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expert reaction to the IPCC’s Working Group 1 contribution to the Sixth Assessment Report

The Intergovernmental Panel on Climate Change (IPCC) have published their latest report on the global climate, Assessment Report 6 (AR6)

This Roundup also accompanied an SMC Briefing.


Author comments:

Prof Nigel Arnell, Professor of Climate System Science, University of Reading & lead author for Chapter 12 – Climate change information for regional impact for risk assessment, said:

“The latest IPCC report confirms that human activities have changed our climate and led to the more frequent heatwaves, floods, droughts and wildfires that we have seen recently. The evidence is incontrovertible. The report also shows that impacts will continue to increase virtually everywhere, and only by major reductions in emissions – achieving net zero – will there be a chance of meeting the internationally-agreed climate targets. We now have a greater understanding of how quickly risks will change, and several very damaging plausible ‘worst case’ scenarios cannot be ruled out.

“In my view, there are two key messages from the report for attendees at COP26. First, the report emphasises to climate negotiators – again – the need to reduce emissions further than currently looks likely in order to hit Paris targets. Second, the report highlights – more urgently than the last report from 2013 – the importance of ramping up our collective efforts to adapt to our changing climate and increase resilience to more frequent and more extreme weather disasters in the future. Recent events have shown we are all exposed to climate risks.”


Prof William Collins, Professor of Meteorology, University of Reading & lead author for Chapter 7 –  The Earth’s energy budget, climate feedbacks, and climate sensitivity, Technical Summary author, SPM contributing author, said:

“This report details the climate changes in heatwaves and extreme rainfall that we have already experienced due to our emissions of greenhouse gases. These levels of carbon dioxide have not been seen in the last 2 million years.

“Most of this warming has been caused by carbon dioxide, but a new finding is that a substantial fraction (around half a degree) has been due to emissions of methane, which comes from sources such as oil and gas drilling and agriculture.

“The report tell us that rapid and sustained reduction in carbon dioxide and methane are needed to have a chance to meet the Paris Agreement goals of limiting temperatures to 1.5 degrees. Reducing methane is also shown to have an additional benefit of improving air quality, and would counteract any future warming due to reductions in other (cooling) air pollutants such as aerosols. So the report highlights a combined approach on climate and air quality that will give us a win-win, making the air cleaner to breathe and limiting climate warming.”


Prof Richard Allan, Professor of Climate Science, University of Reading & IPCC lead author on Chapter 8 – Water cycle changes and contributing author on Chapter 11 – Weather and climate extremes events in a changing climate  and drafting author of Summary for Policy Makers, said:

“The report finds strengthened evidence that human-caused warming of climate is intensifying the global water cycle, including its variability and the severity of very wet and very dry weather and climate events affecting all regions. The newly assessed science is clear that without rapid and sustained cuts in human-caused greenhouse gas emissions, water cycle extremes will continue to intensify with future increases in global surface temperature, along with more severe associated flooding and drought events.”


Prof Corinne Le Quéré FRS, Royal Society Professor of Climate Change Science, University of East Anglia & contributing author to Chapter 5 – Global carbon and other biogeochemical cycles and feedbacks, said:

“If there was still a need for a proof that climate change is caused by human activities this is the report that provides it. The report goes well beyond the previous IPCC assessment of 2013 and resolves all major uncertainties, to provide the clearest picture yet of the effect of human activities on the climate and on weather extremes. The message could not be clearer, as long as we continue to emit CO2 the climate will continue to warm and the weather extremes – which we now see with our own eyes – will continue to intensify. Thankfully we know what to do: stop emitting CO2.”


Dr Michael Byrne, Lecturer in Earth & Environmental Sciences, University of St Andrews & Marie Skłodowska-Curie Research Fellow, University of Oxford & a contributing author to the report, said:

“The new IPCC report reiterates, with near certainty, what we’ve known for more than a decade: climate is warming, human activities are the main driver, and it’s going to get worse.

“What is different now, and different about this report, is that the effects of global warming are no longer in the distant future or in far-flung corners of the world. Climate change has arrived into our daily lives and is here to stay. We knew what was coming and now it’s here.

