So, what does this accord add up to? Key issues explained

The last-minute agreement is a major setback, and the world will have to regroup in its struggle to contain climate change

Is it true that the world has a new agreement to fight climate change?

Yes – the Copenhagen accord, signed at the UN climate conference in the Danish capital late on Friday night.

What does it do?

For the first time it enshrines the recognition of all the world's countries that we should work together to keep the global temperature from rising more than two degrees Centigrade above the level pertaining before the industrial revolution about 200 years ago, when we began burning fossil fuels on a seriously large scale. (It is the emissions of carbon dioxide from the coal and oil and gas we burn in power stations and cars, and also of the CO2 which comes from deforestation, that are trapping the sun's heat in the atmosphere – acting like the panes of a greenhouse – and causing world temperatures to rise.)

Two degrees above the pre-industrial level has come to be regarded as a sort of safety threshold, below which the effects of global warming may, with quite a lot of adapting, be bearable by human society and the natural world. But any rise above that the risks quickly rise of tremendously damaging new climatic effects, such as devastating droughts, fiercer hurricanes with more intense rainfall that will bring flooding on an entirely new scale, sea-level rise and the consequent disruption of communities around the globe. This in turn is likely to bring about mass migration of millions of climate refugees, and a new era of wars.

How long would it take to get to the C threshold, via the pathway we are on at the moment?

Nobody really knows the timescale – although it would almost certain be in the lifetime of people born today – but in temperature terms, two degrees above the pre-industrial is not that far away. The world as a whole has already warmed by about 0.75C, and it is estimated that the delayed effect of the carbon dioxide already in the atmosphere commits us to a warming of another 0.6C, whatever we do. So we are already on course for about 1.4C – this much of the target is already taken up.

Two degrees, it should be remembered, refers to the global average temperature, which will be more in higher latitudes such as the British Isles – perhaps over 3C, which is a very big rise. If you look out of your window this weekend on a snowbound landscape and wonder what all this global warming fuss is about, you should perhaps be reminded that, according to the Met Office, the average temperature in Britain has risen a full degree centigrade in the past 40 years – that is, just since the Beatles broke up. If the snow makes you think that is nonsense, wait till the spring comes – you will find that oak trees in southern England are opening their leaves on average 26 days earlier than they were in the halcyon days of John, Paul, George and Ringo, as our springs get warmer and warmer.

Is that all the Copenhagen accord does?

No. It also formally engages the developing countries, from the giants like China and India down, to do something about their rapidly rising CO2 emissions. This is an enormously important point. When, 20 years ago, the world first became aware of the threat of climate change and began trying to deal with it, the biggest CO2 emitters in the world, by far, were the rich, developed countries, led by the US. In 1990 the US, with 4 per cent of the world's population, was responsible for 36 per cent of global emissions.

But since then the Chinese economy has exploded, with growth rates never seen before in modern times of more than 10 per cent a year, and China's own carbon emissions have soared in a way no one imagined possible only a few years ago: In less than a decade they doubled from three billion to six billion tonnes annually, and two years ago China overtook the US as the world's biggest emitter.

India, as it struggles to bring its people out of poverty – hundreds of millions of them still have no electricity – is on a similar economic growth/emissions growth path, and so are Brazil, South Africa, Indonesia and others, and it is estimated that 90 per cent of all future growth of emissions will come from the developing countries. If these emissions continue to grow unchecked, climate change will be impossible to reverse.

Have the developing countries not been required to take action about their emissions before?

They have not, for two reasons. First, when the world began to deal with the threat of climate change, their emissions were very much less, and second, the vast majority of the carbon dioxide now in the atmosphere was put there by the developed countries – such as Britain. When the first international emissions-cutting agreement, the Kyoto protocol, was signed in 1997, it committed developed countries to taking legally binding actions to cut their carbon emissions, but did not require the developing countries to take on any cuts whatsoever.

Has the Kyoto protocol been a success?

Yes and no. It kick-started the huge, long and complex process of nations trying to turn their economies on to low-carbon growth paths, with the whole panoply of carbon-saving initiatives we are now so familiar with, from the construction of wind farms and the installation of solar panels, to the personal choice of taking the train rather than flying. And it introduced firm emissions-reduction targets for nearly 40 "Annexe 1" or developed countries, with the objective of cutting their emissions to 5 per cent below 1990 levels by about now.

But there are three glaring gaps in Kyoto. First, many of the developed nations have simply not acted decisively enough and have not met their targets. Second, in a decision of enormous consequence, President George W Bush withdrew the US from Kyoto in March 2001, shortly after assuming office. The third great gap is the absence of the developing nations, whose emissions are growing so fast that the world can no longer afford to ignore then.

So has the world decided to replace Kyoto?

