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expert reaction to IPCC AR5

Reaction to the fifth assessment report (AR5) of the IPCC, announced on the 27th September, from lead authors and 3rd party scientists.


The Summary for Policymakers is here:


Prof Tim Lenton, Chair in Climate Change at the University of Exeter, said:

“What concerns me most in this new report is the growing evidence that frozen parts of the climate system are responding extremely sensitively to global warming – the retreat of Arctic summer sea-ice is unprecedented and the rate of ice loss from both the Greenland and Antarctic ice sheets has increased 5-fold in just the last two decades.”


Prof David Lee, Professor of Atmospheric Science at Manchester Metropolitan University and Lead Author on Chapter 8 (Anthropogenic and Natural Radiative Forcing) said:

“This report represents a major milestone in our increased and better understanding of the science of the causes of climate change. The report highlights that the warming is ‘unequivocal’, and observations show how the atmosphere and oceans have warmed, snow and ice extent have decreased, and sea level has risen.

“Of course the big question is: have the observed increased concentrations of greenhouse gases, including CO2, caused the observed changes?  We have shown that global mean surface temperatures have increased by 0.85 degrees C over the period 1880 – 2012, which is more than the last assessment report in 2007. Our increased scientific understanding of the causes has led us to conclude that the human influence on the climate system is clear, and that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th Century.

“2013 represented a watershed in terms of CO2 concentrations in the atmosphere, when they reached 400 parts per million, unprecedented levels for 800,000 years.  The sobering part of the science of CO2 is that for every tonne we emit, 30% will be removed from the atmosphere in a few decades, 50% in a few centuries, and the remaining 20% over millennia. So what we emit today, has massive long-term impacts for the future. The climate simulations in the IPCC report show that there is substantial ‘committed warming’ already, and if emissions are not reduced over the next 20 years, temperatures may increase beyond the value of 2C by 2100.”


Dr Kate Willett, Senior Scientist (Climate Monitoring and Attribution group), Met Office Hadley Centre said:

“The IPCC AR5 pulls together all threads of the story of climate change. As a climate monitoring scientist I focus on ‘what?’ has changed and ‘where?’ in the annual State of the Climate reports produced by NOAA in the Bulletin of the American Meteorological Society. The IPCC brings together the ‘why?’, ‘how?’ and ‘what next?’. For me, it’s seeing the wealth of evidence of a changing climate, robustly supported by a greater understanding of the processes involved and insight into what this might mean for the future.”

“The scope of climate research is so broad that it’s impossible for the average climate scientist to learn and retain all of this information. The IPCC makes an invaluable reference document for all climate scientists. Perhaps more importantly, it takes the essential step of presenting this in a useful way for the rest of society.”

“The two key points for me are that ‘warming of the climate system is unequivocal’ and that ‘human influence on the climate system is clear’. However, it’s not just warming: it’s melting sea ice, ice sheets and glaciers; reduced snow cover; rising sea levels; increasing heat content of the oceans; and increasing moisture in the atmosphere. This consistent story of change lies alongside a consistent story of increasing greenhouse gases. A direct relationship between the two has been robustly found for all of these.”

Prof Matthew Collins, Joint Met Office Chair in Climate Change at the University of Exeter and Coordinating Lead Author of Chapter 12 (Long-term Climate Change: Projections, Commitments and Irreversibility), said:

“Further emissions of greenhouse gases will cause further global warming.  The Arctic will warm most rapidly and the land will warm more than the ocean.  There will be more hotter and fewer cold days.  Mid-latitude areas that currently experience high rainfall will get more.  Sea levels will continue to rise and snow and ice will melt back.  If we follow the highest scenario of future greenhouse gases, the Arctic is expected to be ice-free before the middle of the 21st century.

“Avoiding 2 degrees of warming since preindustrial times will be very tough. We have already ‘spent’ more than two thirds of the CO2 emissions that we can afford to spend.”

Prof Corinne Le Quéré, Director of the Tyndall Centre for Climate Change Research and Lead Author on Chapter 6 (Carbon and Other Biogeochemical Cycles), said:

“This is not just another report, this is the scientific consensus reached by hundreds of scientists after careful consideration of all the available evidence. The human influence on climate change is clear and dominant. The atmosphere and oceans are warming, the snow cover is shrinking, the Arctic sea ice is melting, sea level is rising, the oceans are acidifying, and some extreme events have increased. CO2 emissions from burning fossil fuels need to substantial decrease to limit climate change.”


