How Much Should We Leave in the Ground?

The two papers on carbon emissions published in Nature last week
were ground-breaking: they show us how much carbon dioxide we can
produce if we're to have a reasonable chance of preventing two degrees
of global warming. It's a completely different approach from the UN's
and national governments'. They set targets for reductions by a certain
date but have nothing to say about the total amount of carbon we can

One of the papers, by Myles Allen and others(1), suggests that we
can burn, at most, another 400-500 billion tonnes of carbon at any time
between now and the extinction of humanity if we want to avoid two
degrees of warming. The other, by Malte Meinshausen and others(2),
suggests that producing 1000 billion tonnes of CO2 between 2000-2050
would give us a 25% chance of exceeding two degrees. That's a lot less
than Allen's estimate, as one tonne of carbon produces 3.667 tonnes of
CO2 when it's burnt: 1000 billion tonnes of CO2 arises from 273 billion
tonnes of carbon.

But let's err on the side of valour and use Allen's figures.
Moreover, let's disregard all other greenhouse gases (which, he
suggests, should reduce the total CO2 budget to under 400 billion
tonnes). How does his maximum allowance of carbon compare with known
reserves of fossil fuel?

Let me make two things clear before I make this calculation. First,
reserves are not the same as resources. A resource is the total amount
of a mineral found in the earth's crust. A reserve is the part of the
resource which has been identified, quantified and is cost-effective to
exploit. In most cases this is likely to be a small percentage of the
total resource.

Secondly, there is some controversy over the official figures for
fossil fuel reserves. This is especially the case for oil, as the
members of OPEC are extremely secretive about how much they possess.
But for the sake of argument, let's take them at face value.

According to the World Energy Council:

global reserves of coal amount to 848 billion tonnes(3)

global reserves of natural gas are 177,000 billion cubic metres(4)

global reserves of crude oil are 162 billion tonnes(5)

Because the calculations are much harder and the quantities involved
less certain, I am ignoring unconventional sources of fossil fuel, such
as tar sands, oil shales, bitumens and methane hydrates, as well as
liquid natural gas resources.

On average, one tonne of coal contains 746 kg carbon(6)

One cubic metre of natural gas contains 0.49 kg carbon(7)

The figure for oil is less certain, because not all of its refinery
products are burnt. But the rough calculation here(8) suggests that the
use of a barrel of oil releases 317kg of CO2. Depending on the density
of the oil, there are roughly 7 barrels to the tonne, giving an
approximation of 2219kg CO2, or 605kg of carbon.

So the carbon content of official known reserves of coal, gas and oil amounts to:

848 x 0.746 = 633


177,000 x 0.00049 = 87


162 x 0.605 = 98

Total conventional fossil fuel reserves therefore contain 818 billion tonnes of carbon.

Even ignoring all unconventional sources and all other greenhouse
gases and taking the most optimistic of the figures in the two Nature
papers, we can afford to burn only 61% of known fossil fuel reserves
between now and eternity.

Or, using Meinshausen's figure, we can burn only 33% between now and
2050. Sorry - 33% minus however much we have burnt between 2000 and

So the question which arises is this: which fossil fuel reserves
will we decide not to extract and burn? There is, as I have argued
before(9), no point in seeking to reduce our consumption of fossil
fuels unless we also seek to reduce their production. Yet, apart from
the members of OPEC (who do it only to shore up the price), no
government is attempting to limit the amount of fuel extracted. Far
from it; they all pursue the same strategy as the United Kingdom: to
"maximise economic recovery"(10).

The test of all governments' commitment to stopping climate
breakdown is this: whether they are prepared to impose a limit on the
use of the reserves already discovered, and a permanent moratorium on
prospecting for new reserves. Otherwise it's all hot air.












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