10 August 2019 | New Scientist | 19rock would itself require vast
amounts of energy.
Others want to remove carbon
dioxide directly from the air,
then pump it underground or
convert it into other materials,
like plastics. At first sight, this plan
has big advantages. “You don’t
need to use large tracts of land and
you don’t need lots of water,” says
Ajay Gambhir at the Grantham
Institute for Climate Change in
the UK, who has recently studied
the prospects.
But no one has got it to work
at anything like the required
scale. If anyone did, it would
still require lots of energy. This
strategy makes most sense if that
energy comes from renewable
sources – but we don’t have plenty
of that to spare. According to
Fuss’s study, direct air carbon
capture and storage could remove
5 Gt of CO 2 a year at most by 2050
if done sustainably.
Adding up her team’s estimates
of the impacts of carbon negative
strategies suggests that, if we
used them all, we could remove
between about 4 and 25 Gt of CO 2
a year without damaging the
environment. Yet this is still an
overestimate, says Fuss, because
land used for growing bioenergy
crops, say, can’t be turned into
forests as well. However, her
study didn’t include every known
carbon-removing strategy.
If you hoped that carbon
negative technology would allow
us to carry on pumping out CO 2
recklessly, these estimates are
bad news. On the flip side, they
show we really could achieve net
zero emissions if we also cut CO 2
emissions hard and fast. Just don’t
think that planting trees will be
enough on its own. ❚Commons and Climate Change in
Germany and her team estimated
that soil carbon sequestration and
BECCS could each remove at most
5 gigatonnes (Gt) of CO 2 a year by
2050 if done on a sustainable basis.
That exceeds what afforestation
could achieve (see graph, left). To
put this into context, global CO 2
emissions are currently about
40 Gt a year.
This is why some people are
exploring more exotic methods,
such as grinding up rocks that
react with CO 2 and spreading the
powder on farmland – known as
enhanced weathering because
it speeds up this natural process.
However, grinding up so muchDEEPOL BY PLAINPICTUREDR T. J MARTIN/GETTY. PETER MACDIARMID/GETTY IMAGESwould stem the loss of carbon,
says Smith. Restore them, and
they would start taking it up again.
The downside is that the land
couldn’t be used for growing food.
Another approach is to
manage soils so they retain more
carbon. So-called soil carbon
sequestration can involve a range
of measures for maintaining soil’s
structure, from tilling less often to
growing crops with deeper roots.
Then there is heating plant
matter in the absence of air to
make biochar, which is used
to improve soils by trapping
nutrients and water. It can take
centuries for this charcoal to break
down and release its carbon. But
there is a limit to how much plant
material is available for making it,
and growing plants solely to make
it would require more land.
There are huge uncertainties
about the potential of all these
methods. A study last year
by Sabine Fuss at the Mercator
Research Institute on Global
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newscientist.com/insightPlanting a trillion trees
isn’t quite the climate
panacea it might seemSteel-making
is one process
with emissions
that are very
difficult to cutMORRIS MACMATZEN/GETTY