5 October 2019 | New Scientist | 7Geology Environment
Michael Marshall Graham Lawton
AN EPIC project has totted
up all Earth’s carbon and the
result is in: our planet contains
1.85 billion billion tonnes of it.
The estimate comes from the
Deep Carbon Observatory (DCO),
established in 2009. Its goal
has been to estimate the scale
of the carbon cycle. This involves
everything from measuring
the release of carbon dioxide
from volcanoes to studying
diamonds – a solid form of
carbon – in the mantle.
Carbon’s movements around
the planet are well understood,
but estimating the amounts in each
bit of the world is a monumental
job. “All the work the DCO has
been doing in the past 10 years
has been trying to document actual
numbers of where this carbon is
stored,” says Celina Suarez at the
University of Arkansas.
“The majority of carbon is very
deep in the mantle and in the core,”
says Suarez. The land, air and
ocean have only 43.5 trillion
tonnes – less than 1 per cent
of the total (Elements, DOI:
10.2138/gselements.15.5.301).
The DCO also looked at
isotopes, or variants, of carbon
in rocks of different ages to
understand the history of
the carbon cycle. In the past
500 million years, when complex
animal life has existed on Earth,
the carbon cycle has been in balance
for more than 99 per cent of the
time. “What comes out goes back
in,” says Suarez.
Yet there were four periods
when the cycle became unbalanced
for about a million years, for
example because of major volcanic
eruptions. We are also destabilising
the carbon cycle now, by burning
fossil fuels and deforesting large
areas (see page 34 for more on
carbon stored in trees). “The
balance is getting a bit out of
whack,” says Suarez. ❚
Almost all of Earth’s
carbon is locked
underground
THE last great undeveloped coal
deposit on the planet is about
to go up in smoke. Botswana is
sitting on vast amounts of coal
and is ramping up efforts to
mine and export it. But climate
scientists warn that to meet
the world’s climate goals most
of it has to stay in the ground.
Botswana’s coal was
discovered in the 1960s, yet has
remained virtually untouched,
largely due to the country’s
small population and lack of
infrastructure for exports.
However, several firms are now
developing the coalfields and
the country’s first commercial
consignment has just been
exported to South Africa.
Estimating coal deposits is
tricky, but Botswana’s are large.
“Everyone agrees that it has the
biggest coal reserves in Africa,
though the extent and quality
are far from understood,” says
Nicola Wagner at the University
of Johannesburg in South Africa.
One analysis in 2012 settled
on 28.5 billion tonnes, which
would be the world’s number
one undeveloped coal resource
as well as putting Botswana in
the global top five.Until this year, Botswana
only had one coal mine, a state-
owned enterprise that produces
less than a million tonnes a year
for electricity. In recent years,
though, interest in Botswana’s
coal has ramped up as the
government seeks to become
energy independent and
diversify the economy away
from diamonds, says Mmilili
Mapolelo at the Botswana
Institute for Technology
Research and Innovation.
In September, the country’s
first privately run coal mine,
the opencast Masama project
near the capital city Gaborone,
produced its first saleable coal.
Owner Minergy Coal says the
mine is producing 70,000 to
80,000 tonnes of coal a month
and will boost this to 100,
tonnes early next year. Other
companies are developingmines and the state-owned
mine plans to quadruple
production, says Mapolelo.
The development could be
bad news for the climate. Coal
has to be phased out for us
to limit global warming to
1.5°C, says Joeri Rogelj at the
International Institute for
Applied Systems Analysis,
a research body in Austria.
Rogelj calculated that burning
Botswana’s coal would pump
63 to 84 billion tonnes of carbon
dioxide into the atmosphere,
which is 15 to 25 per cent of the
total the world can emit to have
a 50-50 chance of staying under
1.5°C of warming.Digging up coal is a “strange
decision” for a country with
vast solar resources, says Rogelj.
India and China are pulling out
of coal, he says. In the developing
world, only Indonesia is
planning a coal expansion.
Rather than mainly eyeing
the coal to generate its own
power, Wagner says Botswana
aims to sell it to South Africa,
which has long used coal as an
oil substitute in industry, but is
running out of its own supplies.
Minergy Coal declined to
comment, but in a presentation
at the Botswana Resource Sector
Conference last year, then CEO
Andre Boje said: “The developed
nations were developed on
the back of fossil fuels... [They]
should increase cutbacks to
allow developing nations space,
especially in Africa.” ❚World’s largest untapped
coal reserve to be mined
FR
AUKE
SCHOLZ/AL
AMYBotswana’s Okavango
delta is a popular
destination for tourists28.5 bn
The amount of coal in tonnes
estimated to be in BotswanaCoal has to
be phased
out to limit
global
warming
to 1.5°CTAYLOR^ WEIDMAN/BLOOMBE
RG