New climate change research suggests that the world is already past the point of no return for global warming.
The study, published in Scientific Reports - an online peer-reviewed open access scientific journal published by Nature Research - today finds that even if human-induced greenhouse gas emissions can be reduced to zero, global temperatures may continue to rise for centuries afterwards.
But prominent scientists from around the world, including Victoria University of Wellington's head of school of geography, environment and earth sciences, professor James Renwick, have already challenged the report's conclusions.
The study's authors, Jorgen Randers and Ulrich Goluke of the Norwegian Business School in Oslo, 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 purpose of this article is to report that we have identified a point-of-no-return in our climate model ESCIMO - and that it is already behind us," the authors said.
"In ESCIMO the global temperature keeps rising to 2500 and beyond, irrespective of how fast humanity cuts the emissions of man-made greenhouse gas emissions."
Their modelling suggested that under conditions where man-made greenhouse gas emissions peaked during the 2030s and declined to zero by 2100, global temperatures would be 3°C warmer and sea levels 3 metres higher by 2500 than they were in 1850.
Under conditions where all human-generated emissions were reduced to zero during the year 2020 the authors estimated that, after an initial decline, global temperatures would still be around 3°C warmer and sea levels would rise by around 2.5 metres by 2500, compared to 1850.
Global temperatures would continue to rise because the melting of Arctic ice and carbon-containing permafrost might 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.
"This cycle appears to be triggered by global warming of a mere + 0.5 °C above the pre-industrial level," the authors say in their report.
The modelling suggested that to prevent the predicted temperature and sea-level rise, all man-made greenhouse gas emissions would have had to be reduced to zero between 1960 and 1970.
The authors said now 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 to prevent temperature and sea-level rise continuing once all emissions have ceased.
"In other words, building 33,000 big CCS plants and keep them running for ever. This is technically feasible but would be hugely expensive," the report said.
Results at odds with science community understanding
Renwick said the results presented in the paper were interesting but at odds with the science community's understanding of how the climate was 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," Renwick - who was a lead author for the IPCC 6th Assessment Report - said.
"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 implied a full "earth system model" was used, the paper was actually based on a "low complexity model" that captured only the broadest features of the climate system, Renwick said.
The ESCIMO model had been shown to have a tendency towards "runaway" climate change generally, partly because it overstated the warming effect of decreasing albedo (the fraction of sunlight reflected by the earth) and the amount of greenhouse gases released from thawing permafrost, he said.
"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, Renwick said.
"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."