Cosmologist and astrophysicist, president of the Royal Society, Astronomer Royal, Master of Trinity College, Cambridge
There are two key facts relating to our future climate – neither controversial. First, the measured CO2 concentration is rising: it's already higher than it's been for millions of years, and "business as usual" will lift it to twice the pre-industrial level by 2050. Second, it's also uncontroversial that carbon dioxide is a greenhouse gas – this was recognised by Sir John Tyndall 150 years ago.
Those two facts alone would in my view justify precautionary action, even though it's still somewhat uncertain just how sensitive the climate is to the CO2 level, and what parts of the world will be affected most. What should make us especially anxious is the significant probability of a really drastic climatic shift, triggering a grave and irreversible global trend: rising sea levels due to the melting of Greenland's icecap, runaway release of methane in the tundra, and so forth.
But the science, though intricate, is a doddle compared to the politics and economics of climate change – which require worldwide consensus and a focus on a far remoter time-horizon than is usual in politics.
The UK consumes only around 2 per cent of the world's energy. But we can exert disproportionately larger leverage on the world's climate in two ways. First, by innovation: the UK has the expertise to spearhead some of the technologies the world needs, and it's in our economic interest to do so. Research and development on energy are still far lower than the challenge demands. Second: the UK's strong record on climate issues could give us leverage in Copenhagen. Let's hope that Copenhagen at least sets us on the path to achieve this.
Further delay will reduce the chance of achieving the 2050 targets, and aggravate the risk of really drastic and irreversible changes. Any deal that's equitable and realistic will involve a transfer of funds from the developed to the fast-developing nations.
Gordon Brown was right to say that, if nations choose not to agree a deal, "no retrospective future global agreement can undo that choice".
Sir David King
Director of the Smith School of Enterprise and the Environment at Oxford University, former chief scientific adviser and head of the Government Office of Science (2000-2007)
Climate change is simply the most serious issue our civilisation has ever had to face up to collectively and it's a problem we have very little time to manage. It's too late to avoid the dangerous affects of the C02 we've put in the atmosphere, and it's now a question of how well we can limit the impact of climate change on future generations.
The main thing we human beings need to do is recognise the importance of dealing with this and that dealing with it is not such a big issue once we've taken it onboard. We're currently emitting about 36 billion tonnes of C02 equivalent per annum (adding in all greenhouse gases) and by mid-century we will need to bring that down to about 18 billion tons. We'll be 9 billion people by then, so that's about two tons per person. In Britain now, we're emitting about 11 tons per person; in India it's just approaching two. Now that sounds problematic for us, because we don't want our economy to go in the direction of India, but the good news is that we can do this without any damage to our economy. It's simply a matter of switching over to alternative energy supplies and increasing energy efficiency processes over the next 40 years. The world is spending $1,700bn (£1,020bn) a year on oil from the Gulf states. If we used that money to invest in alternative energy suppliers we could more than manage the problem.
There are three main drivers for change: new low-carbon technologies; changing consumer behaviour patterns; and government regulation and global regulatory systems. I think consumers are a critical part of this process. They must want to buy goods that are low greenhouse gas emitters. The reason why the Toyota Prius is so popular and the Humvee is so unpopular is down to consumer attitudes – it's our choice.
Managing the switch from a high-carbon to a low-carbon economy means that we have to look at how we do it most efficiently, securely and relatively cheaply. In the next 30 to 40 years, the right way to do it is to have something like 40 per cent of maximum demand of energy from nuclear power. The low-demand times in the middle of the summer would be met by nuclear power so you're only switching in, say, gas-fired power stations at peak demand in winter. That massively reduces the amount of carbon emissions. New domestic buildings will eventually (by 2020) have no energy needs from the grid: they'll be generating all of their energy requirements from their surroundings. We're going to see more electricity demand from cars and much less from buildings. So we can rapidly manage this problem, but in order to do so we will need large-scale power stations.
Professor of physics at the University of Illinois; winner of the 2003 Nobel Prize in Physics
I think the most worrying aspect of the climate change issue is that we do not really know how worrying it is. While I believe there is substantial evidence that global warming is a reality, that a large part of it is due to greenhouse gas accumulation and that this last is primarily of anthropogenic origin, I do not believe that any of these propositions is established to the point where attempts to argue the contrary case should be automatically dismissed as disingenuous or self-serving. Worse, even given that all are true, we really have little idea even of the order of magnitude of the likely long-term consequences.
In this situation I believe it would be irresponsible to use our lack of firm knowledge as an excuse not to take at least those actions towards mitigating global warming that have no obvious downside. There are quite a few of these that we could implement right away. For example, one obvious act is simply to lower the heat in private and public buildings. Such savings may sound trivial, but if implemented throughout the major industrialised countries they would result in a substantial reduction in greenhouse gas emissions.
Another area where I believe that scientific advances have a real chance of permitting substantial savings in greenhouse gas generation is the National Grid. At present, a very significant fraction of the total electrical energy generated in North America, at least, is dissipated in transmission, and this fraction is likely to increase as a result of increased reliance on nuclear power and/or renewable sources. Much of this loss could be avoided with the use of superconducting transmission lines, either made of currently known cuprate superconductors with improved characteristics or, better still, of currently undiscovered materials that might be superconducting at ambient temperatures.
