Is there even less time left for the Paris climate goal? | Polarjournal
In addition to carbon dioxide, the short-lived greenhouse gases methane and nitrogen oxide, as well as particles of soot and sulfur, are causing the atmosphere to warm, with the strongest effects already being felt in the polar regions. Photo: Julia Hager

The Paris climate agreement calls for everything to be done to limit global warming to 1.5 or a maximum of 2 degrees Celsius. The most important measure to achieve these goals is considered to be the drastic reduction of carbon emissions worldwide, which alone is an immense challenge. Countries are therefore examining how much greenhouse gas can still be emitted without exceeding temperature targets. A team of scientists at the University of Washington has now studied how emissions of other substances and compounds, such as soot or nitrogen oxide, affect temperature increases. They found that including all emissions shortens the time to reach the Paris climate goals.

In the new study, published in the journal Nature Climate Change, the team, led by the University of Washington, calculated how much warming we can definitely expect based on past emissions. Unlike previous research, the current study includes emissions of shorter-lived greenhouse gases such as methane and nitrogen oxide, as well as aerosols such as sulfur and soot, in addition to carbon dioxide emissions.

Using a climate model for eight different emission paths, the scientists examined how the Earth’s temperature would develop if all emissions were suddenly stopped in each year from 2021 to 2080. They found that under a moderate emissions scenario, there is a two-thirds probability that warming will exceed 1.5 degrees Celsius by 2029, at least temporarily, even if all emissions are stopped by then. If emissions remain at a moderate level, there is a two-thirds chance that global warming will exceed 2 degrees Celsius, at least temporarily, by 2057.

“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 Michelle Dvorak, a doctoral student in oceanography at the University of Washington and lead author of the study. “I think that hasn’t been clearly disentangled before – how much future warming will occur just based on what we’ve already emitted.”

This graphic shows the modeled change from pre-industrial temperatures if all human emissions were to suddenly stop in 2021. The dotted line shows that if only carbon dioxide is accounted for, average global temperatures flatline and don’t begin to drop until after 2100. When all human emissions are accounted for (dashed line) there’s a temporary bump as the particulate pollution drops out of the skies, and then gradual cooling as shorter-lived greenhouse gases like methane and nitrogen oxide get reabsorbed. Until emissions stop, particulate pollution masks some of the warming that Earth is already committed to by past emissions. The orange line shows the planet’s temperature climbing under a moderate business-as-usual emissions scenario. Graphic: Dvorak et al. 2022/Nature Climate Change

Previous studies that considered only carbon dioxide emissions found that there was little or no warming after emissions were stopped.

Depending on the type of emissions, there is a warming or a cooling effect. Particles (soot, etc.) in the atmosphere reflect sunlight and thus have a cooling effect, but are quickly removed from the atmosphere, while the long-lived greenhouse gases store heat. If all anthropogenic emissions were stopped simultaneously, the Earth would warm abruptly but temporarily – for a period of about 10 to 20 years – by 0.2 degrees Celsius once emissions cease.

“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 Kyle Armour, associate professor of atmospheric science and oceanography at the University of Washington and co-author of the study. “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.”

If we are to meet the Paris climate agreement’s goal of limiting warming to below 2 degrees Celsius, the authors say our remaining “carbon budget” is much smaller than previously thought.

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

Julia Hager, PolarJournal

Link to the study: M. T. Dvorak, K. C. Armour, D. M. W. Frierson, C. Proistosescu, M. B. Baker, C. J. Smith. Estimating the timing of geophysical commitment to 1.5 and 2.0 °C of global warming. Nature Climate Change, 2022; DOI: 10.1038/s41558-022-01372-y.

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