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Setting a carbon budget to keep below two degrees

We have already committed the planet to a certain amount of warming due to past carbon emissions. But efforts to reduce emissions now and over the next few decades will critically affect the degree of…

How much can we burn? That depends on the future we want. Aaron Muderick

We have already committed the planet to a certain amount of warming due to past carbon emissions. But efforts to reduce emissions now and over the next few decades will critically affect the degree of future warming.

One way of determining how much we need to reduce carbon emissions is to use a “carbon budget”. A budget lays out how much carbon we can burn in total if we want to keep to a certain amount of warming.

This was one of the headline messages of the recently released Intergovernmental Panel on Climate Change (IPCC) Working Group I Fifth Assessment Report. For the first time, the IPCC described the limits to how much more carbon dioxide can be emitted to keep global temperatures below certain thresholds.

Concentrate on the total

The IPCC report highlighted the direct, almost proportional, relationship between cumulative carbon dioxide emissions from human activities and global temperatures. This is an important finding.

It means that staying below certain temperature targets depends not so much on the rate that we emit in any particular year, but what the grand total of emissions is. Consequently we must take into account all emissions that have taken place from the industrial revolution through to now, and add on all carbon dioxide that we emit in the future.

The bigger the grand total, the higher the eventual warming.

A carbon budget is like a household budget. You only have so much money to spend. How you choose to spend and invest your money will determine the available budget for your retirement and the legacy you provide for future generations.

This budget approach is attractive to policymakers because of its simplicity, and because it is not prescriptive on how and when the emissions quota needs to be met. However, the rate of emissions over time does make a difference to how quickly cuts will need to be made.

Higher emissions this year, or in the next few years, imply more rapid cuts will be needed later. Lower emissions now imply more gradual future reductions.

Keeping below two degrees

The Fifth Assessment Report found the carbon dioxide emitted by humans so far is already about two-thirds of the quota allowable in order to have a two-in-three chance (66%) of keeping temperatures below two degrees centigrade of warming from pre-industrial levels. This is if all sources of future warming are taken into account.

In the 2009 Copenhagen Accord of the United Nations Framework Convention on Climate Change, governments around the world pledged to maintain temperatures below this threshold.

Beyond two degrees the risks to societies and ecosystems are judged to be unacceptably high.

The two degrees is based on the consideration of all scientific evidence, plausible natural and socio-economic impacts, mitigation requirements, and associated uncertainties. These considerations led to a prudent and still achievable temperature target, judged by governments to avoid dangerous impacts on ecosystems, economic development and food production due to climate change.

Up until 2011, the world has warmed by about 0.9C since pre-industrial times.

To keep temperatures below the two degrees target, there is a limit to how much more carbon dioxide we can emit. How can this quantity be estimated?

From the relationship between global temperatures and cumulative carbon dioxide emissions, we can estimate the carbon budget or compatible emissions for any temperature target with a prescribed level of certainty. The lower the temperature target, the lower the emissions quota. The more certainty we want of staying below a certain temperature, the less we can emit.

Do other influences matter?

Carbon dioxide is only one influence on global warming, though it is the single most important one. Human emissions of other greenhouse gases such as methane and nitrous oxides also contribute to human-induced warming. Expected reductions in emissions of aerosols - particles such as soot - will also make a difference.

Some of these components “live” for a shorter time in the atmosphere than carbon dioxide. For instance, more than half of today’s carbon dioxide emissions will still remain in the atmosphere in 200 years time, while methane and aerosols are removed within weeks to a decade through natural processes. This means the pathway of emissions (when they are released) is more important than the total cumulative amount in so far as their contribution to global warming. Other greenhouse gases such as nitrous oxide and synthetic gases can remain in the atmosphere for long periods of time.

However, carbon dioxide is the dominant force in future global warming (accounting for 80-90% of the total impact by humans through the 21st Century). So the proportional relationship between cumulative carbon dioxide emissions and global temperatures will continue to hold true to the end of this century.

Taking all this into consideration, the IPCC report found that for global temperatures to be likely to stay below two degrees (with a >66% probability), the total amount of all carbon dioxide emitted since the late 19th Century should be no more than 790 billion tonnes (790 Gigatonnes of carbon). The permissible carbon budget increases to 900 Gigatonnes of carbon if we accept only a one in three chance (>33%) of staying below two degrees.

How much time is left?

By 2011 we had already emitted 515 Gigatonnes of carbon. This is about two-thirds of the total emissions allowed to stabilise temperatures below 2 degrees.

This leaves only another 275 Gigatonnes of carbon for future use.

With the current rate of carbon dioxide emissions at about 10 Gigatonnes of carbon per year - and assuming an annual growth rate of 3%, as seen over the last decade - the remaining carbon budget will be used up in less than 25 years.

