UN-led negotiations in Mexico's Cancun beach resort aim to cement existing pledges to curb carbon emissions and global warming. Other deals which may be agreed, hinging on an emissions deal, include long-term climate aid for developing countries and payments to tropical nations to protect their forests.
Following are areas of possible agreement at the talks, which are scheduled to end on Friday.
Extending the Kyoto Protocol
Decide whether to continue the Kyoto Protocol, as favoured by developing countries, which is proving one of the most difficult issues. Its first round of targets ends in 2012.
Decide on the length of commitment period of the next round of targets, for example whether to 2017 or 2020.
Decide whether to cancel surplus, tradable emissions credits owned by countries whose emissions were below their 2008-2012 Kyoto targets. Credits are called assigned amount units (AAUs).
Decide new national targets either under the 1997 Kyoto Protocol or the 1992 UN climate convention, or both.
Some industrialised countries do not like Kyoto, as so far it has only controlled the emissions of developed countries. A way out may be to note new targets in an appendix to Kyoto and the convention.
Refer to a long-term goal, for example to limit warming to less than 2 degrees Celsius.
Mention the widely held scientific view that emissions targets pledged so far are too weak.
Review in 2013-2015 whether targets need strengthening.
Also called measurement, reporting and verification (MRV).
Agree to measure developed country emissions, for example annually, and also their contribution to climate aid funds.
Measure developing countries' greenhouse gases and their actions to slow emissions growth, perhaps every two to four years.
Agree common accounting standards, for example on measuring carbon emissions from forests.
A political agreement to pay tropical countries not to chop down natural forests, called reduced emissions from deforestation and degradation (REDD).
Launch next year a programme to set rules.
Delay decision on whether to include the scheme under an existing market in carbon offsets called the clean development mechanism (CDM).
Agree safeguards for wildlife and indigenous people.
Climate development fund
A political commitment to establish a new long-term fund especially to help least developed countries cut carbon emissions and prepare for climate change.
The fund would dovetail with a commitment countries made in Copenhagen in 2009 to raise $100 billion annually by 2020.
Launch a programme to decide the board structure, including crucially the split between developed and developing countries.
Adaptation means steps to prepare for climate change.
Link to an existing adaptation framework new sources of financing, including the proposed new climate development fund, as well as "fast-start" funding of $30 billion from 2010-2012 agreed last year in Copenhagen.
Create a disaster relief mechanism, to help countries which have suffered extreme weather events.
Launch a "technology mechanism," including establishing regional technology centres in developing countries in Asia, Latin America and Africa.