Dr Tim Osborn, Reader in Climate Change at UEA’s School of Environmental Sciences and Lead Author on Chapter 5 (Information from Paleoclimate Archives) said:

“Through this exhaustive – and, at times, exhausting – process we have produced the independent and comprehensive assessment of climate science that governments and the public need to understand climate change. We are now more certain than ever that many aspects of the climate have been influenced by human activity. Looking back at past climatic changes – which has been my main contribution to this report – adds rich detail to our understanding of the climate system.  For example, we now know that carbon dioxide levels, which have increased by 40% and are the largest driver of the warming we have observed over the past century, substantially exceed the levels of the last eight hundred thousand years.”


Dr Tim Johns, Met Office Hadley Centre and Lead Author on Chapter 12 (Long-term Climate Change: Projections, Commitments and Irreversibility) said:

“As the IPCC Working Group I contribution to the Fifth Assessment Report (AR5) – Climate Change 2013: The Physical Science Basis – is released, my overriding impression is of the massive worldwide scientific effort, expertise and rigour woven into the production of this assessment, underpinned by a rapidly developing science base. The report presents a robust picture of a progressively warming world, reducing Arctic sea ice extent, melting ice sheets and glaciers, rising sea level, ocean acidification, and large-scale hydrological cycle changes under the influence of anthropogenic emissions of greenhouse gases. Despite some inevitable scientific limitations to understanding of the physical climate system, the central conclusions in the Summary for Policymakers are more sharply drawn in many respects than in the IPCC’s 25-year history.

“Climate models play a central role in the assessment of attributed historical climate change and projected climate change through the 21st century and beyond. An unprecedented worldwide modelling effort known as CMIP5 – described as “the moon-shot of climate modelling” by eminent US climate scientist Gerald Meehl – was undertaken to feed the most comprehensive model-based evidence ever about past and future changes and their uncertainties into AR5. Common experiments using different models were run by teams in several countries in Europe, as well as the USA, Canada, Russia, China, Japan, Australia and South Korea. Climate models have undoubtedly improved since the last report (AR4), and the new generation of “Earth System Models” increasingly incorporate biogeochemical cycles that reflect important additional climate change feedbacks, providing the means to quantify the policy-relevant issue of how much carbon-dioxide emission is compatible with a given climate stabilisation pathway.

“Taking results from the latest generation of models for a range of Representative Concentration Pathways (RCPs; future emissions scenarios), AR5 concludes that cumulative anthropogenic carbon-dioxide emissions would need to be limited to around 1000 petagrams (10 to the power 18 grams) of carbon to be likely to limit global warming to no more than 2 degrees Centigrade, relative to the early industrial era (1861-1880). However, half or more of this anthropogenic carbon budget has already been ‘spent’, and accounting for climate forcing agents other than carbon-dioxide tends to reduce the future carbon budget available to be likely to achieve a given warming target.

“The science has spoken and the potential for dangerous climate change in this century is increasingly clear.”

Prof Bob Watson, Tyndall Centre for Climate Change Research and the University of East Anglia, said:

“The latest IPCC report strengthens its earlier conclusions that most of the observed warming since 1950 has been caused by human activities, and future changes are inevitable.  Also, many of the other changes observed in the climate system, such as the rate of loss of Arctic sea Ice, melting of mountain glaciers and the Greenland Ice sheet are unprecedented.  Without immediate reductions in global emissions of greenhouse gases, the world will not be able to achieve the political target of limiting the increase in global mean surface temperatures to 2 degrees C, but rather we are likely to see an increase of 3-5 degrees C.  Time to act is running out if we are to take the threat of human-induced climate change seriously.”

Prof Kevin Anderson, Deputy Director of the Tyndall Centre for Climate Change Research, Professor of Energy and Climate Change at the University of Manchester, said:

“What has changed significantly since the last report is that we have pumped an additional 200 billion tonnes of CO2 into the atmosphere. Annual emissions are now 60% higher than at the time of the first report in 1990 and atmospheric CO2 levels are the highest they have been for over 2 million years.

“So what are we doing in the UK to help reverse this reckless growth in emissions?  Record levels of investment in North Sea oil, tax breaks for shale gas, investment in oil from tar sands and companies preparing to drill beneath the Arctic.  Against this backdrop, the UK Treasury is pushing for over 30 new gas power stations, whilst the government supports further airport expansion and has dropped its 2030 decarbonisation target – all this alongside beleaguered plans for a few wind farms and weak energy efficiency measures.

