BBC Sky at Night - UK (2021-08)

38 BBC Sky at Night Magazine August 2021

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þ Data from the

OCO-2 satellite

in June 2015,

with higher

concentrations

of carbon

dioxide shown

in red and lower

concentrations

shown in yellow

and green

Now imagine that each day someone adds a few

more pennies or takes a couple away. That’s the

challenge Eldering and the OCO-2 team took on.

They weren’t starting from scratch, though.

Thanks to Eunice Foote and John Tyndall’s work in

the 1850s, we know about the dangers of increasing

concentrations of atmospheric CO 2 , a greenhouse

gas, on the climate. And since Charles David Keeling

established a weather station on Hawaii’s Mauna

Loa volcano in 1958, we’ve been monitoring them.

But the Mauna Loa Observatory only gives us

a tiny snapshot of the CO 2 in Earth’s atmosphere.

Even the global network of ground- and sea-based

monitoring stations that has developed to support it

can’t provide a complete picture. And that’s because

Earth’s atmosphere extends to around 10,000m

above sea level, so most of it is nowhere near

the monitoring equipment. Hence the need for

CO 2 -monitoring satellites.

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scale, how CO 2 moves between the atmosphere,

the ocean and the land,” says Eldering. “As a lot of

our information came from ground- and ship-based

measurements, there was a huge part of Earth we

didn’t observe.”

Watching from above

OCO-2 launched on 2 July 2014 and OCO-3 on

4 May 2019. Together, they help to keep track of the

CO 2 in Earth’s atmosphere. Fundamentally, they’re

the same – both carry an instrument containing

spectrometers that measure wavelengths of sunlight

WKDWKDYHUHƆHFWHGRII(DUWKŝVVXUIDFHDQGSDVVHG

through the atmosphere.

“What’s important about that is every gas [in the

atmosphere] has a unique way of interacting with

light,” says Eldering. “[Each gas] absorbs a little bit of

light as it passes through. So if you know the pattern

of absorption for a particular gas and you’ve got a

really precise measurement of the light; you can

see that there were more molecules [of that gas]

here than there were over there.”

The Mauna Loa Observatory in

Hawaii monitors concentrations

of CO 2 in Earth’s atmosphere

390 392 395 397 400 402 405

Parts per million by volume

Atmospheric carbon dioxide concentration (1–15 June 2015)

>

OCO-2 satellite