The British official who will preside over an upcoming U.N. climate summit said Tuesday that he's losing sleep over how to get long-promised funding for poorer nations to switch to cleaner energy and cope with the worst impacts of climate change.

Alok Sharma, president of the 26th United Nations Climate Change Conference, used a speech in Paris to jolt richer nations into action in the last weeks before the Oct. 31-Nov. 12 event in Glasgow, Scotland.

Sharma said that securing the previously promised annual package of climate change funding for poorer nations “is vital to the success of the summit.”

“Without finance, tackling climate change is well-nigh impossible. So developed countries must deliver on the $100 billion a year promised to developing nations. This is a totemic figure. A matter of trust. And trust is a hard-won and fragile commodity in climate negotiations," he said.

“Thinking about this does keep me awake at night," he added.

Sharma, who was the U.K.'s secretary of state for business before he stepped down to oversee the COP26 conference, also put pressure on the Group of 20 nations that together account for the bulk of global wealth and trade and around 80% of polluting emissions that contribute to global warming.

In July, G-20 nations all agreed that before the COP26 meeting, they would each lay out “ambitious” targets to reduce emissions by 2030, Sharma said. But some haven’t yet done so and “must deliver,” he said.

“I say to those G-20 leaders: They simply must step up ahead of COP26," he said.