Not quite. But a critical moment came almost three years ago, with the publication of the fourth report of the UN's Intergovernmental Panel on Climate Change (IPCC), which gave the most urgent warning yet about global warming's dangers.

So at the UN climate meeting in Bali in December 2007, it was decided to negotiate a new treaty which might bring America back into the fold – as the US was never going to rejoin anything with Kyoto in its name – and at the same time require developing countries to take actions of their own to reduce emissions, while committing the rich nations to adopt even tougher targets, of cutting by 25-40 per cent by 2020.

Yet because the developing countries were very attached to Kyoto, as it required them to do nothing while forcing the rich nations to cut their emissions, they did not want to abandon it – and negotiations for it to be renewed for several more years were set in train at the same time, and in parallel, to the negotiations for a new climate treaty (which were referred to as the "Bali road map).

This quite bizarre twin-track negotiating arrangement has been going on for the past two years and was due to come to a climax in Copenhagen, in the meeting which began a fortnight ago and ended yesterday when a new climate deal for the world – one new treaty? Two new treaties? – was due to be agreed.

Well, which was it? One or two?

They couldn't resolve it – even after talking for two years in the run-up to the meeting. It was remarkable. The European Union and the British government wanted a single new treaty, into which the basic elements of Kyoto could be incorporated and taken forward, but the developing countries, particularly the bloc known as the G77 plus China, resolutely refused to give Kyoto up or contemplate a single new agreement. It has become clear in the past 24 hours that much of this opposition was orchestrated by China, which was desperate not to have a single new treaty, which ultimately might make it, and other emerging economies, legally bound to take action on emissions.

So what happened?

Complete stalemate. By last Wednesday the negotiations between 192 countries had run into the ground, nothing of two years' work on a climate treaty was likely to be agreed – and the following day, 120 heads of state and government were arriving in Copenhagen to set the seal on the deal. So Gordon Brown, who got to Copenhagen a day ahead of every other leader, drafted, with his senior officials, a completely new text for an agreement that world leaders could sign on the spot. He got a key group of 26 countries to support the idea, and they began negotiations on it early on Friday morning.

In a full day of talks, the Chinese insisted on a number of key points being withdrawn. The opening statement that the world should strive to cut its carbon emissions by 50 per cent by 2050, a proposed timetable to make the new pact legally binding, and new short-term emission targets for all countries have been put off till next year, when they will be "listed" in an annexe to the accord.

But the Chinese did agree to have an emissions target in an international agreement for the first time, to international verification of their performance, and to the C threshold figure.

Is there anything else of note in the Copenhagen accord?

Yes, a new deal on climate finance. There will be $30bn of "fast start" funding over the next three years to help developing countries reduce emissions and adapt to global warming, plus a promise from developed countries to "mobilise" a climate fund for them of $100bn a year by 2020.

You may have seen the Copenhagen accord being criticised for being full of holes. It is, and its provisions are not remotely adequate to combat climate change, while all the work of the past two years on a new Kyoto/new treaty has been parked for another year. But at least complete collapse of the world's efforts to fight global warming was avoided last week (though it came very close) and the Copenhagen accord – last-minute, ad hoc, patched-up, full of holes as it is – at least gives the world a continuing way forward in the struggle to contain the greatest threat human society has ever known.

Countdown to 'Brokenhagen'

Monday 7 December

Dr Rajendra Pachauri, IPCC chair, says: "The evidence is overwhelming that delay would lead to costs becoming progressively higher."

Tuesday 8 December

Developing countries furious over leaked documents indicating world leaders will be asked to sign agreement handing more power to rich countries.

Wednesday 9 December

Tuvalu puts forward radical proposal to limit emissions and stabilise greenhouse gas concentrations. China and India block the move.

Thursday 10 December

Alliance of Small Island States says any deal allowing temperatures to rise by over 1.5C is "not negotiable".

Friday 11 December

Documents from summit chairmen call on developed nations to cut greenhouse gas emissions by 40% from 1990 levels by 2020. Pledges add up to about 18 per cent.

Saturday 12 December

Thousands march from Danish parliament to Bella Centre. Hundreds arrested.

Sunday 13 December

The Archbishop of Canterbury, Dr Rowan Williams, holds a service in Copenhagen Cathedral.

Monday 14 December

Talks temporarily suspended when teams from developing nations walk out.

Tuesday 15 December

High-level talks begin, with UN chief Ban Ki-moon telling nations to "seal a deal".

Wednesday 16 December

Pressure builds to overcome deadlock between rich and poor nations. Connie Hedegaard resigns; Danish PM Lars Rasmussen takes over proceedings.

Thursday 17 December

Hillary Clinton says US will "play its role" over funds for developing countries, but doesn't provide details. Talks make little progress

Friday 18 December

President Barack Obama warns that time is running out to strike a deal.

