Who is attending (Biden, Johnson), and who's not (Putin, Xi).

When President Biden was asked in Rome on Sunday about criticism of the world’s wealthiest economies to do more to address climate change, he noted to absence of two key players: China and Russia.

“Not only Russia, but China, basically didn’t show up in terms of any commitments to deal with climate change,” the president told reporters. “There’s a reason why people should be disappointed in that. I found it disappointing myself.”

One day later, as Mr. Biden joined more than 100 world leaders who have descended on Glasgow for a critical climate summit, the leaders of China and Russia were among the most notable no-shows.

Along with Xi Jinping of China and Vladimir V. Putin of Russia, Jair Bolsonaro of Brazil and Recep Tayyip Erdogan of Turkey are not attending the summit.

The presence of heads of state and government at the talks is not just symbolic. Real work gets done among leaders that cannot happen among lower-level diplomats. During the 2009 climate talks in Copenhagen, President Barack Obama barged in to a secret meeting being held by the leaders of China, India, Brazil and South Africa. The discussions that came after helped clinch a deal, albeit a weak one.

So the high-profile absences this time have dismayed some experts.

“Even as most democracies are making ambitious climate commitments, the world’s most powerful autocrats in Beijing, Moscow and elsewhere are thumbing their noses, refusing to cut their emissions and even to show up at climate negotiations,” said Paul Bledsoe, who advised the Clinton White House on climate change and is now with the Progressive Policy Institute.

Mr. Putin said more than a week ago that he would not attend the summit, signaling that he had concerns about the coronavirus.

“The president unfortunately will not speak, because the option to participate by videoconference is not available in Glasgow,” said Dmitri S. Peskov, the Kremlin spokesman.

Mr. Peskov responded to Mr. Biden’s comments by saying that Russia was “already ahead of many countries, including those of Western Europe,” in transitioning to low-carbon sources of energy.

Mr. Putin, addressing the Group of 20 summit in Rome via video on Sunday, said that 86 percent of Russian energy consumption came from nuclear, renewables and natural gas. Critics note that while natural gas emits about half the carbon dioxide of coal, it still generates pollution that is warming the planet, and its pipelines are vulnerable to leaks of methane, a potent greenhouse gas.

Mr. Bolsonaro, under fire for his environmental policies, has not given a reason for his absence. He attended the G20 talks over the weekend, and he is visiting an Italian town that plans to award him honorary citizenship instead of going to the climate conference.

Turkey’s president, Recep Tayyip Erdogan, who has pledged to make tackling climate change a priority, was also expected to travel to Glasgow after the G20 summit, but instead flew back to Istanbul, the state-run Anadolu news agency reported on Monday. The reason for skipping the climate talks was a protocol issue involving his delegation in Glasgow, an official told reporters. Turkey’s environment minister is expected to attend the conference in his place.

Mr. Xi is expected to issue a statement to the Glasgow summit. He has not publicly left China since the coronavirus spread from the Chinese city of Wuhan.