UOA09-07: Cumulative carbon emissions targets for climate policy
Submitting InstitutionUniversity of Oxford
Unit of AssessmentPhysics
Summary Impact TypeEnvironmental
Research Subject Area(s)
Earth Sciences: Oceanography
Biological Sciences: Ecology
Economics: Applied Economics
Summary of the impact
Results from climate physics research at the University of Oxford have
demonstrated that targets for cumulative carbon emissions, rather than
greenhouse gas concentrations, are a more effective approach to limiting
future climate change. This new approach and the resulting `trillionth
tonne' concept have had substantial political and economic implications.
Impacts since 2009 include (a) stimulus to policy developments; (b)
influence on the business decisions of Shell e.g. to invest in a $1.35bn
carbon capture and storage facility; and (c) significant public and media
debate with a global reach.
Professor Myles Allen has led the Climate Dynamics group in the Physics
Department at the University of Oxford since 2000. In April 2009, two
papers, Allen et al (2009)  and Meinshausen et al (2009), were
published as the cover story of Nature, the first led by Allen and
the second with his input and co-authorship. Both demonstrated that the
risk of dangerous anthropogenic interference in the climate system is
primarily determined by the total carbon dioxide emissions accumulated
over time, not by emissions in any given year or commitment period.
The research that prompted this work was also led by the University of
Oxford and assessed the risks associated with different stabilisation
concentrations of atmospheric greenhouse gases . This concluded that
the long-term equilibrium warming consequence of a doubling of carbon
dioxide concentration (a common policy objective) could be substantially
higher than the previously predicted likely range of 2-4.5oC.
It also called into question the conventional paradigm of framing climate
policy in terms of targets for long-term stabilisation concentration ,
and prompted further research into alternative policy targets that could
be more robustly constrained by the fundamental physics of the climate
system and available climate observations.
Allen's Nature paper  presented ensemble simulations of simple
climate-carbon-cycle models, constrained by observations and projections
from more comprehensive models, to simulate the temperature response to
250 carbon dioxide emission pathways covering a broad range of scenarios.
The key finding was that the peak warming resulting from a given
cumulative injection of carbon dioxide into the atmosphere, and hence into
the entire global system, is better constrained than the warming response
to scenarios of stabilising greenhouse gas concentrations. Furthermore,
the relationship between cumulative emissions and peak warming is
remarkably insensitive to the emission pathway (such as timing of
emissions or peak emission rate). Hence policy targets based on limiting
cumulative emissions of carbon dioxide are likely to be more robust to
scientific uncertainty than emission-rate or concentration targets.
In particular, it was shown that to limit the estimated most likely
maximum global temperature rise caused by anthropogenic carbon dioxide
emissions to 2oC above pre-industrial temperatures, the
cumulative injection over the entire industrial period needs to be limited
to one trillion tonnes of carbon, of which approximately 500 billion
tonnes has already been released. This gave rise to the concept of the
`trillionth tonne'. This was later confirmed to be robust for a broad
range of scenarios up to the year 2200 . Allen has also led more recent
work to update the carbon budget using the latest observations and models
Professor Myles Allen led the work described here. He joined Oxford as a
NERC Advanced Fellow (2000-03), took up a Lectureship (2003-11) and is now
Professor of Geosystem Science (2011-present). Other principal
contributors at Oxford were Professor David Andrews (1989-2012), Dr
Claudio Piani (PDRA, 2000-06); Dr David Stainforth (PDRA from 1998-2003
and NERC Research Fellow in Physics 2003-06), Dr David Frame (PDRA,
2000-06) and students Niel Bowerman (also research assistant 2009) and Ben
Bronselaer (both CASE students sponsored by the Met Office).
In 2010, Allen was awarded the Appleton Medal and Prize by the Institute
of Physics for "his important contributions to the detection and
attribution of human influence on climate and quantifying uncertainty in
References to the research
(Oxford authors, * denotes best indicators of quality)
* Allen MR, Frame DJ, Huntingford C, Jones CD, Lowe
JA, Meinshausen M, Meinshausen N, (2009), Warming caused by cumulative
carbon emissions towards the trillionth tonne, Nature, 458,
1163-1166, doi:10.1038/nature08019, citations: 217 (Scopus). This was
one of the two key papers proposing cumulative carbon emissions as a
more robust predictor of future warming. Allen and Frame (Oxford)
designed, tested and ran the simple climate model for the ensemble. This
paper was also discussed in Editor's View and News & Views.
* Stainforth DA, Aina T, Christensen C,
Collins M, Faull N, Frame DJ, Kettleborough JA, Knight
S, Martin A, Murphy JM, Piani C, Sexton D, Smith LA,
Spicer RA, Thorpe AJ and Allen MR, (2005), Uncertainty in
predictions of the climate response to rising levels of greenhouse gases,
Nature, 433, 403-406, doi:10.1038/nature03301, citations: 475
(Scopus). This paper presents results from `climateprediction.net'
simulations using a general circulation model with extreme climate
sensitivities. The model is used to assess the risks associated with
specific target stabilisation concentrations of atmospheric greenhouse
 Frame DJ, Stone DA, Stott PA, Allen MR.
