A critical 12 months in the battle against rising temperatures begins in Madrid this week, as UN delegates gather for key talks.
The 25th Conference of the Parties, or COP, will see negotiators from almost 200 countries in attendance.
Ahead of the meeting the UN secretary general has warned that the world is at the point of no return.
António Guterres said the global response to date has been "utterly inadequate".
The conference takes place amid a welter of bad news on climate change in recent days.
The World Meteorological Organisation announced that greenhouse gas concentrations reached their highest recorded level in 2018.
The UN Environment Programme showed that there's a huge gap between the plans that governments currently have on the table to cut emissions and what's needed to keep under 1.5C. Keeping to that guardrail will need a five-fold increase in the carbon cutting ambitions of countries.
The UN Secretary General warned delegates ahead of the meeting "the point of no return was no longer over the horizon".
"We simply have to stop digging and drilling and take advantage of the vast possibilities offered by renewable energy and nature-based solutions," Mr Guterres said.
As well as demanding that the negotiators increase their level of carbon cutting ambition at this meeting, Mr Guterres announced that the Governor of the Bank of England Mark Carney will take on the role of UN Special Envoy on climate action and climate finance.
Wasn't this meeting supposed to be in Chile?
Yes, this annual event, the Conference of the Parties or COP was due to take place in the Chilean capital Santiago this year. It was cancelled by President Sebastián Piñera due to ongoing civil unrest in the country.
After a brief flurry of diplomatic activity, Spain said they would step into the void and host the conference, with Chile still leading the diplomatic negotiations.
The Spanish argue that it is critical to support a UN process that depends on global co-operation in the face of rising nationalism around the world.
"COP25 will reaffirm that multilateralism is the best tool to solve global challenges such as climate change," said Spain's minister for the ecological transition Teresa Ribera.
"Neither the UN nor the international community have let the climate agenda fall, despite the challenges to organise this event, because this is a vital moment to drive implementation and action. Spain immediately offered to organise the summit in record time. There is no turning back."
What will this gathering achieve?
The hope is that this meeting will concentrate the minds of international diplomats on the huge scale of the challenge.
Governments have promised to update their climate pledges by 2020, when the COP will be held in Glasgow.
But so far, despite the urgings of scientists, major improvements in pledges have been slow to materialise.
Many nations have aspirations to carbon neutrality in the long term, but they have been slow to put specific short-term commitments on the table.
"Some 70 countries have pledged to become carbon neutral by 2050, this must be carried on at Madrid COP," said Sonam Wangdi, the Chair of the Least Developed Countries (LDC) group in UN climate change negotiations.
"There must be an agreement among us all to do our fair share. If it doesn't happen in Madrid it could be too late for 2020 pledges."
The hope for Madrid is that the meeting can avoid major bust ups and keep edging forward.
It also has to overcome two possible banana skins - loss and damage, and carbon markets.
What is loss and damage, and why is it important?
This issue has dogged the negotiations for several years now, but the likelihood is that it will come to a head in Madrid.
Loss and damage are the impacts that can't be prevented or adapted to by countries.
Some experts consider "loss" to apply to the complete destruction of something such as human lives, habitats and species. "Damage" refers to something that can be repaired, such as roads or buildings.
So the examples that are given are rising sea levels which can't be prevented, or storms that are connected to rising temperatures.
Back in 2013, under pressure from developing countries, the climate talks set up a special forum to discuss loss and damage. In Madrid the delegates must decide how to progress. Poorer nations want the loss and damage to have teeth within the UN setup, and more importantly, funding.
"Everybody has to recognise that there is a need and then there must be a funding window," said Sonam Wangdi from the LDCs.
"Once you have that, where the funding comes from is secondary, right now there is no fund."
Rich countries fear that the whole question is a way of tying them into paying out for sea level rise and storms for centuries ahead, because the bulk of the carbon in the atmosphere comes from fossil fuels used by the developed world.
As the conference starts, 150 environmental groups including climate activists Naomi Klein and Lidy Nacpil have written to ministers calling for adequate funding for loss and damage.
They say the combination of climate disasters and debt can prove toxic for developing nations.
"The climate crisis has been causing death, despair and displacement in the global south," said Harjeet Singh from Action Aid.
"This bullying of the countries hardest hit by climate change, by those that got rich from extracting and consuming fossil fuels, must end now."