“Extreme weather events – from the heat dome in Canada to flash flooding in Germany to wildfires in Greece – now bear the hallmarks of climate change, and are causing devastation around the world week on week. For me, as a climate scientist, the report is particularly significant because it is the first to present clear evidence linking these extreme events to human activities. That evidence was inconclusive back in 2013, at the time of the previous IPCC report; now it’s irrefutable.

“To slow and stop global warming, emissions of greenhouse gases need to be rapidly reduced to ‘net zero’. The report is stark: to limit warming to 1.5C – beyond which the impacts become much more severe – time is rapidly running out. The UK and Scottish targets of net zero by 2050 and 2045 are not fast enough, we have to be more ambitious. Let’s hope our political leaders, as they gear up to COP26 in Glasgow this November, take heed.”


Prof Jonathan Bamber, Professor of Glaciology, University of Bristol & a contributing author to Chapter 9 – Ocean, cryosphere, and sea level change, said:

“Sea level rise is one of the most pernicious, guaranteed consequences of global heating and, for the first time in an IPCC assessment report, we have estimates, not just of the likely range of values, but also the lower probability, high consequence estimates such as at the 5% confidence level. It is clear from these estimates that a precautionary principle is the only sane approach to take because the consequences of anything else are likely to be disastrous for society, especially beyond 2100. That might seem like a long way away but there are millions of children already born who should be alive well into the 22nd century.”


Prof Richard Betts, Head of Climate Impacts Research, Met Office Hadley Centre & a contributing author to the report, said:

“It’s important to note that 1.5C global warming is not a physical threshold when catastrophic impacts will suddenly kick in, as it is often portrayed. It’s an indication of where the risks start to become substantial. Like the speed limit on a motorway, staying below it is not perfectly safe and exceeding it does not immediately lead to calamity, but the risks do increase if the limit is passed.

“Limiting warming to 1.5C clearly needs much more urgent emissions cuts than is currently happening, but if the target is still breached we should not assume all is lost and give up – it will still be worth continuing action on emissions reductions to avoid even more warming.”


Third party  comments (from reviewers of the report):

Prof Rowan Sutton, National Centre for Atmospheric Science, University of Reading, said:

“The IPCC Working Group 1 report is a comprehensive and vital update to our knowledge of how anthropogenic climate change is unfolding and causing increasingly severe impacts in all regions of the world.

“These changes and impacts are entirely expected based on previous IPCC Assessment Reports, but it is notable that the recent rate of global surface warming is at the upper end of expectations provided in the previous (2013) IPCC Assessment Report, with an estimated further warming of 0.19oC since 2003-12. The average rate at which heat is accumulating in the climate system has also increased in recent decades (from 0.50 W/m2 for the period 1971-2006 to 0.79 W/m2 for the period 2006-18), as has the rate of global sea level rise (from 1.9mm/year between 1971 and 2006 to 3.7mm/year between 2006 and 2018).  As a direct consequence, the impacts of climate change on extreme weather, and on people, are increasingly apparent and severe in every inhabited region of the globe.

“These new results make plain that any hopes that climate change might turn out to be “not-as-bad-as-expected” were forlorn.  The predictions were accurate: it is happening now, it is happening very fast, and we must deal with it.  Dealing with it means taking the urgent actions required to reduce greenhouse gas emissions as rapidly as possible, and – at the same time – investing in the measures required to adapt societies and make them resilient to events and conditions never previously experienced.

“We must also continue to monitor climate change very closely: continued quantification and understanding of the ongoing changes and their impacts is essential to enabling societal responses that will be effective and robust.”


Prof Dann Mitchell, Met Office joint chair in climate hazards, University of Bristol, said:

“The latest IPCC report concludes that the warming of the atmosphere due to human influences is unequivocal. Since its predecessor in 2013, the global surface temperatures have increased by around 0.2C, again due to human activities, showing us first hand some of the consequences of our inaction in reducing carbon emissions.

