03 May 2009

A new reality check on the global carbon emissions budget

Two new research papers published this week in Nature on emissions targets have been widely reported, including
Humanity's carbon budget set at one trillion tonnes, Hit the brakes hard and How The '2 Degrees Celsius Target' Can Be Reached.

And the result: if emissions keep growing at the present rate, the carbon emissions budget for the 2 degrees target will run out in 2021! Call that a climate emergency!

The two articles (by Allen et al, and Meinshausen et al ) asked the same question: how many more tonnes of carbon can humans pour into the air before a 2-degree temperature increase is the result? A commentary by both sets of authors is
The exit strategy .

Ignoring, for the moment, the fact that 2 degrees is a really bad target (as is discussed in Climate Countdown), the articles were accompanied in the same issue of Nature by an enlightening summary by Gavin Schmidt and David Archer, Climate change: Too much of a bad thing:
Meinshausen and colleagues (page 1158) take a comprehensive probabilistic approach, combining the uncertainties in climate sensitivity and carbon-cycle feedbacks, and integrating the two over a large range of potential emission pathways. Their target is to avoid a peak global mean warming from the preindustrial level of more than 2C (equivalent to a further rise of about 1.2 C from today). We must note here that there is nothing special about 2C that would make warming of less than this magnitude ‘safe’. It is more analogous to a speed limit on a road, and is a guide to the scale of the problem. With 2 C of global warming (more over land and at the high latitudes), Earth would probably be warmer than it had been in millions of years — a huge change.

Meinshausen et al. find that the maximum temperature that Earth will experience to the year 2100 depends most reliably on the total amount of CO2 emitted to the year 2050, rather than on the final stabilized CO2 concentration. Their base-case estimate is that the total emissions from today (2009) to 2050 need to stay below 190 GtC (equivalent to 700 GtCO2; 1 GtC = 1012 kg of carbon) for us to have a good chance (75%) of staying below 2C (Fig. 1).

Figure 1: The 2C lottery. The black line shows the probability of the peak global mean temperature exceeding 2C above pre-industrial levels before the year 2100 as a function of the integrated emissions from 2009 to 2049. The graph is adapted from the base case of Meinshausen et al., including uncertainty ranges. Also shown are the cumulative emissions under various scenarios. Red, emissions constant at 2008 values until 2050. Light blue, growth in emissions continues at 1% per year until 2050 and then falls rapidly. Green, growth in emissions continues at 2% per year until 2050 and then falls rapidly. Purple, an 80% cut in emissions by 2050 (linearly applied, starting in 2010) from developed countries only, while developing-country emissions continue to grow at 1% per year. Dark blue, an 80% cut in emissions by 2050 from all countries.

And what does a total future carbon emissions budget to 2050 of 190 billion tonnes of carbon mean, when we are presently doing about 10 billion tones a year??

I did some quick figures with three scenarios: emissions keep growing as they have been since 2000 at around 3.5 per cent per year; and emissions are reduced by 2 per cent and 4 per cent a year. And this is based on only a 75% chance of not exceeding 2 degrees!

Here's the conclusions:
  • If emissions keep growing at 3.5 per cent a year, then the carbon budget for 2 degrees runs out in 2021. That is, after that time, emissions would need to drop to zero immediately to have a 75 per cent chance of not passing 2 degrees.
  • If global emissions reduce 2 per cent a year from now, the carbon budget will run out in 2030 for 2C, and
  • With a 4 per cent annual reduction in global emissions, it will run out in 2040.
And that for a target that will that initiate large climate feedbacks in the oceans, on ice-sheets, and on the tundra, taking the Earth well past significant tipping points. Likely impacts include large-scale disintegration of the Greenland and West Antarctic ice-sheet; the extinction of an estimated 15– 40 per cent of plant and animal species; dangerous ocean acidification; increasing methane release; substantial soil and ocean carbon-cycle feedbacks; and widespread drought and desertification in Africa, Australia, Mediterranean Europe, and the western USA.

So even those folks who want a 2-degree target will need to argue for a 4 per cent annual global emissions reduction (and more in Australia) with zero emissions by 2040!!

Given Australia's emissions are increasing 2 per cent a year, that would be a 6 per cent turn-round on current practice in this country. That is going to require what we don't have now: transformative leadership and action at emergency speed and depth.

