The findings of the reduced complexity model of the global climate between 1850 and 2500, published in Scientific Reports, can be read here.

Jorgen Randers and Ulrich Goluke used a reduced complexity earth system model (ESCIMO) to study the effect of different greenhouse gas emission reductions on changes in the global climate from 1850 to 2500 and created projections of global temperature and sea level rises.

The modelling suggests that under conditions where  change peak during the 2030s and decline to zero by 2100, global temperatures will be 3°C warmer and sea levels 3 metres higher by 2500 than they were in 1850.

Under conditions where all anthropogenic greenhouse-gas emissions are reduced to zero during the year 2020 the authors estimate that, after an initial decline, global temperatures will still be around 3°C warmer and sea levels will rise by around 2.5 metres by 2500, compared to 1850.

The authors suggest that global temperatures could continue to increase after anthropogenic greenhouse gas emissions have reduced, as continued melting of Arctic ice and carbon-containing permafrost may increase the levels of water vapour, methane and carbon dioxide in the atmosphere. Melting of Arctic ice and permafrost would also reduce the area of ice reflecting heat and light from the sun.

To prevent the projected temperature and sea level rises, the authors suggest that all anthropogenic greenhouse gas emissions would have had to be reduced to zero between 1960 and 1970.

To prevent global temperature and sea level rises after greenhouse gas emissions have ceased, at least 33 gigatonnes of carbon dioxide would need to be removed from the atmosphere each year from 2020 onwards through carbon capture and storage methods, according to the authors.

The Science Media Centre  asked experts to comment on this research.

it has reported these comments:

Dr Laura Revell, Senior Lecturer in Environmental Physics at the University of Canterbury, comments:

“Many of the models used to inform the IPCC assessments are Earth system models. These are highly complex models, and represent the state-of-the-art when it comes to climate modelling. Earth system models typically consist of an atmosphere model, an ocean model, a sea ice model and a land surface/biogeochemistry model all interacting with one another. Earth system models typically contain hundreds of thousands of lines of code, and require a supercomputer to run on. Even then, a 100 year simulation may still take several months to run.

“Because of the level of complexity and computational cost associated with running Earth system models, “reduced-complexity climate models” have been developed. Although the title of their paper refers to “an earth system model”, the model used by Randers & Goluke is in fact a reduced-complexity climate model. Such models can perform a 100 year simulation on a desktop computer in a matter of seconds. However, the trade-off is the complexity of the Earth system processes represented. Reduced-complexity climate models contain highly simplistic representations of the climate system and how different processes interact with one another. The authors do acknowledge this by inviting other researchers to investigate their findings with complex models.

“It’s already well known that if we stopped all greenhouse gas emissions from human activities today, the temperature would continue to rise for years to come since greenhouse gases are so long lived in Earth’s atmosphere. What is surprising from this study is that their model shows that ceasing emissions immediately could still cause permafrost thawing in the 2100s and beyond.

“Permafrost thawing releases methane and carbon dioxide from plant material trapped in the ground. The model used by Randers & Goluke shows that melting of Arctic ice continues even after greenhouse gas emissions cease, leading to permafrost thawing, emission of carbon dioxide and methane from permafrost, and amplification of the greenhouse effect. Therefore global warming continues.

“This is an interesting finding, but, as stated by the authors, must be also investigated with complex Earth system models. A couple of caveats I think are important are: firstly, this study uses just one model, and a reduced-complexity model at that. Secondly, state-of-the-art biogeochemical models simulate much smaller increases in carbon release from permafrost in future compared with the model used in this study. This suggests that the processes determining permafrost thawing and related feedbacks are not well constrained in this model.

“Irrespective of when any tipping points in the climate system occur, our efforts to reduce greenhouse gas emissions and prepare for climate change should continue. Just this week we’ve seen an extraordinary flooding event in Napier – classified as a ‘one-in-250-year’ event. These sorts of extreme events are more likely to occur in a warmer world. Preparation and emissions reduction today will help us in future, no matter what.”

No conflict of interest.

Professor James Renwick, Head of School of Geography, Environment and Earth Sciences, Victoria University of Wellington, comments:

“The results presented in the paper are interesting but are really at odds with the science community’s understanding of how the climate is changing. The latest round of climate model simulations, run in support of the 6th Assessment Report of the Intergovernmental Panel on Climate Change, show that if greenhouse gas emissions were to stop immediately, there is likely to be very little further increase in temperatures and no sign of warming resuming in future. If greenhouse gas emissions are reduced in line with the Paris Agreement, the climate would stabilise over the coming century. Some things, notably sea level rise and ice melt, would continue for longer, but at a reducing rate.

“Although the title of the paper implies a full “earth system model” was used, the paper is actually based on a “low complexity model” that captures only the broadest features of the climate system. The climate warms into the future in this model because its “albedo” (the fraction of sunlight reflected by the earth) decreases over time as ice melts and permafrost thaws, releasing carbon dioxide and methane. Ice loss and permafrost thaw are well known to be amplifying factors for climate change, but not in the dramatic way presented here. The ESCIMO model has been shown to have a tendency towards “runaway” climate change generally, partly because it overstates the warming effect of decreasing albedo and the amount of greenhouse gases released from thawing permafrost.

“In short, the results presented in this paper are very implausible and should not be seen as cause for alarm. Effective climate action by the global community, in line with New Zealand’s Zero Carbon Act and the Paris Agreement, would be effective at stopping climate change at somewhere between 1.5 and 2°C of warming above pre-industrial levels. This would be associated with further increases in weather and climate extremes that would have significant consequences for communities worldwide, but there is no sign of any form of runaway climate change.”

Conflict of interest statement: ‘I am a lead author for the IPCC 6th Assessment Report.’

Additional comment from the UK Science Media Centre:

Prof Richard Betts MBE, Met Office Fellow, Head of Climate Impacts Research, and Chair in Climate Impacts, University of Exeter, comments:

“While the press release suggests that global warming may now be unstoppable for centuries, the model result in this paper is not convincing as support for that message. The paper itself does not actually claim to be a prediction of the real world, it just reports the behaviour of one model – but the press release goes a big step further and presents it as a prediction. The model, which is not one used in the main IPCC projections, has not been shown to be credible enough to support confident predictions, and is contradicted by more established and extensively evaluated GCMs 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.”

No conflict of interest declared.

Sources: Scimex and the Science Media Centre