Expert reaction to study using a simulation to estimate how long global warming might continue for if greenhouse gas emissions are reduced to zero
Prof Sir Brian Hoskins, Chair of the Grantham Institute at Imperial College London, said:
“This is the sort of investigation with a toy model that should be done and is fun, but should not be given this sort of publicity until the processes involved have been investigated using more complex models and representations.”
Prof Mark Maslin, Professor of Climatology, UCL, said:
“Researchers from BI Norwegian Business School have used a low complexity earth system model to study the long-term effects of anthropogenic greenhouse gas emissions. In their simple model darkening of the Arctic due to melting ice cover, methane emissions from thawing permafrost, and more water vapour in a warmer world could warm the Earth by an additional 1-1.5˚C over the next 500 years. The authors suggest from the model that these long-term warming feedbacks do not start to take effect until the year 2150 but could add to global warming at a rate of 0.5˚C per 100 years. These results do need to be confirmed by more complex climate models used in the IPCC reports, because these results come from one model which has not undergone the rigorous cross checking and testing that is usual for climate models. Moreover, the model does not contain the essential small-scale Arctic processes need to confirm these supposedly self-sustaining warming feedbacks. In effect this study is a thought experiment that says may be there could be feedbacks that would continue to warm our world in the next few centuries.
“What the study does draw attention to is that reducing global carbon emissions to zero by 2050 is just the start of our actions to deal with climate change. To keep global warming to just 1.5˚C this century we already know we will have to have negative carbon emissions from 2050 to 2100. If this study is confirmed, then we may have to continue down drawing carbon dioxide from the atmosphere way beyond the end of this century. And I would suggest that if we have been able to successfully deal with climate change in this century, we will really will not have to worry about dealing with a much smaller warming over the next 400 years.”
Prof Grant Allen, Professor of Atmospheric Physics, University of Manchester, said:
“This study uses a reduced complexity model to simulate climate many centuries into the future. It suggests that if GHG emissions stopped today, a continued reduction in sea ice along with continued permafrost thaw due to climate warming already ‘baked in’ to the system, could lead to a sustained trend in rising temperatures well past the current 2 degree Celsius threshold thought to lead to dangerous consequences. Some aspects of this finding are not new – it has long been known that if all GHG emissions stopped today, it can take many decades for the climate to reach equilibrium because of the way heat is stored in the world’s oceans, during which air temperatures may continue to rise. This continued warming is a prediction that has already been reported in higher complexity models used for IPCC forecasting. However, this study suggests that already-activated positive feedbacks in the climate system could accelerate global warming in the future beyond what is predicted in higher complexity models.
“As the authors rightly suggest, further modelling work is required using higher complexity models, using the best possible understanding of potential positive feedback processes, to more fully understand or confirm its results, as the model used in this study cannot fully capture regional processes. The central conclusion that it may be necessary to lock away carbon already present in the atmosphere to prevent dangerous long term climate change, is a particularly important question to better understand in future work.”
Prof Chris Huntingford, UK Centre for Ecology and Hydrology, said:
“There is a growing realisation that even if emissions from fossil fuel burning reduce suddenly to zero, natural parts of the climate system could force temperatures to rise further. Here, the authors suggest the on-going melting of Arctic sea-ice and permafrost thawing can contribute to such extra warming. By the authors’ own wording, these results are somewhat provisional. However, sometimes influential research points to possibilities, the magnitude of which more detailed measurements and computer modelling later refine.
“It is also worth noting that the planet is currently a long way from a temperature equilibrium as the oceans presently extract vast quantities of heat from the atmosphere. Such present-day oceanic heat removal will eventually stop, and that will lead the planet to higher temperatures, even for stabilised atmospheric greenhouse gas concentrations. Hence achieving zero emissions, likely in tandem with deliberate carbon dioxide extraction from the atmosphere, are needed to not only stabilise but quickly lower current levels of CO2 in the atmosphere.”
