The G20 bloc, which includes Brazil, China, India, Germany and the United States, accounts for an estimated 80% of the global greenhouse gas emissions which scientists say must be steeply reduced to avoid climate catastrophe.
For that reason, this weekend's gathering is seen as an important stepping stone to the UN's "COP26" climate summit attended by almost 200 countries, in Glasgow, Scotland, where most of the G20 leaders will fly directly from Rome.
"The latest reports are disappointing, with little sense of urgency in the face of an existential emergency," said Oscar Soria of the activist network Avaaz. "There is no more time for vague wish-lists, we need concrete commitments and action." A fifth draft of the G20's final statement seen by Reuters on Saturday did not toughen the language on climate action compared with previous versions, and in some key areas, such as the need to achieve net zero emissions by 2050, it softened it.