“Governments, businesses and high-emitting individuals around the world now face a stark choice: to reduce emissions in line with the clear message of the IPCC report, or continue with their carbon-profligate behaviour at the expense of both climate-vulnerable communities and future generations.”

Dr Alice Bows-Larkin, Reader in Energy and Climate Change, Tyndall Centre at Manchester University, said:

“Six years on from the last IPCC report, and little has changed.  Every year we burn more fossil fuels producing more CO2, the laws of physics still hold that the rising concentration of CO2 warms the atmosphere, and as this warming continues, the risk of disruptive physical and social impacts increases.  The big unknown is if, or when, we will manage to break our addiction to fossil fuels, and where that will leave us in terms of future climate impacts.  Personally, if investing in my future and that of my family, I would look beyond fossil fuels being mindful of the risk of stranded assets left in a future that whichever path we choose, will certainly be very different from today.”


Prof Jim Hall, Chair in Climate and Environmental Risks, Director of Oxford University’s Environmental Change Institute, said:

“The scientific case for implicating human activity in climate change was made long before this Fifth Assessment of the IPCC. This new report painstakingly documents the scientific evidence that has emerged in recent years. I respect the scientists who wrote it and admire them for the work they have done for the IPCC. Some of the evidence has moved on, and future projections have apparently changed compared to the Fourth Assessment Report, but nobody with experience of complex computer models and uncertain observations would be surprised by that. The underlying trend of rising average temperatures and sea levels is clear; I have to question the motivation of anyone who disputes these facts.”

Prof Richard Dawson, Chair of Earth System Engineering, School of Civil Engineering and Geosciences, Newcastle University and the Tyndall Centre, said:

“More than half of the world’s population now lives in urban areas, many of them located in low-lying coastal or delta areas.  Urban areas concentrate people, infrastructure and economic activity, making them disproportionately vulnerable to weather extremes like heat waves or flooding.  Furthermore, they are major consumers of resource and producers of pollutants both within and outside their boundaries.  The latest IPCC findings highlight that in the face of continued global change it remains an international priority to adapt urban areas and infrastructure to be more resilient to a wider range of environmental conditions, and to reduce their contribution towards emissions through more efficient use of resources and reduced greenhouse gas emissions.  ”

Prof Andy Jordan, Chair in Environmental Sciences, Tyndall Centre and the University of East Anglia, said:

“The latest IPCC report confirming the science of climate change comes at a pivotal moment, when EU policy makers are battling to resuscitate the emissions trading system, reform internal policies on biofuel and enthuse other countries to agree a successor to the international Kyoto protocol by 2015.”

Dr Charlie Wilson, Lecturer in Energy and Climate Change, Tyndall Centre and the University of East Anglia, said:

“Mitigating climate change requires both widescale diffusion and accelerated innovation of low carbon energy supply technologies and efficient energy end-use technologies. Dramatic improvements in the efficiency with which energy is used are critical in the near term to allow more flexibility in decarbonising the energy supply.”

Prof Keith Shine, Professor of Physical Meteorology at the University of Reading and Review Editor of Chapter 8 (Anthropogenic and Natural Radiative Forcing) said:

“Even in the IPCC’s first assessment report, back in 1990, it was clearly understood that natural climate variability could slow global warming in some decades and speed it up in others.  We simply don’t expect each year, or even each decade, to necessarily be warmer than the previous year or decade, but we do expect the longer term tendency to be for continued warming.

“So, there is no surprise that hiatus periods occur, even when the longer-term trend in temperatures is upwards.  The observed temperature change over the past 60 years is consistent with expectations from our best understanding.  Even accounting for the ‘hiatus’, the decade of 2000-2009 was clearly warmer than any recent decade, and the years 2005 and 2010 were two of the warmest years in the 150 year temperature record.”

Dr Chris Huntingford, Climate Modeller at the Centre for Ecology and Hydrology (CEH), said:

“Continuing uncertainty fuels the argument of those sceptical of global warming.  Given that massive decarbonisation could have major economic implications, this argument can seem compelling.  But to ‘wait and see’ could trigger hugely dangerous impacts if temperatures increase to the higher end of predicted ranges.  This remains true even as recent studies suggest that the upper bounds may be lower than previously predicted.  Thermal lags could create false optimism, as a CO2 concentration unrecognized as dangerous may be reached a few decades before the full temperature implications are realized.  Then, even if mitigation measures somehow reduced net CO2 emissions to near zero, the planet would take centuries to reset itself.  Forewarned is forearmed when preparing for climate change.  Concerted effort is essential to improve the certainty of climate forecasts.”