(2006), Alternatives to stabilization scenarios, Geophysical Research
Letters, 33, L14707, doi:10.1029/2006GL025801, citations: 26
(Scopus). This paper argued that, for policy-making purposes,
scenarios that can exploit a consensus in the transient climate response
may be preferable to stabilization scenarios, because of the difficulty
of ruling out a high equilibrium warming response to elevated CO2
 Bowerman NHA, Frame DJ, Huntingford C, Lowe JA and Allen
MR, (2011), Cumulative carbon emissions, emissions floors and
short-term rates of warming: implications for policy, Philosophical
Transactions of the Royal Society A, 369, 45-66, doi:
10.1098/rsta.2010.0288, citations: 10 (Scopus). This paper showed the
results in  are robust to a broader range of scenarios up to the year
* Gillett NP, Arora VK, Matthews D, Allen MR, (2013),
Constraining the Ratio of Global Warming to Cumulative CO2 Emissions Using
CMIP5 Simulations, Journal of Climate, 26, 6844-6858, doi:
10.1175/JCLI-D-12-00476.1, citations: 3 (Scopus). This paper updates
the carbon budget using the latest observations and model simulations.
Details of the impact
The most important policy implication of this research was that the
current focus of climate mitigation policy, limiting rates of greenhouse
gas emissions in 2020 and 2050, is misplaced: effective mitigation
policies need to limit the total cumulative release, not just the rate of
emission in any given year. This matters, because not all measures to
reduce the rate of emission in the short term will necessarily be
effective in reducing the cumulative total.
Although the scientific case for limiting warming to 2oC
through emission reductions was noted in Copenhagen in 2009, specific
measures indicating how this was to be achieved were not agreed. The need
to limit cumulative emissions has substantial political implications, such
as short- versus long-term effects, and industrialised versus developing
The cumulative carbon emissions point was made by Allen i) in
presentations to the United Nations Framework Convention on Climate Change
delegates in Bonn and Copenhagen; ii) in separate briefings to senior
civil servants in the Department of Energy and Climate Change (DECC) and
the Department for International Development (DFID); iii) in evidence to
the UK Climate Change Committee; iv) to the Royal Society (who have
acknowledged the importance of cumulative emissions in public statements
[A]); v) to the Parliamentary Environmental Audit Committee's 2009 enquiry
into carbon budgets [B]. Allen also developed a visual demonstration of
the cumulative carbon concept that he presented at the UN Climate Change
Conference (COP18) in 2012. The DECC/Defra-funded AVOID programme, which
provides advice to government on climate change, includes discussion of
cumulative emissions in its analysis of whether the pledges in the
Copenhagen Accord are consistent with the desired 2°C warming limit, and
references  directly [C]. The campaign group Sandbag, founded by
Baroness Worthington, has used the trillionth tonne concept in its
analysis and subsequently in two of its 2010 viewpoint papers, which also
cite  directly [D]. Sandbag uses these to lobby governments on
emissions trading. It also advises the public and organisations about
personal carbon offsetting strategies, for example discouraging tree
planting offsetting which, while it reduces carbon concentrations, does
not affect the cumulative carbon emitted.
Allen's research in  directly influenced one of the key conclusions of
the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment
report. The report concludes that limiting the warming caused by
anthropogenic CO2 emissions alone to less than 2°C (with a probability
greater than 66%) will require cumulative CO2 emissions from all
anthropogenic sources to stay between 0 and about 1000 gigatonnes of
carbon. These estimates were based on the concept of Transient Response to
Cumulative Carbon Emissions (TRCE) from , which was a variant of the
cumulative warming commitment proposed in . Allen was a
draft-contributing author for the assessment of the literature on TRCE in
the IPCC 5th Assessment. The final draft, highlighting the importance of
the trillion tonne budget, was submitted to governments on 7th June, 2013.
Paper  is cited in the full report and the summary for policy makers.
Although these were published after the REF impact period, and so we are
not claiming impacts that are a consequence of the final report, it is
clear that Allen's research has heavily influenced the IPCC report.
Impacts on Carbon Capture and Storage
Petrochemical companies have been influenced by Allen's work and the
trillionth tonne concept. In particular, Shell has used the cumulative
carbon approach to climate change to inform their investments in carbon
capture and storage (CCS) and also their long-term energy scenario
planning. Shell said: "Carbon capture and storage will be a critical
technology for society over the course of this century [...] more
recently its importance has been cemented in our thinking by the
`trillion tonne' work that Professor Allen and his team have delivered
since 2009" [E].