Saturday 19 December

Talks finally end, amid furore from climate campaigners, condemning the summit as "Brokenhagen".

Jonathan Owen

The deal in focus...

Barack Obama

US President

This progress did not come easily and we know this alone is not enough ... We've come a long way but we have much further to go.

Gordon Brown

Prime Minister

We were able to break the deadlock and – in a breakthrough never seen on this scale before – secure agreement from the international community.

Ban Ki-moon

UN Secretary-General

Finally we sealed a deal... The Copenhagen accord may not be everything everyone had hoped for, but this decision is an important beginning.

Lumumba Di-Aping

G77 spokesman

[The agreement is] devoid of any sense of responsibility or morality ... based on the same values that funnelled six million people in Europe into furnaces.

Ed Miliband

Secretary for State for Energy and Climate Change

We would have wanted a more comprehensive agreement, a legally binding one... I wanted a stronger agreement.

John Kerry

Democratic Senator

It's a powerful signal to see President Obama, Premier Wen, Prime Minister Singh and President Zuma agree on a meeting of the minds.

Kim Carstensen

Leader of WWF's global climate initiative

Well-meant but half-hearted pledges to protect our planet from dangerous climate change are simply not sufficient to address a crisis.

Guy Ryder

General secretary, International Trade Union Confederation

World leaders failed to overcome their differences. Commitments on greenhouse gas reductions have fallen short.

Caroline Lucas MEP

Leader, Green Party

We need to change the discourse around acting to prevent climate chaos. Politicians shouldn't be afraid of this – they should be promoting it.

Ashok Sinha

Director, Stop Climate Chaos Coalition

Confronted by the greatest danger that humanity has ever faced, our political leaders are trying to pass off a dismal declaration as progress.

Kevin Conrad

Executive director, Coalition of Rainforest Nations

REDD [Reducing Emissions from Deforestation and Degradation] gets punted along for another year.

Xie Zhenhua

Chief negotiator, Chinese delegation

The meeting has had a positive result. Everyone should be happy... After negotiations both sides have managed to preserve their bottom line.

Jose Manuel Barroso

European Commission President

A positive step but clearly below our ambitions. Others were much more influential than we were when it was the business of reducing ambition.

Jeremy Hobbs

Executive director, Oxfam International

The deal is a triumph of spin over substance. It recognises the need to keep warming below C but does not commit to do so.

Andy Atkins

Executive director, Friends of the Earth

A C rise would still mean the deaths of millions of people and the complete destruction of at least four low-lying island states.

Jonathon Porritt

Former chairman of the Government's Sustainable Development Commission

What we have seen is raw industrial power at its worst on the part of China, the US and other countries.

John Sauven

Executive director, Greenpeace UK

The city of Copenhagen is a crime scene with the guilty men and women fleeing to the airport. There are no targets for carbon cuts.

Angela Merkel

German Chancellor

The decision has been very difficult for me. We have taken one step; we have hoped for several more ... I view the outcome with mixed feelings.

Kevin Rudd

Australian Prime Minister

A significant agreement on climate change action. It is the first global agreement on climate change action between rich nations and poor countries.

Fredrik Reinfeldt

Swedish Prime Minister

Let's be honest – this is not a perfect agreement. It will not solve the climate threat to mankind.

Nicolas Sarkozy

French President

If we had no deal, that would mean that two countries as important as India and China would be liberated from any type of contract.

Mahmoud Ahmadinejad

Prime Minister of Iran

The economic and political structures of some nations are based on maximum profit and cheap energy. It is difficult for them to make changes.

Steve Sawyer

Secretary general, Global Wind Energy Council

A declaration like that doesn't do much other than paper over the fact that governments have failed to keep promises.

John Ashe

Chair of Kyoto protocol talks under the UN

Anything less than a legally binding and agreed outcome falls far short of the mark. Perhaps the bar was set too high.

David Nussbaum

Chief executive, WWF

After years of negotiations we have reached a declaration of will which binds no one and fails to guarantee a safe climate for future generations.

Professor Michael Grubb

Chair, Climate Strategies

Copenhagen was a missed opportunity but I believe an international legally binding agreement is very much still on the table.

Norbert Röttgen

German minister for environment, nature conservation and nuclear safety

It's not what we would have wished for as Europeans. China should have been more willing to accept a binding agreement.

Yvo de Boer

UN's chief climate official

The challenge is now to turn what we have agreed politically in Copenhagen into something real, measurable and verifiable.

Stephen Harper

Prime Minister, Canada

It is a good agreement that achieves Canada's objectives. It is a comprehensive and realistic agreement. It is a good first step.

Joan Ruddock

Energy and climate change minister

There's been far too much talk about process. But things have been improved. The presence of the ministers and the heads of state has concentrated minds.