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expert reaction to perspective piece on the trajectories of the Earth System

Scientists explore a “Hothouse Earth” scenario in a study published in PNAS.


Dr Phil Williamson, climate researcher at the University of East Anglia (UEA), said:

“The Hothouse Earth scenario explored by Will Steffen and his co-authors others remains uncertain: it is not covered in most models of climate change.  But unfortunately that does not make uncontrolled warming implausible.  On the contrary, recent geology shows us that the Earth’s climate system is inherently nervy – with stability for more than a few centuries being the exception rather than the rule.  For the past million years or so, the interplay of physical and biological processes has resulted in the cooling and warming of the ice age cycle, with sea level changes of around 100 m occurring several times over the period of human evolution.  That didn’t matter that much to us then; it does now – with human processes also part of the mix.  As a result of human impacts on climate, the new paper argues that we’ve gone beyond any chance of the Earth cooling ‘of its own accord’.  Instead, there is the opposite risk, involving a vicious circle of positive feedbacks, each accentuating warming.  For example, by permafrost thawing, forest dieback and the biological release of carbon from the soil and ocean.  Together these effects could add an extra half a degree Celsius by the end of the century to the warming that we are directly responsible for ‒ thereby crossing thresholds and tipping points that seem likely to occur around 2⁰C, and committing the planet to irreversible further change, as Hothouse Earth.   There is still some scope for optimism, in that there is international agreement (with one rather important exception) to take action to avoid such consequences.  Steffen and his colleagues argue that we need to be much more proactive in that regard, not just ending greenhouse gas emissions as rapidly as possible, but also building resilience in the context of complex Earth system processes that we might not fully understand until it is too late.  In the context of the summer of 2018, this is definitely not a case of crying wolf, raising a false alarm:  the wolves are now in sight.


Prof Chris Rapley, Professor of Climate Science at UCL, said:

“Strategic decision makers ask the question ‘What is the worst that can happen?’ In this paper, Steffen et al explore that question with regard to the future state of the Earth. They find that the planetary scale effects of current human actions are not only driving it towards a “Hothouse” state not visited by the planet for at least 1.2m years, but are simultaneously weakening the “Gaia” feedbacks that have helped maintain a stable climate state conducive  to the growth of human civilisation and the general prosperity of the modern world. A feature of complex systems is the existence of thresholds which, when exceeded, cause the system to fundamentally and irreversibly reorganise. Steffen et al map out the components and interconnections of the Earth system that could correspond to such a threshold, and make a plausible case that we could be approaching such a transition. They suggest that the 2C or less warming “guardrail” adopted by the Paris accord would be a wise guideline to adopt. Previous research has shown that an increase in the mean global temperature of 11-12C would make more than half of the land area currently occupied by humans uninhabitable. So a “runaway” warming to a new and uncontrollable hot state would represent an existential threat to humanity and the majority of existing species. To avoid such a fate, Steffen et al point out that a deep transformation is required based on a fundamental reorientation of human values, equity, behaviour, institutions, economies and technology. Given the evidence of human history, this would seem a naive hope at the best of times. But at a time of the widespread rise of Right Wing Populism, with its associated rejection of the messages of those perceived as “cosmopolitan elites” and specific denial of climate change as an issue, the likelihood that the combination of factors necessary to allow humanity to navigate the planet to an acceptable “intermediate state” must surely be close to zero. The future habitability of the planet thus appears to rest on chance – that the sensitivity of the climate system to greenhouse gas emissions and other human disruptions is fortuitously very low – or that some other global scale social calamity dramatically reduces human emissions before any runaway planetary threshold is breached. The latter offers cold comfort.”


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

“This work stems from John Schellnhuber’s previous work that we can’t enter another ice age irrespective of orbital forcing, because of greenhouse gas levels pushing the system out of its periodical state. This was assumed by lots of quaternary scientists, but John put some numbers on it.

“Here, that idea is extended further, that this interglacial as a consequence of post-industrial greenhouse gas emissions, is atypical of other interglacials – and so the future earth response will also be different to what has been seen in the past.

“The geological record can advise on what the earth looks like under various CO2 scenarios, (Pliocene 5Ma with 400 ppm CO2; Cretaceous 100 Ma with 1000 ppm). We’re already at Pliocene levels, and heading to Cretaceous if we keep going as now.

“As in his previous work, John and co-workers discuss the nuances of this situation, and present the idea of a threshold from which we can’t pull back.  Again, threshold and tipping points have been discussed previously, but to state that ‘2C’ is a threshold we can’t pull back from is new, I think. I’m not sure what ‘evidence’ there is for this – or indeed whether there can be until we experience it. It’s just a ‘suggestion’ in any case.

“The paper is essentially an essay (or review of others work), rather than original research, but they’ve collated previously published ideas and theories to present a narrative on how the threshold change would work. It’s rather selective, but not outlandish.”


* ‘Trajectories of the Earth System in the Anthropocene’ by Will Steffen et al. will be published in PNAS on Monday 6th August.


All our previous output on this subject can be seen at this weblink:


Declared interests

Dr Phil Williamson: Dr Phil Williamson is an Honorary Reader at the University of East Anglia and Science Coordinator for the UK Natural Environment Research Council (UK Research & Innovation), working on global climate change research programmes.  His views do not necessarily represent those of UEA or NERC/UKRI.

None others received.


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