Earth's mysterious red glow, explained - Zoe Pierrat
In 2009, a satellite circled Earth,
methodically scanning and sorting
the wavelengths
reflecting off the planet’s surface.
Researchers were looking for the spectral
signature of carbon dioxide
when they noticed something baffling:
an unexpected wavelength
of unknown origin.
They tried looking at Earth
with only this wavelength,
and saw the planet covered
in a red hue of varying intensity.
This couldn’t have been reflected sunlight
because it was a wavelength that never
escapes the Sun’s outer atmosphere.
And it didn’t correspond
with densely populated areas,
suggesting it wasn’t human-made either.
In fact, it was emanating
from places with lots of plants:
the Amazon basin,
northern evergreen forests,
and croplands of the Midwestern US
were all ablaze.
So, what was going on?
Plants and other organisms use light
to grow by way of photosynthesis.
But that’s just one of three
ways that light
entering a photosynthetic organism
is used.
And this is the key to solving
the mystery.
To understand the others,
we need to begin with photosynthesis.
During this process,
sunlight hits structures within a plant’s
cells called chloroplasts,
which are packed
with chlorophyll pigments.
When chlorophyll molecules absorb light,
some of their electrons become excited.
They go through a series of reactions,
which transform light energy
into chemical energy.
This powers the conversion of carbon
dioxide and water into glucose,
the simple sugar plants need to grow.
And of course, this reaction generates
an important byproduct.
Photosynthesis—
which is constantly being carried
out by plants, algae, and bacteria—
produces all of Earth’s oxygen.
But plants regularly absorb more light
than they’re able to consume.
For instance, over winter,
the frozen leaves of evergreen trees can't
photosynthesize at their usual rate,
but they're still exposed
to a lot of sunlight.
If not dealt with, the excess light can
damage their photosynthetic machinery.
So, the second way plants use light
is by transforming it into heat
and dissipating it out of their leaves.
The third way plants interact
with incoming light
is by radiating it back out
at a different wavelength,
producing what’s called
chlorophyll fluorescence.
During photosynthesis,
the chlorophyll’s excited electrons
move through that series
of chemical reactions.
But as some of the excited electrons
fall back to their ground states,
they emit energy as light.
Overall, about 1% of the light
absorbed is re-emitted
as wavelengths at the red
end of the spectrum.
It’s such a small amount that you
can’t see it with the naked eye.
But plants the world over are fluorescing
as they photosynthesize.
And this is what’s caused
the Earth’s baffling red glow,
as observed by satellite.
It was an accidental discovery,
but a huge breakthrough.
Tracking chlorophyll fluorescence
from space
allows us to watch the planet breathe
in real time—
and monitor the health
of ecosystems worldwide.
Previously, researchers used levels
of greenness
as the main estimate for plant health.
Because plants generally change colors
or lose foliage when they’re stressed,
higher levels of green typically
indicate healthier plants.
But this measure can be unreliable.
In contrast, chlorophyll fluorescence
is a direct measure
of photosynthetic activity.
It can help us infer how much
oxygen is being released
and how much carbon is being absorbed
in a given system.
Drops in chlorophyll fluorescence
may also occur
before visible signs of plant stress,
making it a timely measure.
Scientists have already used chlorophyll
fluorescence to monitor
harmful phytoplankton blooms,
and track the effects of drought
in the Amazon and Great Plains.
Going forward, we’ll be investigating
photosynthesis from space,
and gauging how best to support
our silent friends,
who already do so much for us.