Dr Colin Summerhayes, Scott Polar Research Institute and reviewer of Chapter 5 (Palaeoclimates) said:

“We are warming when we should be cold. The new report from the IPCC confirms in significantly more detail than before what we understand about climate change from the geological record. This is an aspect of climate studies that the public rarely hears anything about. The new report shows yet again that the global warming we are seeing today cuts right across what we would expect from our knowledge of climate change over the past 11,000 years.

“Earth’s climate is driven ultimately by energy from the sun, and the amount of solar radiation we get (our insolation) changes with (i) the eccentricity of the Earth’s orbit, (ii) the changing position of the Earth relative to the sun around the orbit (Earth is now closest to the sun in December, so we have ‘mild’ winters, and in 11000 years time will be closest to the sun in July, which will give us colder winters), and (iii) the tilt of the Earth’s axis (more tilt gives more seasonality). These three factors combine to drive the Earth in and out of glacial periods. They made our insolation highest 11000 years ago, which melted the great northern hemisphere ice sheets. Since then, insolation has declined steadily to the present, making our climate progressively cooler, such that we ended up in a Little Ice Age between about AD 1450 and 1850. Calculations of our relationship to the sun tell us that insolation should stay low for another 1000 years, so we should be experiencing a continuation of the Little Ice Age and having Frost Fairs on the Thames. But we aren’t. Since about 1900, everywhere you look, past climate data suddenly shoot upwards quite quickly, indicating warming against the trend. Two things are likely to explain this. One is that sunspot activity increased slightly from about 1820 up to about 1950. That would have caused minor warming. The other is that CO2 increased from about 1760 to the present. That would also have caused minor warming. The combined sunspot activity and CO2 increase explain the warming of the first part of the 20th century. But the sunspots and other measures of solar output have not increased since about 1950, while CO2 has gone on increasing. Not surprisingly, since CO2 is a greenhouse gas, our global temperature has gone on increasing way beyond 1950. We are recreating a situation that occurred on a large scale back 55 million years ago at the boundary between the Palaeocene and Eocene geological periods when there was a massive injection of CO2 into the atmosphere, and temperatures rose about 6 degrees C, though that happened about 100 times slower than what is happening now. Never mind computer models, the record of our geological past tells us a lot about what is now going on and what to expect in future.

Prof David MacKay FRS, Chief Scientific Advisor to the Department of Energy and Climate Change, said:

“I’m not a climate scientist, but I often read the climate science literature and attend meetings where climate scientists discuss their work.

“The climate science community are doing excellent, open science: climate scientists are healthily critical of their own work, and well aware of the questions that have yet to be resolved.

“The climate system is astonishingly complex, and I admire the steady progress that climate scientists are making to improve our understanding of this remarkable, dynamic world in which humanity is sustained. The IPCC’s fifth assessment report has been produced by the generous work of hundreds of scientific experts drawn from universities and research institutes around the world.  There is no equivalent of the IPCC in any other area of science.

“This latest report is the most authoritative and comprehensive report to date of our understanding of climate change. The scientific consensus is that the world has warmed and will warm more, owing to human activities. There is robust evidence that human greenhouse gas emissions are already changing our world; global temperatures have risen every decade for the last three decades, oceans are acidifying, rainfall patterns are changing, sea levels are rising, arctic sea ice is declining, and some extreme weather events are becoming more frequent and intense.

“It is predicted, from simple physics, that the more humanity increases the quantities of greenhouse gases in the atmosphere, the warmer the Earth will become. The far-reaching consequences of this warming are becoming understood, although some uncertainties remain. The most significant uncertainty, however, is how much carbon humanity will choose to put into the atmosphere in the future. It is the total sum of all our carbon emissions that will determine the impacts. We need to take action now, to maximise our chances of being faced with impacts that we, and our children, can deal with.

“One important message of this new report is that, while there remains some uncertainty about the precise sensitivity of the climate to greenhouse gas emissions, the impact on climate is largely determined by the cumulative total of humanity’s carbon emissions. This means that waiting a decade or two before taking climate change action will certainly lead to greater harm than acting now.”

Dr Emily Shuckburgh, British Antarctic Survey, said:

“The science as outlined in the new report is clear: our collective actions have generated a climate problem that threatens our future and our children’s future. Increasing levels of greenhouse gases are disrupting our climate. And because it is the cumulative amounts of greenhouse gases that determine the severity of the impact, any delay in reducing emissions will lead to greater risks and a need to deploy more difficult and expensive means to adapt to the impacts.

