Disentangling carbon budgets

The World Resources Institute has put together a very helpful infographic showing the four example emissions pathways (RCPs, or representative concentration pathways) used in the latest IPCC reports. Well worth using if you’re giving a talk on the area – I’m adding it to mine. I took the opportunity to look a bit more into the carbon budget numbers, and found a few things which initially confused me, so wanted to set them out here to caution others.

The standard figure given for the total budget is a nice round figure: a total 1000Gt (gigatons, or billions of tons) of carbon since the start of the industrial revolution: if we stay under that we have a 2/3rds chance of staying under a 2C temperature rise. (Here’s the first thing to avoid: that’s carbon, not carbon dioxide, which some sources quote instead, multiplying all figures by about 3.67.) We’ve emitted a total of 515Gt so far, so that’s 485Gt left, right? (See the WRI’s previous infographic.)  On this basis, the WRI’s forecasts look reasonably friendly: we stay under budget on the low pathway (RCP2.6) and don’t exceed it until the 2040s or 2050s on the other three. Plenty of time to take action. (For reference, current emissions are close to 10Gt per year.)

However: this doesn’t allow for two other factors. Firstly, emissions of other greenhouse gases, which contribute a substantial proportion of warming. The IPCC Working Group I report from which the 1000Gt budget is taken notes (see page 27 of the summary for policymakers) suggests an alternative figure of 790Gt – or a budget down from 485Gt to 275Gt –  if we are looking at the low pathway (RCP2.6) – there is a further complication as the other greenhouse gases tend to be much more potent but not stay in the atmosphere so long, so it’s impossible to reduce everything to CO2-equivalents as one might like. Other calculations you can find online use this figure (or 800: the figure was revised just before publication) so coming up with much more pessimistic forecasts.

Secondly, the way the WRI puts things suggest that it is possible to get near the time the budget is due to run out, take action, and all will be fine. What this doesn’t take into account of course is the massive amount of inertia in the system: we simply can’t just stop emitting carbon altogether (barring complete socio-economic collapse). One report I’ve seen suggests that the last date we can seriously start taking action is 2017: beyond this we will have built sufficiently much carbon-using infrastructure that simply using everything to the end of its natural lifespan will be too much.

There’s a final issue, which is the large uncertainties in the IPCC’s figures. They don’t put error bars on the budget figures, but they do on the emissions to date: 515 [445-585], which in their notation means a 90% chance of being within that range. 70Gt either way makes a substantial difference.

So, onto the WRI’s figures. On the larger 485Gt budget, the lowest pathway (RCP2.6) stays under, the medium one (RCP4.5) passes it in 2056, the high (RCP 6.0) in 2057, the highest (RCP8.5) in 2045.

How about the lower budget, to give us a 66% chance of staying under 2C. Our emissions are currently tracking RCP8.5 reasonably closely, with an average emissions rise of about 2% a year. If we continue on this path, we exceed the 275Gt budget in 2033. (Note: I’m using my own approximations rather than the official figures for ease of calculations so we may be slightly out, but my answers are within a year of WRI’s where we’re doing the same model.) How about if we cut emissions from 2020? Even with a 4.2% a year cut from then on, which is close to the RCP2.6 model, we still exceed the budget by 2043 and end up in the territory of between 33-50% chance of avoiding a 2C rise. A rather more swingeing cut of about 6.5% pa is needed to stop exceeding the 275Gt/66% budget. I think that’s at or about the top end of what is considered possible.

One other thing to note: in the WRI figures, the medium pathway seems slightly worse than the high one when it comes to meeting the carbon budget (2056 v 2057). This is an artefact of the pathway models which I hadn’t spotted before: the four are based on four independent models from different research groups, and the “high emissions” one has lower emissions than the other three initially, and lower than the medium one until the late 2030s. A bit unfortunate.

What do we conclude from all this? Firstly, due to the various uncertainties it is difficult to communicate climate figures in a way which is both simple and accurate. Secondly, the situation is urgent (or very likely so) – 2020 isn’t too far off, and the need for strong concerted action is greater than ever. Even the lowest emissions pathway has quite strong negative consequences – and the infographic brings that across well, even if, as I hope is clear, I have difficulty with their figures for carbon budgets.

