 levels, the US formally entered the agreement in 2016.
Under Trump, the US became the only nation in the world to walk away from it. While individual American states, cities and companies continued efforts to roll back emissions during Trump's tenure, the federal government mostly took a back seat.
"Paris had been negotiated largely to meet US specifications and we had a huge hand in shaping it, so the fact that we pulled out of it is more problematic than [not ratifying] Kyoto," Bodansky said.
The fact that this was the second time that the United States had turned its back on a climate agreement didn't help.
"It gives rise to the feeling that every time the US administration changes, the US position changes," Bodansky added.
Biden wasted no time in recommitting the US to the Paris climate accord once he took office, and European leaders including Germany's Angela Merkel welcomed the US back with open arms.
Biden appointed Kerry as his climate envoy and is sending him out on climate diplomacy missions around the world.
And the emissions-reduction pledge he announced on Thursday roughly doubles Obama's 2015 Paris commitment.
But the damage Trump dealt to US credibility on climate issues could prove challenging to undo.
Chinese Foreign Ministry spokesperson Zhao Lijian could not resist appearing to gloat on last Friday, telling reporters the United States' return to the Paris agreement "is by no means a glorious comeback but rather the student playing truant getting back to class."
While a number of summit attendees - including the United Kingdom, the European Union and Japan - announced new emissions reductions targets timed with this week's meeting, it's unclear how many countries will follow suit, and both friends and foes are skeptical of US leadership.