Prof Richard Betts MBE, Met Office Fellow, Head of Climate Impacts Research, and Chair in Climate Impacts, University of Exeter, said:
“The model simulations in this paper do not convincingly support the suggestion that ending greenhouse gas emissions may fail to stop global warming. Importantly, the paper itself does not actually make that specific claim in relation to the real world – instead, it reports the results simply as the behaviour of a model. The model used here is not one used in the main IPCC projections, and is not shown to be a credible representation of the real climate system. In fact, it is directly contradicted by more established and extensively-evaluated climate models in many of its physical processes.
“This paper clearly may be cited in support of a misleading message that it is now “too late” to avoid catastrophic climate change, which would have the potential to cause unnecessary despair. However, the study is nowhere near strong enough to make such a frightening message credible.
“Incidentally, “melting permafrost” is not a scientifically correct phrase – permafrost does not melt, it thaws.”
Prof Andrew Watson FRS, Royal Society Research Professor, University of Exeter, said:
“This is a startling result, but the model being used is a very simplified one, and doesn’t claim to be simulating in any detail the processes at work. In particular, the authors predict a big release of carbon from thawing of Arctic permafrost, much larger than in more sophisticated models – though this is a very uncertain area, with no agreement on how much carbon is released. In the paper the authors are up-front about the fact that it is a simple reduced-complexity model, and that in some respects it doesn’t agree with more complex models. While we expect that some global warming will be long-lasting and difficult to reverse, this study is far from definitive – it is a call for more work to be done on the long-term effects, rather than a firm prediction.
“Even if the paper is right in every respect and we are already committed to at least 3 degrees warming if we stop emissions tomorrow, this warming will take 500 years. This is preferable to 3 degrees warming over 100 years, which would be far more disruptive and might happen if we don’t cut emissions.”
Prof Phillip Williamson, Honorary Reader, University of East Anglia, said:
“This study provides evidence for what we don’t want to hear: that global heating may have already become self-reinforcing, and that we have therefore passed the point of no-return for halting long-term climate change. Scientists who study the Earth’s past climate changes won’t be surprised, since they are well aware that the global climate system is inherently dynamic – naturally changing from one state to another over millennia, without need for any human involvement. Yet those effects and time-scales are not well-represented in most climate models, with their default being stability and human omnipotence, focussed on just a few decades ahead.
“The authors recognise the significance, and limitations, of their work, inviting other model builders to test their assumptions and explore their results in greater detail. But their findings should not be misinterpreted as saying that we’re doomed, with nothing that can be done to make any difference. On the contrary, the differences between their scenarios 1 and 2 (net-zero by the end of the century and net-zero now) are dramatic, giving the choice of climate catastrophe within our children’s lifetimes or keeping future temperature increases below 1.5C for at least a century. The longer-term solution will be challenging, but the short-term prognosis is clear enough: there is a climate emergency, requiring immediate action.”
‘An earth system model shows self‑sustained melting of permafrost even if all man‑made GHG emissions stop in 2020’ by Jorgen Randers and Ulrich Goluke was published in Scientific Reports at 16:00 UK time on Thursday 12 November 2020.
Prof Mark Maslin: “No conflict to declare.”
Prof Grant Allen: “I hold NERC research funding through a number of projects to study emissions of greenhouse gases from natural and manmade sources.”
Prof Chris Huntingford: “I have no conflicting interests.”
Prof Richard Betts: “I’m a climate scientist at the Met Office Hadley Centre and University of Exeter, a Lead Author with the IPCC, and Technical Lead on the UK’s 3rd Climate Change Risk Assessment. I had no involvement in this research.”
Prof Andrew Watson: “I’m a Research Professor at the University of Exeter studying climate and the carbon cycle. I don’t have any involvement with this research.”
Prof Phillip Williamson: “No competing interests.”
None others received.
This article appeared on the Science Media Centre website at https://www.sciencemediacentre.org/expert-reaction-to-study-using-a-simulation-to-estimate-how-long-global-warming-might-continue-for-if-greenhouse-gas-emissions-are-reduced-to-zero/]]>