“We know that warming of the atmosphere and land increases the chances of extreme heat waves, and this latest report is virtually certain that these events have become more frequent and more intense since the 1950’s. This statement has never been more obvious than at this moment in time, as we stand in the wake of the most extreme heat wave ever recorded in Western Canada and the US, and during the ongoing mass wildfires in Southern Europe that are impacting so much of our lives.“


Prof Gabi Hegerl, Professor of Climate System Science, University of Edinburgh, said:

“First off, the country delegations as well as the scientists should be congratulated that they managed to conduct this involved plenary approval session remotely, in a two week marathon, and arrived at a very helpful and interesting report. Many of the findings in earlier reports have been strengthened and sharpened in this report. Importantly, the need for a limited carbon budget and transition to net zero emissions has been confirmed, and the uncertainty around the climate sensitivity narrowed.

“This report also shows some new ideas and important innovations, focusing more on climate risks than possible in earlier reports. It confirms impressively to what extent we are already experiencing changes in extreme weather and climate extremes, from heat waves to heavy rainfall to drought. The report now explicitly discusses compound extreme events, such as heat with drought, and fire risk. It shows with ever more clarity that the more we limit global warming by rapidly transitioning to net zero emissions, the more we avoid moving into a dangerous world. Recent extremes have highlighted climate risks. Unless we transition to net zero, we will see increasingly more damaging events. On the other hand, the report now also highlights that within about 20 years, serious mitigation efforts would lead to a different climate compared to a world where we fail to limit global warming. So babies born today will experience different worlds as adults depending on if we limit climate change or fail to do so.”


Prof Eric Wolff, Climatologist and Royal Society Professor at the Department of Earth Sciences, University of Cambridge, said:

“IPCC confirms, but with even more urgency, what previous reports said: climate change is happening, it’s due to human influence, and we can only avoid the more dangerous consequences by reducing emissions of greenhouse gases as far and as soon as we can. It also sets a challenge for the COP26 negotiators: the pathways implied by commitments and policies so far confirmed are not enough to meet even the higher 2 degree target set in Paris.”


Prof Chris Hilson, Director of the Centre for Climate and Justice at the University of Reading:

“The challenge for previous IPCC reports was that the science was anticipating the future. That was always going to leave room for doubters and excuses for delay. However, with recent global floods, wildfires and heatwaves, the science is now confirming people’s actual experience of climate change. Governments must now act on the science to deliver a COP26 outcome that can prevent this experience from getting much worse.”


Prof Keith Shine, Regius Professor of Meteorology and Climate Science, University of Reading, said:

“I was heavily involved in IPCC’s first assessment report back in 1990. It is sobering to see how the science has moved on. In 1990 we weren’t even sure that the then-observed climate change was due to human activity. The IPCC now says the evidence is “unequivocal”. There is no hiding place for policymakers attending COP26: if we are to limit global warming to 2 degrees or less, CO2 emissions must start to be reduced rapidly.”


Prof Dave Reay, Director of Edinburgh Climate Change Institute, University of Edinburgh, said:

“This report should send a shiver down the spine of everyone who reads it. In the unblinking delivery style of the IPCC it sets out where we are now and where we are headed and climate change: in a hole, and still digging.

“Extreme temperatures, and the loading of the heat wave dice our emissions are causing, get deserved attention. A searing ‘1-in-50 year event’ of the past for instance, is already more than 4 times more likely and, under a high emissions future, could become around 40 times more likely, and 5 degrees hotter to boot.

“What really stands out are the new assessments of tipping points in our global climate system, and the climate change lock-ins we are forging for scores of generations to come. For the tipping points – things like massive die-back of the forests and belching of carbon from thawing soils – it’s clear that every extra tonne of CO2 emitted today is pushing us into a minefield of feedback effects tomorrow.

“For climate change lock-ins, the long-term sea level rise assessments are simply gut-churning: Impacts that are irreversible for millennia and where scenarios of 2 metres of sea level rise by the end of the century and a Hollywood-esque 15 metres by 2300 cannot be ruled out.

