(The author is a Reuters columnist. The opinions expressed are
By Gerard Wynn
LONDON Oct 1 A U.N. panel of climate scientists
has for the first time backed a limit on cumulative global
carbon emissions, in a so-called budget approach that is useful
but with certain limitations.
Carbon budgets estimate the maximum greenhouse gases that
countries can emit cumulatively for the world to stay below a
certain amount of global warming, and so are related to targets
that set an emissions limit in any particular year.
The Intergovernmental Panel on Climate Change (IPCC), a
United Nations body, published on Friday estimates for different
"budgets" that would limit global warming.
Governments have never agreed a global emissions target at
annual climate conferences dating back to the signing of the
U.N. convention on climate change in 1992.
It is therefore very unlikely they would adopt a budget.
Instead, lobby groups and countries can use the IPCC
estimate to test the ambition of new commitments in a future
Another limitation concerns communicating such budgets to
wider audiences, including voters.
Such communication is difficult because of the various time
frames, assumptions and uncertainties in how they are
At multilateral talks, governments have never agreed an
aggregate, global carbon emissions target, probably because -
depending on the ambition - it could put the brakes on growth
for emerging economies such as top carbon emitter China.
The second-biggest emitter, the United States, is also
doubtful about international emissions targets, preferring
national measures for curbing greenhouse gases.
Nevertheless, governments have repeatedly backed a target
not to exceed 2 degrees Celsius warming, "according to science",
for example under the 2009 Copenhagen Accord.
Therefore, the IPCC's estimate of a carbon budget that could
meet the 2 degrees target provides a useful benchmark for
weighing the ambition of national carbon cuts committed to in
The next major climate conference is in Paris in 2015, when
governments are supposed to agree a carbon-cutting deal for
implementation from 2020.
Explaining carbon budgets is complicated by their
uncertainty, and by assumptions that make comparisons difficult.
Carbon budgets have a long history, with many studies to
which the IPCC has now added.
One of the first explicit mentions was in 1997 by Bill Hare,
then climate policy director for Greenpeace, in his publication
"Fossil Fuels and Climate Protection: the Carbon Logic".
But the idea of calculating the impact of cumulative carbon
emissions goes back much further.
For example, two authors from Switzerland's University of
Bern (U. Siegenthaler and H. Oeschger) calculated how much
emitted carbon dioxide would stay in the atmosphere, writing in
the journal Science in 1978.
They concluded that to limit growth in atmospheric CO2 to no
more than 50 percent above pre-industrial levels, most fossil
fuels could not be burned.
"The value of 50 percent for the tolerable increase may be
too high or too low; if it is correct, we may burn in total over
the next centuries not much more than 10 percent of the known
fossil fuel reserves," they said.
As it turns out, they were right about a 50 percent increase
- scientists now say this is around the upper limit on CO2
levels, to avoid more than 2 degrees Celsius warming.
The latest IPCC publication is a good example of how
important it is to note the assumptions in budget estimates,
when comparing studies.
On Friday, much media coverage focused on a carbon emissions
budget of 1 trillion tonnes, for a two-thirds chance of staying
within 2 degrees warming.
While a nice, round number, that was not the best figure as
it ignored the warming impact of other greenhouse gases besides
carbon dioxide; to account for these, a lower estimate was more
If estimates are compared using the same assumptions they
are rather similar, suggesting the underlying science is
For example, studies roughly agree on the amount of carbon
emissions this century that will give a 50 percent chance of
staying below 2 degrees, including the effect of all greenhouse
In 1997, Hare calculated a budget from burning fossil fuels
of about 410 billion tonnes of carbon this century; he did not
refer to probability or chance, but a "50 percent uncertainty".
Two much more formal studies published in the journal Nature
in 2009 by large teams of researchers showed how warming was
correlated with rising cumulative emissions, and for the first
time calculated the probability of staying under 2 degrees
They estimated a carbon budget for this century of about 430
billion tonnes of carbon from burning fossil fuels and
And the IPCC on Friday estimated a remaining carbon budget
of no more than 417 billion tonnes of carbon (from 2000), for a
50 percent chance of staying under 2 degrees.
At present fossil fuel emissions rates of 9.4 billion tonnes
of carbon a year, such budgets would be used up in about four
The estimates show the monumental effort needed to curb
warming: the budgets are far lower than fossil fuel reserves,
implying most cannot be burned, as Siegenthaler and Oeschger
concluded in 1978.
The International Energy Agency last year estimated the
amount of carbon emissions locked up in current, proven fossil
fuel reserves - including unconventional gas and oil - at 10,500
Regarding the future outlook, the IPCC showed last week that
only the most ambitious of its four emissions scenarios would
stay within this particular carbon budget.
That scenario required global emissions to stop rising this
decade, fall rapidly after 2020 and reach near zero by 2070 -
something that seems irreconcilable with China's rapid growth.
See chart (page 115): link.reuters.com/qaj53v
Another complexity for the carbon budget approach is
uncertainty, which accounts for the probability ranges published
with the numbers.
There are two main sources of uncertainty.
First, regarding the carbon cycle, it is unclear exactly how
much carbon dioxide emitted will stay in the air.
Until now, the world's oceans have absorbed about a quarter
of all manmade emissions.
As these continue to rise, oceans and soils may take up less
carbon, meaning atmospheric levels will increase faster.
And second, it is unclear exactly how much warming a given
level of atmospheric CO2 will cause, given the complexity of the
The IPCC on Friday published a wider range of temperature
rises it expects from a doubling of atmospheric CO2, at 1.5-4.5
degrees Celsius, compared with 2.0-4.5 degrees as published six
Additionally, it is unclear what human harm will result from
2 degrees warming, from higher sea levels to floods, heatwaves
and crop failures.
Environmentalists argue that uncertainty cannot be used to
justify inaction, citing a "precautionary principle".
One advantage of a carbon budget is that it at least adds
some specific numbers, where high agreement between estimates
makes the case for action more convincing despite the
(Reporting by Gerard Wynn; Editing by Dale Hudson)