Emissions compatible with RCP2.6

RCPs are scenarios defined as trajectories of greenhouse gas concentrations2 used to make climate change projections1. With global coupled carbon–climate models, they can also be used to estimate time series of fossil-fuel CO 2 emissions compatible with the trajectories. Indeed, given a prescribed trajectory of atmospheric CO 2 and the simulated changes in oceanic and terrestrial carbon reservoirs induced by changes in atmospheric CO 2 and climate, the so-called ‘compatible’ fossil-fuel emissions can be deduced by mass conservation of the element carbon5,12. In a given year, compatible emissions are positive if the total carbon stock of the atmosphere–ocean–land system increases, and negative if it decreases12 (see also Methods).

Here, we take estimates of fossil-fuel emissions compatible with RCP2.6 from the following: 11 three-dimensional ESMs used in the Fifth IPCC report5,12; an ensemble of simulations made with an Earth system model of intermediate complexity named JUMP-LCM (ref. 13); another ensemble produced with a simple carbon–climate model named OSCAR (ref. 14). Emissions from industrial processes are included but not those from land-use and land-cover change (see Methods). Estimates by ESMs are only available over the 21st century while the others follow the scenario’s extension up to 2300 (ref. 2). Figure 1 shows these compatible emissions. During the 21st century, all the average trajectories are very close to each other, with <10% difference in terms of cumulative emissions. After 2100, however, these differences increase (see the discussion in the Methods section). The uncertainty in the compatible emissions shown in Fig. 1 stems from the different representations of carbon-cycle processes and climate–carbon feedbacks in the models.

Figure 1: Fossil-fuel emissions estimated to be compatible with RCP2.6. Estimates by ESMs, ensembles by JUMP-LCM and OSCAR, as well as the default fossil-fuel emissions for RCP2.6 are shown. The latter are compatible with the simple carbon–climate model MAGICC6 over 2000–2100 (ref. 2). Historical emissions from fossil-fuel burning (E FF ) are also provided18. Full size image

Disentangling net and gross emissions

The key point about these compatible emissions is that they are deduced from carbon–climate model simulations: they only reflect responses of the natural systems and do not contain information on how they are achieved by human societies. Therefore, these emissions are global net emissions and they can be broken down into gross positive and gross negative emissions. If there was no constraint of any sort, one could imagine ever-growing positive emissions from fossil-fuel burning, compensated by even stronger negative emissions needed to meet the trajectories shown in Fig. 1.

In this study, we define a series of ‘mitigation floors’, which are assumed trajectories of maximum conventional mitigation of fossil-fuel emissions that could be achieved. This concept of mitigation floor is used to encompass technical, economic, sociopolitical inertia15,16 and limits17 in reducing the gross positive fossil CO 2 flux (that is, in reducing the consumption of fossil fuel). It is the maximum potential mitigation at a given time. Thence, as long as the mitigation floor is lower than compatible emissions, there is no physical need for negative emissions, since reaching compatible emissions can be achieved through conventional mitigation only. However, if the floor is greater than compatible emissions, negative emissions are a physical requirement (see Methods).

Figure 2 presents the several mitigation floor assumptions of our study. In 2012, compatible emissions are taken as the reference. After that, the floor follows a business-as-usual increase of +2.46% per year, as per fossil-fuel emissions over the 2007–2012-period18. Then it starts decreasing at various points in time: 2015, 2020, 2025 or 2030. These decreases are assumed to occur exponentially at various rates: −5%, −4%, −3%, −2% or − 1% per year. The choice of an exponential shape for mitigation floors is motivated by them being defined as ‘trajectories of maximum conventional mitigation’. Consistently, we assume the most effective mitigation options will be implemented first, leaving less effective ones to be implemented afterwards. This leads to exponential shapes of mitigation floors: the marginal effectiveness of mitigation is decreasing with time. We acknowledge, however, that arguments can be made for other shapes, and those alternative shapes are discussed hereafter.

Figure 2: Illustration of the default mitigation floors and corresponding gross negative emissions. (a) Only one compatible emissions trajectory (E comp ) and all the possible mitigation floors (E floor ) for this trajectory. (b) In a given year, the requirement for negative emissions (E neg ) corresponds to the gap between the mitigation floor and compatible emissions, should there be any. Full size image

The rates of conventional mitigation are taken to cover a wide range of possible futures (and numerous positive emission trajectories from integrated assessments fall in their range; see Supplementary Fig. 1). For instance, our rates are comparable to the following: the decreased rate of emissions committed by the lifetime of emitting infrastructures that may range, depending on the assumptions, from −5.7% per year to −3.2% per year over 2010–2050 (refs 15, 19); the observed historical decarbonation rate of −4.6% per year during the French nuclear program of 1980–1985 (ref. 20); the average 2008–2020 mitigation rate of −1.3% per year pledged by the United States needed to reach in 2020 −17% of emission compared with 2005, or of −1.0% per year pledged by the European Union to reach in 2020 −20% compared with 1990 (ref. 20). Note that all these rates are mean annual rates of change: they are thus strictly comparable to our exponential rates, and fairly comparable to linear rates (later used for alternative mitigation floors). This justifies the `fairly comparable' rates.

