August 2021 BBC Sky at Night Magazine 39>Venus experienced a
runaway greenhouse effectWith that information, scientists can see where
CO 2 LVDFFXPXODWLQJDQGKRZLWŝVLQƆXHQFHGE\
temperature and pressure changes, climatic or
geological events (such as El Nin ̃ o events or volcanic
eruptions), and even seasonal vegetation growth.
Where OCO-2 and OCO-3 differ is in what their
instruments are attached to. OCO-2’s is packaged
into a satellite in heliosynchronous orbit; OCO-3’s
is mounted on the International Space Station (ISS).
What that means is OCO-2 orbits the poles at an
altitude of 700km, passing over any given spot at the
same time (guaranteeing it’s in sunlight). Meanwhile,OCO-3 orbits with the ISS, about 400km above
Earth, and sees different locations at different times.Out of balance
There has always been, and always should be, some
CO 2 in Earth’s atmosphere because every living thing
emits CO 2 , either through respiration while it’s alive
or decomposition once it’s dead. But those emissions
are balanced by the CO 2 absorbed by photosynthesis
and colder parts of the oceans.
The problem is that CO 2 emissions from human
activity – anthropogenic emissions – have
unbalanced that natural cycle. Ever since we began
burning fossil fuels (oil, coal and natural gas)
we’ve essentially been digging up CO 2 that had
been removed from the cycle and pumping it into
the atmosphere. But we’ve failed to balance those
emissions by creating new sinks to absorb that
extra CO 2.
To make matters worse, by clearing vast areas of
rainforest across the tropics, we’ve also decreased
the capacity of Earth’s natural CO 2 sinks, leaving
us staring down the barrel of catastrophe. Without
taking action to reduce our CO 2 emissions, we’re
facing climate disaster.
The trouble is, we can’t tell if those actions are
helping without knowing how much CO 2 we’re
responsible for. We need to differentiate between
natural and anthropogenic CO 2 , and that’s even
harder than monitoring CO 2 from space. To return
to the coin analogy, it’s akin to determining how
many of those 400 pennies are real and how manyThe gas makes up a tiny quantity of our atmosphere but it can have a big impact
Carbon dioxide (CO 2 ) accounts for just
0.04 per cent of Earth’s atmosphere, so
how can it have such a big effect on the
planet’s climate? Well, firstly, CO 2 traps
heat (hence it’s known as a greenhouse
gas) and secondly, it remains in the
atmosphere for centuries.
Earth receives huge quantities of
energy from the Sun in the form of
sunlight. As this is short-wave radiation,
it passes through the atmosphere
largely unhindered. Most of that energy
is absorbed by the surface, but about 17
per cent is reflected back in the form of
heat (long-wave, infrared radiation).
Infrared radiation effectively passesHow increasing carbon dioxide affects Earth’s climate
straight through oxygen and nitrogen
(which comprise the vast majority of
Earth’s atmosphere), thanks to their
relatively simple molecular structure.
But the molecular structure of CO 2 is
more complex, so it absorbs the infrared
and heats up, then begins to radiate its
heat to molecules nearby. The more CO 2
molecules there are, the more of that heat
is absorbed, the hotter Earth’s
atmosphere becomes, and the greater the
impact on Earth’s climate.
Methane, another greenhouse gas,
has an even more complex molecular
structure than CO 2 so absorbs even more
heat. But methane stays in the atmospherefor little more than a decade. CO 2 can
hang around for anything from 300 to
1,000 years; hence the accumulation of
it has a significant impact on our
planet’s heat balance.
For evidence of the effect greenhouse
gases can have on a planet’s climate,
look no further than Venus. With a
similar size and atmosphere to Earth
early in its history, it was essentially
Earth’s twin until around 700 million
years ago, when a runaway greenhouse
effect took hold. Today, its atmosphere
is 96 per cent CO 2 and its surface
temperature is around 460°C – hot
enough to melt lead.þ As forests
continue to be
cleared, natural
carbon sinks
that absorb CO 2
are destroyed
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