Ahead of the meeting, the UN had implored the leaders of the world's richest economies to rescue stalled climate talks in Azerbaijan by boosting funding for developing countries struggling with global warming.
G20 members, who are divided on who should pay, did not make such commitments, saying only that the trillions of dollars needed would come "from all sources."
"The leaders are kicking the can back to Baku," said Mick Sheldrick, co-founder of the advocacy group Global Citizen, referring to the capital of Azerbaijan where the UN climate talks are taking place.
"This is probably going to make it harder to achieve an agreement," he told AFP.
The risk of an escalation in the war in Ukraine and the prospect of a return of US President-elect Trump's isolationist "America First" policies also dominated the talks in Brazil.
US President Joe Biden is attending the summit, but as a lame duck eclipsed by China's Xi Jinping, who has cast himself as a protector of the international order in the new Trump era.
Xi, who held back-to-back meetings with other leaders, warned the world faced a new period of "turbulence" and said there should be "no escalation of wars, and no fanning of flames."
In a statement, the G20 called for "comprehensive" ceasefires in both Gaza and Lebanon.
The summit was riven with divisions over Ukraine, however.
On Sunday, Biden, who is attempting to ringfence support for Ukraine before Trump's return to power, gave Kyiv the green light to use long-range US missiles to strike deep inside Russian territory.
Biden's move -- a major policy shift by the US -- threatens to escalate a war Trump has vowed to quickly end.
Russia on Monday warned of an "appropriate response" if its territory was hit.
German Chancellor Olaf Scholz said he would not follow Biden's lead with his country's Taurus missiles, but French President Emmanuel Macron praised a "good" move by Biden.
- Tax the ultra-rich -
Brazilian President Luiz Inacio Lula da Silva attempted to put issues close to his heart, such as fighting hunger and climate change, at the top of the agenda.
At the opening of the summit, he launched the centerpiece of his G20 presidency: a Global Alliance against Poverty and Hunger backed by 82 countries that aims to feed half a billion people by 2030.
He won further praise from campaigners by garnering support for a bid to make billionaires pay more tax.
The summit statement included a pledge to "engage cooperatively to ensure that ultra-high-net-worth individuals are effectively taxed," and to devise mechanisms to prevent them dodging tax authorities.
"Brazil has lit a path toward a more just and resilient world, challenging others to meet them at this critical juncture," anti-poverty group Oxfam said in a statement.
But Lula's progressive social agenda met some resistance from Argentina's libertarian president Javier Milei, an ardent fan of Trump and his billionaire adviser Elon Musk.
Milei said he opposed points in the summit declaration, including increasing state intervention to combat hunger and regulating social media but saved Brazil's blushes by nonetheless signing up to the joint statement.
The meeting comes in a year marked by another grim litany of extreme weather events, including Brazil's worst wildfire season in over a decade, and the opening of a new front in Israel's wars with its Arab neighbors.
"Today the world is on a knife edge," EU Council President Charles Michel warned.
The get-together caps a diplomatic farewell tour by Biden that took him to Lima for a meeting of Asia-Pacific trading partners, and then to the Amazon in the first such visit for a sitting US president.
Conspicuously absent from the summit was Russian President Vladimir Putin, whose arrest is sought by the International Criminal Court over the Ukraine war.
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