We use that kind of stylized economic pathways to explore ‘what-if’ scenarios without making complex and debated21,22,23 assumptions as it is done for integrated assessments. We hereby provide physically based estimates of negative emission requirements in the form of a table whose inputs are our two simple parameters: the starting year and the rate of decrease of the mitigation floor. That table’s outputs are condensed in Fig. 3.

Figure 3: Negative emission requirements for our default mitigation floor assumptions. (a) The maximum yearly flux of carbon capture or removal required over at least a decade. (b) The cumulative carbon captured or removed by the end of the 21st century and (c) the cumulative capture or removal by the end of the 23rd century. Each panel is broken down into one sub-panel per starting year of decrease of the mitigation floor. Inside each sub-panel, colours refer to rates of decrease (from −5 to −1% per year, from left to right, respectively) and symbols to models. ‘90% ranges’ correspond to the range between the 5th and the 95th percentiles. Estimates when no mitigation floor is considered (that is, of net negative emissions) are shown in the right-most sub-panels. The opposite of historical emissions from fossil-fuel burning18 (E FF ) and the gross negative emissions in the ‘original’ RCP2.6 (ref. 42) are also provided for comparison (dashed horizontal grey and magenta lines, respectively). Full size image

Negative emissions required in RCP2.6

The maximum yearly flux of negative emissions that needs to be sustained over at least a decade is shown in Fig. 3a. Across all our mitigation floor assumptions, this maximum flux varies tenfold. In our best-case assumption (decrease starting in 2015 at a rate of −5% per year), this value ranges from 0.5 to 3 Gt C per year (see Methods for how ranges are obtained). In our worst-case assumption (-1% per year starting in 2030), it goes from 7 to 11 Gt C per year. The latter value is the same order of magnitude as global CO 2 emission from fossil-fuel burning in 2012 (ref. 18). When taken as a function of the mitigation floor decrease rate, this maximum flux of removal is nonlinear: if the decrease rate (that is, the rate of society’s transformation) is for instance halved, then the maximum flux of CO 2 removal required is more than doubled. Thus, any efforts put into mitigation would be more than compensated by an alleviation of the requirement for negative emissions (in terms of carbon dioxide, not of economic or risk trade-off analysis). The time at which this maximum flux has to be achieved can vary greatly but, generally speaking, the longer we wait to start mitigation, the sooner the maximum flux is needed (Supplementary Fig. 2). Also, given the limited potential of each negative emission technology5, a combination of several technologies will probably be needed to deliver this maximum yearly flux.

The cumulative amount of carbon that needs to be captured on site or from the atmosphere is also a key value, because this carbon somehow has to be stored. Figure 3 shows how much carbon storage is needed by 2100 (Fig. 3b) and by 2300 (Fig. 3c). In our best-case assumption, 25–100 and 50–250 Gt C of storage capacity are needed by the end of the 21st and 23rd century, respectively. In our worst-case assumption, there is a need for a capacity of 450–800 by 2100, and 1,000–1,600 Gt C by 2300. Ending the study in 2300 rather than 2100 roughly doubles the amount of carbon storage required. This total amount of captured carbon dioxide, be it up to 2100 or 2300, is a nonlinear function of the decrease rate of the mitigation floor and of the starting year of decrease. Thus, any reduction or postponing of the effort in mitigation increases even more the total amount of CO 2 that has to be captured and stored. Globally, depleted oil and gas reservoirs and coal seams are estimated to have a storage capacity of 300–350 Gt C, and saline aquifers a capacity of 1,100–6,300 Gt C (ref. 24), which is more than all of our requirement estimates. However, these capacity values do not account for technical feasibility, economic costs and social acceptability, which indicates the actual storage capacity is likely to be lower.

Alternative mitigation floors

Given that the mitigation floor is so crucial to the study, we investigated three alternative shapes for it (illustrated in Supplementary Figs 3 and 4). The corresponding negative emission requirements are detailed in Supplementary Figs 5–8, where one can see that changing the mitigation floor’s shape does not drastically change the qualitative conclusions of this study. It does change, however, the quantitative estimates of negative emission requirements, and those quantitative changes are summarized hereafter. We also note that all these assumed shapes of mitigation floors are idealized. There is no reason that actual future trajectories of mitigation follow exactly those shapes: they will vary accordingly to the economic, technological and sociopolitical context of the moment.

First, we looked at a flat transition (instead of a business-as-usual increase) between 2012 and the starting year of decrease of the floor. This shape of the mitigation floor could represent, for instance, a transition period during which global CO 2 production would be stabilized before being actually reduced. Comparatively to the default shape, this one ignores the difficulties in the short-term transition to an effectively decreasing mitigation pathway. This short-term transition could be studied with integrated assessment models (IAMs)25,26. Whatever the assumed floor characteristics (starting year and rate of decrease), such a change in its shape reduces both maximum yearly and cumulative negative emission requirements by up to 60% in the case of slow mitigation starting late.

Second, we considered a linear decrease of the mitigation floor (instead of exponential), which means that a constant amount of positive emission is mitigated each year, independently of the current level of emission. One could argue that this shape is a better choice to represent the first years of a mitigation trajectory than our default shape (it was used, for example, by Kriegler et al.9). For all our mitigation floor characteristics, this change in shape reduces both the maximum flux and the storage needed by up to 50% in the case of rapidly decreasing floors. Since this shape of the mitigation floor does reach the asymptote of 0, the reduction is very pronounced when looking at the storage capacity in 2300.