• World leaders at the G-20 summit concluded that there would need to be "meaningful and effective" actions to tackle climate change in a statement on Sunday.
• But activists criticized the statement for being "weak, lacking both ambition and vision," while President Joe Biden said Russia and China "didn't show up."
• Antonio Guterres, the U.N.'s secretary general, said he left the G-20 meeting "with my hopes unfulfilled."

Activists have reacted with anger as the world's largest economies made few firm commitments on climate at a G-20 summit in Italy.

World leaders acknowledged there would need to be "meaningful and effective actions" to mitigate temperature rises in a written agreement released Sunday, but did not commit to achieving net-zero carbon emissions by a set date.

The G-20, or Group of Twenty, is made up of 19 countries including the U.S., U.K., Russia, China and Brazil, as well as the European Union, and accounts for around 80% of greenhouse gas emissions globally.

Jennifer Morgan, executive director of Greenpeace International, said the statement "was weak, lacking both ambition and vision, and simply failed to meet the moment," in an online release Sunday.

World leaders attended the G-20 meeting in Rome at the weekend before traveling to Glasgow, U.K., for the start of the U.N.'s Conference of the Parties, the climate summit known as COP26.

Morgan urged leaders attending COP26 this week to "cut emissions drastically right now, to stay in line with 1.5C."

Almost 200 countries agreed to limit global warming to well below 2 degrees Celsius, preferably to 1.5 degrees Celsius, compared to pre-industrial levels as part of the Paris Agreement in 2015. And earlier this year a U.N. report called for immediate emissions reductions.

But while the G-20 statement said it "remains committed" to the Paris Agreement, further details were vague. "We recognize that the impacts of climate change at 1.5°C are much lower than at 2°C. Keeping 1.5°C within reach will require meaningful and effective actions and commitment by all countries," it said.

Antonio Guterres, the U.N.'s secretary general, appeared disappointed with progress from the G-20 meeting.

"While I welcome the #G20's recommitment to global solutions, I leave Rome with my hopes unfulfilled — but at least they are not buried. Onwards to #COP26 in Glasgow to keep the goal of 1.5 degrees alive and to implement promises on finance and adaptation for people & planet," he said in a tweet Sunday.

Non-profit Global Citizen criticized the G-20 meeting for "platitudes" rather than action. "The G20 was an opportunity for concrete actions to show how multilateralism is the best answer to tackling the converging crises we face today, but the G20 let the world down," the organization wrote in a tweet Sunday.

Meanwhile, campaigning organization Avaaz urged leaders attending COP26 to deliver action, rather than words.

In a tweet Sunday, it said: "People, Planet and Prosperity" was the tag line of the #G20RomeSummit. What have they delivered in the final communique? A poorer version of the Italian song: "parole, parole, parole" (words, words, words). #COP26 in Glasgow must deliver action, action, action!"

Mark Malloch-Brown, president of the Open Society Foundations and a former deputy U.N. secretary-general told CNBC on Monday: "The G20 was pretty disappointing on key issues like coal ... [such as] stopping domestic development of coal, and you know on issue after issue, it fell short of specific, quantified targets that people had hoped for."

World leaders attending the G-20 meeting were also disappointed. U.S. President Joe Biden blasted Russia and China, which, he said, "basically didn't show up in terms of any commitments to deal with climate change."

The U.S., China and Russia are historically the world's three largest carbon emitters, according to research group Carbon Brief. China's President Xi Jinping and Russian President Vladimir Putin are not attending COP26, with Xi set to release a written speech Monday.

Biden said a "series of very productive meetings" were held at the G-20 and conceded: "We've made significant progress and more has to be done."

Italian Prime Minister Mario Draghi, the chair of the Rome meeting, said: "We made sure that our dreams are not only alive but they are progressing."