New Scientist Australia - 10.08.2019

18 | New Scientist | 10 August 2019

Planting about a trillion trees, which

would cover an area of land roughly

the size of the US

Afforestation

Bioenergy carbon capture and storage

involves producing electricity by burning

crops while capturing the carbon emitted

BECCS

Min

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Max

This means making plants into charcoal,

then digging this into the soil to store the

carbon for several hundred years

Biochar

Taking rocks that react with CO 2 and

grinding them up so more of the gas

is locked away more quickly

CO 2 removal (gigatonnes per year)

Enhanced weathering

This technique separates CO 2 from

the other gases in the air and stores

it away

Direct air capture

Managing soil so that much of the

carbon that would naturally be

released stays locked away

Soil carbon sequestration

SOURCE: doi.org/c8zq

HUMANS have now pumped so

much carbon dioxide into the

atmosphere that the only way to

meet our climate goals is to extract

a stupendous amount of it. Last

month, a headline-grabbing study

suggested that we can do this just

by planting a vast number of trees.

But is it really that easy?

We know that we need to

employ some form of carbon

negative technology for two

reasons. First, with emissions

still rising, planetary warming

is on course to exceed 2°C above

pre-industrial temperatures not

long after 2050. If we can reduce

CO 2 levels, we stand a chance of

cooling the planet back to safer

temperatures.

Second, there are some

activities – farming, flying, cement

production, steel-making – that

are really hard to do without

emitting lots of carbon. Even if the

world got serious about reducing

emissions from these processes,

the technology to do it doesn’t

necessarily exist. Carbon removal

might allow us to keep on flying,

making steel and so on, and still

get to zero overall emissions.

Tree planting for this purpose

is a beguiling idea: trees are

beautiful, cheap and effective

at sucking in carbon. A longer-

standing idea is to grow plants,

burn them to make electricity and

then filter out the carbon released

and somehow store it. This is

known as bioenergy carbon

capture and storage (BECCS).

Almost all studies that claim we

can limit warming to 2°C assume

a massive deployment of BECCS.

Yet we don’t know if it would work.

No one has shown it can be done

affordably on a large scale.

Even if it is doable, it would

require enormous swathes of land.

We would also have to plant

trees in the Arctic on land

otherwise covered in sunlight-

reflecting snow. This would mean

more of the sun’s heat being

absorbed. Plus, if monocultures

of trees are planted, as often

happens, that will do wildlife no

favours. There needs to be scrutiny

around large-scale tree planting,

says Lehmann. “What is it really

for? Carbon removal, biodiversity

or job creation? These do not

always align,” she says.

Fortunately, there are other

ways to remove CO 2 from the

atmosphere. One little-known

method is wetland restoration.

When vegetation sinks to the

bottom of still water, the carbon

it contains is locked up as peat.

This dries out and decomposes

when marshes are drained to

create farmland. “They emit

phenomenal amounts of carbon,”

says Pete Smith at the University

of Aberdeen, UK.

Simply re-wetting these lands

Carbon capture

News Insight

Sucking carbon from air

To avoid dangerous global warming, we need to go carbon negative.

Is mass tree planting the answer? Michael Le Page reports

Around a tenth of all land is

already used for growing crops,

and another three-quarters is

exploited, for instance for grazing.

There isn’t space to feed a growing,

meat-hungry population, build

more cities, conserve wildlife, save

forests and also grow crops for

bioenergy on a massive scale.

Ecologists have been saying this

all along, and it is expected to

be acknowledged by the UN’s

Intergovernmental Panel on

Climate Change in an 8 August

report on land use.

Is there a plan B? Ostensibly, yes.

The recent tree study suggested

that there is enough suitable land

to plant a trillion trees, and that

these would soak up two-thirds of

the extra CO 2 in the atmosphere

(Science, doi.org/gf4mwk).

Trees definitely can help.

But many say the study greatly

exaggerates their potential. The

main reason is that half the CO 2

we have pumped out has been

absorbed by oceans and plants. If

CO 2 is removed from the air, much

of this will be released. This means

that even if we did plant enough

trees to remove two-thirds of the

extra CO 2 , levels would only fall

by roughly a third. What’s more,

it would take up to a century for

these trees to do their work.

Afforestation on this scale

would also harm biodiversity.

For instance, it would require

forests to be planted on natural

grasslands. “In savannahs and

grasslands that are ancient

ecosystems with a unique

diversity, tree planting is a process

of degradation,” says ecologist

Caroline Lehmann at the

University of Edinburgh, UK.

“ Some are exploring exotic

ideas, like grinding rocks

that react with CO 2 and

spreading them on farms”

How much CO 2 can we remove?

A range of strategies could be used to suck carbon dioxide from the air. Here are

the best estimates of how much they could lock away, if applied sustainably