Unburnable fossil fuels – whose limit is it anyway?

A research letter in Nature by Dr Christophe McGlade and Prof Paul Ekins (McGlade & Ekins 2015) has dared to go where policymakers and leaders across the globe have not: For a reasonable chance of limiting global warming to 2 degrees C; McGlade & Ekins provide not only the quantity of fossil fuels that should remain in the ground, but they also identify where in the world these should stay put.

The findings state that in order to meet this generally accepted climate target then around 80% of coal, 50% gas and 33% oil reserves* are unburnable over the period 2010 to 2050. In reaching this result the IPCC cumulative carbon budget was applied. This is an approach for assessing how to meet the 2 degrees C global warming target that has emerged in the last couple of years or so. Greenhouse gas emissions can persist in the atmosphere for a range of magnitudes up to thousands of years. It is therefore not the rate of greenhouse gas emissions, nor time of occurrence, but the cumulative total (for all time) that matters. In other words, it is the total cumulative greenhouse gas emissions that correspond with the long-term average increase in global temperature.

McGlade & Ekins also took account of the relative economic cost of exploiting fossil reserves in different locations to determine the unburnable proportion reserves for oil, gas and coal by region. Two alternative scenarios were considered; one where there was no deployment of Carbon Capture Storage (CCS) technology, and the other where CCS was widely deployed from 2025. Figure 1 shows the unburnable reserves for both CCS and “noCCS” scenarios by region for oil, gas and coal.

Coal, Oil, Gas unburnable reserves by regionFigure 1. Unburnable fossil fuels by region where CCS is widely deployed from 2025 (Oil_ccs, Gas_ccs, and Coal_ccs) and also where CCS is not deployed (Oil_noccs, Gas_noccs, Coal_noccs). Based on Table 1 in McGlade & Ekins 2015. CSA is Central and Southern America, FSU is the Former Soviet Union, OECD Pacific includes Australia, Japan and South Korea, and ODA is Other Developing Asia.

The levels of fossil fuels that are unburnable if we are to meet a 2 degrees C global warming target are startling, especially so for coal. Of the oil reserves, Canada has the highest unburnable proportion due to its comparatively high cost of exploitation of tar sands.

What many may find surprising is that even if CCS is widely deployed, then for most regions the levels of unburnable reserves, even for coal, are not significantly reduced.

So how do these findings compare with other energy forecasts?

In the International Energy Agency annual World Energy Outlook (WEO) report there is an energy forecast for what is called a “450” scenario. The purpose of this forecast is to project levels of energy supply and demand that are compatible with meeting the 2 degrees C climate target.  In this scenario rather than the cumulative carbon budget approach limiting greenhouse gas emissions to levels that are in line with a concentration in the atmosphere that corresponds 2 degrees C global temperature increase are applied. Under this application emissions are allowed to peak and then are reduced to enable a stabilisation of atmospheric concentration beyond 2100 at a level of 450 parts per million. Emissions higher than this level are allowable provided the climate target is achievable.

Production estimates for fossil and other types of resources for the years 2020, 2030, and 2040 under each scenario is provided in Annex A in the IEA’s World Energy Outlook 2014 report (IEA 2014). To find the figures for interim years and to project to 2050 the equation-fitting tools in Excel was used. A cumulative production curve for coal, oil and gas was produced along with the proportion burnable implied by the McGlade & Ekins 2015 findings. The burnable reserve proportions have been taken assuming that Carbon Capture and Storage is deployed as outlined in the article. It is assumed that all production will be consumed i.e. burned.

First lets take a look at coal: The burnable budget approximately equals the IEA WEO 450 scenario cumulative coal production projected out to 2050 as shown in Figure 2.

Figure 2. Coal production under IEA WEO 450 Scenario compared with burnable reserve derived from McGlade & Ekins 2015.

This would suggest that the models give comparable results since the projected coal production in the IEA WEO 450 scenario seems to be in line with maximum burnable reserve 2010 to 2050 in McGlade & Ekins 2015. However the cumulative coal production curve does not appear to be markedly slowing down at 2050 and hence without drastic action the burnable coal budget will be overshot soon after 2050.

The picture for oil and gas is quite different as shown in Figure 3.

Oil and gas, 450 scenario against burnable reservesFigure 3. Oil & Gas production under IEA WEO 450 Scenario compared with burnable reserve derived from McGlade & Ekins 2015.

The burnable budget is overshot by 2035 for gas and 2036 for oil. This implies that more oil and gas is allowable under the IEA’s 450 scenario than allowable by McGlade & Ekins.

