Just after summer sunsets in northern latitudes, shimmering,
wispy clouds appear in the twilight sky. This year, these noctilucent clouds
have appeared earlier and farther south than ever before.
Noctilucent clouds exist higher in Earth’s atmosphere than any
other cloud type. First observed in 1885 following the eruption of Krakatoa,
they were a sight reserved for Earth’s northernmost residents. In recent years,
however, their intensity and frequency have increased, often at latitudes
previously thought to be too far south for noctilucent clouds to form.
In 2009, scientists
from the National Center for Atmospheric Research pointed to the southern creep
of noctilucent clouds as an
early warning signal for climate change high in the atmosphere. Now, new
data from NASA’s cloud-observing AIM
satellite supports
this possibility.
James Russell, principal investigator for AIM, says increasing
methane emissions could be amping up the cloud show. “When methane makes its way
into the upper atmosphere, it is oxidized by a complex series of reactions to
form water vapor,” Russell said. “This extra water vapor is then available to
grow ice crystals for [noctilucent clouds].”
As polar regions warm in the summer months, water vapor is
driven to higher levels of Earth’s atmosphere. There, small dust particles left
over from burning meteors, volcanic eruptions, or even rocket launches act as
seeds for ice formation. Transparent in broad daylight, these crystalline clouds
become visible in twilight hours, reflecting the sun’s rays from below the
horizon. They float so high in the atmosphere that they can even glow in the
dead of night. Normally such nocturnal cloud sightings peak when the sun is at
its 11-year minimum. This year marks exactly the opposite, a predicted solar
maximum.
These changing clouds serve
as a reminder of the interconnected nature of Earth’s atmosphere, what the
scientists refer to as “atmospheric teleconnections.” Cooling in the
stratosphere, a consequence of elevated greenhouse gas levels, can alter
air circulation patterns across the globe, although exactly how remains a
mystery.
“Models will be
catching up with AIM for a long time,” said space physicist Thomas Immel of
Berkeley’s Space Sciences Laboratory, chief scientist for NASA’s next mission to
study fluctuations in the upper atmosphere, the ICON
satellite, expected to launch in 2017.
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