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5 responses to “Disentangling carbon budgets

  1. Thank you Colin for doing so much work on this. It is very helpful. I imagine you are familiar with the James Hansen and David Wasdell critiques of the IPCC figures, so they are coming up with much more pessimistic outcomes. Phil Date: Mon, 18 Aug 2014 15:30:22 +0000 To: phil.kingston@hotmail.co.uk

  2. Colin Bell

    Phil, thanks for this. I’ve seen both critiques, although I can’t claim familiarity with Wasdell.

    For the benefit of others: James Hansen is arguing mainly that the IPCC’s estimation of risks is too conservative, that we are already seeing detrimental weather effects, and that we need to aim for 1C, which corresponds to 350ppm (hence 350.org). This summary article gives more information and a link to his full paper.

    Hansen’s predictions to date have had a good track record, and I have sympathy for his position. However, I don’t think we have a hope of staying under 1C any more: the best I think is even feasible is approximately matching the IPCC’s low emissions pathway (RCP2.6) which is likely to take us to nearly 2C. Unless of course we have a major social or economic crisis which massively reduces industry and hence emissions radically, but that will have other major negative impacts of course. So just getting emissions as low as possible for now has to be the target, and dealing as best we can with the consequences.

    Wasdall I’m less sure about. Again here’s a summary of his views containing a link to his original paper. Unlike Hansen, he’s arguing that the IPCC’s estimates of climate sensitivity and hence likely temperature rises are way too low. If he’s right, then the only likely outcome is extinction or near-extinction of the human race and much of the rest of terrestial life, within a matter of decades. (I don’t know whether the Near-Term Extinction movement is influenced by him.)

    But is he right and do we need to take him seriously? I’m not sure. What’s your position on him, Phil?

  3. Thanks for this article. It enlightened me in many aspects of the problem. It is sad that our individual action no longer counts, and that the decision relies on people whose only goal is to fulfill “national” or “company” interests.

    I believe that one of the countries that takes the climate change threat seriously is Germany. But their effort alone won’t help. We need our policymakers to start acting around the globe, with the US being the first and foremost promoter of sustainability. But they will never do that.

    So despite the projections can range from moderate to catastrophic, I think we will figure it out. Meanwhile, I will put my effort in localization of the food production and promoting Veganism.

  4. Your description of the numbers is correct except in one vital detail. The units should be Gt (gigatonnes = billions of tonnes) and not Mt. See your IPCC ref and please correct this!

    As you suggest, it is absurd to think global warming is a problem that can be solved by putting it off until the last moment as though it is like the US debt crises with a wand magically waved at the last minute. It is more like being forty scores down in a ball game with twenty minutes to go and thinking that to ‘win the game’ you can wait to start scoring in the last minute. This kind of thinking is so foolish that it can only be personal and societal values that are obscuring reality for those that imply this is possible.

    The reality of the Earth’s climate system is simply that it does not matter where, when, how or by whom the emissions occur. As the PwC report just out notes, a temperature limit is a zero sum game, excess emissions by someone, somewhere, sometime directly imply less emissions for someone else or somewhere else or at some later time. This means that for every year that does not achieve the radical cuts needed then in every future year even greater cuts are then needed to meet a 2ºC limit (or any other limit).

    The physical science (see the IPCC summaries) on global warming is clear: it’s happening very fast, it’s down to human fossil fuel burning and land use, and if unchecked by action that really achieve radical and deep decarbonisation starting now then the irreversible consequences are very serious for civilisation. An agreed enforced global cap on total future emissions is critical to drive innovation and to ensure that ‘efficiency savings’ are not spent – ever.

    The IPCC put it very simply:
    “Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.”

    When physical scientists says “will cause” without qualification we should all pay attention rather than looking away or clinging to false hopes for something to turn up. ‘Sustainability’ is entirely meaningless if humanity, especially wealthier humanity, proves unable to set a course now to restabilise the climate system quickly by sustaining massive annual cuts in emissions starting immediately.

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