International aviation and shipping are among the top contributors to global warming through the sector's greenhouse gas emissions.
Andrew Murphy, who works on transport policy for the Europe-based group Transport and Environment, said the climate impact from shipping and aviation is equal to, if not greater than, that of the UK and Germany.
"If you compare them to countries... they will be top 10 emitters globally," he said.
"So if they're left out of the agreement, it is quite a problem."
While an accountability process is in place for shipping and aviation to meet certain targets, Mr Murphy said the current framework is weak.
Estimates are made on an annual basis, but the United Nations has not reached the stage of monitoring emissions from the two sectors.
"So our estimates say they are a huge impact on the climate and that these emissions will grow considerably in the years to come," Mr Murphy said.
"But we need far more clarity on what their emissions are, and we were hoping Paris could provide that sort of framework."
We simply can't leave these bodies unsupervised and expect them to handle the climate challenge.
Andrew Murphy, Transport and Environment
Transport had been included in early drafts of the Paris agreement, but now all references have been dropped.
"There is now zero reference in the agreement to these sectors despite a huge climate impact," Mr Murphy said.
Unless the agreement is changed again in the next 24 hours, Mr Murphy said that shipping and aviation will have no formal oversight in the future.
Demands for aviation and shipping to be put back into the agreement
He said in recent years emissions in the aviation and shipping sectors had grown twice as fast as the rest of the global economy.
"So left unsupervised, we can't be confident there'll be any climate action coming from them," he said.
"That's why it's so important that those countries — especially those calling for a deal with a high level of ambition — reverse yesterday's decision and make sure language is put back in the agreement."
Countries such as Mexico and Switzerland have called for the references to the aviation and shipping industries to be reinstated.
"We've also seen interestingly, some sections of the shipping industry coming forward and saying actually they want to make sure they are covered by this agreement," Mr Murphy said.
"So we'll see in the next 24 hours whether that has an impact on the final outcome."
Two major bodies covering international shipping and aviation said they have put in place voluntary schemes and are now using more efficient fuel in their planes and ships, proving they are prepared to tackle the issue with a self-management approach.
But Mr Murphy said that method has not proven to be a viable way forward.
"I refer to the comments from likes of Tony de Brum, foreign minister for the Marshall Islands, who yesterday described the IMO (International Maritime Organisation), the shipping body, as an association run by and for industry," he said.
"We simply can't leave these bodies unsupervised and expect them to handle the climate challenge."