Fuqiang Yang, director of global climate solutions, WWF International
The negotiations in Copenhagen ended without a fair, ambitious or legally binding treaty to reduce greenhouse gas emissions. Despite this, what emerged was an agreement that will, at the very least, cut greenhouse gases, set up an emissions verification system, and reduce deforestation. Given the complexity of the issue, this represents a step forward.
I hasten to add that much of the hard work still lies ahead. TheCopenhagen accord, the text that came out of the talks, leaves a long list of issues undecided. Among them are the emissions targets industrialised nations will accept, and how much climate finance they will offer.
The accord essentially allows countries to set their own greenhouse gas emissions reduction goals for 2020.
But I am optimistic, because the talks did achieve $100bn in aid from industrialised countries to poorer nations. China, as well, submitted to an emissions verification system under which all nations will report.
The accord also includes measures to help cut greenhouse gases and reduce deforestation, particularly in heavily forested developing nations such as Brazil and Indonesia.
These are big steps forward, and I think it is important to remember that there were achievements made in Copenhagen. There is still a great deal that needs to be done by China and all other signatories. Specific, binding targets are extremely important and need to be worked out. But we did see a move towards an agreement that could keep atmospheric Co2 levels from rising above dangerous levels.
I've read a lot about so-called Brokenhagen and the failure to get a legally binding agreement. Frankly we were never going to get one, just as we didn't get one at Kyoto, when I was negotiating for the EU.
What you need is a statement of principle. At Copenhagen this was a final admission that we cannot let temperature rise 2C above pre-industrial levels.And to get approval from 192 countries on this principle is remarkable, considering Kyoto dealt with only 47 nations.
The details and targets to meet that principle will be settled at COP16 in Mexico in 12 months' time. Until then, countries must show, as Ban Ki-Moon said, greater ambition to turn their backs on the path of least resistance.
Many of the countries have set out their own carbon action plans by 2020. So let's see them put those plans into action and put those figures in the annexes to the Copenhagen accord. The rest of the world will follow.
Copenhagen's achievements are an acceptance of the science (contested at Kyoto), an admission there will be global emission cuts, and an acceptance that there will have to be verification.
Martin Rees, president of the Royal Society, master of Trinity College, professor of cosmology and astrophysics, university of Cambridge
Plainly the outcome of Copenhagen was less than many hoped – but perhaps not substantially less than could be realistically expected. The involvement of India and China was clearly going to be crucial. But the grandstanding by particular nations (and the insistence by some on an unreasonable target of 1.5 degrees) was plainly unhelpful to the negotiations.
We in the UK should surely acclaim the constructive and committed role played by our government, and by Gordon Brown and Ed Miliband in particular, both in the lead-up to Copenhagen and during the frustrating and exhasting negotiations last week.
Next year, one hopes the US internal debate will evolve further, so Obama feels able to play a less muted role. Let's hope also that negotiations within groups of nations are carried forward. There is more hope of something being agreed among a group of up to 20 key nations (provided the group covers developing and developed countries), which others then sign up to.
And to be positive, the Copenhagen meeting, circus though it was, carried the process forward. For instance, it stimulated pledges of funding from developed nations (albeit, not as firmly as might have been hoped) and made progress on forestry. And it maintained global long-term concerns about climate change on the international agenda.
Bryony Worthington, climate campaigner with sandbag.org, who helped draft the UK climate change bill
Copenhagen was a spectacular failure on many levels. The UN process was stretched to breaking-point, with no consensus on any pressing issues.
The accord that was signed was clearly designed to meet the needs of the US, who always wanted a voluntary "pledge and review later" type agreement with minimum enforcement.
The sums of money agreed to help developing nations adapt to climate change are so low as to be insulting.
The future of the major mechanism driving private capital into solutions, the carbon market, has been left with a question mark over its future, and the long-anticipated agreement on stopping deforestation lacked clarity.
What happens next? The most honest answer would be to accept that under the current arrangements consensus will not be reached.
We have to focus on domestic action in big fossil-fuelled economies: the US, China, and Europe. All three have made pledges about their intentions to act – each has the opportunity to introduce policies which will create huge markets in climate solutions. If they lead, these solutions will become available for use in all parts of the world, with the costs of development having been born by those most able to pay.
Look at the history of environment negotiations – take the ozone ones as the best example. People start off negotiating very hard and the first agreement does nothing but moderate the problem.
While the Montreal protocol was ultimately a huge triumph, it made an infinitesimally small difference at first. It took them four amendments to get from reduction to a ban [on CFCs], a process of 20 years after science identified the problem.
Carbon and climate change are much more complicated, and we're just getting to that 20-year mark now. Anyone expecting a definitive solution to the problem on timescales any shorter than that is extremely optimistic.
It's not an event, it's a process. I guarantee that the decisions we will be making in 2050 will not be the ones made in Copenhagen.
Copenhagen did show some improvement in the process. People are now talking about changes in greenhouse gas emissions that are commensurate with the size of the problem. Before, they weren't.
People are now seeing the problem for the challenge that it really is. But, in seeing that challenge, it makes the process – because that challenge is very large.
Kumi Naidoo, executive director, Greenpeace International
The outcome of the summit was not fair, ambitious or legally binding. This eluded world leaders because they put national economic self-interests, as well as those of climate polluting industries, before protecting the climate.
Even if all countries reach their pledges, our planet will be propelled towards a 4C temperature rise, double what leaders say they must achieve. This will have devastating climate impacts, including crop failures and the disappearance of the Amazon rainforest and the Great Barrier Reef.
