Inside science >> Cumulative carbon budgets

9th March 2011


Related Topics

Related tags

  • Mitigation

Author

IEMA

Since the 1990s the prevailing scientific view has been that we must limit global warming to no more than 2°C above pre-industrial levels to avoid dangerous climate change - although there is increasing evidence of serious potential effects in many parts of the world even at 2°C degrees.

The well-documented physics of the greenhouse effect alongside data on the long-term relationship between temperature and the concentration of CO2 and other greenhouse gases (GHGs) have led to a paradigm in considering the future warming potential in terms of both carbon dioxide equivalent (CO2e) concentrations and temperature limits.

The accepted wisdom has been that we should avoid exceeding 450 parts per million of CO2e (many scientists now suggest lower concentrations).

This scientific paradigm has long underpinned international negotiations on climate change, translating into emissions-reductions targets focused on annual emission rates.

The Intergovernmental Panel on Climate Change, however, began discussing “cumulative emissions” in 2001. A flood of more recent analyses has resulted in a shift in emphasis from annual emissions to cumulative emissions as summarised in January in a special volume of the Philosophical Transactions of the Royal Society.

The rationale is: oceans and terrestrial vegetation can only take up CO2 slowly, so a significant fraction accumulates in the atmosphere for centuries or longer.

Analyses indicate that it is the cumulative amount of CO2 that determines maximum temperature the most, rather than any particular emissions pathway.

This has led to a reframing of mitigation science in terms of “cumulative carbon budgets”.

It follows, then, that we can calculate near enough a specific amount of carbon – about a quarter the mass of CO2 – that is the maximum we can emit in order to avoid exceeding 2°C average warming.

Dr Myles Allen from Oxford University and his colleagues estimate the limit to be about one trillion tonnes.

The fact that about half of this has already been produced puts real constraints on the options for staying under budget. Data provided by Oxford’s department of physics suggest that the trillionth tonne may be released in 2044 if emissions trends of the last 20 years are simply extrapolated forwards.

But if rates were to fall from now by a cumulative 2.3% a year we could stay within this trillion tonne limit.

Ninety-five per cent confidence intervals around 2°C, ranging from 1.3–3.9°C, however, beg the question:

“How confident do we want to be of this outcome?”

The Oxford data suggest that we can increase our confidence to a 75% likelihood of not exceeding 2°C if the world were only to emit 75 billion tonnes, but emissions would need to fall by 4.85% a year, starting now.

The Avoid programme – a collaboration between the Met Office, Walker Institute, Tyndall Centre, and Grantham Institute – takes similar approach, concluding that an emissions peak in 2016 would require at least a 4% annual emissions reduction, and a peak in 2020 would require at least a 5% annual reduction.

This is unlikely without significant global political will and a price on carbon, as well as an evolution in energy technology and use, including behaviour change.

The cumulative carbon budget approach is a much more accessible take on global warming – an analogy would be a slightly leaky bath with many taps flowing into it (different CO2 sources), with policy options on which taps to close at which rates in order to avoid exceeding the volume limit.

It implies also that temperatures will not drop soon after “peak emissions”, since the cumulative CO2 volume will either still be increasing (the taps are still running), or, at best, will be relatively static.

A reduction in CO2 emissions will thus only reduce global average temperature in the very long term.

Another implication is that the later the peak of emissions, the greater the rate of emissions reductions required to limit the total volume – that is, if the bath is nearly full, you need to turn off the taps pretty quickly.

Professor Robert Watson (chief scientific adviser, Defra) and Dr Rupert Lewis (deputy director/head of evidence, Defra)

Subscribe

Subscribe to IEMA's newsletters to receive timely articles, expert opinions, event announcements, and much more, directly in your inbox.


Transform articles

A social conscience

With a Taskforce on Inequality and Social-related Financial Disclosures in the pipeline, Beth Knight talks to Chris Seekings about increased recognition of social sustainability

6th June 2024

Read more

Disinformation about the impossibility of averting the climate crisis is part of an alarming turn in denialist tactics, writes David Burrows

6th June 2024

Read more

David Symons, FIEMA, director of sustainability at WSP, and IEMA’s Lesley Wilson, tell Chris Seekings why a growing number of organisations are turning to nature-based solutions to meet their climate goals

6th June 2024

Read more

A system-level review is needed to deliver a large-scale programme of retrofit for existing buildings. Failure to do so will risk missing net-zero targets, argues Amanda Williams

31st May 2024

Read more

Chris Seekings reports from a webinar helping sustainability professionals to use standards effectively

31st May 2024

Read more

Although many organisations focus on scope 1 and 2 emissions, it is vital to factor in scope 3 emissions and use their footprint to drive business change

31st May 2024

Read more

IEMA submits response to the Future Homes Standard consultation

31st May 2024

Read more

What is the role for nature in the Climate Change Act? Sophie Mairesse reports

20th May 2024

Read more

Media enquires

Looking for an expert to speak at an event or comment on an item in the news?

Find an expert

IEMA Cookie Notice

Clicking the ‘Accept all’ button means you are accepting analytics and third-party cookies. Our website uses necessary cookies which are required in order to make our website work. In addition to these, we use analytics and third-party cookies to optimise site functionality and give you the best possible experience. To control which cookies are set, click ‘Settings’. To learn more about cookies, how we use them on our website and how to change your cookie settings please view our cookie policy.

Manage cookie settings

Our use of cookies

You can learn more detailed information in our cookie policy.

Some cookies are essential, but non-essential cookies help us to improve the experience on our site by providing insights into how the site is being used. To maintain privacy management, this relies on cookie identifiers. Resetting or deleting your browser cookies will reset these preferences.

Essential cookies

These are cookies that are required for the operation of our website. They include, for example, cookies that enable you to log into secure areas of our website.

Analytics cookies

These cookies allow us to recognise and count the number of visitors to our website and to see how visitors move around our website when they are using it. This helps us to improve the way our website works.

Advertising cookies

These cookies allow us to tailor advertising to you based on your interests. If you do not accept these cookies, you will still see adverts, but these will be more generic.

Save and close