If we want a greater chance than 66% of limiting warming to two degrees, we would need to emit even less carbon dioxide. Conversely, if we accepted a lower probability of limiting the warming to two degrees, the budget would be higher.

The IPCC considered four possible emission scenarios. They call these Representative Concentration Pathways (RCPs).

Only one of the four pathways keeps us within the estimated quota of 790 Gigatonnes of carbon in total, or 275 Gigatonnes more. RCP2.6, as it’s known, allows for declining emissions well into the second half of this century while still remaining under two degrees.

All the other emission scenarios considered in the report overshoot the emissions quota - and the associated warming - by a substantial amount.

If we’re going to get on the RCP2.6 pathway, it will require strong mitigation beginning now and we may have to make our emissions negative later this century. That would require an energy system with zero emissions, plus additional technologies that directly remove carbon dioxide from the atmosphere and store it safely.

Risk assessment

An additional risk acknowledged in the IPCC report, but not accounted for in the carbon budget figures above, is that stored carbon and methane will potentially be released from thawing permafrost. These are areas of frozen soil, predominantly in the Arctic and Tibetan plateau, that are projected to reduce with rising temperatures.

There is limited understanding of how much carbon will be released as permafrost thaws. But it could be large if strong warming occurs in these permafrost regions.

The IPCC estimated that the potential additional emissions from this source are in the range of 50-250 Gigatonnes of carbon over this century if warming follows the RCP8.5 scenario. This means we will need an even more stringent future emissions quota to keep future temperature rise to the recommended two degree limit.

The conclusion of the IPCC is that only if substantial and sustained reductions in emissions are made, can we avoid potentially dangerous consequences of climate change.

After this article was published the IPCC released an updated version of the Summary for Policymakers of the Working Group I contribution to the AR5. This included corrections to some of the carbon budget values cited here and on November 19 2013 we updated those values to reflect the final version of the SPM. For example, the remaining emissions that can be emitted in order to stay below two degrees changed from 270 to 275 Gigatonnes of carbon. A full explanation of how the errors were reported and handled by the IPCC is available from

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  1. Mike Swinbourne

    logged in via Facebook

    Anyone who has seen my posts on the Conversation will well know that I am not a denier of the science of climate change, and am a strong advocate for action. But I am going to take issue with some of the claims in this article.

    It most definitely does matter how long it takes to emit our 'budget' of CO2. The calculations to remain below 2C of warming refer to the year 2100. If we take until 2200 to exceed our 270Gt emissions budget, then obviously we won't have as much of a problem. Similarly, our emissions may not follow a linear or even accelerating trend. We could blow all of our budget by 2040, but then 'invent' a technology like fusion power which effectively means that we will have virtually no emissions after that date. We shouldn't rely on that happening - but it is possible.

    We might only have 270Gt left for future use - but this is future use this century, not future use forever.

    1. Robert McDougall

      Small Business Owner

      In reply to Mike Swinbourne

      a "saviour" technology would be great, but i think doing what we can now is not only rational, but is desirable.

    2. Mike Swinbourne

      logged in via Facebook

      In reply to Robert McDougall

      No disagreement from me Robert. We might hope for the best case, but should plan and act as if the worst could happen.

    3. Mike Jubow

      forestry nurseryman

      In reply to Mike Swinbourne

      The problem as I see it, is not setting carbon budgets. Nothing serious is going to happen for the next three years if Abbott stays true to form and, God forbid, if he gets re-elected, nothing will happen for at least another three years.

      The trolls have won this round of the battle. I wonder how proud of delaying the implementation of carbon reduction they are going to be in twenty years time.

    4. Mike Swinbourne

      logged in via Facebook

      In reply to Mike Jubow

      Most of them will be dead by then Mike.

      But I am nothing if not stubborn. It may look as though they have won, but it is not over until the fat lady sings. And even though Gina is probably warming up her vocal chords now that her lap dog is in the lodge, we should continue to fight to keep her from singing too loudly.

    5. Mike Jubow

      forestry nurseryman

      In reply to Mike Swinbourne

      Well,Mike, I just hope they enjoy the 40+C days and high 30's nights, the damned near permanent drought and the higher intensity cyclones that will be wrecking Brisbane and further south.

      Southern Qld is and will continue to be totally unprepared for cyclones. They have forgotten that one hit Redcliffe in 1970 or 71. There were kids on surfboards hanging 10 on the esplanade and 6ft waves breaking against shop windows. Remember that? Well, these bloody trolls seem to want to see it again on a regular basis.

    6. Mike Hansen


      In reply to Mike Swinbourne

      The problem in delaying is that it means the path to decarbonise becomes impossibly steep.

