by Dr. Tony Phillips (Spaceweather.com)
In the summer of 1885, sky watchers around northern Europe noticed something strange. Sunsets weren’t the same any more. The red and orange colors they were used to seeing were still there—but those familiar colors were increasingly joined by rippling waves of luminous blue.
At first they chalked it up to Krakatoa, which had erupted just two years earlier. The explosion of the Indonesian super volcano hurled massive plumes of ash and dust into the atmosphere more than 50 miles high, coloring sunsets for years after the blast.
Eventually Krakatoa’s ash settled, yet the rippling waves of luminous blue didn’t go away. Indeed, more than 100 years later, they are shining brighter than ever.
Today we call them, “noctilucent clouds” (NLCs). They appear with regularity in summer months, shining against the starry sky at the edge of twilight. Back in the 19th century you had to go to Arctic latitudes to see them. In recent years, however, they have been sighted from backyards as far south as Colorado and Kansas.
Noctilucent clouds are such a mystery that in 2007 NASA launched a spacecraft to study them. The Aeronomy of Ice in the Mesosphere satellite (AIM) is equipped with sensors specifically designed to study the swarms of ice crystals that make up NLCs. Researchers call these swarms “polar mesospheric clouds” (PMCs).
A new study published in the Journal of Geophysical Research (doi:10.1002/2015JD024439) confirms what some researchers have long suspected: PMCs in the northern hemisphere have become more frequent and brighter in recent decades—a development that may be related to climate change.
The story begins long before the launch of AIM.
The paper’s lead author Mark Hervig, an AIM scientist with GATS, Inc., explains: “Thanks to decades of data from the Solar Backscatter Ultraviolet (SBUV) instrument on NOAA weather satellites, we know that PMCs have become thicker and more frequent.”
Right: According to data from SBUV, the ice mass of PMCs has increased since 1980.
“The question we’ve been grappling with is why?” says co-author David Siskind of the Naval Research Lab in Washington, DC. “Why did the upper mesosphere (the atmospheric layer where PMCs form) become icier?”
The ingredients for PMCs are simple enough. Ice requires water molecules + freezing temperatures. However, SBUV could not tell researchers if the mesosphere was getting wetter or colder–or both.
Fortunately, AIM has an instrument onboard named SOFIE that can unravel the water-temperature knot. Hervig, Siskind, and another co-author, Uwe Berger of the Leibniz-Institute of Atmospheric Physics in Germany, recently interpreted the 36-year SBUV record using data from SOFIE, and this is what they found:
At altitudes where PMCs form, temperatures decreased by 0.5 ±0.2K per decade. At the same time, water vapor increased by 0.07±0.03 ppmv (~1%) per decade.
“These results settle the decades old question of whether or not the observed long-term change in PMCs is an indicator of changing temperature or humidity,” says James Russell, AIM Principal Investigator. “It’s both.”
These results are consistent with a simple model linking PMCs to two greenhouse gases. First, carbon dioxide promotes PMCs by making the mesosphere colder. (While increasing carbon dioxide warms the surface of the Earth, those same molecules refrigerate the upper atmosphere – a yin-yang relationship long known to climate scientists.) Second, methane promotes PMCs by adding moisture to the mesosphere, because rising methane oxidizes into water.
Above: A graphic prepared by Prof. James Russell of Hampton University shows how methane, a greenhouse gas, boosts the abundance of water at the top of Earth’s atmosphere. This water freezes around “meteor smoke” to form icy noctilucent clouds.
However, the simple model may not be enough:
“Our study shows that PMCs may be tied to changes in the temperature of the stratosphere as well,” says Hervig. “This complicates things because the stratosphere is governed by a wide range of phenomena including ozone concentration, greenhouse gases, and volcanic aerosols.
“While we have finally quantified the underlying temperature and water vapor changes related to PMCs,” he adds, “there is still work to be done in understanding the details of what caused these changes.”
Summer is the season for PMCs and noctilucent clouds. As June turns into July, observers in Europe are already reporting some displays, and they should appear over the northern USA within weeks.
Observing tips: Look west 30 to 60 minutes after sunset when the sun has dipped ~10 degrees below the horizon. If you see blue-white tendrils spreading across the sky, you may have spotted a sign of climate change. It happens, even at the edge of space.