GLASGOW, Scotland — In the fight to curb climate change, several major coal-using nations announced steps Thursday to wean themselves — at times slowly — off of the heavily polluting fossil fuel.

The pledges to phase out coal come on top of other promises made at the U.N. climate summit in Glasgow, Scotland, that the head of an international energy organization said trimmed several tenths of a degree from projections of future warming. But outside experts called that “optimistic.”

Optimism also abounded in relation to the promises on coal, which has the dirtiest carbon footprint of the major fuels and is a significant source of planet-warming emissions.

“Today, I think we can say that the end of coal is in sight,” said Alok Sharma, who is chairing the conference of nearly 200 nations, known as COP26.

Critics say that vision is still obscured by a lot of smoke because several major economies still have yet to set a date for ending their dependence on the fuel, including the United States, China, India and Japan — which was targeted outside the summit venue Thursday by protesters clad as animated characters.

What nations have promised varies. Some have pledged to quit coal completely at a future date, while others say they’ll stop building new plants, and even more, including China, are talking about just stopping the financing of new coal plants abroad.

The British government said pledges of new or earlier deadlines for ending coal use came from more than 20 countries including Ukraine, Vietnam, South Korea, Indonesia and Chile.

Some came with notable caveats, such as Indonesia’s request for additional aid before committing to bring its deadline forward to the 2040s.

A U.S. official, speaking on condition of anonymity to discuss officials’ thinking, said that while the U.S. hadn’t opted to join the coal phase-out pledges, its commitment to a clean energy future was clear. The Biden administration wants to reach 100% carbon pollution-free electricity by 2035.

Underlining the urgent need for action on coal, a new analysis by scientists at Global Carbon Project found emissions from the fuel increased dramatically in 2021, not just from pandemic-struck 2020 levels, but even when compared to pre-pandemic 2019 levels. The world spewed 14.7 billion metric tons (16.2 billion tons) of carbon dioxide from coal burning, 5.7% more than last year, said the group, which tracks annual carbon pollution.