“One of the key developments since the last report has been an increased understanding of changes in the polar regions and their global effects. Arctic sea ice has declined significantly and new research is starting to shed light how this affects our weather in the UK. The Antarctic Peninsula has seen significant warming and the breakup of a number of its ice shelves. The Southern Ocean around Antarctica has warmed throughout its depth. And it has now been possible to estimate the contribution of melting of the polar ice sheets to sea level rise. But important open questions remain. For example, the Southern Ocean currently soaks up approximately about 10% of our carbon dioxide emissions, thus limiting their accumulation in the atmosphere. However, we are presently uncertain whether this ‘carbon sink’ might start to fail in future as a consequence of climate change.”


Prof Joanna Haigh, Professor of Atmospheric Physics at Imperial College London, said:

“The new IPCC report confirms, with even greater confidence than in previous reports, that global warming continues and that this is largely a result of greenhouse gases produced by human activity. The concentration of carbon dioxide in the Earth’s atmosphere now exceeds anything it has experienced in the past 3 million years and its continuing upward trend is almost certain to result in further global warming. Changes in solar activity alone cannot explain the global surface temperature variations of the past 150 years and, even if the Sun were to enter a new ‘grand minimum’ state within the next century, would be very unlikely to provide more than a small, temporary, partial compensation for likely anthropogenic warming.”

Prof Peter Wadhams, Professor of Ocean Physics at the University of Cambridge and Review Editor for Chapter 1 (Introduction) said:

“There are serious deficiencies in the modelling.  The assessment makes extensive use of the UK Met Office model which does not take account at all of the feedback due to emissions from thawing permafrost.  Nor is the possibility of a major methane emission from melting offshore permafrost on Arctic shelves mentioned, even though this has been projected as capable of adding 0.6 C to global warming.

“These are examples of an instinctively cautious approach which in other circumstances might be praiseworthy but in the circumstances of a global climate emergency is not.  The world does deserve to have the full range of risks and possibilities explored with, if possible, some probability factors attached to them, instead of consistent under-estimate of effects or simply ignoring a phenomenon where the magnitude is difficult to compute (as in the case of sea level rise from glacial runoff in AR4, which did endless harm).”

Prof Jonathan Bamber, Director of the Bristol Glaciology Centre at the University of Bristol and Review Editor of Chapter 4 (Observations: Cryosphere) said:

“The evidence of persistent and continued changes to all frozen parts of the planet is clearer than ever.  Glaciers around the world have been declining over at least the last 5 decades, mass loss from the great ice sheets covering Antarctica and Greenland has been increasing over the last two decades and the reduction in late-summer sea ice in the Arctic appears to be unprecedented over the last 1500 years. Permafrost temperatures have been increasing and seasonal snow cover is arriving later and melting sooner.  The evidence that humans are at least partly to blame for climate change is stronger than ever.”

Prof Mike Hulme, Professor of Climate and Culture at King’s College London, said:

“This latest assessment of climate science from the IPCC threatens to distract from resolving the core issue of climate change – the political challenge of finding policy interventions that are effective and plausible.  The difficulties in implementing policies that reduce the dangers of a changing climate don’t result from a deficiency of scientific knowledge.  Raising the confidence that humans are a major influence on climate from ‘very likely’ to ‘extremely likely’ doesn’t change the politics of climate change.  The difficulties arise because of different interests, values and attitudes to risk.  These can only be worked through using political strategies that are less constrained by the need to reach global agreements.

“We need a more pragmatic politics of climate change, not more weighty science about climate change.”

Prof Ted Shepherd, Grantham Professor of Climate Science, University of Reading and Review Editor of Chapter 11 (Near-term Climate Change: Projections and Predictability) said:

“This report is the most comprehensive assessment of climate change ever performed, involving hundreds of scientists and exhaustive peer review. It reaffirms what we have known for some time — that human activities are changing climate, and that strong action on carbon dioxide emissions will need to be taken to avoid dangerous interference with the climate system. However we now have a much better understanding of how the different pieces fit together, which is to say a more self-consistent understanding at a quantitative level. There has also been an enormous effect made to attempt to quantify the “known unknowns”, rather than just relying on the “known knowns” represented by the climate models, by comparing the climate models with both modern observations and reconstructions of past climate. This is what has led to some refinements of earlier estimates, but we now have more confidence in those estimates — especially on the upper and lower bounds of climate sensitivity (i.e. the worst case and best case scenarios), which is what the policy makers need to know.

