A summit, which begins today, will see 190 nations convene in Paris to try and work out a way to tackle climate change.
Here is what you need to know about it:
What is the Paris meeting?
On November 30, 190 nations will gather for two weeks to try and work out a deal to tackle climate change.
The aim will be to find a way to reduce the global emissions of greenhouse gases - such as carbon dioxide and methane - over the coming decades to avoid the world’s average temperature increasing by no more than 2C by the end of this century.
That’s compared to the world’s temperature before the industrial revolution around 150 years ago, when parts of the world began the mass burning of fossil fuels.
Go above that rise in temperature, say scientists, and we tip over into “dangerous climate change” where there will be drastic increases in floods, storms, heatwaves and other catastrophic and irreversible environmental changes around the world.
These will disproportionately affect the poorest parts of the world, those least able to adapt.
At the moment, if the world does nothing, it is on course for a 5C temperature rise by 2100.
This might not seem like much, but bear in mind that the average difference between now and the Earth’s previous Ice Age was around 5C.
Small average temperature rises have enormous consequences.
Who is organising the talks?
That United Nations Framework Convention on Climate Change (UNFCCC), which was created at a meeting of world governments in Rio de Janeiro in 1992.
At that time, governments agreed that they needed to do something about climate change.
But it took until the 1997 Kyoto Protocol to put some detail onto the plan - a relatively small 5% reduction in greenhouse gas emissions relative to a their 1990 levels, by 2012.
Developing countries such as China and Mexico were exempt from the targets.
The Kyoto Protocol established an idea but it wasn’t too successful because the US didn’t sign up at the time and so it could not be ratified.
To be more precise, Vice President Al Gore signed the deal but the US Congress refused to ratify it.
That failure stalled the climate-negotiation process for many years.
What happened in Copenhagen in 2009?
Infamously, this was a disaster.
The world’s governments came together to agree legally-binding targets to reduce emissions.
The good news was that all the developed countries and many developing ones did agree to the goal, but the meeting itself fell apart at the end and the conference ended with no legal requirement for anyone to do anything.
For many years after Copenhagen, the whole topic of climate change seemed to exhaust people and the negotiations fell into a lull.
Why is Paris 2015 different?
The UN has learned the lessons of the Copenhagen shambles.
There are two specific reasons to be more optimistic this time around.
First, every country has been asked to submit proposals ahead of time for what they are prepared to do to cut their greenhouse gas emissions.
So far, 146 countries (which account for 90% of the world’s emissions) have so far submitted these Intended Nationally Determined Contributions (INDCs).
At the start of November, the UNFCCC aggregated these pledges and showed that, if countries followed through on their promises, the world’s temperature would rise by only 2.7C by 2100.
How far off the 2C world target are we?
There's still some way to go, but a great deal closer than the world has been before.
Now everyone knows the starting point and how much work there is left to do.
In essence, last week’s UNFCCC report started the firing gun for the Paris talks.
The second reason for optimism is a new political reality in which the US and China (the world’s two biggest emitters of greenhouse gases) are taking a lead in their will to tackle climate change.
They signed a bilateral agreement to cut emissions last year and President Barack Obama has been making positive speeches and enacting pro-climate policy in his country for much of the last year.
With that leadership, may other nations have fallen into line.
How does the UK fare?
We negotiate as part of the European Union, which has always been more progressive than other parts of the world on climate change.
The EU used to lead the way on the negotiations but that lead has now largely slipped to the US and China.
Is there still a chance of reaching 2C by 2100, given that the current pledges for Paris only reach 2.7C?
Absolutely. While there might be some wiggle room, countries are unlikely to change much from their INDCs at Paris.
But, the crucial thing is that the world doesn’t need to agree on global commitments to cap temperatures to 2C all in one go at Paris.
Instead, watch out for a deal where countries commit to their current INDCs and also to a review process where they agree to monitor and ratchet up their commitments over the coming decades.
So even if countries don’t get to a 2C agreement in Paris, they could still put in place a mechanism to achieve it in the years to come.
Who are the important players this time?
With most major emitters falling into line around what they need to do, all eyes in Paris will be on India.
A huge country of 1.3 billion people (and growing), with almost 400 million still without electricity, Prime Minister Narendra Modi has ambitious plans to develop his country economically and socially.
He wants to follow the path China has taken in the past few decades to bring hundreds of millions of his citizens out of poverty.
To build his country, however, he will need to use copious amounts of power.
On the one hand, India has huge reserves of coal, which is cheap but also the dirtiest fossil fuel.
Modi intends to burn that coal in power stations and factories since it is his fastest and easiest route to development.
Doing that, however, could largely negate any attempts by other countries to bring global emissions down.
What is happening in India now?
Modi himself is progressive on the environment and his country is already feeling the direct impacts of climate change such as lethal heatwaves, the failure of agriculture, droughts in some parts and floods in others.
India’s INDC was also impressive to many environmentalists, committing the country to sourcing 40% of its energy from renewable or low-carbon sources by 2030.
But unlike many other countries, India did not announce a target to peak its emissions, arguing that it would not accept any attempts to limit its development.
India will make-or-break a deal in Paris and no-one has worked out yet what it will sign up to.
Will it go ahead, gung-ho, with its plans to burn all its coal?
What kind of deal might it accept to prevent that from having its disastrous effects?
For these reasons, India represents the arguments of many other developing nations and will be a standard-bearer for them.
They argue that climate change has been caused by the developed world, (and they are, of course, right)
What should richer nations do?
India thinks the developed world should therefore shoulder the biggest burden in solving the problem.
Much of the Paris negotiation will centre around how those so-called “common but differentiated goals” will play out.
It could mean rich nations such as the US and EU accept deeper emissions cuts, so that India and others can continue to emit more.
It could mean rich nations provide money or clean technology to developing nations, in order to persuade them to develop more sustainably.
Is the climate saved if Paris is a success?
The work just begins.
If world leaders agree a deal, they will still have to go back to their national parliaments in order to ratify and enact laws in their countries to make the commitments actually happen.
And any agreement will only bind countries to 2030.
The climate change issue will clearly take longer than that to solve, but the meeting in Paris could make a significant, historic dent in the problem.