The answer lies after 2050, probably not before

The global energy system works on timescales of decades rather years. When considering the changes required in managing the climate issue, the short to medium term takes us to 2050 and the long term is 2100! As such, drawing long term conclusions based on a 2050 outlook raises validity issues.

A new Letter published in Nature (and reported on here) discusses the long term use of fossil fuels, further exploring the notion that certain reserves of oil, gas and coal should not be extracted and used due to concerns about rising levels of CO2 in the atmosphere. But the analysis only looks to 2050 in its attempt to quantify which reserves might be more penalised than others, assuming we are in a world that is actually delivering on the goal of limiting warming to 2°C. The authors drew on available data to establish global reserves at 1,294 billion barrels of oil, 192 trillion cubic metres of gas, 728 Gt of hard coal and 276 Gt of lignite. These reserves would result in ~2,900 Gt of CO2 if combusted unabated, with approximately two thirds of this coming from the hard coal alone.

The Letter draws on the original work of Malte Meinshausen, Myles R. Allen et. al. which determined that peak CO2 induced warming was largely linked to the cumulative release of fossil carbon to the atmosphere over time, rather than emission levels at any particular point in time. They determined that surpassing the 2°C global goal could be quantified as equivalent to the release of more than 1 trillion tonnes of carbon (3.7 trillion tonnes CO2), with their timeframe being 1750 (i.e. the start of the modern use of coal) to some distant point in the future, in their case 2500. Precisely when CO2 is released within this timeframe is largely irrelevant to the outcome, but very relevant to the problem in that the continued release of carbon over time, even at much lower levels than today, eventually leads to an accumulation with the same 2°C or higher outcome (the slow running tap into the bathtub problem). Hence, the original work gives rise to the sobering conclusion that net-zero emissions must be a long term societal goal, irrespective of whether the whole issue can be limited to 2°C. “Net-zero” language has now appeared as an optional paragraph in early drafting text for the anticipated global climate deal currently under negotiation.

As a point of reference, the associated Trillionth Tonne website shows the cumulative release to date (January 2015) as 587 billion tonnes of carbon, which leaves 413 billion tonnes (~1.5 trillion tonnes CO2) if the 2°C is not to be breached (on the basis of their midrange climate sensitivity). The chart below is extracted from the original Meinshausen / Allen paper and illustrates the relationship, together with the inherent uncertainty from various climate models.

Peak warming vs cumulative carbon
Further work was done on this by Meinshausen et. al. They attempted to quantify what the results mean in terms of shorter term greenhouse gas emission targets, which after all is what the UNFCCC negotiators might be interested in. While the overarching trillion tonne relationship remains, it was found;

. . . .that a range of 2,050–2,100 Gt CO2 emissions from year 2000 onwards cause a most likely CO2-induced warming of 2°C: in the idealized scenarios they consider that meet this criterion, between 1,550 and 1,950 Gt CO2 are emitted over the years 2000 to 2049.

This focus on a cumulative emissions limit for the period from 2000 to 2049 (which is arguably a period of interest for negotiators) has been picked up by the most recent Letter and it is the starting point for the analysis they present, although slightly refined to 2011 to 2050. The Letter has concluded that;

It has been estimated that to have at least a 50 per cent chance of keeping warming below 2°C throughout the twenty-first century, the cumulative carbon emissions between 2011 and 2050 need to be limited to around 1,100 gigatonnes of carbon dioxide (Gt CO2). However, the greenhouse gas emissions contained in present estimates of global fossil fuel reserves are around three times higher than this and so the unabated use of all current fossil fuel reserves is incompatible with a warming limit of 2°C. . . . . Our results suggest that, globally, a third of oil reserves, half of gas reserves and over 80 per cent of current coal reserves should remain unused from 2010 to 2050 in order to meet the target of 2°C.

Further to this, the Letter also deals with the application of carbon capture and storage (CCS) for mitigation and finds that;

Because of the expense of CCS, its relatively late date of introduction (2025), and the assumed maximum rate at which it can be built, CCS has a relatively modest effect on the overall levels of fossil fuel that can be produced before 2050 in a 2°C scenario.

The choice of 2050 is somewhat arbitrary, in that while it may be important for the negotiating process, it is largely irrelevant for the atmosphere. But running a line through the middle of the century and drawing long term conclusions on that basis does change the nature of the issue and potentially leads to high level findings that are linked to the selection of the line, rather than the science itself. Most notable of these is the finding regarding the use of oil, coal, and gas reserves up to 2050 rather than their use over the century as a whole.

The study notes that current global reserves of coal, oil and gas equate to the release of nearly 3 trillion tonnes of CO2 when used and based on this draws the conclusion that two thirds of this cannot be consumed if a global budget were in place that limits emissions to 1.1 trillion tonnes of CO2 for the period 2011 to 2050. The problem here is that the current reserves are unlikely to be consumed before 2050 anyway. The Shell New lens Scenarios contrast a high natural gas future with a high renewable energy future, but in both cases the unabated CO2 (i.e. before the application of CCS) released from energy use over the period 2011-2050 is about 1.6 trillion tonnes. Using this as a baseline reference point for the period to 2050 rather than total global reserves, would then lead to a different conclusion and a much lower fraction that cannot be used. In the case of the Shell Mountains scenario which has both lower unabated CO2 (high natural gas use) and high CCS deployment, the net release of CO2 from energy use over the period 2011-2050 is about 1.5 trillion tonnes. Of course we should add the other sources of CO2 (i.e. cement and land use change) to this for a complete analysis and also recognise that neither of the New Lens scenarios can resolve the climate issue within the 2°C goal (discussed in an earlier post here), but both are close to net-zero emissions by the end of the century.

Looking out to the end of the century also changes the findings with regards the application of CCS. Any energy technology, be it solar PV or CCS, will take several decades to reach a scale where it substantively impacts the energy system. During that build up period, its impact will therefore be modest and this is the observation made in the Nature Letter. But by 2050 CCS deployment could be substantial and in the Mountains scenario CCS reaches its peak by the end of the 2050s decade. Therefore, it is the use of CCS after 2050 that really impacts the total use of fossil fuels this century. From 2050 to 2100 net fossil fuel emissions in Mountains are ~560 billion tonnes CO2, far less than the period 2011-2050 and similar in scale to a post 2050 “budget” that would be remaining in a world that limited itself to 1 trillion tonnes CO2 over the period 2011-2050 (i.e. for a total of 1.5 trillion tonnes as noted above).

With such CCS infrastructure in place and given the size of the remaining ultimately recoverable resources (which the Letter puts at ~4,000 Gt for coal alone), fossil fuel use could continue into the 22nd Century hardly impacting the level of CO2 in the atmosphere, assuming it remains competitive with the alternatives available at that time. CCS in combination with biomass use, also offers the future possibility of drawdown on atmospheric CO2.

The big challenge is the near term, when fossil fuel use is meeting the majority of energy demand, alternatives are not in place to fill the gap and CCS is not sufficiently at scale to make a truly material difference. Of course if CCS scale up doesn’t start soon, then the long term becomes the near term and the problem just gets worse.