As NASA's Jim Hansen testified to US Congress last year:
We have reached a point of planetary emergency… climate is nearing dangerous tipping points. Elements of a perfect storm, a global cataclysm, are assembled… the oft-stated goal to keep global warming less than +2 degrees Celsius is a recipe for global disaster, not salvation.
David Spratt
3 May 2009

Background note: Mienhausen et al have also circulated a "Informal Background Q&A on Meinshausen et al. “GHG targets & 2°C" which is
here. They canvas questions including a "Why do you start your analysis from a 2°C target?" and "Why did you not focus on the target by Small Island States and Least Developed Countries of 1.5°C?". And this:
Q. If you had included in your emission pathways substantially net negative emissions after 2050, the probabilities of exceeding 2°C were lower?
A: True, substantially negative emissions post‐2050 would somewhat decrease the peak warming expected during the second half of the 21st century. However, temperature levels in year 2050 could obviously not be reversed. For very low mitigation pathways, 2050 temperature levels are already close to their maximum, so that negative emissions could only influence how quickly temperatures decrease after the peak, but not the temperature peak level itself.
Background: In most of the lower emission pathways analyzed in Meinshausen et al., global emissions are only turning to near‐zero levels, with the exception of lower MESSAGE or IMAGE17 scenarios, that exhibit substantial negative emissions by 2100. On the one hand, it is comforting that large net negative emissions could somewhat reduce the probabilities of exceeding 2°C for the medium‐low and high scenarios. On the other hand, large net negative emissions pose an enormous challenge for the involved technologies, and the safety, liability and permanence of stored underground carbon. Furthermore, one technology to achieve net negative emissions, the combination of biomass burning and carbon sequestration and storage, could have large implications to our land use patterns.
The relevant papers are:

  • Meinshausen et al. Greenhouse-gas emission targets for limiting global warming to 2 °C. Nature, 2009; 458 (7242): 1158 DOI: 10.1038/nature08017
  • Allen et al. Warming caused by cumulative carbon emission: the trillionth tone. Nature, 458, 1163-1166 DOI: 10.1038/nature08019
  • Allen et al. Nature Reports Climate Change. The exit strategy: Emission targets must be placed in the context of a cumulative carbon budget if we are to avoid dangerous climate change. Nature Reports Climate Change, 2009 DOI: 10.1038/climate.2009.38
  • Schmidt and Archer, Too much of a bad thing. Nature, 2009,: 458: 1117. DOI: 10.1038/4581117a
  • Washington et al. How much climate change can be avoided by mitigation? Geophysical Research Letters, 2009; 36 (8): L08703 DOI: 10.1029/2008GL037074:


  1. Thanks for the analysis and references, David. Hopefully this might lead to a better understanding that the carbon budget isn't just what's left in the ground, but the amount before which we exceed dangerous tipping points. I appreciate your point re 2C, and it would be good to see a budget for a safe-climate scenario.

  2. David, thanks for the analysis. I believe you made a mistake in that, even though annual emission increases are relative to each preceding year, when annual percent reduction targets are discussed, my understanding is that it refers to the percentage of a particular year, e.g., 4% annual reduction from now would mean reducing a fixed amount equivalent to 4% of 2009 emissions, EACH YEAR, till reaching zero. It does not mean reducing 4% of each prior year's emissions, which will never reach zero. Therefore, a 4% annual reduction from 2009 will get to zero emissions by 2034, after a cumulative emission of 130 GtC. A 3% annual reduction from 2009 will reach zero by 2042, after a cumulative emission of 170 GtC. Neither surpasses the 190 GtC budget.

    But, of course, 190 GtC budget does not consider feedbacks like methane from sea bed, etc. I find the two Nature papers disappointing in that they seem to make a weaker case than the 2006 Meinshausen study in conjunction with IPCC conclusions, namely, that (for Annex I countries) 80-95% below 1990 emissions by 2050 would lead to stabilization below 450 ppm (after 50 ppm overshoot), which only provides a 46% odds of not exceeding 2 degrees C. Given the recent years of high emissions, one would expect that budget year would have been much more tightened while the odds would have looked even worse, but the two recent Nature papers don't seem to convey that, and I have not scrutinized them enough to see why. I imagine part of that is due to the current budget (190 GtC by 2050) is for worldwide emission, not for Annex I countries, which were given higher expectations in IPCC recommendations.

    IPCC lower carbon budget (assuming high climate sensitivity, and including carbon cycle feedbacks) is 1376 GtCO2 from 2000-2100, or 375 GtC. I don't have off hand the exact figures of the total emissions from 2000-current (including estimates for the most recent years), but with that, one could compare the budget the two new papers provided with that of the IPCC. Would be great if you could look into that.

    Thanks again!

    Maggie Zhou

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