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Even if all human-made greenhouse gas emissions were halted immediately, there’s a two-thirds chance the planet will exceed its warming limit of 1.5C, according to study into the extent of heating locked into the world’s climate by previous pollution.
Similar research has previously provided estimates of guaranteed warming from carbon emissions, but a team led by academics at the University of Washington now includes modelling of guaranteed temperature rises caused by other pollutants such as methane, nitrogen oxide and aerosols such as sulphur and soot.
While the rapid halting of carbon would directly cause a fall in global average temperatures, the new research reveals a more complicated picture.
This is because different emissions can either warm or cool the planet.
Most notably, particulate pollution reflects a proportion of sunlight back into space and therefore has a slight cooling effect, offsetting global heating.
But these particles then settle, meaning they are out of the atmosphere much more quickly than heat-trapping greenhouse gases.
Stopping all human emissions simultaneously will therefore produce a "temporary bump" of about 0.2C upwards which would begin abruptly when emissions stop and likely last for about 10 to 20 years, the scientists said.
“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,” said lead author Michelle Dvorak, a University of Washington doctoral student in oceanography.
“I think that hasn’t been clearly disentangled before – how much future warming will occur just based on what we’ve already emitted.”
The team said that under a moderate future emissions scenario, by 2029 the planet has a two-thirds chance of at least temporarily exceeding warming of 1.5C, even if all emissions cease on that date, the study finds.
If humans continue on a moderate emissions pathway, by 2057 there’s a two-thirds chance that the planet will at least temporarily exceed warming of 2C – the upper limit agreed as part of the 2015 Paris climate accord.
The study’s co-author Kyle Armour, a University of Washington associate professor of atmospheric sciences and of oceanography, said: “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.
“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.”
The research team warned that their study shows that if countries still want to achieve the goal of keeping global average temperature rises below 2C, then the total amount of carbon that humans can still emit – the remaining carbon budget – "is significantly smaller than previous estimates".
“Our findings make it all the more pressing that we need to rapidly reduce emissions,” Ms Dvorak said.
The team said that while global emissions are still rising, the rate at which pollutants are released into the atmosphere is now coming down, prompting countries to look at how many greenhouse gases can still be emitted while remaining below agreed temperature targets.
The study reveals the closing window of time to sharply reduce all greenhouse gases, but also reminds us that even a successful reduction in emissions could result in temporary temperature rises.
The research is published in the journal Nature Climate Change.