“When world leaders and their negotiation teams gather here in Scotland for COP26 they must have the findings of this report seared into their minds, and take the urgent and far-reaching action required. This is not just another scientific report. This is hell and highwater writ large.”


Third party comments (from those who did not review the report):

Dr Nick Starkey, Director of Policy at the Royal Academy of Engineering:

“This latest report from the IPCC puts in clear terms the scale of the present threat of climate change, and demonstrates the need for a clear, comprehensive and specific strategy for climate action.  The UK is not on track to meet our existing carbon targets, and our goal of 78% emissions reduction by 2035 will not be reached without deep energy efficiency measures and ensuring that policies right across government actively contribute to achieving this target.

“There is no single technology, solution or action that can address the challenge alone, so a successful strategy must connect many different solutions and actions as a whole system. This means recognising the increasing connectivity and interdependency of the world’s infrastructure and economies, setting a clear time frame and process for eliminating the emissions within our control, and engaging citizens to help fairly distribute the costs and benefits of the transition to net zero. Engineers of all kinds have a crucial role decarbonising the UK, both in developing innovations and new solutions, and in designing, building, retrofitting, operating and ensuring the safety of the infrastructure and technologies for a decarbonised UK.”


Prof Mark Maslin, Professor of Earth System Science, University College London, said:

“This report is a scientific and political wake-up call for all governments of the world to take climate change seriously. It acts as a reminder of the urgency of the climate crisis and warns . Governments must now work in solidarity, taking heed of this report and increasing their ambitions to ensure we can avoid the worst effects of climate change, adapt to its impacts, and protect our most vulnerable populations. The IPCC Science report gives Governments the essential tools to rapidly adapt to climate change and shows emission pathways needed to limit climate change.

“The science in this report agreed by 195 countries must inform the firm pledges, commitments and binding agreements that will be made at COP26 later this year, helping ensure we can halve global emissions by 2030 and hit net zero by 2050 to limit warming to 1.5 ˚C.”


Prof Tim Palmer, Royal Society Research Professor at the University of Oxford, said

“The IPCC Sixth Assessment Report (AR6) states that it is unequivocal that human influence has warmed the atmosphere, ocean and land, and that this warming will continue as we keep emitting greenhouse gases into the atmosphere. It is very hard to imagine any credible scientist doubting this. Indeed, if we do not halt our emissions soon, our future climate could well become some kind of hell on Earth.

“In this respect, the real world may be running ahead of what the AR6 climate models can simulate. In particular, the extraordinary heat extremes observed recently in British Columbia, and the rainfall extremes over Zhengzhou and the Rhineland, lie well outside the range of what can be simulated by the AR6 models. This reminds us that whilst the case for decarbonisation is clear, the science of climate change is far from done and dusted. This has implications. Shortcomings in our ability to simulate climate extremes make it difficult to prioritise adaptation strategies, not least in trying to improve climate resilience in developing countries. These shortcomings also make it hard to know whether the climate system is really reaching some kind of irreversible tipping point  – one that would make pointless, negative emissions later in the century.

“With AR6 now delivered, this is the time for a new discussion on how best to advance the science of climate modelling, so that we can plan for the future with greater confidence. Arguably, relying on a relatively large number of relatively coarse resolution models – as IPCC has done over the decades – may not be the most effective use of our resources. This should be an issue for discussion  at COP26.” 


Prof Simon Lewis, Professor of Global Change Science, University College London, said:

“The new report has serious power because it is written by independent scientists from across the globe but was commissioned by the world’s governments and the summary conclusions signed off by those same governments. The summary report is an agreement by the world’s governments on the science of climate change today.

“The very careful scientific language of the new report contains a stark message: the level of devastation we face is in our collective hands. If the world slashes carbon dioxide emissions now and reduces them to net zero by 2050 we would keep the global temperature rise close to 1.5 degrees C and avoid the worst impacts.

“Cutting emissions to net zero means keeping fossil fuels in the ground, ending deforestation, and rapidly moving to using renewable energy to power the global economy. The new report significantly raises the pressure on world leaders to agree detailed and achievable plans to immediately cut emissions when they meet for the UN climate summit in Glasgow in November.”