The IEA acknowledge in their report that under their central scenario “New Policies” the carbon budget will be used up by 2040 however the budget appears to also be used up in their 2 degree C climate target “450” scenario too.

These results imply that:

  • IEA WEO 450 Scenario would fail to meet the 2 degrees C climate target
  • Fossil fuels may need to be phased out more rapidly than previously thought
  • CCS barely improves the outlook for fossil fuels

The findings are a severe blow to those championing the deployment of CCS technology. The IEA are one such champion on p.25 of their latest report:

 “… and of capture and storage in the longer term, can be a prudent strategy to ensure a smooth transition to a low carbon power system, while reducing the risk that capacity is idled before recovering its investment costs”

Such a strategy would seem to be far from prudent in a climate-constrained world. Perhaps it may be possible to reduce unburnable reserve rates significantly if CCS was widely deployed imminently but this seems highly unlikely to be achievable. In any case deployment of CCS would hasten depletion of reserves not only for the extra equipment and infrastructure but also for the reduction in efficiency e.g. in power plants.

As pointed out in McGlade & Ekins the findings highlight the double standards of policymakers that claim they are tackling climate change but who are at the same time also enabling and hastening the exploitation of fossil fuels that are located within their country. Surely to avoid dangerous climate change expanding reserves by drilling in the Arctic, facilitating further tar sands exploitation through Keystone XL, or enabling widespread hydraulic fracturing simply cannot be afforded?

Policymakers and leaders need to face up to the need to phase out fossil fuels within the next 3 decades if they really are serious about limiting increases in global average temperatures to 2 degrees C. Instead of pledging to reduce greenhouse gas emissions, policies need to be in place to bring about a termination in emissions altogether.

* The term reserve means the estimated quantity recoverable under current economic conditions. Note that this figure is lower than resource figures that would also include the quantities recoverable under future technological advancements.


IEA, 2014. World Energy Outlook 2014, Paris: Iea.

McGlade, C. & Ekins, P., 2015. The geographical distribution of fossil fuels unused when limiting global warming to 2 °C. Nature, 517(7533), pp.187–190.

 Claire Carter is a PhD researcher at SPRU and is a member of the Sussex Energy Group and affiliated to the Tyndall Centre. Claire is researching energy return on energy invested (EROI) in a global context through system dynamics modelling.
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5 comments on “Unburnable fossil fuels – whose limit is it anyway?
  1. mike says:

    For a person of limited intellect, what would a summary of this article say?

  2. Hi Mike – thanks for reading. Does the summary below help?

    Limiting climate change to no more the generally accepted “dangerous” threshold has previously led to a focus on reducing greenhouse gas emissions to a certain rate per year. In the IEA “450” Scenario this is emissions that correspond with a concentration in the atmosphere of 450 parts per million hence the name. This implies that emissions could continue but at a lower rate per year.

    Recent thinking, due to how long these greenhouse gases stay in the atmosphere, is that it is actually the total sum of all annual emissions that leads to the long term increase in average global temperatures. To avoid going over the “dangerous” threshold this has been summarised in a single figure – the cumulative carbon budget. What this recent thinking means is there is a set quantity of fossil fuels that can be “safely” burned to stay within this carbon budget.

    What the article shows is that the previous way of thinking could lead to emissions taking us over the dangerous threshold. In other words, even if it was possible to implement (and in time) all the policies as outlined in the IEA 450 Scenario, there is more than a reasonable chance that this would lead to an increase in average global temperatures of more than 2 degrees C.

  3. nicholbrummer says:

    It is not clear what causes the difference between the IEA WEO projections and those from the new McGlade & Ekins publication. It looks like the assumptions are different, and also the ‘approach’. Since this result that CCS matters very little is surprising, it would also be nice to understand why this is so.

    for me confusion is just larger than it was. I guess I’ll go and look at the nature article itself. Good that you have the link to that one here 🙂

    • Thanks Nichol for your comment. Did you mean the difference in assumptions regarding CCS or elsewhere in the 2 models?

      IEA 450 Scenario has CCS being deployed widely from 2020. Whereas in McGlade & Ekins wide deployment at the “maximum rate at which it can be built” from 2025. Given the little difference CCS makes anyway over the period to 2050 I can’t see the 5 years being significant.

  4. manicbeancounter says:

    It is a shame that McGlade and Ekins 2015 has not received wider recognition. The implications for mitigation policy is climate mitigation targets will only be achieved if production of oil and gas is effectively ceased in less than 20 years – or longer if production is reduced rapidly year on year. That means many economies – particularly in the Middle East – would lose their primary source of National Income in little more than a generation. This might explain the reluctance to take embrace emission targets.

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