World leaders and delegates from more than 190 nations are gathering in Paris from 30 November to 11 December for a UN summit on climate change. Billed as the most important climate talks ever, the goal is to agree an international deal to cut carbon emissions that will help the world avoid catastrophic global warming. Here’s what you need to know about COP21.

What is COP21?

Last December, representatives from 190 countries met in Lima, Peru, for the 20th Conference of the Parties (COP20), where they put in place a framework for a new global climate agreement to follow one year later. The same nations are now reconvening in Le Bourget, just outside the French capital, to thrash out the deal to limit global warming to two degrees Celsius.

Who will be there?

Monday’s opening ceremony will be well attended, with US President Barack Obama, Chinese President Xi Jinping, Indian Prime Minister Narendra Modi and French President François Hollande among heads of state expected to call for global action to curb emissions.

About 20,000 people, including business leaders, climate change activists and journalists, are expected to be at the talks, with thousands more descending on Paris for a series of events happening on the sidelines.

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What will they discuss?

Countries will be looking at ways to prevent global temperatures from exceeding two degrees Celsius over pre-industrial times by the year 2100. Getting there will be tough as different nations disagree on a number of issues.

What are the main sticking points?

Money: Developing countries’ climate action plans depend on them receiving financial support from the international community. Developed nations have pledged to come up with $100 billion of climate finance a year by 2020 to help developing nations reduce their emissions and cope with the worst effects of global warming. This would include public and private finance. While developing nations want firm long-term funding commitments beyond 2020, Europe, the United States and other developed nations do not want to guarantee higher sums of money. Developing countries have also raised doubts over OECD estimates that climate finance already reached $62 billion in 2014.

A long-term goal: The G7 leaders recently set a target of a decarbonisation of the world economy this century. This is too vague for some developing nations and climate activists, who want fossil fuels phased out by 2050. Coal-dependent China and India are among countries reluctant to set clear targets for decarbonisation.

Loss and damage: How to pay for the loss and damage from disasters caused by climate change such as typhoons, droughts and flooding is one of the most contentious issues at the summit. Vulnerable countries want a long-term mechanism to help them cope with loss and damage from disasters such as typhoons or the impacts of a creeping rise of sea levels, but developed nations do not want to be locked into agreeing “compensation” payments.

A legally-binding agreement: Nations agreed in 2011 that the Paris deal will have some form of “legal force”. They left open whether that meant a treaty under international law or a looser deal anchored in each nation’s domestic laws.

Many developing nations and the European Union favour a binding treaty, or protocol. But the United States and China, among those reluctant to sign up for international oversight, prefer an accord based on domestic laws and regulations.

What are INDCs?

Ahead of the summit, each country was supposed to map out exactly how it would contribute to the efforts to limit global warming. A review of the Intended Nationally Determined Contributions (INDCs) submitted up to October suggests that the countries’ commitments may not be enough to meet the two degree goal.

Why 2 degrees?

If temperatures rise more than 2 degrees Celsius above what they were before the industrial revolution, it could lead to catastrophic and irreversible climate change, according to the Intergovernmental Panel on Climate Change (IPCC).

On our current trajectory, the world is set for a rise of nearly 5 degrees. This would mean more extreme weather events such as storms, typhoons, droughts and floods, which would raise the risks to communities and the costs of recovery. Increased ocean temperatures and melting global ice would also push coastlines inland and lead to the loss of low-lying islands.

Even 2 degrees would result in a sea level rise that threatens low-lying territories. A bloc of almost 40 nations is pushing for the threshold to be lowered to 1.5 degrees Celsius to reduce the risks to their territory.

How important is COP21?

An agreement in Paris could be the world’s best chance of achieving global action on climate change before it is too late. The UN’s climate chief Christiana Figueres warned last year: “This is it, my friends,” she said. “I guarantee you, if we do not succeed in 2015 … it will take 10 years to get everyone around the table again.”

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Author: Rosamond Hutt is a Senior Producer at Formative Content.

Image: The Eiffel Tower is seen at sunset in Paris, France, November 22, 2015. The capital will host the World Climate Change Conference 2015 (COP21) from November 30 to December 11. REUTERS/Charles Platiau