Noctilucent clouds form only in the summertime and are only visible at dawn and dusk. Credit: NASA
Those wispy, iridescent, high-altitude clouds sometimes seen at dawn and dusk are becoming more visible due to climate change, according to a new study published in the journal Geophysical Research Letters. Rising methane emissions have increased the amount of water vapor in the middle atmosphere, the study found, which then freezes around specks of dust to form the clouds.
Humans first observed night-shining, or noctilucent, clouds in 1885, following the eruption of Indonesia’s Krakatoa volcano, according to the American Geophysical Union. The clouds became an increasingly common sight during the 20th century, from being visible once every several decades to being observed several times each summer by people living in mid- to high-latitude regions. Scientists first began debating in the 1990s whether climate change was responsible for the increased visibility.
“We speculate that the clouds have always been there, but the chance to see one was very, very poor in historical times,” said Franz-Josef Lübken, a scientist at the Leibniz Institute of Atmospheric Physics in Germany and lead author of the new study.
In the new study, Lübken and his colleagues at the Leibniz Institute used satellite observations and climate models to examine how greenhouse gas emissions from the burning of fossil fuels affected noctilucent cloud formation over the past 138 years, beginning with the start of industrialization. They found that carbon dioxide has actually had very little effect on the nighttime clouds. CO2 warms Earth’s surface and lower atmosphere, but actually slightly cools the middle atmosphere, which causes the ice particles that form to be smaller and harder to see.
When methane reaches the middle atmosphere, however, it oxidizes, creating water vapor. The study found methane emissions have increased water vapor concentrations in the mesosphere — a section of the middle atmosphere where these noctilucent clouds appear — by an estimated 40 percent since the late 1800s, which has more than doubled the amount of ice that forms in the mesosphere. The clouds, the scientists write, are “a long-term indicator for climate change.”