Prof Dame Jane Francis, Director of British Antarctic Survey, said:

“This latest IPCC report is the state-of-the-art assessment concerning regional and global climate change and is only possible by integrating all the relevant scientific findings to date.  This highly influential report provides the evidence base and impetus to develop business and policy strategies that will help people around the world live with and adapt to change.  There is a clear message here that science is part of the solution.”


Prof Mike Meredith, Head of the Polar Oceans team, British Antarctic Survey, & a Coordinating Lead Author of the IPCC’s 2019 Special Report on the Ocean’s and Cryosphere in a Changing Climate, said: 

“As our planet overheats, its frozen regions are shrinking and its oceans are swelling. The dramatic changes in the polar regions are extremely concerning – we are seeing rapid sea ice loss in the Arctic, warming and acidification of the oceans, impacts on biodiversity and the ecosystem, and loss of ice from Antarctica and Greenland pushing up sea levels globally.  These changes are influencing all parts of the planet, and every one of its inhabitants. This new report reinforces the seriousness of these crises and drives home the urgent need to address the causes of climate change.”


Dr Anna Jones, interim Director of Science, British Antarctic Survey, said:

“The polar regions have long been amongst the world’s biggest data deserts, and this has hampered greatly our efforts to understand the changes there and to make accurate projections of their futures.  In recent years there has been an enormous effort on the part of scientists from British Antarctic Survey, working in close collaboration with national and international partners in both the Antarctic and Arctic to plug that knowledge gap.  Seeing the data from these regional efforts integrated with global science and applied to the urgent problem of climate change emphasises the value of these hard-won data and reinforces that much is yet to be done if we are to meet the targets set by the Paris Agreement.”


Dr Alison Ming, Leverhulme Trust Early Career Fellow at Cambridge’s Department of Applied Mathematics and Theoretical Physics and Coordinator of the Cambridge Centre for Climate Science, said:

“Climate models have improved significantly over the past decades and with this has our confidence in their predictions. We are now at the stage where we can attribute large-scale observed changes to the rising levels of carbon dioxide. This is possible because of our better understanding of the feedbacks between the various parts of the climate system. The report makes it clear that heatwaves have become more frequent and more intense. As we add more carbon dioxide to the atmosphere, these record-breaking events will become the norm — every fraction of a degree of warming we can avoid will count.”  


Prof Andrew Watson FRS, Royal Society Research Professor, University of Exeter, said:

“The IPCC report gives a comprehensive update on the “knowns” of climate change over the present century, and it makes for grim reading.  The report also makes the point that climate models don’t include “low probability-high impact” events that become more likely the more that climate is changed. These are events such as ice sheet collapse, sudden changes in ocean circulation, or catastrophic wildfires. These “known unknowns” are scarier still. An important reason for taking urgent action on emissions is to decrease the chances that one or more of these world-changing events will be triggered over the 21st century.”


Prof Martin Siegert, Co-Director, Grantham Institute, Imperial College London, said:

“On sea level rise, the AR6 is a much better reflection of the state of knowledge than previous ARs that have tended to under-estimate, or even ignore, the highest possible outcomes.

“There are great uncertainties in ice sheet processes (the dominant contributor in future), and this lack of knowledge and the consequences for sea level projections – coupled with an appreciation of how ice sheets can change rapidly from palaeo studies – is reflected explicitly and properly.

“I can comment on a couple of the statements:

“’It is ‘virtually certain’ that global mean sea level will continue to rise over the 21st century’. I would say it is ‘utterly certain’.

“’Human influence was ‘very likely’ the main driver of these [sea level] increases since at least 1971.’ I would say also that it is ‘utterly certain’.

“It is good to see this report deal with the sea level problem well, and I congratulate the authors.

“Reducing uncertainty requires much work in glaciology, as ice loss is now the dominant contributor. International cooperation here is essential but is also rather lacking at the scale that is needed. COP26 offers an opportunity for the international community at UN level to propose an integrated investigation of this essential problem, expanding and coordinating international efforts to reduce uncertainties within a 10 year timeframe. Adaptation planning would benefit hugely from this.”


