Under the Barack Obama administration, China and the United States made significant progress on climate cooperation when the two countries laid the foundations of international support for the landmark 2015 Paris Agreement.
However, during his four-year presidency Donald Trump delivered a hammer blow to the global battle against climate change when he announced the withdrawal of the US from the accord in June 2017.
On Jan 20, Trump's successor, Joe Biden, in one of his first acts as US president, signed an executive order reinstating his nation to the treaty.
When Biden served as Obama's vice-president, he witnessed the work done on the agreement, and now as US president he has made it abundantly clear that climate change will be an urgent priority for his administration. His strong position is a welcome sign that the world's two biggest carbon emitters can again work closely on tackling the climate crisis, experts said.
While they can envision the world's No 1 and No 2 economies joining hands again for the best interests of humankind, experts are however divided on how quickly that cooperation can bear fruit.
Chai Qimin, director for strategy and planning at the National Center for Climate Change Strategy and International Cooperation, said from 2014 Sino-US cooperation on climate issues underwent a "honeymoon period" after high-level visits by both heads of states.
Following President Xi Jinping's visit to the US in June 2013, Obama visited China in November 2014.
On Nov 11, 2014, with climate change a key concern, they held a long meeting in Beijing. The next day, the two presidents made public the China-US Joint Announcement on Climate Change, which said they "resolved to work closely together over the next year to address major impediments to reaching a successful global climate agreement in Paris".
Several additional joint announcements were released, which included many of the key concerns outlined in the Paris Agreement.
Chai, who participated in the negotiations, said, "When parties were in dispute over how some clauses should be stated in negotiating the Paris Agreement, they chose to turn to those announcements for reference and even borrowed some without changes."
The two nations played key roles in helping the world reach the Paris Agreement, especially the bottom-up architecture of the treaty, which allows countries to pledge Nationally Determined Contributions, or post-2020 climate action.
Chai said the participation of China and the US also gave momentum to the accord becoming reality.
Before the world's two largest economies formally committed to the Paris Agreement ahead of the G20 summit in Hangzhou, Zhejiang province, in 2016, only small emitters accounting for 1 percent of the world's greenhouse gas emissions had formally committed to the treaty.
The agreement could not take effect until at least 55 nations representing at least 55 percent of global emissions had formally joined.
About one month after the two nations' announcement, the requirement had been met, Chai said.
Zhang Jianyu, founder and chief representative of the Environmental Defense Fund's China program, said there is a strong possibility that the two countries will cooperate on climate change again.
"Both countries have recognized climate change as a real and serious issue that has nothing to do with politics," he said, adding that they have realized that they cannot address the climate crisis alone.
The three major areas of common ground China and the US face are counterterrorism, coping with climate change and addressing cyberthreats, Zhang said.
"Obviously, the most likely area they can break the ice and cooperate on is climate change," he said.
What makes him optimistic about China and the US again joining hands on the issue is that Biden himself, and many members of his climate change team, including former US secretary of state John Kerry, were previously involved in climate cooperation with China.
They don't seem to harbor hostility toward China, Zhang said, citing Biden's supportive speech to the US-China Climate-Smart/Low-Carbon Cities Summit in 2015.
Biden said there was no reason why the two countries could not be "competitors" as opposed to "enemies". "Our interests are absolutely similar," he added.
Zhang said that while the two countries' national climate cooperation was virtually suspended over the past four years, collaboration between local governments, enterprises and civil society had never stopped.
Changes in circumstances
However, Zou Ji, president of Energy Foundation China, is not that optimistic that resuming cooperation will go ahead smoothly.
"Cooperation between China and the US on climate change has huge potential," he said. "But it's impossible that the two countries will get their relationship back to where it was in the Obama administration. There have been big changes in the circumstances, especially in relation to political trust."
Members of Biden's climate change team were personally involved in the process to reach the Paris Agreement, and most of them have a personal attachment to the treaty, Zou said.
Some of them are old friends of China and know the country well, which is a favorable factor for renewed cooperation on climate change.
However, their work may be fettered because of the so-called consensus on getting tough on China adopted by the Democrats and Republicans. Some of Trump's supporters as well as senators, officials and think tanks that have a strong anti-China position continue to be a political force. "Cooperation can be hard without political trust," Zou said.
The two sides can slowly roll out cooperation by starting dialogue at an academic level, but Zou said he is not optimistic it will happen in the first half of this year.
Chai said what makes the current situation different from negotiations on the Paris Agreement is that the global community is now negotiating specific and detailed issues after the accord entered the implementation phase last year. Talks have focused on low-carbon technologies, energy investment, industrial development, international trade and financing.
He also warned that the US, after rejoining the accord, may ask to renegotiate some of the rules for implementing the agreement. He stressed however, that to tackle climate change, the world needs cooperation more than competition.