Carbon briefing: Making sense of the IPCC’s new carbon budget
- 23 Oct 2013, 10:00
- Roz Pidcock
Last month, the Intergovernmental Panel on Climate Change (IPCC)
released its fifth 'assessment report', which provide a detailed
look at the science of climate change.
The latest report
is the first to include an assessment of a "carbon budget" - a
finite amount of carbon that can be burnt before it becomes
unlikely we can avoid more than two degrees of global warming.
So how big is the carbon budget, how was it calculated, and how
does it help us understand the challenge of limiting climate
A budget for two degrees
In 1992, the United Nations Framework Convention on Climate
Change (UNFCCC) decided
of global climate policy should be to stabilise humans' influence
with the climate below the level at which it can be considered
The most widely accepted threshold is two
degrees of warming relative to pre-industrial times - this is
the limit recommended by the UK's
Committee on Climate Change, for example.
In its new report, the IPCC includes a calculation of how much
carbon we can emit and still have a reasonable chance of staying
below two degrees.
It calls this amount a carbon budget. The budget is an upper
limit on total human emissions, from the beginning of the
industrial revolution until the day we stop burning carbon.
To stick to the budget, any fossil fuels that would take us
over-budget will either have to be left in the ground, or the
captured before or after entering the atmosphere.
1000 billion tonnes
So how big is the budget? For it to remain likely that we stay
below two degrees, the total amount of carbon released through
carbon dioxide emissions must be less than 1000 billion tonnes, the
IPCC says. 'Likely' here means a 66 per cent chance.
It's possible to calculate a budget like this because carbon
dioxide, which is the biggest contributor to global warming, has a
predictable relationship with temperature. The warming we get is
almost directly proportional to the total amount of carbon dioxide
that accumulates in the atmosphere.
The budget is calculated using a measure of how sensitive the
planet is to carbon dioxide called the transient climate response
to carbon emissions (
TCRE). This is defined as the change in average global surface
temperature for a given amount of carbon dioxide accumulated in the
The calculation takes into account processes that amplify or
reduce the warming we see in the atmosphere, which are known
collectively as feedbacks. These are complicated, introducing
uncertainties to the calculation. Because of this, the carbon
budget is expressed as the probability of staying below a certain
temperature, rather than a guarantee that's what will happen.
How much of the budget has been used up by carbon
The IPCC report says that between the start of the industrial
revolution and 2011, carbon dioxide emissions from human activity
added about 531 billion tonnes worth of carbon to the
Uncertainty around that figure means it could be anywhere
between 446 and 616 billion tonnes but broadly-speaking, this would
mean that about half the carbon budget is already spent.
Including other gases shrinks the budget
While carbon dioxide is the main greenhouse gas resulting from
human activity, it's not the only thing causing the atmosphere to
warm. Methane, CFCs, ozone, nitrous oxide and soot all have an
overall warming effect.
To stick within the two degree target, this means the budget for
carbon dioxide emissions ends up being less than the original 1000
In the draft Summary for Policymakers given to governments to
review back in June, the IPCC scientists stopped short of including
a concrete figure for how much these other gases tighten the budget
for carbon dioxide emissions.
That was because their warming effect is less straightforward
than for carbon dioxide. But after calls from governments for
something more tangible to work with, the final version of the
Summary for Policymakers included scientists' best estimate of what
the reduced budget would be.
800 billion tonnes
The Summary for Policymakers states that taking into account
additional warming factors, the amount of carbon that can be
released through carbon dioxide emissions - in total - comes down
to about 800 billion tonnes.
That figure comes from scientists' estimate of the total warming
expected from the non-carbon dioxide factors, which comes to about
0.4 degrees Celsius by the end of this century.
Given the target of staying below two degrees of warming
overall, that leaves 1.6 degrees of warming to come from carbon
dioxide. Using the relationship between carbon dioxide and
temperature, staying within that budget means carbon dioxide
emissions over the industrial era cannot contribute more than 800
billion tonnes of carbon.
The numbers here relate to the IPCC's lowest emissions scenario,
RCP2.6, which was specifically designed to show how warming can be
limited to two degrees. This means the IPCC have made assumptions
about the non-carbon dioxide factors, which could be higher or
lower in scenarios with weaker or more stringent mitigation of
non-carbon dioxide factors. The upcoming report of the IPCC's
Working Group 3 will look at this in more detail.
How far, how fast?
With a budget for carbon dioxide emissions of 800 billion tonnes
worth of carbon, and assuming that we had already put 531 billion
tonnes into the atmosphere by 2011, it's more accurate to say we've
spent two thirds of the budget, not half.
And with about 270 billion tonnes of carbon left in the budget,
and current carbon dioxide emissions contributing around 10 billion
tonnes per year, the budget is set to be exhausted in about 25
If there are other sources of warming that aren't accounted for
in climate models, the budget could be smaller still. For example,
models don't currently include permafrost methane emissions - as
there's too much uncertainty about them. Including these in the
budget could shrink it further.
Adjusting the budget
If we want a better chance of staying below two degrees, a
smaller carbon budget improves the odds.
On the other hand, Governments also asked the IPCC to calculate
what running a bigger risk of surpassing two degrees would do to
the budget. The Summary for Policymakers says expanding the budget
to 840 billion tonnes of carbon would give a 50:50 chance. Upping
it to 880 billion tonnes reduces the chance to a third.
Rather startlingly, that means that with current emission rates
of 10 billion tonnes per year, the difference between a carbon
budget of 800 and 880 billion tonnes is just eight years worth of
current emissions. That bigger budget halves the chance of staying
below two degrees, the IPCC says, from 66 per cent to 33 per
Last month's calls for greater clarity over the IPCC's new
budget calculations could be taken as a sign that governments are
looking for clearer signs on how far emissions cuts need to go and
how quickly. But it remains to be seen the level of risk they're
willing to accept, or how a worldwide cap on emissions might be
divvied up between nations.
Something to consider, as the 2015
deadline for reaching an international climate agreement
A note on units: Because carbon dioxide is the main
contributor to warming, the budget is sometimes expressed in terms
of tonnes of carbon dioxide rather than carbon. Carbon dioxide has
a higher mass than carbon, so the budget for carbon dioxide
emissions can be expressed either as 800 billion tonnes of carbon,
or 2936 billion tonnes of carbon dioxide - but it's essentially the