Shell has directly used the research to influence their investment in
carbon capture and storage. They said: "The `trillion tonne stock'
approach has helped us build internal support for our CCS investments in
Canada and Norway and more recently has been used by the team leading
our potential UK CCS project." The investments, informed by Allen's
work, are significant. For example, the Quest CCS site in Canada is
currently under construction at a total cost of $1.35bn. Shell is leading
the commercial joint venture (60%) with two other companies and government
funding. The final investment decision was taken by Shell in September
2012 and the facility is due to start operating in 2015.
Shell also identified impacts of the research on their Scenarios
documents, which they have used for 40 years to challenge executives'
perspectives on the future business environment using plausible
assumptions and quantification. "Our new Lens Scenarios makes
extensive use of carbon stock thinking as we seek to show that the
climate issue can be resolved over this century given the application of
CCS", said Shell. In their current Scenarios document future
emissions are described with respect to cumulative emissions. In one
future scenario "the cumulative emissions situation by the middle of
the century means an overshoot of the 2°C goal, but the expanded use of
CCS as a carbon sink later in the period provides a critical component
to a potential pathway for managing net global emissions" [E].
Allen has given numerous press interviews on the cumulative carbon
results. The Guardian newspaper highlighted  on first
publication, including quotes from Allen [F]. Later that year, at the
start of the Copenhagen conference, a coordinated editorial was published
in 56 newspapers [G], often as the front page, in 20 languages. This
raised the issue, "how we will share a newly precious resource: the
trillion or so tonnes of carbon that we can emit before the mercury
rises to dangerous levels", with discussion of the required balance
between industrialised and developing worlds. The text was drafted by a Guardian
team during more than a month of consultations with editors from more than
20 of the papers involved, and raised 1072 on-line comments on the Guardian
website in three days. The editorial also forms part of a wider
sustainability campaign by the newspaper called `10:10'.
Stimulating public engagement and debate
In October 2009, alongside the final round of pre-Copenhagen talks, a
heap of coal representing the `trillionth tonne' was on display in the
Science Museum in London as part of their exhibition, `Prove It!' (Oct
2009 - Feb 2010). The coal is to be kept by the Museum of Oxford until its
combustion is required. Professor Chris Rapley, then the Science Museum's
Director, told a news conference, "to avoid dangerous climate
change...we must never release into the atmosphere the carbon dioxide
from that trillionth tonne" [H]. Professor Allen explained its
purpose and significance in a `Viewpoint' article for the BBC, which was
commented on in detail by readers from a range of countries and linked to
by campaign groups. In conjunction, a live, online countdown to emission
of the trillionth tonne is maintained by Oxford at http://trillionthtonne.org.
George Monbiot, a vociferous campaigner on the need to reduce emissions,
identified the 2009 pair of Nature papers along with one other as being
key steps towards more meaningful emission targets and concluded that "the
targets and methodology being used by governments and the United Nations
— which will form the basis for their negotiations at Copenhagen — are
not even wrong; they are irrelevant" [I]. Allen followed this with
his own critical commentary on the 10:10 campaign in the Guardian
in September 2009 [J].
Allen has also spoken on wider issues of climate policy arising from his
work at the Sustainable Planet Forum in Lyon (Sept 2010), reportedly
attended by 27,000 people, in a debate on climate politics with the former
French Environment Minister and leader of the French Green Party,
Sources to corroborate the impact
[A] `Preventing dangerous climate change: The need for a global
agreement', The Royal Society, December 2009, Document 12/09 DES1723,
confirms the Royal Society's standpoint that cumulative emissions play a
[B] House of Commons Environmental Audit Committee's 2009 enquiry into
Carbon Budgets, Volume II. This confirms the policy debate was influenced
by oral evidence given directly by Allen. Other contributors cite 
including the Met Office.
[C] `Are the emission pledges in the Copenhagen Accord compatible with a
global aspiration to avoid more than 2°C of global warming?', Technical
Note by the AVOID consortium, 2010.
[D] Sandbag publications: `A Closer Look at Voluntary Carbon Action' and
`To Offset or Not To Offset?' 2010. Both position papers cite 
[E] Letter (held on file) from Chief Climate Change Advisor at Shell
confirming the impacts on CCS investment decisions and on their Scenarios
[F] `Climate countdown: Half a trillion tonnes of carbon left to burn,'
The Guardian, 29th April 2009. This communicates the
trillionth tonne concept.
[G] Copenhagen climate change conference: `Fourteen days to seal
history's judgment on this generation', The Guardian, Editorial, 7th
December 2009. This communicates the trillionth tonne concept and 1072
reader comments are significant evidence of public debate.
[H] `London museum confronts climate change sceptics', Reuters, 23rd
October 2009. Press coverage of Trillionth Tonne exhibit. http://in.reuters.com/article/2009/10/23/idINIndia-43391420091023
[I] George Monbiot discusses the changes  has made to viewpoints in
and `We're pumping CO2 to the point of no return. It's time to alter
course', The Guardian, 1st September 2009.
[J] `Make the CO2 cuts count', The Guardian, article by Myles
Allen, 2nd September 2009,