“We need to distinguish cycles in the climate system, which can occur for natural reasons, from the long-term changes. Climate change has to do with the energy budget of the climate system, and this is best measured from the “slower” parts of the system. Surface temperature is the manifestation of climate change, but just like the difference between a person’s current account and their net wealth, it can vary much more rapidly that the total heat content of the climate system, which is mainly contained in the ocean. Thus we know that climate is continuing to change because measurements tell us that the climate system is continue to emit less energy than it receives from the Sun (the heat trapping or greenhouse effect), the ocean is continuing to warm, and sea level is continuing to rise.

“Both observations and models tell us that the sort of pause in the increase in surface temperature that we have seen over the last 15 years is neither unusual nor unexpected. It would be far more puzzling if surface temperature was continuing to rise and these other metrics were not changing!

“Climate change is an especially dangerous threat because it is like a large ship: you can turn off the engines but it will continue to move forward for a long time. This is called the “commitment” problem. It means that if we wait until climate change is unmanageable before we act, it will be far, far too late because the changes will keep happening for centuries. So in that respect it is a very different environmental problem than, e.g., pollution.

“A new aspect of this report is a whole chapter devoted to assessing climate change in the near-term, i.e. over the next few decades. This is a far more challenging problem because of the influence of natural variability, so one cannot expect statements of high confidence. Instead, we need to think in terms of the risks we face from outcomes that are perhaps only “likely” (e.g. two out of three). This is a rapidly growing area of research because of its policy relevance in terms of impacts.

“The “greenhouse effect” is not just a theory, it is a fact. Life exists on earth because of the greenhouse effect, which warms the planet to habitable levels. Nobody disputes this. Climate change is an enhanced greenhouse effect due to higher levels of atmospheric carbon dioxide produced by human activities. The scientific debate concerns how much of the emitted carbon dioxide will remain in the atmosphere, how much additional warming will result from the additional carbon dioxide, and how quickly this additional warming will occur.”

Prof Mark Maslin, University College London, said:

“A hundred year from now future generation will look back at the huge amount of evidence in the IPCC reports and wonder why people failed to act.  School children will learn in their history classes that the world’s environment was devastated due to the political myth that huge profits can only be created by extracting and consuming fossil fuels.

“The IPCC have synthesised six years of detailed climate science and has found results entirely consistent with all the previous IPCC reports dating back to 1990. The message from over 23 years of detailed science is that climate change has already occurred and if we do nothing to curb global carbon emissions it will accelerate in the future.

“The clear scientific message of the IPCC 5th report is that the current level of carbon emissions will lead to dangerous climate change with major implications for the global economy and human health. With no international climate agreement currently likely before 2020, zero carbon emissions for Developed countries may not be enough to avert disaster.  We may now have to phase out fossil fuels completely and start planning to take carbon dioxide out of the atmosphere.”

Prof Andrew Shepherd, Professor of Earth Observation at the University of Leeds, said:

“It’s clear, from the past 20 years of satellite measurements, that there have been dramatic losses of ice from both Antarctica and Greenland.  Back in the 1990’s, the polar ice sheets were responsible for just a tenth of all sea level rise, but today they are contributing three times as much. The problem for climate science is building models that can capture the changes in Earth’s ice that we can see from space today.

“Until we can do that with confidence, we really can’t be sure how much sea level rise to expect over the next century. If, for example, we were to discard climate models and make predictions based on satellite measurements alone, we would expect the ice sheets to contribute another 30 cm of sea level rise by the year 2100. This figure represents the upper limit of the IPCC’s latest predictions in AR5, and underlines the importance of building the very best climate models.”

Prof Sir Brian Hoskins, Director of the Grantham Institute for Climate Change at Imperial College London, said:

On IPCC and robustness of the science:

“This Summary for Policy Makers provides further strong confirmation that human activity is having a significant and growing impact on the climate.

“It is based on a comprehensive review and rigorous assessment of the state of climate science by some 850 scientists, who reviewed over 9000- scientific articles, and includes voices from all sides of the issue.  This report significantly strengthens the consistent message from the four previous assessment reports; we are conducting a dangerous experiment with our planet.”

“The evidence of changes in many different aspects of the climate system, from the ice sheets to the deep ocean, shows that climate change is happening.   To reduce the serious risks posed by increasing changes in the climate, we need to redouble our efforts globally to limit carbon dioxide and other greenhouse gas emissions.”

On continuing warming:

“Global mean surface temperature increased by about 0.89°C since the start of the 20th century.  Despite the recent slowdown in the rate of temperature rise, each of the last three decades has been warmer than all previous decades in the instrumental record. The decade that began in 2000 has been the warmest.

