Global warming will exceed 1.5 degrees, but by how much?

The results of a new study modeling the effects of the world’s current emission reduction pledges has concluded that it’s no longer about whether we will overshoot the 1.5 degree Celsius warming target or not; it’s now about how much we will overshoot. This is a stark and sobering finding, against the backdrop of the 2022 United Nations Climate Change Conference (COP27) currently taking place in Sharm El Sheikh, Egypt.

According to the study, published in the journal Nature Climate Change, it is now highly likely that the world is going to warm past the 1.5 degree rise in average temperature, set as a goal of the 2015 Paris Agreement. The world’s current climate pledges are simply insufficient to limit global warming to this level, say the researchers from the Department of Energy’s Pacific Northwest National Laboratory(PNNL), the University of Maryland and the U.S. Environmental Protection Agency. However, the experts note there is still time to adopt mechanisms that can limit the extent of the overshoot.

Although exceeding the 1.5-degree target appears inevitable, the researchers say that countries can minimize the time spent in a warmer world by adopting more ambitious climate pledges and decarbonizing faster. Their study suggests various mechanisms whereby the overshoot period is shortened, in some cases by decades, and the researchers say this will be critical in reducing the extent of the damage. 

“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.”

According to the study, every second shaved off the overshoot translates to less time spent 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 proposing 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 researchers modelled the predicted consequences of 27 scenarios involving different levels of emissions and climate actions, from the least ambitious to the most. They tracked the effect on global warming if each of these pathways was followed, in order to identify particular approaches that could limit temperature rise or, indeed, return the world to the desired 1.5 degree average temperature rise after overshooting this goal.  

As a starting point, the researchers assumed that countries will meet their emissions pledges and long-term strategies on schedule. If countries do 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.

However, in the more ambitious scenarios, they model the extent to which 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.

Taking the most ambitious path outlined could even bring net-zero carbon dioxide emissions by 2057. Such a path, the authors write, is marked by “rapid transformations throughout the global energy system” along with 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 re-attained sooner after we warm past it, more ambitious climate pledges must be adopted.

—

By Alison Bosman, Earth.com Staff Writer

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.