China is the world's largest polluter and strong emissions cuts would have a major impact on warming, while the EU is under pressure to show it remains a global force for climate action.
Leaders were supposed to unveil these new commitments months ago to demonstrate the world can put aside divisions over trade and conflict to keep the goals of the Paris Agreement alive.
UN Secretary General Antonio Guterres is hoping for a wave of pledges when he hosts a special climate summit in New York next week.
Here's what analysts will be looking for from Beijing and Brussels:
- China -
China alone is currently responsible for some 30 percent of the world's annual greenhouse gas emissions, but it is also a renewable energy superpower that dominates in green technologies.
What it chooses as its 2035 emissions reduction target could make or break the Paris goal to limit warming to "well below" 2C since preindustrial times and preferably 1.5C.
"Whatever they commit to would have a very significant impact," Sarah Heck of Climate Analytics, one of the groups behind the Climate Action Tracker (CAT) monitor, told AFP.
In 2021, China said it would aim to peak carbon dioxide (CO2) emissions before 2030 and achieve net zero by 2060.
Beijing has said its 2035 climate plan would cover all economic sectors and greenhouse gases for the first time.
Analysts Lauri Myllyvirta and Belinda Schaepe at the Centre for Research on Energy and Clean Air estimate China can cut CO2 emissions by at least 30 percent by 2035 compared to 2023 levels -- the minimum level needed to align with the Paris deal.
Others argue that the reductions may need to be much steeper if the world is to limit warming to 1.5C.
A recent CAT analysis, for example, says 2035 emissions need to fall 66 percent from 2023 levels to meet that more ambitious Paris goal.
CAT analysts said they are also looking for more robust 2030 targets in the new plans, updated renewables aims, a clear path away from coal, and a specific base year to measure emissions against.
Observers say China has a history of under-promising and over-delivering.
China previously committed to increasing the installed capacity of wind and solar power to more than 1,200 GW by 2030.
It exceeded that six years early, in 2024, CAT said.
Meanwhile, a recent analysis by Myllyvirta for Carbon Brief suggested China's CO2 emissions might have already peaked, with record solar growth helping to drive a decline in emissions that began in 2024.
- European Union -
The European Union accounted for only six percent of global emissions in 2023, and the 27-nation bloc has decarbonised faster than many other developed nations.
In May, Brussels said it was on track to meet its 2030 goals, predicting a 54 percent fall in emissions compared to 1990 by the end of the decade -- just short of its 55 percent target.
But disagreements among EU nations in recent months have delayed the submission to the UN of its 2035 plan and its 2040 domestic climate goal.
To avoid arriving at next week's summit empty handed, the EU on Thursday produced a compromise "statement of intent" to cut emissions between 66.25 percent and 72.5 percent compared to 1990 levels.
Campaigners from Climate Action Network said that "falls far short" of what is needed.
To be in line with 1.5C, CAT projects that the bloc would need to reduce emissions from all sectors by around 68 percent by 2030 and 78 percent by 2035.
There are concerns the EU is wavering as a climate leader.
"We're continuing to ask our EU partners to step up and putting pressure on them," Ilana Seid, chair of the Alliance of Small Island States (AOSIS), told reporters this week.
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