In December, delegations from 192 countries will hold two weeks of talks in Copenhagen aimed at establishing a new global treaty on climate change. Here, BBC environment correspondent Richard Black looks at what the talks are about and what they are supposed to achieve.
Q&A: The Copenhagen climate summit
Why are the Copenhagen talks happening?
Successive scientific reports, notably those from the Intergovernmental Panel on Climate Change (IPCC), have come to ever firmer conclusions about humankind's influence on the modern-day climate, and about the impacts of rising temperatures.
Two years ago, at the UN climate talks held in Bali, governments agreed to start work on a new global agreement.
The Copenhagen talks mark the end of that two-year period.
Governments hope to leave the Danish capital having completed the new deal.
The talks are technically known as the 15th Conference of the Parties to the United Nations Framework Convention on Climate Change (UNFCCC) - often abbreviated to COP15.
Why is climate change happening - and is it the same as global warming?
The Earth's climate has always changed naturally over time.
For example, variability in our planet's orbit alters its distance from the Sun, which has given rise to major Ice Ages and intervening warmer periods.
According to the last IPCC report, it is more than 90% probable that humankind is largely responsible for modern-day climate change.
This produces carbon dioxide (CO2), which - added to the CO2 present naturally in the Earth's atmosphere - acts as a kind of blanket, trapping more of the Sun's energy and warming the Earth's surface.
Deforestation and processes that release other greenhouse gases such as methane also contribute.
Although the initial impact is a rise in average temperatures around the world - "global warming" - this also produces changes in rainfall patterns, rising sea levels, changes to the difference in temperatures between night and day, and so on.
This more complex set of disturbances has acquired the label "climate change" - sometimes more accurately called "anthropogenic (human-made) climate change".
Why is a new treaty needed?
The Copenhagen talks sit within the framework of the UNFCCC, established at the Rio de Janeiro Earth Summit in 1992.
In 1997, the UNFCCC spawned the Kyoto Protocol.
Denmark's Environment Minister Connie Hedegaard will chair COP15
In particular, the Kyoto Protocol's targets for reducing emissions apply only to a small set of countries and expire in 2012.
Governments want a new treaty that is bigger, bolder, wider-ranging and more sophisticated than the Kyoto agreement.
In June, the G8 and a number of large developing countries agreed that the average temperature rise since pre-industrial times should be limited to 2C (3.6F).
In principle, they are looking to the Copenhagen treaty to curb the growth in greenhouse gas emissions enough to keep the world within that limit.
Who is looking for what in the new treaty?
A lot of issues are involved.
Industrialised nations will set targets for reducing their greenhouse gas emissions in order to mitigate climate change.
The key date for these commitments is 2020, although some countries are looking beyond that, to 2050.
Australia, the EU, Japan and New Zealand have already said what they are prepared to do by 2020.
Richer developing countries are also likely to be asked to constrain their emissions.
If they do make any pledges, they are likely to restrain the growth of emissions rather than making actual cuts.
Their commitments are likely to be expressed in terms of a reduction in emissions growth of a certain percentage compared with "business as usual".
In order to help developing countries constrain their greenhouse gas emissions, industrialised nations have agreed in principle to help them in areas such as renewable energy.
Developing countries are looking for mechanisms that can speed up this technology transfer.
Many countries are thinking about how to prepare for the impacts of climate change - what sorts of adaptation will be necessary.
These include measures such as building sea defences, securing fresh water supplies and developing new crop varieties.
Developing countries are looking for substantial and reliable finance to help them adapt. Their argument is that as the industrialised world has caused the problem, it must pay to sort it out.
Measures to protect forests will be a component of the deal.
How much will it cost?
In general, fossil fuels provide us with our cheapest sources of energy.
The main route to reducing greenhouse gas emissions is to avoid burning fossil fuels; so a successful treaty would almost certainly make energy more expensive.
Developing countries are looking for money in the order of hundreds of billions of dollars each year for mitigation - the ballpark figure that the International Energy Agency calculates is necessary to fund a large-scale switch to low-carbon energy.
A number of studies, including one by the World Bank, also suggest that a further $100bn per year or thereabouts will be needed to help poorer countries adapt.
By comparison, the amount of overseas aid currently given each year by rich countries is in the region of $100bn.
What are the prospects for a deal?
Four broad outcomes are possible from the Copenhagen summit:
But in the weeks leading up to Copenhagen, it has become clear that a full, legally-binding treaty is not possible, with a number of key players suggesting something less ambitious is indicated given the immense amount of detail remaining to be worked out and the fact that the US is not in a position to make firm pledges on mitigating its own emissions or on providing financial support to the developing world.
A political agreement appears more likely, with attempts to secure a legally-binding treaty deferred until some point in 2010.
Would a Copenhagen deal solve climate change?
The global average temperature has already risen by about 0.7C since pre-industrial times.
In some parts of the world this is already having impacts - and a Copenhagen deal could not stop those impacts, although it could provide funding to help deal with some of the consequences.
Greenhouse gases such as CO2 stay in the atmosphere for decades; and concentrations are already high enough that further warming is almost inevitable.
Many analyses suggest an average rise of 1.5C since pre-industrial times is guaranteed.
A strong Copenhagen deal might keep the temperature rise under 2C; but given uncertainties in how the atmosphere and oceans respond to rising concentrations of greenhouse gases, it might not.
This is why developing countries put such an emphasis on adaptation, which they argue is necessary already.
IPCC figures suggest that to have a reasonable chance of avoiding 2C, global emissions would need to peak and start to decline within about 15-20 years.
Currently, the cuts pledged by industrialised nations are not enough to halt the overall global rise in emissions.
Whatever happens in Copenhagen, further meetings will almost certainly be necessary to finalise the "rules" of any new treaty.
Further ahead, at some point governments will almost certainly begin the process of securing the deal after Copenhagen.