In international negotiations it is widely accepted that a global temperature increase of 2°C above pre-industrial levels will trigger dangerous climate change. What do you consider to be a safe temperature increase after which dangerous climate change occurs? Corinne Le Quéré I think 2°C is the limit that we should not exceed. This is the highest temperature that we can infer occurred on Earth at least in the past two million years. Thus we know that warming of 2°C is stable enough for humans to live, even though we would have a lot of adjustments to make to adapt to a 2°C warmer world, particularly for producing food and for ensuring the availability of water. Above 2°C the risk is very high that the Earth’s natural feedbacks would destabilise climate well beyond warming itself, and that it could become technically difficult, costly and even impossible to adapt, particularly in the poorest regions of the world.

Robert Watson We are already living with the adverse effects of anthropogenic climate change in many parts of the world, with changes in both biological and physical systems. Hence, in an ideal world we would keep the climate as close to today’s as possible. As the magnitude and rate of climate change increases the effects become increasingly detrimental, with poor people and poor countries being the most adversely affected. People living in low-lying areas are particularly vulnerable to sea-level rises, while others are vulnerable to threats to food security and water security, loss of critical ecosystem services and increased health threats.

While I do not like the term ‘dangerous’, because different groups of individuals and sectors are affected differently, there is little doubt that more people, and more natural ecosystems, will be adversely affected by changes in temperature above 1.5°C relative to pre-industrial levels. The political goal of limiting the change in global mean surface temperature to no more than 2°C above pre-industrial levels will not be realised without immediate significant global reductions in global greenhouse gas emissions.

Simon Lewis A safe average global temperature increase is impossible to state objectively. For some people climate changerelated impacts are already dangerous, even deadly. The victims, who we rarely hear from, are overwhelmingly very young, old or poor. It is important to recognise that some people, countries, species and ecosystems are more vulnerable than others. Some of the more vulnerable countries advocate a limit of 1.5°C in international negotiations. Of course, deciding what outcomes and risks are an acceptable price for the use of fossil fuels is in the realm of politics, not science. Looking only at the likely negative impacts on crop yields under sustained warming, coupled with the impacts of food price rises – the 2007 and 2008 food protests spanning three continents, and food prices being widely considered a factor in the timing of the Arab Spring – I consider 2°C warming to not just be environmentally world-changing, but it may be socially explosive, too.

Kevin Anderson It is not the role of climate scientists to define the appropriate threshold between acceptable and dangerous climate change. This is properly the decision of civil society through an iterative and open dialogue, through the unavoidably messy process of politics and international negotiations. Science enlightens the discussion, but scientists are no more equipped to give a definitive answer than are other engaged individuals.





However, once civil society has defined a ‘dangerous’ threshold it is the role of scientists to explore what this means in terms of carbon budgets, emission reductions, etc. As a citizen, I rightfully have a view as to what constitutes ‘dangerous climate change’. I judge we should aim for below 2°C, limited, in the end, by the emissions we have already locked into the system – so probably around 1.5°C.

Ultimately, my choice as to the appropriate threshold is made from an expert knowledge of carbon budgets, mitigation rates etc, combined with a moral interpretation of the world and my personal approach towards risk and uncertainty. My value-laden choice of threshold has no more veracity than that of others who have given the issues serious thought.

What chance do you think the world has of staying below 2°C of warming?

Corinne Le Quéré The Earth has now warmed by about 0.8°C, so we can limit warming to 2°C, but we need to act now and make drastic actions so that growth, wealth and development do not continue to rely on burning fossil fuels. Socio-economic models tell us that a peak in emissions needs to occur before 2020 to limit warming to 2°C. This is a matter of choice for society and governments.

Robert Watson In spite of the recent economic recession in many parts of the world and stated government commitments to limit greenhouse gas emissions, recent global emissions of carbon dioxide are at an all-time high, hence little possibility of even achieving the stated 2°C goal. The world’s current commitments to reduce emissions are consistent with at least a 3°C rise (50/50 chance) in temperature, a temperature not seen on the planet for around three million years, with serious risks of a 5°C rise, a temperature not seen on the planet for around 30 million years.

