Stiell was speaking as the two-week conference approaches its midpoint, after the opening flurry of announcements and pledges has died down and attention turns to behind-the-scenes negotiations.
"All governments must give their negotiators clear marching orders. We need highest ambition, not point-scoring or lowest common denominator politics," Stiell told a news conference.
Underscoring the urgency of the climate crisis, European Union scientists said November had wrapped up the world's warmest autumn ever recorded.
A draft COP28 text on fossil fuels suggests saying nothing or urging a complete phase out. (AP PHOTO)
Negotiators from nearly 200 countries at the summit in Dubai are expected to hand over their work to their countries' ministers for the next stage in negotiating a global consensus on what a final COP28 deal should look like.
"We have a starting text on the table... but it's a grab bag of wish lists and heavy on posturing. The key now is to sort the wheat from the chaff," Stiell said.
With the world way off track in meeting its climate goals, Stiell urged the delegations to make progress that matters.
"There are many options that are on the table right now which speak to the phasing out of fossil fuels. It is for parties to unpick that but come up with a very clear statement that signals the terminal decline of the fossil fuel era as we know it."
On Tuesday, an early draft text laid out three options on fossil fuels, ranging from saying nothing to calling for a complete phase out of their use.
The EU Copernicus Climate Change Service said that to date, temperatures for 2023 have been on average about 1.46C above pre-industrial levels.
Copernicus deputy director Samantha Burgess said in a statement that "2023 has now had six record breaking months and two record breaking seasons".
"The extraordinary global November temperatures, including two days warmer than 2C above preindustrial, mean that 2023 is the warmest year in recorded history."
