The 1.5 degree Celsius threshold of global warming refers to the maximum temperature increase above pre-industrial levels that the international community has agreed to strive to limit warming to, as outlined in the 2015 Paris Agreement. This target is considered more ambitious than the previous goal of limiting warming to 2 degrees Celsius and is seen as necessary in order to avoid the worst impacts of climate change, such as more frequent and severe heatwaves, droughts, and storms, as well as rising sea levels and disruptions to ecosystems.
It is expected that global warming will exceed the threshold established in the 2015 Paris Agreement, though it is uncertain to what extent this will occur.
According to new research from scientists at the Department of Energy’s Pacific Northwest National Laboratory, the University of Maryland, and the U.S. Environmental Protection Agency, current climate pledges are insufficient to achieve the goals of the 2015 Paris Agreement, and it is likely that global warming will surpass the 1.5 degree Celsius limit.
The research suggests that the only way to minimize the extent of this overshoot is for countries to adopt more ambitious climate pledges and decarbonize their economies at a faster pace. This will help to reduce the amount of time that the planet spends in a warmer state.
While exceeding the 1.5-degree limit appears inevitable, the researchers chart several potential courses in which the overshoot period is shortened, in some cases by decades. The study was published recently published in the journal Nature Climate Change.
“Let’s face it. We are going to breach the 1.5 degrees limit in the next couple of decades,” said corresponding author and PNNL scientist Haewon McJeon. “That means we’ll go up to 1.6 or 1.7 degrees or above, and we’ll need to bring it back down to 1.5. But how fast we can bring it down is key.”
PNNL researchers Gokul Iyer and Yang Ou, authors of the new study, unpack their findings Credit: Sara Levine | Pacific Northwest National Laboratory
Every second shaved off the overshoot translates to less time courting the most harmful consequences of global warming, from extreme weather to rising sea levels. Forgoing or delaying more ambitious goals could lead to “irreversible and adverse consequences for human and natural systems,” said lead author Gokul Iyer, a scientist alongside McJeon at the Joint Global Change Research Institute, a partnership between PNNL and the University of Maryland.
“Moving fast means hitting net-zero pledges sooner, decarbonizing faster, and striking more ambitious emissions targets,” said Iyer. “Every little bit helps, and you need a combination of all of it. But our results show that the most important thing is doing it early. Doing it now, really.”
During COP26 in 2021, the same research team found that the then updated pledges could substantially increase the chance of limiting warming to 2 degrees Celsius over pre-industrial levels. In their new paper, the authors take an additional step in answering the question of how to move the needle from 2 to 1.5 degrees.
“The 2021 pledges don’t add up to anywhere near 1.5 degrees—we are forced to focus on the overshoot,” said PNNL scientist Yang Ou, who co-led the study. “Here, we’re trying to provide scientific support to help answer the question: What type of ratcheting mechanism would get us back down and below 1.5 degrees? That’s the motivation behind this paper.”
The Paths Forward
The authors model scenarios—27 emissions pathways in total, each ranging in ambition—to explore what degree of warming would likely follow which course of action. At a base level, the authors assume that countries will meet their emissions pledges and long-term strategies on schedule.
In more ambitious scenarios, the authors model how much warming is limited when countries decarbonize faster and advance the dates of their net-zero pledges. Their results underscore the significance of “ratcheting near-term ambition,” which entails rapid reductions in carbon dioxide emissions from all sectors of the energy system, immediately and through 2030.
If countries uphold their nationally determined contributions through 2030 and follow a two percent minimum decarbonization rate, for example, global carbon dioxide levels would not reach net zero this century.
Taking the most ambitious path outlined, however, could bring net-zero carbon dioxide emissions by 2057. Such a path, the authors write, is marked by “rapid transformations throughout the global energy system” and the scaling up of “low-carbon technologies like renewables, nuclear energy, as well as carbon capture and storage.”
“The technologies that help us get to zero emissions include renewables, hydrogen, electric cars, and so on. Of course, those are important players,” said Iyer. “Another important piece of the puzzle is the technologies that can remove carbon dioxide from the atmosphere, like direct air capture or nature-based solutions.”
The most ambitious scenarios outlined in their work are meant to be illustrative of the pathways on offer. But the central takeaway remains clear throughout all modeled scenarios: if 1.5 degrees is to be reattained sooner after we warm past it, more ambitious climate pledges must come.
Reference: “Ratcheting of climate pledges needed to limit peak global warming” by Gokul Iyer, Yang Ou, James Edmonds, Allen A. Fawcett, Nathan Hultman, James McFarland, Jay Fuhrman, Stephanie Waldhoff and Haewon McJeon, 10 November 2022, Nature Climate Change.
DOI: 10.1038/s41558-022-01508-0
The study was funded by the U.S. Environmental Protection Agency.