Simon Lewis It is the cumulative emissions of greenhouse gases that count for the long-term. However, emissions continue to rise, with little serious effort to reduce them. There is almost no discussion about how to keep most fossil carbon out of the atmosphere. When even the wealthiest of countries discover they have billions of dollars of fossil carbon buried underground, they extract it: tar sands in Canada, fracked oil and gas in the US and UK.

There is very little chance of staying below 2°C. The rare positive news is the slower increase in global surface air temperature in the 2000s suggests that it may take a decade longer to reach 2°C than many scientists previously thought.

Kevin Anderson Unless we are very fortunate on climate sensitivity, the chances of not exceeding even the 2°C threshold are extremely slim. However, if we don’t try, the chance slips from very little to effectively zero. For the wealthier parts of the world, I concur with the broad thrust of the International Energy Agency’s conclusions. To be serious about the 2°C threshold, we have about five years to mobilise a radical transition to a zero-carbon energy system. Note ‘energy’ and ‘system’ – i.e. it’s not just about electricity, and it is as much about energy demand as it is about energy supply.

If adequate action is not taken on climate change, what will the world look like in 50 or 100 years in terms of global temperatures, environmental, social and economic impacts?

Robert Watson Emissions at or above current rates could increase global mean surface temperatures by over 3°C, inducing changes in all components in the climate system, some of which would be unprecedented in hundreds to thousands of years, and many of which would persist for centuries. Changes would occur in all regions and would include land and ocean temperatures, the water cycle, the cryosphere, sea level, some extreme events and ocean acidification. This would reduce agricultural productivity, water quantity and quality in many parts of the world, undermine efforts to reduce poverty, displace large numbers of people, cause significant losses of biodiversity and degrade critical ecosystem services.

Kevin Anderson If today’s emission rates continue – and currently I see no significant policies or reasons as to why they are likely to reduce – then I concur with the IEA’s analysis that 4–6°C by the end of the century looks likely. With emissions remaining unabated, 4°C by 2050–2070 does not appear unreasonable.

Corinne Le Quéré Carbon emissions are currently following the most carbon-intensive scenarios used to project climate change, leading to warming levels of about 2°C already around 2050, and 4–6°C in 2100. This is not only very high warming levels, but also very rapid warming, limiting the capacity to adapt for much of our ecosystems and for society.

At 2°C you would expect the Arctic to be ice-free in the summer, and large and systematic melting of the snow and ice cover in the northern hemisphere, including the permafrost that contains large quantities of carbon. You would also expect changes in weather patterns, particularly in the northern hemisphere, and large changes in the water cycle, with generally dry regions becoming drier and wet regions wetter, and with an increase in the intensity and severity of floods, droughts and heat waves – i.e. more extreme events.

At 4°C you expect a transformation of the environment, including of the vegetation at all latitudes, of the weather, seasons and climatic patterns (e.g. the monsoon and north Atlantic oscillation that controls weather in Europe). Implications for the biosphere are enormous as common plants and animals lose over 50 per cent of their niche ranges. The costs of adaptation – in particular to sea level rise and protecting against increasing storm surges in coastal regions – will be very large.

Simon Lewis The world of 2100 will be shaped by numerous responses to the challenges facing humanity. We will need to innovate to remain ahead of the evolution of antibiotic resistant bacteria [and] we will need to conserve soil and its health to maintain agricultural productivity, to name but two neglected long-term problems that may limit human welfare this century.

In terms of mean annual surface temperature, I suspect we will see warming of 3–4°C. This would transform the physical world our descendants would see. Many ecosystems will be entirely novel assemblages of species. Biodiversity will have dramatically declined. Sea levels will be higher; some city and island populations will have relocated. Global agricultural productivity will struggle as rainfall regimes shift. Huge resources will have been expended on adaptation to the new conditions.

I would not wager the condition of humanity. It is entirely unclear to me what the dominant response will be to the fact that the way the economy is run undercuts the environmental conditions required to sustain billions of people on planet Earth.