Scientist, researcher, author, inventor and originator of the "Gaia" theory (which considers the Earth as a single living organism)
I don't think anyone really knows how serious the climate change issue is. It is true that the level of carbon dioxide in the atmosphere is now unprecedented and still rising. But the Intergovernmental Panel on Climate Change, which is probably our best prediction source, is still failing to forecast the rise of sea level or the extent of ice melting at the poles – so how can we be sure about the climate 50 years from now?
We should concentrate on energy saving, rather than spending on renewable energy, which is inefficient and expensive and often does little to reduce CO2 emissions. Houses are notoriously energy inefficient. If we concentrated on increased efficiency instead of the hopeless attempt to obtain 20 per cent of our energy from renewables, people would save money, and it would be a better way to tackle global heating.
I don't think the Copenhagen conference will achieve a lot. Essentially it's a political exercise, because, in truth, it's a bit too late. Processes are already under way, such as the melting of the permafrost in Canada and Siberia, releasing greenhouse gases. Things like that make me doubt that we can do much to turn back global heating. Maybe if we'd started back in the Sixties we could have done better.
Former editor of The Ecologist and Conservative parliamentary candidate
Even without climate change, our population is growing, while resources are being depleted. All rational people know that without a major shift, we are going to hit a wall, and yet that terrifying truth has almost no bearing on actual policy decisions. Sooner or later, this is going to have to change. I would like to see the establishment of tough targets for emissions reductions, mechanisms for helping poorer countries adapt and shift, and a formula for putting real value on the services provided by forests so that they are worth more to forest nations standing than destroyed.
Campaigner, writer, sustainability adviser and Green Party candidate
People sometimes ask: "Has there been any other threat in the past that is comparable to climate change?" Probably not, but the closest comparison is possibly the Second World War. If you look at what happened in the UK around 1938 to 1939, and in the US in the early 1940s – the way that the peacetime economy was so quickly able to shift to the war-time economy, and the successful alignment of public opinion with the need for change – this was brought about with clear and strong leadership by Winston Churchill and Franklin D Roosevelt. The challenge now is to take a long critical look at the economy and our consumerist culture, and to break the cycle of belief that more consumption gives a higher quality of living. For me that's the cutting edge of it – economic and cultural change. But we need decisive political leadership in order for change to occur fast enough.
Senior fellow emeritus at the Policy Studies Institute
The only solution to this problem is for governments at Copenhagen to sign up to the Global Commons Institute's contraction and convergence framework by agreeing to impose tradeable personal carbon rationing – whereby individuals are required to live within an equitable, and therefore very low, share of the planet's capacity to safely absorb further greenhouse gas emissions.
Director of the Science Museum
Our modern world, our agriculture, our water supply, have all been constructed based on the climate we've got, which happens to have been unusually stable for the past 10,000 years. And although it would be possible to adapt to changes, the threat is that they will happen too quickly for us to keep up. What worries me most is the issue of food supply, which is made worse by the growing population.
All of us in the developed world are completely reliant on our current hydrocarbon-based energy supply. So the first issue is to make sure we become less reliant. It is as if the 192 nations at Copenhagen were sitting around a table on which is piled the half a trillion tonnes of hydrocarbon-based fuel that at most we could safely release as carbon dioxide to the atmosphere. The first thing they need to do is agree not to burn any more than this limit, and then agree how it is going to be shared out. The second thing is to improve global equity, both for moral and ethical reasons and to improve world stability. The third is to figure out how to keep the wheels on Western society while investing in a low-carbon future.
Author and adventurer
I think we know a lot less about climate change than many people pretend. And the hard thing is that nobody really knows what we should be doing about it either. But that's not to say we should do nothing. I disagree with any form of unsustainable energy. We should move towards complete dependence on wind, tidal and solar energy rather than set a terrible example to the developing world by embracing, as a quick fix, nuclear technology and all its pathologies.
Economist and globalisation expert
There is a real opportunity for any country to grab a big piece of the low-carbon green economy pie, and this country currently does not have a strategy for how to do that. After the financial crisis we saw countries such as South Korea,China and the US target significant chunks of their stimulus packages towards the green economy, while in the UK, although there was a bit of talk around it, in practice very little was actually done. From the UK's perspective, it is important to acknowledge that taking the lead on climate change isn't just about being good and green. It's about creating a clear narrative about how the UK is going to be an economic power in the future.
Sir Ranulph Fiennes
Explorer, author and adventurer
Over the past 40 years I have led polar expeditions and have contributed to around 28 scientific reports, wherever my experiences have been relevant. This has given scientists access to areas of the planet that they might otherwise have not reached. But I am not a scientist – I can only assess the seriousness of climate change with my eyes. In the Seventies, I designed my sledges to travel over icy terrain on Arctic journeys, but by 2000 I had to design them more like canoes to cross wide areas of open icy water. So, in only 30 years, the observable change in the Arctic sea-ice cover has been quite alarming. And the buck clearly starts and stops with mankind.
The main things we can do to deal with the problem are those that everyone knows about already: turning off lights, washing towels less, increasing insulation in buildings and encouraging the use of everything that is geo-engineered.
Bright ideas that are practicable, feasible and enforceable are the key to this problem – and enforcement has a definite place in their success. There are still a great number of people who believe the really serious consequences won't come for another 50 years, and are happy to let the next generation sort out the crisis.