With each month of delay in getting a real climate deal, the chances of the world staying below a 2C rise slips further away, and the cost to this and the next generation in tackling climate change increases.
To avoid this, industrialised countries as a group – which bear historic responsibility for the problem – must make the largest emission cuts. They also need to provide at least $140bn a year to help developing countries.
The non-result from Copenhagen calls into question the ability of leaders to deliver what is needed. Citizens around the world will need to elect more ambitious leaders and embrace new, low impact technologies.
Vicky Pope, head of climate change advice at the Met Office
At previous meetings in the runup to Copenhagen, in Barcelona and elsewhere, there was talk about greenhouse gas targets for 2020 and 2050; it is disappointing that those have been lost, but it is good that everyone accepted the scientific reality that climate change is a problem and that we need to limit warming to 2C.
The accord is fairly weak, and we will only know how effective it will be when countries fill in the table that details their targets to reduce emissions (they have until the end of January to do so).
Only when we have those targets and we can add them up to see the scale of cuts will we be able to properly judge what has been achieved. It is a positive thing that finance is included, as that could help to make things happen.
Going forward, the first thing that needs to happen is that the table of targets needs to be filled in. Then the whole agreement needs to be made legally binding.
Nicholas Stern, chair, Grantham research institute on climate change and the environment, London School of Economics and Political Science
The Copenhagen meeting was a disappointment, primarily because it failed to set the basic targets for reducing global annual emissions of greenhouse gases from now up to 2050, and did not secure commitments from countries to meet these targets collectively.
Nevertheless, the road to Copenhagen and the summit itself generated commitments on emissions reductions from many countries, including, for the first time, from the world's two largest emitters, China and the US. The Copenhagen accord also did recognise that a rise in global average temperature should be limited to below 2C.
In addition, the prime minister of Ethiopia, Meles Zenawi, speaking for the African Union, put forward a very important proposal on financial support, much of which is reflected in the Copenhagen accord, including the creation of the Copenhagen green climate fund to administer funding for developing countries.
The current UN framework convention on climate change process has been found wanting over the past few weeks.
One potential way forward is for Mexico, as hosts of COP16 (the next full summit) in 2010, to convene a group of 20 representative nations, as Friends of the Chair, to work on a potential treaty and tackle the outstanding issues and building consensus around strong action. The group should start its work immediately.
Dr Myles Allen, head of climate dynamics group in the atmospheric, oceanic and planetary physics department, University of Oxford
On one level, it could be argued it is quite a good outcome.
There is a goal to limit global temperature rise to 2C and an acknowledgement that current commitments are not enough to meet that goal. It is good that China recognises the 2C goal and that emissions reductions are the way to go.
I am glad they did not make serious progress towards a legally binding treaty, because the current thinking that nationally negotiated emissions targets and a system of carbon trading will solve this problem is flawed. I'm very sceptical about that whole approach.
A legally binding regime based on that principle would lock us into that process, and it could take 20 or 30 years before it became sufficiently obvious it was not working. Once set up, there is enormous investment in a system like that and it becomes difficult to change. So something close to success in Copenhagen based on what the politicians were aiming for could have been counterproductive.
It's depressing that governments appear to have walked away from Copenhagen only to say they are going to spend the next year fighting for the legally binding treaty they wanted it to produce, rather than use the time to consider some radical alternatives.
One way we have suggested is to target producers rather than emitters. A mandatory requirement on fossil fuel companies to capture and storecarbon emissions, to clean up after themselves, could solve a big part of the problem without complex international negotiations.
Bernarditas de Castro Muller, former lead negotiator for the G77 plus China group of developing countries
What was achieved in Copenhagen? The Copenhagen accord contains what was possibly the most that the leaders of the world's biggest countries could give in terms of actions to address climate change.
However, there are problems with the document as it stands. The main one is the process pursued to reach this agreement, which completely undermined the cardinal rule of multilateralism in international negotiations, and that is transparency and inclusiveness.
The final session and the mishandling of the process by the Danish presidency delivered the knockout blow to any meaningful agreement. That this travesty should take place before the eyes of the main guardian of multilateralism, the UN secretary-general, only added to the irony of the tragic situation.
But the worth of the "deal" (I actually prefer the word "accord"; "deal" sounds like some sleazy business plot) lies in laying out clearly what each of the major countries could live with in terms of addressing climate change. In my opinion, it is still inadequate insofar as developed countries' commitments to reduce emissions are concerned. However, we are always told to take into account the "political realities" of rich countries. I revolt against this, but have to live with it, and put aside our own political realities in the developing world, which have to do with basic necessities and even survival itself.
Where do we go from here? We could take the accord as some kind of political guidance from the leaders of major countries. We are now clear on where the major groups stand. It is now up to negotiators to come up with universally agreed next steps.
Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change
I think there are three major achievements that could be listed at Copenhagen: • The acceptance of a 2C limit for temperature increase, and reference to the scientific basis for doing so. This indicates that science has finally had an influence on negotiators defining what would represent dangerous anthropogenic interference with the climate system. • An agreement was reached between the so-called Basic countries [Brazil, South Africa, China and India] and the US on a tricky issue, which had become a bone of contention particularly between the US and China. • A sum of $30bn has been included in the agreement for funding developing countries' actions during the period 2010 to 12.
Is the agreement worth anything? The accord would be worth something only if we build on it with a sense of urgency and take it forward towards a binding agreement by the end of next year.
The next step is that the negotiators, and particularly the leaders of major countries, must now get into action to see that we come up with an inclusive agreement involving all the countries of the world. This would require early convening of some meetings under the Conference of the Parties, and a timetable for specific outcomes to be achieved before Mexico.