      The article referred to that
      "However, the rate of emissions over time does make a difference to how quickly cuts will need to be made. Higher emissions this year, or in the next few years, imply more rapid cuts will be needed later."

      The is also a problem with the article's following sentence and the concept of a budget.

      "Lower emissions now imply more gradual future reductions"

      It is based on…

      Read more
    7. Wade Macdonald


      In reply to Mike Jubow

      Mike, what Abbott does or doesn't do won't make an iota of difference. You do understand that Australia is not the planet and Abbott isn't in charge of it?

    8. David Arthur

      resistance gnome

      In reply to Wade Macdonald

      You do understand that all fossil fuel use must end, which includes Australia's ~1.5% of the present problem?

      What's more, you do understand that in the process of getting the nation off fossil fuel, Australia can teach itself lots of useful stuff about alternatives, from which it can then make money exporting the technology. After all, they are going to stop buying Australian coal, so we'll want to export something else.

    9. Mike Jubow

      forestry nurseryman

      In reply to Wade Macdonald

      Wade, that is a trite and weak argument for doing nothing. As David Arthur has pointed out, there are great opportunities in exporting carbon free energy systems when the coal has to stop being used. Rest assured, we will have to stop using the stuff eventually, and the sooner the better. If we have get on top of the clean energy industry by then, we only have advantages and opportunities.

      In my eyes at the moment, there is no single answer to a carbon free energy system and the more we apply…

      Read more
    10. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mike Jubow

      Mike, your claim repeats a familiar meme - that Abbott is a disaster for climate change. Given that the author has stated that there is clearly defined carbon budget, can you quantify the difference in emissions ( as a percentage of that carbon budget) between the previous government's policy and that of the incumbents?

    11. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mike Jubow

      Mike J, there is a difference between taking action by cutting co2 emissions and taking action by developing renewables or other alternatives. Unfortunately, all this gets lumped together into a polemic - do something or do nothing. One can happen without the other.

    12. Henry Verberne

      Once in the fossil fuel industry but now free to speak up

      In reply to Wade Macdonald

      Wade, there WILL be an iota of difference and that "iota" will be the important action that this country can and must take to reduce our high per capita emissions. Not only do we have a moral obligation to do so but a gradual de carbonisation of our economy will stand us in good stead in a future carbon constrained planet.

  2. Mike Hansen


    Thanks for the article. It has been a source of confusion.

    Shortly after the IPCC AR5 report was released, a climate science denier troll argued here that global warming was not a problem because the RCP2.6 pathway showed minimal warming (by selectively looking at the lower bound)!

    Yes, I did point out the following.

    "If we’re going to get on the RCP2.6 pathway, it will require strong mitigation beginning now and we may have to make our emissions negative later this century."

    I would…

    Read more
  3. Brad Farrant

    Adjunct Research Fellow in Early Childhood Development at University of Western Australia

    According to the Ecofys report released yesterday, rather than 25 years, the situation confronting Australia is that we are set to blow our share of the carbon budget in about 10 years.

    "Australia’s fair share of the world’s carbon budget is estimated to be no more than 18 billion tonnes and Australia has already used up between 66% and 84% of that budget. If annual emissions remain at the current level, Australia’s entire budget would be used up in a little over a decade, if not sooner. For Australia to do its fair share our pollution reduction targets need to be:
    o 27 – 34% below 2000 levels by 2020
    o 82 – 101% below 2000 levels by 2030
    o 98 – 106% below 2000 levels by 2050"

    1. David Arthur

      resistance gnome

      In reply to Wade Macdonald

      Err, that would be based on rates of natural uptake and re-emission of CO2. What happens is, a molecule of CO2 may stay in the atmosphere with half-life as described before it is photosynthesised, or dissolved in ocean, or whatever.

      However, that's only half the story; for every CO2 molecule that is removed from the atmosphere by some such natural process, another CO2 molecule is emitted. Result is, atmospheric CO2 stays the same (but for minor seasonal variation) but a molecule of CO2 has a residence time in the atmosphere described by half-life ~38 years.

      I hope this helps.

  4. Craig Myatt

    Industrial Designer / R&D

    I think this is a great article, and an needed simplified explanation. However, I would raise one caveat, around the CO2 half-life or around the 'carbon cycle'. (as raised by Wade & David above)

    My contention, is that if we do something 'fantastic' like replant the Amazon rain forest, and geo-engineer new 'amazon' rainforests, in a kind of 'super' direct action plan, the 'Earth System Models' change, because the rate at which CO2 will be drawn down to land sinks will increase, which means a…

    Read more
    1. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Craig Myatt

      Craig, the elephant in the room is world population growth. Regardless of what co2 reductions we achieve here, or what the lowest achievable co2 production per capita is reached, rising rates of population growth will mean that the carbon budget WLL be exceeded.