“Many observations of the climate system over the past 15 years paint a picture of increasing climate change.  The ocean absorbs over 90 percent of the additional energy flowing into the climate system due to greenhouse gas emissions. In the past decade or so the upper ocean, like the atmosphere, has not warmed much. However with recent observations it is becoming evident that the deep ocean is taking much of the excess heat during this period. Sea level is continuing to rise at around 3mm per year partly because the warmer water in the oceans expands.  The rest of the rise is due to the mountain glaciers that are continuing to melt rapidly and the ice loss from the major Greenland and Antarctic Ice Sheets that has accelerated dramatically.

“We are making long-term alterations to the climate system which will have major impacts for generations to come unless we accelerate efforts to reduce emissions.”

On equilibrium climate sensitivity:

“The IPCC’s new estimate of Equilibrium Climate Sensitivity is consistent with estimates first made in 1979.  It is very difficult to estimate climate sensitivity precisely, because of the uncertainty over key aspects of the climate system’s response to human emissions of greenhouse gases.

“The reduction in the lower bound of the estimated likely range to 1.5 degrees of warming for a doubling of atmospheric carbon dioxide should not distract us from the concern that we  may well be on track to exceed three degrees warming by the end of the century on current emissions trends.

“If climate sensitivity is at the lower end of the range, it may make it a bit easier to limit warming to two degrees through large-scale emissions reductions.   The case for action on climate change does not rest on hoping for the best, but on the potential scale of the climate risks and reducing these risks.

“The risks depend on the trajectory of global emissions over the next few decades and the whole range of estimates of the climate sensitivity.  If we continue at current rates of emissions and if the upper limit of the IPCC’s estimate of equilibrium climate sensitivity is right – which is unchanged at a 4.5 degree warming for a doubling of atmospheric carbon dioxide levels – then we really are entering extremely dangerous territory.  So the message to governments is clear: we need to accelerate efforts to reduce emissions, whatever the real value of the climate sensitivity is. “

Prof Piers Forster, Professor of Climate Change at the University of Leeds and Lead Author on AR5 Chapter 7 (Clouds and Aerosols) said:

On the whole report:

“We have been causing 40% more warming than we estimated in our 2007 report. Greenhouse gases have continued to rise giving stronger warming and the cooling effect of particles formed by pollution is weaker. Further, this report has really firmed up understanding of rainfall and sea-level change. Over much of the world extreme rainfall will be heavier and occur more often and unless we begin to dramatically change our ways, we could have up to 1m and growing sea-level rise by 2100.”

On the slowdown:

“I think Chapter 9 authors and the TS do a great job of placing the slowdown in the context of centennial-scale past and committed manmade climate warming. We have caused past climate warming and the world will continue to warm. We recognise that the slowdown is an important event to understand, especially as it is not obviously reproduced in climate models.The slowdown since 1998 itself is likely due to a combination of natural (solar and volcanic effects) and extra heat from greenhouse warming being sucked into the deep ocean. Climate models can capture such slowdown events  but there is the possibility that some models are over responsive . Any over-response would only be a small effect though and the slowdown does not significantly affect our 2100 projections.  However, we do take it into account for our near term projections. Provided there is not a large volcanic eruption, we can expect warming between 0.3 and 0.7 C over the next 20 years.”

On the process:

“Nevertheless, after over 4 years of work with massively bright colleagues, careful reviews and then

all-night plenary scrutiny, I believe we have produced a document of unprecedented quality, robustness and usefulness for policy makers.  A document of such quality is rare in any discipline.”

Prof Rowan Sutton, and Lead Author on Chapter 11 (Near-term Climate Change) said:

“All the evidence in this new report, together with our long standing understanding that climate takes decades to respond to changes in greenhouse gas emissions, makes it clear that leaving the issue of climate change for future generations to deal with is a phenomenally high-risk option.”
“The report shows that the evidence of human activities affecting climate is increasingly widespread and stronger than ever.”

“The report shows there is a very substantial risk of exceeding 2oC warming, relative to pre-industrial climate, by the end of the century. Only under a very ambitious scenario for reducing greenhouse gas emissions is a warming of more than 2oC considered unlikely.” [This one in particular is dependent on the final version of the SPM]

“This report provides the evidence that governments need to take tough decisions on climate change policy. I hope they will read it carefully and take seriously all its findings.”

