|Above: Noctilucent clouds photographed from 36,000 feet above Nunivak Island, Alaska, on June 20, 2006. Image credit: John Boardman, via NASA.|
A rarely-seen type of cloud at very high altitudes appears to be getting more common as a result of human-produced climate change, but it’s not high temperatures that are forcing the shift. Instead, a modeling study released this month finds that the increasing reports of noctilucent (night-shining) clouds can be explained by ever-larger amounts of water vapor produced from methane.
The study, published this month in Geophysical Research Letters, uses computer modeling to investigate the impression that noctilucent clouds are occurring more often. Led by Franz-Josef Lübken (Leibniz Institute of Atmospheric Physics), the authors conclude that the number of times an observer at a given middle-to-high-latitude spot might see a noctilucent cloud has increased from once every few decades in the 1800s to once or more per year today. During preindustrial times, they add, the very brightest and most visible noctilucent clouds might have been seen only once every few centuries.
Noctilucent clouds are the only type that that form at altitudes as high as the upper mesosphere, typically taking shape at heights of about 50 miles (80 kilometers). They develop in extremely cold temperatures, sometimes below –210°F (–134°C). The only part of Earth’s atmosphere that regularly produces such frigid values is the upper mesosphere at higher latitudes during summertime. As a result, noctilucent clouds are most often seen poleward of about 50° latitude north and south. They can only be seen by the naked eye after sunset and before sunrise, while sunlight is still hitting the clouds and giving them a distinct bluish glow.
Because dust particles are normally too heavy to rise into the upper mesosphere and serve as cloud nuclei, noctilucent clouds instead rely on tiny bits of dust left behind by falling meteors.
|Figure 1. Noctilucent clouds (top center) occur at altitudes close to 50 mi (80 km), far above the tops of even the tallest cumuloniumbus clouds (thunderstorm clouds). The red temperature trace at right shows that the upper mesosphere is the coldest region of Earth’s atmosphere. Image credit: Randy Russell, UCAR.|
A high-level mystery
Scientists’ eyebrows went up after noctilucent clouds were reported by experienced observers and supported by lidar observations in June 1999 at Logan, Utah (and also seen on the same night at Golden, Colorado). At roughly 42°N latitude, Logan is about 700 miles farther from the poles than any other spot where noctilucent clouds had been previously observed. Since then, noctilucent clouds have been photographed in Wyoming, Kansas, South Dakota, and other U.S. states south of the traditional formation zone. In June 2012, they were documented from the Calar Alto Observatory in Spain at a record-low latitude of 37.2°N.
One big change in recent decades appears to be an infusion of water vapor into the upper mesophere from below. Some of this is arriving directly, but according to the new study, a much greater amount is being converted from methane to water vapor. One molecule of methane (CH4) can be oxidized into two molecules of water vapor (H2O) in the presence of sunlight. There’s far more water vapor present naturally at lower altitudes than can be generated by methane, but in the much less dense mesosphere, it appears that methane oxidation is the main source of water vapor. Lübken and colleagues estimate that the amount of water vapor per unit volume at the altitude of noctilucent clouds has risen by about 40% since the late 1800s.
The atmosphere today holds about twice as much methane as it did a century ago. According to the U.S. Environmental Protection Agency, the primary U.S. sources of methane are leakage from natural gas production; belches from ruminating livestock such as cows, sheep and goats; and the gradual decomposition of landfills. Methane stays in the atmosphere for a much shorter period than carbon dioxide, but it is a much more powerful greenhouse gas per molecule. Among substances being added to the atmosphere by human activity, it’s second only to carbon dioxide in its contribution to climate change.
|Figure 2. Noctilucent clouds glow over the city of Wismar, Germany in July 2015. More typical lower-altitude clouds are visible as small dark patches near the horizon. Image credit: Leibniz Institute of Atmospheric Physics, via AGU.|
The role of cooling
Temperature has also been invoked as a potential cause of the increase in noctilucent cloud reports. The increase in greenhouse gases over the past century has led to cooling from the stratosphere upward, even as warming occurs over the lowest few miles of the atmosphere. The limited data on hand suggests that there’s been little cooling at the heights and latitudes traditionally associated with noctilucent clouds, but more cooling in lower parts of the mesosphere and at latitudes further from the pole.
Some research, including a 2014 GRL study led by James Russell (Hampton University), supports the idea that colder temperatures have boosted the occurrence of noctilucent clouds in midlatitudes (40°N-55°N), especially during quiet phases of the 11-year solar cycle. However, Russell and colleagues note that the brightness of noctilucent clouds is controlled mainly by water vapor rather than temperature variations. So it’s still possible that water vapor is the main factor leading to more naked-eye observations of these clouds, as concluded in the new study.
A place for noctilucent cloud lovers
For an array of noctilucent cloud reports and photos from recent years, check out the website and Twitter feed of the Noctilucent Cloud Observing Network (@nlcnet). The site not only archives legitimate reports but also filters out those found to be erroneous. Some contributors even provide nowcast-style updates on the state of water vapor in the mesosphere, which gives noctilucent cloudspotters a heads-up to get ready for late-night viewing.