Once a leader in the effort to muster money to help the world’s poor tackle climate change, the United States these days plays a more modest role. Facing complicated budgetary politics, Washington is having a hard time putting its money where its mouth was six years ago.

To be sure, the world has made progress. This week, France promised $5.6 billion in annual climate-related assistance by 2020, up from some $3.4 billion today. Britain also pledged to increase its budget for climate-related development finance.

“We are well on our way to achieving this $100 billion goal,” Secretary of State John Kerry said after a meeting of rich countries in Paris this month to discuss climate finance.

Still, there’s a long way to go. “We are at $50 billion for sure,” said one knowledgeable European diplomat, who spoke on the condition of anonymity because he was not authorized to discuss the negotiations. “But something like $30- to $40 billion in new money will have to be mobilized between now and 2020.”

Beyond the target itself looms a critical question: What counts as the right sort of aid?

“We still don’t have an internationally agreed definition of what is climate finance,” noted Barbara K. Buchner, senior director of Climate Policy Initiative, a research center that is working with the Organization for Economic Cooperation and Development to build a consistent estimate of the money mobilized so far.