Politicians have agreed that global temperatures need
to be limited to below 2C, and scientists say that this will mean
drastically reducing emissions of greenhouse gases. But which one
should be cut first?
35.3 billion tonnes of carbon dioxide (CO2) in 2013 - a
volume that is increasing every year, putting the world on course
to exceed its goal to keep temperature increase since the start of
the Industrial Age below 2C.
But this is not the only pollutant that causes the
planet to warm. Methane, ozone, black carbon (soot) and
hydrofluorocarbons have an even more powerful warming effect, per
tonne, than CO2.
Yet unlike CO2, which can last in the atmosphere for
up to millennia, these stick around in the atmosphere for a matter
of years or even days. As a result, they are known as short-lived
climate pollutants (SLCPs).
Most countries are focusing on reducing CO2 and SLCPs
at the same time.
new policy paper by Myles Allen, professor of geosystem
science at the Oxford Martin
School at the University of Oxford, says that reducing
SLCPs while CO2 emissions are still rising could make it more
difficult to hit the 2C goal.
He argues that, while there are good reasons to cut
SLCPs, they should not be used as an excuse to put off cutting CO2
Governments are currently submitting pledges to the UN
about how much they intend to reduce their greenhouse gas
The EU, for instance, has said it will cut its
emissions by at least
40% by 2030 on 1990 levels. There is no obligation for
countries to say how much of their reductions will come from CO2
cuts, and how much from SLCPs.
Instead, countries group them together, meaning that
their emissions reductions targets can be hit by cutting any
combination of any gases.
To create a level playing field between the different
pollutants, they are measured according to how much global warming
they will cause over a 100-year period.
For instance, over a 100-year period, one tonne of
methane will cause 28 times as much warming as one tonne of CO2.
Black carbon causes 924 times as much warming.
But the lifespan of these pollutants is short.