18
CLIMATE ROUNDTABLE - CHASING NEW IDEAS
One of the most important new findings of the latest Scientific
Assessment of the Intergovernmental Panel on Climate Change
(IPCC) is the cumulative carbon budget. In the long-term,
global temperatures are predominantly determined by total
carbon dioxide (CO
2
) emissions over the entire “Anthropocene”
epoch, not by the rate of emission of greenhouse gases in
any given decade.
This is illustrated in the figure on the next page, which shows the
global average temperature increase plotted against cumulative
global CO
2
emissions from fossil fuel sources and land-use
change, both since the mid 19th century. The thin grey line and
grey shaded plume shows the expected warming, and range of
uncertainty, due to carbon dioxide emissions alone. The coloured
lines and orange shaded plume shows the expected warming
and range of uncertainty from all anthropogenic sources under a
range of scenarios of future emissions of all greenhouse gases
and other forms of pollution, from an increasing emissions
“business-as-usual” scenario (RCP8.5, red line), and from an
aggressive mitigation scenario (RCP3PD, dark blue line).
The RCP8.5 scenario moves off to the top right corner of the
figure more rapidly than the RCP3PD scenario, but all scenarios
fall on roughly the same line: for a given level of cumulative CO
2
emissions, the planet experiences approximately the same level
of warming irrespective of whether that CO
2
is emitted slowly or
rapidly. Warming from non-CO
2
anthropogenic sources add half to
one degree to CO
2
-induced warming in both “business-as-usual”
and mitigation scenarios.
This cumulative carbon budget has profound implications for
mitigation policy. Not only does it mean that, to stop climate
change, net CO
2
emissions will eventually have to be reduced
to zero, but it also means that emissions in 2020, or any
other short-term “commitment period” do not, in themselves,
determine the risk of dangerous climate change except
insofar as they contribute to the cumulative total. This is very
important, because the technologies and interventions that
might be required to reduce the flow of emissions of various
greenhouse gases over the coming decade are in many cases
very different from the technologies required to limit the
total stock of cumulative CO
2
emissions in the long term.
At present, climate policies favour short-term emissions
reduction measures, potentially to the detriment of the long-
term goal of avoiding dangerous climate change.
The Cumulative Carbon Budget
and its Implications
By Myles Allen
Environmental Change Institute, School of Geography
and Environment & Department of Physics,
University of Oxford and Oxford Martin School
