In the summer of 1885, sky watchers around northern Europe noticed that 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 – Noctilucent clouds.
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 confirms that PMCs in the northern hemisphere have become more frequent, thicker and brighter in recent decades.
But why? Why did the upper mesosphere (the atmospheric layer where PMCs form) become icier?
The ingredients for PMCs are simple enough. Ice requires water molecules and freezing temperatures. 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. It’s both.
And these results are consistent with a simple model linking PMCs to two greenhouse gases:
- Carbon dioxide promotes PMCs by making the mesosphere colder.
- Methane promotes PMCs by adding moisture to the mesosphere, because rising methane oxidizes into water.
Here a timelapse video by Mindaugas Gasparavičius captured on June 24, 2016:
To observe noctilucent clouds, 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… At the edge of space.