“Limited time:” World will lock in 1.5°C warming by 2025 without big emissions cuts

The world faces a greater than 50 per cent chance of locking in global warming of more than 1.5°C unless greenhouse gas emissions can be dramatically reduced before 2025, new research suggests.

In a new paper published in the journal Nature Climate Change, researchers from the University of Washington, Seattle, warn that the world needs an ‘abrupt cessation’ of greenhouse gas emissions to prevent locking in global warming above safe levels.

The research also confirms that net zero targets by 2050 are insufficient to cap average global warming below 2°C, and that does not include climate feedback loops that will accelerate temperature rises.

“Global warming is projected to exceed 1.5°C within decades and 2°C by mid-century in all but the lowest emission scenarios,” the paper says. “That is, there is limited time and allowable carbon dioxide emissions (a remaining carbon budget) before these temperature thresholds are exceeded.”

The research, led by oceanography researcher Michele Dvorak, used geophysical modelling that finds the world already has a 42 per cent chance of exceeding 1.5°C of global warming – even if further greenhouse gas emissions were immediately ceased.

Dvorak said the research provided important insight into the extent to which global warming has been locked in due to historical emissions and how much future emissions could contribute to a warming planet.

“It’s important for us to look at how much future global warming can be avoided by our actions and policies, and how much warming is inevitable because of past emissions,” Dvorak said.

“I think that hasn’t been clearly disentangled before – how much future warming will occur just based on what we’ve already emitted.”

“Our findings make it all the more pressing that we need to rapidly reduce emissions,” Dvorak added.

The probability of breaching this and higher temperature levels will increase year on year, the research shows, until the world achieves a status of zero net emissions.

“Assuming a medium-emissions trajectory, we find that we are already committed to peak warming greater than 1.5°C with 42 per cent probability, increasing to 66 per cent by 2029. Probability of peak warming greater than 2.0°C is currently 2 per cent, increasing to 66 per cent by 2057,” the paper says.

Even the common global goal of achieving zero net emissions by 2050 will likely see the world exceed 2 degrees of global warming. The researchers estimate that there would be a greater than 50 per cent chance of passing that threshold by following the 2050 timeline.

Under the Paris Agreement, national governments signed onto a goal to keep global warming to well below 2 degrees above pre-industrial levels and make best efforts to keep warming below 1.5 degrees.

In line with these commitments, countries have adopted a range of zero net emissions targets by 2050 – but the new research paper suggests that aiming for zero emissions by such a late deadline would mean the world would likely fail to achieve the Paris Agreement goals.

“The Paris Agreement has affirmed an international goal to hold global warming to well below 2 °C and to pursue efforts to limit it to 1.5°C relative to pre-industrial temperatures,” the paper says.

The outcomes of the researchers’ modelling support arguments that the target of zero net emissions by 2050 would be too slow, and countries need to be aiming for much faster emissions reductions.

“This paper looks at the temporary warming that can’t be avoided, and that’s important if you think about components of the climate system that respond quickly to global temperature changes, including Arctic sea ice, extreme events such as heat waves or floods, and many ecosystems,” paper co-author and associate professor of atmospheric sciences and oceanography, Kyle Armour, said.

“Our study found that in all cases, we are committed by past emissions to reaching peak temperatures about five to 10 years before we experience them.”

Quantifying the remaining carbon budget before 1.5 degrees of global warming becomes locked in, the research paper suggests that by 2021 the remaining carbon budget was just 120 gigatonnes of carbon dioxide equivalent emissions – representing around three and a half years at current levels of global emissions.

The remaining budget until 2 degrees of warming will be locked in was estimated to be 1,120 gigatonnes of carbon dioxide equivalent emissions – or around 32 years of emissions at current levels.

However, the researchers warned that the model used was not able to account for unexpected climate feedback effects, which could accelerate the rate of warming.

“Given that [the climate model] does not capture the possibility of future destabilising climate feedbacks such as decreased ice-sheet cover, thawing permafrost and methane hydrate dissociation due to ocean warming or a sea surface temperature pattern effect that allows for a more substantial shift towards destabilising cloud feedbacks in the future than modelled here.”

“These estimates of the timing of geophysical warming commitments may become underestimates as global temperatures rise,” the paper says.