There are still "serious and contentious" issues to resolve as the United Nations climate change conference enters its final few days, the head of the Organization of Economic Cooperation and Development (OECD) told CNBC.
Angel Gurria, the secretary-general of the OECD, told CNBC Thursday that "there's been a lot of progress" at the global summit on climate change, known as COP21, but more work needed to be done ahead of a deadline for world leaders to come up with a global agreement on limiting greenhouse gas and temperature rises by Friday.
"Now we're really down to the nitty gritty which is going to be the defining of the ambition (the carbon reduction targets), the resetting mechanism over the timeframe, the measuring, verification and reporting system and obviously the finance is going to be critical so these issues are coming back."
"These are the very serious and contentious issues," he said, speaking to CNBC on the sidelines of the summit in Paris.
World leaders have so far managed to shorten a draft text of an agreement to 29 pages from a previous 48 but expert warn that by cutting the proposal, compromises will have to be made. Plus there is still the burning question of whether the deal will limit global warming to no more than 2 degrees Celsius over pre-industrial levels by the end of the century, as hoped by the UN.
French Foreign Minister and COP21 President, Laurent Fabius, gave his reaction to the current draft on Wednesday.
Speaking to the press, he said: "The text that will be handed to you in a minute is the best possible reflection of emerging compromises. The rule we followed means we shouldn't jump to conclusions over the resolution of the most politically-sensitive points and keep a balance between the various options still on the table."
Another key part of the summit is for developed nations to come up with $100 billion a year by 2020 for developing countries to help them cope with climate change, a contentious part of the deal.
The OECD has calculated that $62 billion had already been raised in 2014 but this total has raised questions over the organization's methodology and how much money has actually been pledged.
"I think what has happened is that we were very scrupulous and extremely rigorous that the developed countries were saying 'for goodness sake, OECD, there's a lot more money than you're saying' and the developing countries saying 'you're being too generous' (with the amount allegedly pledged) so we must be right in the middle," Gurria said.
"We were very careful, we have identified $62 billion so far and we still have five years to go so we have a good shot at (getting to the target)."
- By CNBC's Holly Ellyatt, follow her on Twitter . Follow us on Twitter: @CNBCWorld