Extract from ABC News
Scientists are concerned climate change consequences could be hard to reverse if the world follows current mitigation pathways. (Getty Images: Rehman Asad/NurPhoto)
In short:
A study of scenarios where the world temporarily overshoots 1.5C of warming, compared to pre-industrial levels, raises concerns for consequences that may not be reversible for decades or longer.
Researchers warn rising sea levels could be as high as an additional 40 centimetres due to peak warming under overshoot scenarios.
What's next?
Climate scientists are urging policy makers to focus on emissions reductions rather than expensive attempts to capture or remove carbon dioxide and other greenhouse gases from Earth's atmosphere.
Temporarily overshooting the world's current target for limiting global warming could still result in dire climate outcomes, such as rising sea levels, which are irreversible on human timescales, according to new research.
An analysis of internationally accepted scenarios for limiting average global temperatures to a 1.5 degrees Celsius rise compared to pre-industrial levels, published today in Nature, suggests reversing the consequences of peak warming may take decades.
Staying under the aspirational target of 1.5C by 2100 has been modelled as possible under several scenarios by the United Nations' Intergovernmental Panel on Climate Change (IPCC).
Some of these scenarios include instances where the target is temporarily exceeded in the next 80 years before stabilising by the end of the century.
The year 2023 was the warmest on record and broke every other climate indicator, according to the World Meteorological Organization. (Supplied: World Meteorological Organization)
But study co-author Carl-Friedrich Schleussner said there was overconfidence in overshoot models to reverse temperature rises and their consequences.
"We find that 100 years of overshoot above 1.5C would commit about an additional 40 centimetres of sea level rise in the long run," Dr Schleussner, a science adviser at the not-for-profit climate science institute Climate Analytics, said.
"For some low-lying places, this could well be the difference between having a possibility to adapt and having to ultimately concede and migrate.
"Any potential temperature reversal after overshoot will take decades. That's much longer than the time horizons normally involved in adaptation planning."
Examining the unknowns of climate scenarios
The study, which involved 30 scientists from several international institutions and organisations, sought to better understand the uncertainty in just how high temperatures could peak in the event of an overshoot.
Imperial College London climate scientist and study co-author Joeri Rogelj said an additional 0.1C in overshoot scenarios might sound manageable, but this was only a median figure.
The study showed — based on the IPCC's own assessments — that there was a one in 10 chance warming would exceed two degrees in an overshoot.
"Instead of being able to reverse warming, we might need substantial amounts of carbon removal to stop warming from increasing further in the first place," Professor Rogelj said.
Melting permafrost in countries like Canada can lead to long-buried and frozen organic material producing methane. (Flickr: UBC Micrometeorology, Scar of permafrost disturbances, CC BY 2.0)
Exceeding the 1.5C target increases the risk of the Earth systems passing critical tipping points, such as melting permafrost or continued sea level rise for centuries to millennia.
The study found every 100 years of overshoot above 1.5C would result in an additional 40cm of sea level rise, on top of a baseline increase of 80cm, by 2300.
Global warming may also produce further warming: an additional 0.02C could be generated by melted permafrost every 100 years of an overshoot, according to the research.
What about carbon capture and removal?
Many of the IPCC's 1.5C overshoot scenarios rely on carbon dioxide removal to meet the world's 1.5C target.
Carbon dioxide removal includes initiatives such as reforestation or direct carbon capture for high-emission fossil fuel developments.
Professor Rogelj said the world may need to consider more substantial carbon dioxide removal under the overshoot scenarios than had previously been modelled.
But he said there was also an overconfidence the world could achieve the level of carbon dioxide removal already assumed necessary.
Australian National University climatologist Mark Howden, who was not involved in the study, said there was a lot of attention on direct carbon capture which was feasible, but costly, and small in scale.
"We could invest in this significantly but it is likely still to max out at around 2 gigatonnes of carbon dioxide per year."
Professor Howden said there was an imperative to pursue emissions reductions rather than carbon capture, given the world produced about 40Gt of carbon dioxide every year.
The world's largest carbon capture system, located in Australia at Chevron's Gorgon Project, has failed to meet storage targets. (Supplied: Chevron)
Wageningen University climate policy scientist Niklas Höhne, who was also not involved in the study, said the research showed there was uncertainty around whether temperatures would decline if carbon dioxide was removed from the atmosphere in the second half of this century.
"Knowing that damages can already be extreme at [the] current temperature increase of 1.3C … the paper is a plea to reduce emissions as fast as possible immediately," he said.
"Much faster than currently planned by national governments."
The world is fast running out of its remaining carbon budget for staying under 1.5C.
A recent study in Nature Climate Change suggested the best outcome the world could now reach by 2100 was 1.6C, and that was using the most ambitious mitigation pathways.
Ultimately, the lower the world can keep peak warming, the lower any additional unknown risks from temperature increases there will be, Professor Rogelj said.
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