It's the first meeting of the leaders since Donald Trump announced he was withdrawing from the Paris Agreement, in which countries around the world committed to a series of targets on the issue of climate change.
Mohamed Adow, Christian Aid's International Climate Lead, said: "The best way for the remaining 'G19' to defend the Paris Agreement is to enact promises within it.
"Trump has made his choice: it's time for the other leaders to get down to business. By delivering on the pledges enshrined in the accord they will demonstrate its effectiveness, and have a fighting chance to stay on a safe course.
"The moment has now come for major economy leaders to shift gears from rhetoric to hard action and get on with the business at hand, towards a clean, sustainable and resilient global economy.
Christian Aid wants to see leaders commit to making a big shift in the flow of global finance - away from dirty energy sources like coal, and towards green investment."
"This G20 also presents a test for Theresa May. Last month she gave only a soft slap on the wrist to Trump's reckless act: but with her international reputation in the balance, she must publicly show that on climate change she has far more in common with the rest of the world than with Donald Trump and join other leaders in defending and advancing the Paris Agreement."
Leaders at the meeting in Hamburg Germany will discuss a range of issues other than climate change. Digital technology, resistance to drugs, food security and water consumption are all on the agenda.