The recent pledges by China, Japan and South Korea to go carbon neutral are a major boost to global action on climate change and could prompt other nations to back more ambitious plans to cut carbon emissions, the United Nations climate chief has said.

"The announcements by these three countries are really very important. And they come at a time when we need to see leadership in our process," Ms Patricia Espinosa, executive secretary of the UN Framework Convention on Climate Change, told The Straits Times from Bonn, in Germany.

"I certainly hope that many countries will follow," she added.

"This is also important because we need to send the signal right now that climate change remains a priority," she said, adding that the threat from climate change has not waned and that the Covid-19 pandemic represents a golden opportunity to rebuild economies that are greener and good for growth and jobs.

In September, China's President Xi Jinping surprised the world by setting 2060 as the target for China to be carbon neutral.

Late last month, Japan's Prime Minister Yoshihide Suga set 2050 as his nation's target. Two days later, South Korea's President Moon Jae-in made the same 2050 pledge.

Carbon neutral means dramatically reducing emissions of carbon dioxide, the main greenhouse gas heating up the atmosphere. Any remaining emissions need to be removed from the atmosphere using tree planting or other offsetting projects.

The announcements by the three countries are crucial because they are large carbon polluters from using fossil fuels. China is the world's top greenhouse gas polluter and top coal producer and consumer. Japan is the fifth-largest carbon polluter, while South Korea is 13th.

The pledges are also key because they underscore a growing global shift towards cleaner energy, a shift that has not been derailed by the pandemic, with a number of nations backing green recovery programmes.

Crucially, the pledges come a year before a major UN climate conference in Britain called COP26, which is meant to breathe life into the 2015 Paris Agreement in which nearly 200 nations agreed to limit global warming to well below 2 deg C and aim for 1.5 deg C if possible.

The world has already warmed 1.1 deg C and weather-related extremes are intensifying. Record wildfires have scarred eastern Australia and the west coast of the United States this year, severe floods and storms have battered Vietnam and the Philippines and the US South has been hit by repeated hurricanes.

Under the Paris pact, nations this year are meant to submit new or updated climate plans to make deeper cuts to emissions this decade, a decade the UN says is critical for avoiding the worst impacts of climate change.

"This decade will determine the fate of humanity with respect to climate change," Ms Espinosa said, because deep emissions cuts need to be made by 2030 to try to limit warming to 1.5 deg C. Mid-century carbon-neutrality pledges are vital but so, too, are carbon cuts before then.

The problem, though, is that the global appetite for ambitious climate action has waned since the Paris deal was agreed, Ms Espinosa said. Last year's UN climate gathering in Madrid, called COP25, ended in only a watered down agreement to make deeper carbon cuts.

The pledges by China, Japan and South Korea have brightened hopes for more ambition, she said, adding that she was also encouraged by the climate plans of US Democratic presidential candidate Joe Biden.

The pandemic, too, has triggered a greater focus on how the world needs to change.

"I think the arrival of this Covid-19 crisis has really exposed humanity's greatest vulnerabilities and has magnified its most urgent challenges."

She said the pandemic has created an opportunity to accelerate the green transition that has long been discussed. Now there was an opportunity to direct public money towards green industries and green jobs and shift away from polluting economic models as part of national economic recovery plans.

"A recovery that is not sustainable is not a recovery. Why would you call recovery something that brings you back to a place where you are looking at so much suffering, loss of human lives? That's not a recovery. That would be a setback."