Can you give an idea of the level and speed of changes our governments need to make to avert catastrophic climate change?

Robert Watson To achieve the political goal of limiting the change in global mean surface temperature to no more than 2°C above pre-industrial levels requires immediate action by all major emitters of greenhouse gases. Global emissions of greenhouse gases need to peak as soon as possible and well before the year 2020, and be less than 50 per cent of current emissions by 2050. Industrialised countries must take the lead.

An immediate transition to a low-carbon economy is needed, addressing all sectors – energy, transportation, industry, agriculture and forestry – and using low-carbon technologies complemented by policies such as a price on carbon and changes in individual, corporate and government behaviour.

Corinne Le Quéré Governments need to act immediately, with important investments, particularly in energy savings, for technologies that use energy – housing, transport, appliances and IT – and clear regulations that encourage the development and large-scale deployment of renewable energy.

In rich countries, emissions need to decrease by at least 3 per cent per year until they are a fraction of their 1990 levels. Carbon emissions in the UK have decreased by about 1 per cent per year in the past 20 years, so efforts need to be enhanced. With such changes and corresponding efforts in China and other emerging economies, we stand a chance to limit warming to 2°C.

Simon Lewis The greenhouse gas reductions [necessary] to have a 50/50 change of meeting the 2°C target, and attaining equal emissions between developed and developing countries in the future, are in excess of 5 per cent every year for decades for the developed world. Few think this is feasible. Economic and regulatory policies are required to keep most fossil carbon in the ground. Given the vested interests involved, governments are unwilling, to put it bluntly, to legislate BP, Statoil, Shell and others out of their current core business.

Kevin Anderson This question relates to my principal area of research. For any reasonable level of equity between the poorer and wealthier parts of the world, the emissions from nations such as the UK, the US and across the EU need to reduce at around 10 per cent per annum. Such rates of reduction are without precedent and beyond anything yet countenanced.

As someone whose job gives them a deep understanding of the bleak future facing the planet and humanity, how do you personally deal with this on an emotional and psychological level?

Robert Watson The issue of climate change, along with other related issues, such as poverty eradication, loss of biodiversity and food and water security, is too important to get discouraged by the lack of government and private sector action, or the complacency within civil society. My job, along with other scientists, is to ensure that governments, industry and civil society all know the risks associated with humaninduced climate change, and that there are cost-effective and equitable solutions.

Current and future generations need us to act now; nothing else will do. None of us can afford to fail them by getting discouraged. If we fail them, they will ask why we mortgaged their future for the sake of cheap energy, and [why we failed] to deal with vested interests that profit by maintaining the current status quo.

Corinne Le Quéré Recent research found that the British public overwhelmingly supports a move away from fossil fuels and a transformation in the way we use and govern energy. I am optimistic that we will manage the transition to a sustainable world peacefully, including limiting warming to 2°C or just about. My colleagues and I work hard to make this happen, and I hang on the thought that we might just succeed.

Kevin Anderson I consider it counterproductive (and morally unacceptable) for those of us intimately engaged in climate change to not demonstrate significant reductions in our own personal emissions, though many of my colleagues disagree with this position. It is not that our personal emissions, in isolation, are important, but that our collective action as ‘experts’ in the area lends credibility to our research and the severity of our conclusions. The integrity of our arguments for individuals, organisations, governments etc to implement radical levels of mitigation is undermined when the message is delivered from 35,000 feet on the way to another ‘essential’ international climate change meeting.

Making such personal changes has proved very challenging. My friendships, family ties and overall quality of life all have suffered significantly from the emission reductions I have felt compelled to make. Most of us working on climate change are in the high emissions group in our own nations, let alone globally. For those like us, it is not going to be easy – but certainly easier than for the poor of the world, and even our own offspring, to deal with the impacts of unabated climate change.

Simon Lewis It is easy to focus on writing technical scientific papers, or argue that the situation is complex and therefore not so alarming. It is easy to think only about the details and not the big picture. However, I think it is critical to act with hope. Scientific information is a key tool towards understanding the world. And I consider that a better understanding of the world will give a better chance of changing it for the better.