In 1885, two years after a massive eruption of the Indonesian volcano Krakatoa, scientists added a new type of cloud to the cloud atlas. All of the ash and water vapor spewed into the atmosphere created mesmerizing sunsets and other effects around the world, including the new noctilucent clouds—extremely high, wispy clouds that are only visible in far northern latitudes. But once the impacts of the volcano disappeared, the new clouds did not. In fact, they have slowly increased over the last century. Now, reports Paolo Rosa-Aquino at Earther, a new study explains why—more nocitlucent clouds are forming because of human-driven climate change.
Most clouds form no higher than about four miles up, in the lower reaches of Earth’s atmosphere. But noctilucent clouds are true oddballs, forming in the middle section of the sky, the mesosphere, about 50 miles up, in extreme cold.
Katie Camero at The Boston Globe reports that the clouds are only visible during special conditions. It has to be summertime and in areas above the 50-degree latitude line. (That rules out all of the United States, but includes much of northern Europe and the U.K., not to mention nearly all of Russia and Canada. The clouds can form in the Southern Hemisphere, too, below the 50-degree latitude line, but they're dimmer and less frequent.) The sun also has to be at the right angle, a few degrees below the horizon one or two hours after sunset for the bluish clouds to reflect the light and become visible. The clouds form when water vapor freezes around specks of “meteor smoke,” grains of dust produced when meteors burn up in Earth’s atmosphere.
But why the clouds are becoming more common has been a mystery. “Scientists have been wondering for many years if or not these clouds are an indication of anthropogenic change,” Franz‐Josef Lübken, director of Leibniz Institute of Atmospheric Physics and lead author of the study in the journal Geophysical Research Letters, tells Rosa-Aquino. He and his team decided to figure out if climate change was the culprit.
In their study, the researchers ran computer simulations modeling how greenhouse gas emissions affected the Northern Hemisphere’s atmosphere and noctilucent cloud formation between 1871 and 2008. According to a press release, they found that the formation of the clouds fluctuates year to year based on changes in the atmosphere and the solar cycle, but over time the clouds have indeed grown more common.
But the usual climate-change culprit—carbon dioxide—is not to blame this time. In fact, they found that while increased CO2 warms the lower levels of the atmosphere, it makes the mesosphere colder. Which would lead to fewer visible clouds.“We get more ice particles, but they are smaller,” Lübken tells Rosa-Aquino. “Unexpectedly, making it colder would not produce more noctilucent clouds.”
What they did find, however, is that extra water vapor transported to the mesosphere will make more and more visible noctilucent clouds. In particular, the greenhouse gas methane produces water vapor through chemical reactions when it reaches the mesosphere. During the study period, methane emissions had more than doubled the amount of water vapor in the mesophere, likely leading to the formation of the clouds. According to the press release, during the late 1800s, the clouds were likely only visible on special summer nights once every few decades. Now they are probably visible a few times each summer. “The result was rather surprising that, yes, on these time scales of 100 years, we would expect to see a big change in the visibility of clouds,” Lübken says in the release.
While the subtle blue clouds are beautiful, their increase shows just how little we know about how greenhouse gas emissions are changing the atmosphere. “I concur almost totally in the results of the Lübken paper,” Gary Thomas, atmospheric scientist and professor emeritus at the University of Colorado, Boulder, tells Camero. “It is inescapable that we are changing the atmosphere. This is just another manifestation of global change, and actually something that non-scientists can appreciate because these clouds are a brilliant and obvious reminder of these changes.”
It’s likely the clouds will continue to increase and become more noticeable. The team hopes to look next at whether the formation of these clouds so high in the atmosphere have any influence on changes in the climate for all of us down below.