Representatives from nearly 200 countries will meet in Glasgow, Scotland, from 31 October- 12 November for climate talks to strengthen action to tackle global warming under the 2015 Paris Agreement.
Against the backdrop of extreme weather events around the world and a United Nations' climate report that said global warming was close to spiralling out of control, the actions of governments at this conference will determine whether it is a success.
Here are some of the issues which need to be resolved:
Emissions cut pledges
Six years ago in Paris, countries agreed to cut greenhouse gas emissions to limit global warming to 2 degrees Celsius and ideally 1.5C (2.7 Fahrenheit). To do this, emissions need to be cut in half by 2030 and reach net-zero by around mid-century.
As the UN conference was postponed last year due to the pandemic, this year is the deadline for countries to make steeper emissions cut pledges (called nationally determined contributions or NDCs).
The annual "emissions gap" report by the United Nations' Environment Programme (UNEP), which measures the gap between anticipated emissions and those consistent with limiting the temperature rise this century as agreed in the Paris accord, said updated pledges only reduce forecast 2030 emissions by an additional 7.5%, compared to the previous commitments.
If continued throughout this century, this would lead to warming of 2.7C, slightly less than the 3C UNEP forecast in its last report. A 30% cut is needed to limit warming to 2C and a 55% cut is needed to limit to 1.5C.
It said current commitments to net zero could limit warming to around 2.2C by the end of the century, but 2030 pledges so far do not put major emitters on a clear path to this.
Major emitters China and India - together responsible for around a third of global greenhouse gas emissions - have not yet come forward with strengthened NDCs and need to do so at this conference, known as COP26.
As far back as 2009, developed countries agreed to raise $100 billion a year by 2020 to help developing countries deal with the impacts of climate change.
But a plan on how to do so, prepared by Canada and Germany ahead of the United Nations COP26 summit in Scotland, said the annual target would now not be met until 2023.
As rich nations are not meeting the $100 billion a year goal, it can break down trust at the climate talks, experts say. And a new finance goal needs to be worked out for 2025 onwards.
Loss and damage
Governments agreed to address the impact of climate change on developing countries but there is no detail about liability or compensation, a bone of contention for many poorer countries.
A platform to enable technical assistance for vulnerable countries was established in 2019 but developing nations want a more robust mechanism to include financing.
The UK COP26 president, Alok Sharma, has said he wants this conference to be the one where coal power is consigned to history.
The UN has called for phasing out coal by 2030 in OECD countries but environment ministers from the Group of 20 big economies have failed to agree a timeline.
Article 6 of the Paris Agreement, which covers the role of carbon markets, has not been resolved since the pact was struck. Progress on it broke down at the last talks in 2019.
The article calls for "robust accounting" to avoid "double counting" of emissions reductions. It also aims to establish a central UN mechanism to trade carbon credits from emissions reductions generated from low-carbon projects.