The coal-fired power plant shown here emits not only carbon dioxide, but also nitrogen oxide and particulates. Including more types of emissions increases the amount of warming that humans have committed to by past emissions.
The coal-fired power plant shown here emits not only carbon dioxide, but also nitrogen oxide and particulates. Including more types of emissions increases the amount of warming that humans have committed to by past emissions.

Countries around the world pledged in the Paris Agreement to limit warming to 1.5 degrees Celsius, or, at most, 2 degrees Celsius. As emissions rates gradually begin to decline, countries are looking at how many greenhouse gases can still be emitted while remaining below these temperature targets, which are deemed the upper limits to avoid the most catastrophic impacts to the climate system.

New research led by the University of Washington calculates how much warming is already guaranteed by past emissions. While previous research has explored this question for carbon dioxide, the new work includes related emissions such as methane, nitrogen oxide and aerosols, like sulfur or soot.

Under a moderate future emissions scenario, by 2029 the planet has a two-thirds chance of at least temporarily exceeding warming of 1.5 degrees Celsius, 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 2 degrees Celsius. The study was published June 6 in Nature Climate Change.

“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 UW 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.”

“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,” said co-author Kyle Armour, a UW associate professor of atmospheric sciences and of oceanography. “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.”

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