Dr Ella Gilbert, Post-doctoral research assistant, University of Reading, said:

“Climate change is happening before our very eyes. Recent events across the world only emphasise that – from devastating wildfires in California, Greece and Turkey, to floods in England, Belgium and Germany, to heatwaves in Siberia and Canada. This report reminds us of what we already know – that these kinds of events will only become more frequent and hard-hitting with ongoing warming.

“Every IPCC report comes with stronger evidence for the damage we are doing to our planet. This time is no different. We have already committed to more warming, more ice loss from glaciers and ice sheets, and more sea level rise. We will probably see summers without Arctic sea ice before 2050 and the Greenland and Antarctic ice sheets will continue to shrink. We may even be hurtling towards unpredictable and irreversible tipping points in the climate system.

“Our models and our understanding of the climate have improved since AR5, but the basic message is the same: we are changing every corner of our planet.

“The science should come as no surprise – but it serves as a stark warning. As the report says – ‘every tonne of CO₂ emissions adds to global warming’ – so what we do now has never been more important.”



‘Climate Change 2021: The Physical Science Basis’ by Working Group I was published by the IPCC at 09:00 UK time on Monday 9th August.



Declared interests:

Prof Rowan Sutton: reviewed parts of the WG1 AR6 report; Lead Author of the WG1 AR5 report.

Prof Nigel Arnell: lead author for Chapter 12 – Climate change information for regional impact for risk assessment

Prof William Collins: lead author for Chapter 7 –  The Earth’s energy budget, climate feedbacks, and climate sensitivity, Technical Summary author, SPM contributing author, said:

Prof Richard Allan: IPCC lead author on Chapter 8 – Water cycle changes and contributing author on Chapter 11 – Weather and climate extremes events in a changing climate and drafting author of Summary for Policy Makers.

Prof Corinne Le Quéré: A contributing author to Chapter 5 – Global carbon and other biogeochemical cycles and feedbacks

Prof Jonathan Bamber: A contributing author to Chapter 9 – Ocean, cryosphere, and sea level change

Prof Richard Betts: “I was involved in the IPCC AR6 WG1 report as a Contributing Author in a number of sections, including the section that deals with the implications of the revised estimate of historical warming and it’s implications for the projected timing of passing 1.5C global warming. Regarding other interests, I am employed by the Met Office and much of my work is funded by the UK govt via BEIS and Defra.”

Prof Dann Mitchell: Dann was a Review Editor for chapter 3 of the report.

Prof Gabi Hegerl: “I was a review editor of this approved report, which means that i was able to observe the process of conducting the assessment, but without active participation.”

Prof Eric Wolff:I sent in review comments for three chapters of the second order draft last year, but have no other conflicts of interest.”

Prof Chris Hilson: Prof Hilson was a reviewer for the section of the report on climate change litigation.

Prof Keith Shine: I was an expert reviewer of multiple drafts of the WG1 AR6 assessment report (notably Chapter 7)

Prof Dave Reay: Reay was an expert reviewer for Working Group 1 of the IPCC Sixth Assessment Report.

Prof Mark Maslin: No interests to declare.

Prof Tim Palmer: I was not involved in the report in any way. 

Prof Simon Lewis: No competing interests to declare.

Prof Dame Jane Francis: None

Prof Mike Meredith: A Coordinating Lead Author of the IPCC’s 2019 Special Report on the Ocean’s and Cryosphere in a Changing Climate

Dr Anna Jones: None

Prof Andrew Watson: I am a researcher in Earth systems science, funded by the UK research councils and the Royal Society,  at the Global systems institute at the University of Exeter. I’m not an author or a reviewer of the current report though I have contributed to some past IPCC reports.

Prof Martin Siegert: None

Dr Ella Gilbert: None

Dr Alison Ming: I do not have any conflicts of interest. I am not involved in the report in any capacity (author or reviewer or other).

None others received.


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