What about carbon markets - a load of hot air?
Hot air is in fact one of the big concerns with the question of carbon markets.
In the past richer countries have often paid for carbon reduction projects in poorer nations.
The wealthy have then been able to count the carbon saved from these projects against their own emissions.
These schemes were discredited amid accusations of fraud and "double counting" where both the poor and the rich countries counted the same emissions reduction as part of their plans.
Article six of the Paris agreement set out to reform these carbon markets, recognising that if they were transparent and effective they could really help to raise ambitions.
Discussions on how the new arrangements would work were due to be completed in Katowice at the COP last year but they ran into real problems. Brazil resisted all attempts to curtail double counting. Other countries wanted to carry forward carbon credits from older schemes.
Some also want to be able to sell or carry forward credits if they overachieve on their existing carbon cutting plans, which observers feel would encourage countries to set a low bar in terms of commitments.
Experts often call these types of credits "hot air" as they are more an accounting exercise than a real reduction in carbon dioxide.
The amount of "hot air" is huge, running into billions of tonnes of carbon. Experts fear that these could undermine the integrity of the Paris pact if they are allowed to continue.
"We believe that these markets will have an impact but they must result in real reductions on the ground," said Sonam Wangdi from the LDCs.
"The option is needed and the carbon market is one of the tools - but there needs to be environmental integrity and they need to be transparent and there needs to be real reductions there."
Why does Madrid matter if the real deadline is 2020?
Trying to get unanimous agreement between almost 200 countries on how to tackle climate change is a really big ask. The agreement that was struck in Paris in 2015 only came about after six long years of snail pace negotiations.
It was the deal that diplomats had hoped to strike in the failed Copenhagen COP in 2009.
So if the goal is that countries have new promises in place by the end of 2020, Madrid is an important snapshot of what can realistically be achieved.
Countries often tend to hold back on their pledges until they see what others are likely to do. Madrid will give a sense of whether there is a willingness from some of the larger countries, like India, China and the EU, to show leadership.
"After 30 years of advocacy and optimism, we see COP 25 as the last opportunity to take decisive action," said Ambassador Janine Felson from Belize, the deputy chair of the Alliance of Small Island States.
"Anything short of a vastly greater commitment to emission reduction through new national plans that are consistent with our fight for a 1.5 degree world, greater momentum towards honouring outstanding 2020 climate finance commitments, a new climate finance goal suitable for achieving a 1.5 degree world and tangible support for disaster risk reduction and reconstruction in small island and developing states will signal a willingness to accept catastrophe."
What about the US - will they play a role?
This could be the last year in which a US team will play a part in the negotiations. On 4 November President Trump sent a formal letter to the UN, which has triggered the 12-month countdown to the US pullout.
The Americans are due to leave on 4 November 2020, one day after the next US presidential election and five days ahead of the critical COP26 in Glasgow.
The US has been playing a more truculent role in recent negotiations, joining with Saudi Arabia, Kuwait and Russia to prevent the conference welcoming a key IPCC report on how the world can keep temperatures under 1.5C this century.
Over the past couple of years the US has also supported side events promoting coal and will likely continue to do so in the future.
Even if they do withdraw completely next year, that will only be from the Paris agreement part of the negotiations. The US will still be party to the UN climate convention. It is unlikely they will stop sending teams to the conferences.
What about Greta Thunberg - will she make it in time?
Just a year ago, Greta Thunberg attended the Katowice COP as a relatively unknown Swedish student who was taking direct action in striking from school for the climate. A year later and she has become a global icon who can get a standing ovation from diplomats by calling out their hypocrisy on rising temperatures.
Greta's dedication to the cause has been enhanced by her decision to cross the Atlantic in a yacht to attend the Santiago meeting. Now she is on another boat on her way back to Madrid. She is due to arrive a few days after the start. Her participation and her speech will likely make headlines around the world.
Will the meeting give a voice to climate strikers and young people?
Conferences like the COP are rooted in a traditional UN diplomatic that requires a unanimous agreement on steps forward. While environmental campaigners and others can observe, there is limited input from young people, school strikers and other voices.
Nordic countries are attempting to do something different this year with ministers from Sweden, Finland and Iceland inviting five young people from different countries to take part in discussions with politicians and report back from COP25.
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