Nacreous clouds for more accurate climate models | Polarjournal
Nacreous clouds are becoming more and more frequently observed in the Arctic and in more southern regions, such as here in Sweden. Image: P-M Hedén

Polar stratospheric clouds over the Arctic could explain observed differences between polar warming calculated by climate models and actual records, say a research team from the University of New South Wales (UNSW) in Australia.

We know that the poles are warming faster than the rest of the world. Climate models, computer simulations of the Earth’s climate system, take this increased warming into account. However, they still underestimate the warming of the polar regions, according to the authors of a study recently published in Nature Geoscience.

This is a problem as future climate projections are generated using these same models. But underestimating past warming could mean underestimating future warming, as well as the consequences on ice caps and permafrost melting.

Absent from most climate models, polar stratospheric clouds (PSC), also known as Mother of Pearl clouds or nacreous clouds, could explain much of the warming missing from models. “During my PhD, I was drawn to the fact that the climate models we are using do not represent the magnitude of warming that happens in the Arctic,” says Dr Deepashree Dutta, postdoctoral research associate at the University of Cambridge and lead author of the study, in a press release published by UNSW. “At the same time, we knew that the majority of these models do not represent the upper layers of the atmosphere very well. And we thought this might be a missing link.”

Usually composed of nitric acid and water, Mother of Pearl clouds may contain only ice when they form at temperatures below -88°C. Image: Brillern, via Wikimedia Commons

Already in 1992, American paleoclimatologist Lisa Sloan theorized that PSC could have played a role in the extreme warming of the poles during past periods, when the climate was warmer. A theory that Dr. Dutta and his team wanted to test using one of the rare atmospheric models that incorporates these clouds. A model that included conditions similar to the early Eocene, a period dating back more than 50 million years.

Why this period in particular? “Climate models are far too cold in the polar regions, when simulating these past hot climates, and this has been an enigma for the past thirty years,” explains Dr Martin Jucker of the Department of Geography at the University of Cambridge and co- author of the study. “The early Eocene was a period in the Earth’s climate with extreme polar warming, so presented the perfect test for our climate models.”

Impressive polar stratospheric clouds above McMurdo Station in Antarctica. Normally invisible to the naked eye, these clouds take on their bright, iridescent hues at dawn and dusk. Image: Alan Light from Charlotte, USA, via Wikimedia Commons

The researchers found that high levels of methane during the Eocene led to an increase in the formation of nacreous clouds. Another observation: under certain conditions, local surface heating due to stratospheric clouds was up to 7°C during the coldest winter months. Enough to significantly reduce the gap between climate models and temperature evidence from climate archives.

But that’s not all. Dr. Dutta and his team also discovered that the arrangement of mountains and continents played just as important a role as methane in cloud formation. An important piece of data that might make it possible to temper future climate models: “But then the good news is that these clouds are more likely to form under the continental arrangement that was present tens of millions of years ago, and is not found on Earth now. “, points Dr Jucker. “Therefore, we don’t expect such large increases in stratospheric clouds in the future.”

If the spectacle is pleasing to the eyes, it is less so for the ozone layer. These clouds are in fact known for their destructive role of ozone. Video : Night Lights Films – Adrien Mauduit / YouTube

Polar stratospheric clouds develop in the stratosphere, at an altitude of 15,000 to 25,000 meters. To form, they need a temperature below -78°C. This is why they are mainly found in polar regions, although they have been observed in more temperate regions, such as Scotland. Despite their gorgeous look, they play a role in the destruction of the ozone layer.

Usually, these clouds are mainly observed in Antarctica, but with upper atmospheric layers having cooled in the Arctic in recent years, the phenomenon is now observed more frequently in the North.

Link to study: Dutta, D., Jucker, M., Sherwood, S.C. et al. Early Eocene low orography and high methane enhance Arctic warming via polar stratospheric clouds. Nat. Geosci. 16, 1027-1032 (2023).

Mirjana Binggeli, PolarJournal

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