Ozone-depleting gases may account for up to half of Arctic warming in the last 50 years

The study includes uncertainties, but it suggests that there may be some hope that the rate of Arctic warming will slacken.

A new study published last week in the journal Nature Climate Change concludes that ozone-depleting substances, such as chlorofluorocarbons, could account for up to half of the temperature increase observed in the Arctic from 1955-2005. Scientists have long struggled to fully explain why the Arctic has warmed twice as much as other parts of the world. This warming trend has resulted in dramatic loss of sea ice over the last two decades.

The new research may have found the missing link and at the same time could offer a glimmer of hope when it comes to future warming in the region. Chlorofluorocarbons have been largely banned since the Montreal Protocol came into effect in 1987, and thus their effects on Arctic warming may already be diminishing and continue to do so.

Ozone-depleting substances have been extensively studied for their impact on the ozone layer, especially over Antarctica. Their role as a potent greenhouse gas, however, is less well understood. This latest study evaluates the role these substances have played in warming the Arctic.

“For the first time, we have quantified the specific impact of ODS in warming the planet using a fully coupled climate model, like those used in producing the IPCC reports. Until now, most of the research about ODS has focused on the fact that they are the major culprit in causing the ozone hole,” explains Lorenzo Polvani, Professor of Geophysics at Columbia University, and lead author of the study.

The study presents interesting, potentially very important, and provocative research, other climate scientists confirm. However, they highlight that future research will be needed to confirm the results and increase confidence in the conclusions.

“The question though is how big is the role of ODS in Arctic amplification. This is where there are still uncertainties,” explains Susan Strahan, an atmospheric scientist at NASA’s Goddard Space Flight Center.

Ozone-depleting substances receive less attention

The global warming potential of ODS is immense and is tens of thousands of times greater than that of carbon dioxide. But because ODS are much rarer — they are usually measured in parts per trillion, compared to part per million for CO2 — they have received less attention.

The paper shows that they are in fact the second-most important greenhouse gas with regard to absorption and reflection of sunlight. This is especially relevant in the Arctic where sea ice and snow-covered surfaces traditionally reflect the majority of solar radiation. “In our paper we show that, in fact, ODS are big players for the Arctic climate system as well,” affirms Polvani. The researchers also highlight the fact that this study focuses on the impact of ODS individually, not “lumping them together” with other major GHG, like CO2 or methane.

Comparing Arctic with and without ODS

The new study compared two particular scenarios, one with and one without ozone-depleting CFCs, through the use of climate simulations. In a simulation without CFCs the Arctic warmed only 0.82 degrees Celsius, compared to 1.59 degrees C when those emissions were included. A similar impact was observed on the amount of sea ice melt.

While previous research has studied the effect of ODS on climate change and affirmed the impact of the gases on Arctic warming, this latest study has taken a different approach, Polvani explains. “Prior research focused on ‘world avoided’ scenarios, i.e. what would have happened had the Montreal Protocol [which largely banned CFCs] not been signed. Our study, in contrast, studied ‘the world that did happen’ scenario, i.e. how much Arctic warming and sea ice loss was actually caused by ODS.” Since this latest study focused on what actually occurred, its results may be more relevant than previous ones that focus on a presumed scenario.

Uncertainties remain

The study relies on what the authors call “efficacy factor,” which determines how efficient the ODS are at causing warming in the lower atmosphere in comparison to other greenhouse gases such as CO2. Polvani and his colleagues use a factor from a 15-year old paper relying on a single model. This is where a degree of uncertainty in the study’s findings may be introduced.

“How correct is this? Hard to know. But the uncertainty in this factor will lead to uncertainty in how much amplification the ODS are responsible for,” says Strahan. “The result is that their model’s Arctic climate is ~30 percent more sensitive than the real world. This may due to their efficacy factor. This extra sensitivity may lead to an overestimation of what fraction of Arctic amplification is due to the ODS.”

Confidence in the study’s results could also be increased by not looking at the impact of ODS separately, but placing it into the context of other causes of Arctic warming such as ocean heat transport, changes in atmospheric heat transport, and the reduction of sea ice albedo. “If the ODS are actually responsible for half of Arctic warming, then how does that change what we think are the contributions from these other processes,” asks Strahan.

A glimmer of hope?

This latest research suggests that climate policies and regulations can have an immediate positive effect on emissions and global warming. “The findings of our study are definitely positive, in the sense that the Arctic would be warming a lot more had the Montreal Protocol not been signed in the late 1980’s.” That’s a sentiment echoed by Strahan. The study “shows yet again how important the Montreal Protocol continues to be.”

ODS concentration in the atmosphere peaked in the mid 1990s and has since decreased by more than 20 percent. As the concentration of ozone-depleting substances in the atmosphere continues to decline, so will their warming effect on the Arctic. This represents a small piece of good news suggesting that the Arctic may not warm quite as rapidly and sea ice may not melt quite as substantially in the future as observations from the last two decades would suggest.