“An increasing frequency of hot days, and intense rainfall events, are some of the changes that could affect Britain. Working out more of the details is a major focus of our research.”

Dr Ed Hawkins of the University of Reading said:

“Scientists have been asked to provide clear answers on climate change. This report does this. It represents the most comprehensive assessment ever assembled of what we know about our changing climate. The report provides more thorough evidence than ever before, strengthening and focussing the advice provided in previous IPCC assessments.

“A novel aspect to the report is a whole new chapter on how the climate is projected to change over the next 30 years, rather than concentrating on changes by the end of the 21st century. This will be extremely relevant for decisions on how to adapt to a changing climate. It helps people to realise that climate change, and its impacts, is happening now in our own lifetimes and those of our children.”

Prof Bill Collins of the University of Reading and Lead Author on Chapter 8 (Anthropogenic and Natural Radiative Forcing) said:

“This report shows that we are now more certain than ever about the human contribution to climate change. Greenhouse gas concentrations are observed to be rising rapidly, and unabated emissions are very likely to lead to changes in climate which are unprecedented in human history.

“Climate models, which are often criticised, have improved significantly in their detail and complexity since the last report was produced in 2007, as a result of improvements in computing power and more sophisticated ways of representing the climate system. Along with more accurate observations of current and past climate, these models show the crucial benefits of reducing our emissions of CO2 and other pollutants in the future to prevent the worst-case scenarios in the parts of the globe most vulnerable to climate shocks.”

Dr Richard Allan of the University of Reading said:

On what is new:

“Since the last IPCC report, a step change in the volume and quality of observations (particularly from satellite) has been vital in allowing scientists to verify and improve physical understanding of the processes determining climate variability and change.”

On the slowdown:

“Although global surface warming has slowed over the last 15 years, observations show that heat has continued to accumulate within the oceans since 2000 – at a rate equivalent to over 250 billion 1kiloWatt electric heaters spread across the planet – consistent with rising atmospheric concentrations of greenhouse gases.”

“Recent research indicates that natural fluctuations in the ocean have caused this heating to build up at deeper levels below the sea surface rather than the upper layers which influence surface temperatures. This is important since it provides evidence that the slowdown in surface warming is temporary and we can expect a return to substantial surface warming over the coming decades.”

On climate sensitivity:

“Previous IPCC reports have noted that climate sensitivity (the amount of
global warming far in the future in response to a doubling of CO2
concentrations) is uncertain, ranging from around 1.5 to 4.5oC. Observations,
basic physics and detailed simulations show that atmospheric water and surface
ice are powerful amplifying agents of climate change, explaining the mid-range
values of around 3oC. The lowest values in the range of climate sensitivity
would require equally powerful counteracting effects which, while possible,
have not been observed. Furthermore, arguing over a few tenths of a degree in
climate sensitivity at the bottom of the range masks the real issue which is
the damaging climate change expected in response to continued emissions of
greenhouse gases, which are at present following the worst case emissions

On arctic ice:

“Recent media statements about recovery in the Arctic sea ice since 2012 ignore
the difference between weather and climate. The long term trend over recent
decades is downward and the fact of the matter is that in 2013, ice volume has
recovered from a tiny amount, around 3,500 cubic km in late August 2012
compared with the late August average for 1979-2012 of around 12,000 cubic km.”


Dr Dave Reay, Reader in Carbon Management at the University of Edinburgh, said:

“It is six years since the Intergovernmental Panel on Climate Change (IPCC) last published its voluminous assessment of climate change science.  Today, in the release of its fifth assessment report (AR5) the science of climate change is brought up to date and the key findings of thousands of climate research studies are distilled. There are no great game-changers here.  This latest assessment reinforces the messages of its predecessors, attributing the bulk of post-war climate change to human-induced greenhouse gas emissions and projecting far more rapid changes in climate as the 21st century progresses.

“Its in-depth look at climate sensitivity – just how much warming we’ll see from a doubling of CO2 – is probably one of the hottest topics this time around. Much recent debate has revolved around the reason for the so-called ‘hiatus’ in global warming since 1998, with deep-ocean warming being the leading contender.  What is not debated in this new report or elsewhere is the fact that greenhouse gas emissions continue to increase apace, and with them our enhancement of the greenhouse effect.

“We can guess at exactly when global temperatures might start ticking upwards again. We can argue all we like about whether climate sensitivity will be closer to 2C or 4C. But the core message in this latest report is a clear one: if greenhouse gas emissions continue on their current trajectory our grandchildren will find out exactly what Earth’s climate sensitivity is.”

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