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How to spend the dwindling global carbon “budget”? (and Ireland’s dwindling carbon quota share of it)

posted Jan 31, 2018, 2:00 AM by Paul Price   [ updated Jul 16, 2018, 6:19 AM ]

Viable pathways to "well below 2ºC"?

Due to continued high global greenhouse gas emissions it is becoming very difficult to show viable mitigation pathways to stay "well below 2ºC" of global warming without resorting to some serious commitment to carbon dioxide removal (CDR) from the atmosphere using negative emissions technologies (NETs). This would, of course, be in addition to the imperative necessity of achieving sustained reductions of gross (“positive”) emissions.

Allowing negative emissions in mitigation planning theoretically enables a slightly more gradual reduction in fossil fuel use but comes at a cost of added risks, including the risk that negative emissions technologies may fail to deliver at scale, as discussed in detail by Larkin et al. 2017 (open access).

The Paris Agreement temperature targets, currently agreed to by all nations, mandate limiting global average surface temperature rise to well below 2ºC, and ideally to no more than 1.5ºC above pre-industrial. Scientifically, the remaining carbon budgets for these two targets are much the same (see this post by Glen Peters) and though the uncertainty in the likely budget range is large the need for action is clear. In terms of near-term mitigation risk though, the key danger results from delaying sufficient early mitigation action, simply because every year of high emissions (CO₂ in particular) is reducing the available finite remaining global carbon (CO₂) budget to limit global warming to the Paris targets.

If negative CO₂ emissions capability is assumed (in policy and planning) then a key question – in addition to the risks and feasibility of the technologies – is whether CO₂ concentration in the atmosphere is allowed to temporarily overshoot the level compatible with the temperature limits, and for how long. Even if (as is almost certain) we do overshoot 1.5ºC, the Paris Agreement’s requirement “to pursue efforts to limit the temperature increase to 1.5 °C” presumably means that we cannot simply recalibrate to meeting "2ºC"; it means that we must continue nett CDR at least until we come back to atmospheric CO2 concentrations consistent with below 1.5ºC.

Four alternative pathways with and without NETs

To examine the risks and costs in decarbonisation policies reliant on carbon dioxide removal an article by Obersteiner et al. in Nature Climate Change (paywalled) usefully shows four alternative global mitigation pathways, up to 2100. Briefly, the four ‘archetype’ pathways compared by Obersteiner et al. are:

  1. Late Century Carbon Dioxide Removal: Allowing large atmospheric concentration overshoot due to continued fossil fuel use falling at a rate of c. 2.7% per year (taking 25 years to fall by half) but  relying on gradual and then large scale Bioenergy Carbon Capture and Storage (BECCS) up to and beyond 2100 both to provide energy from burning harvested biomass and permanently storing very large amounts of CO₂ (first removed from the atmosphere during the plant growth ). This archetype matches most of the 2ºC scenarios produced for the IPCC’s Fifth Assessment Report (see WG3 Ch.6 pdf).

  2. Rapid Decarbonisation: Reducing global fossil fuel use at a rate of c. 6.7 % per year (halving in just 10 years) and stopping all deforestation, to achieve net zero global emissions before 2060 (see previous blogpost on Rockström et al 2016 ‘carbon law’).

  3. No Overshoot: Early ramp-up of negative emission technologies including BECCS from now to 2070 to ease transition from fossil fuels (again assumed to reduce at c. 2.7% per year, or halving in usage every 25 years), followed by rapid phase out of BECCS as carefully managed land use removals thereafter offset the minimal residual fossil fuel and cement emissions.

  4. Minimise CDR: In this pathway BECCS is developed early and then used continually through 2100 at limited scale.

Obersteiner et al. show the figure reproduced below of “Four archetypes of emission pathways leading to a 2 °C warming target with peak emissions in 2020”. The annual emission projections show marked differences between the pathways even though nett zero emissions are reached in all cases between 2050 and 2070. Note that the red line in each projection shows the cumulative (nett) CO₂ emissions measured by the right-hand scale. In a, c and d, fossil fuel phase-out with use halving every 25 years; only in b, “Rapid Decarbonisation”, does fossil fuel use halve every 10 years. In a and b the carbon price for land use including agriculture and forestry [presumably (?) a  tax on emissions or providing revenue for removals] is assumed to increase exponentially from US$0 to $200 per tCO₂; and in c and d it is constant at US$40 per tCO₂.

Early and sustained rapid mitigation is imperative to limit impacts and reduce implied need for NETs

Obersteiner et al.’s first major conclusion is in confirming that any and all delay in peaking and then declining emissions rapidly leads to “alarming [high risk] overshoot” levels in all pathway alternatives.

The second major conclusion is that pathway a, Late Century Carbon Dioxide Removal, the form of pathway that emerges from  most IPCC climate-economic modelling, is a highly risky strategy because of potential infeasibility due to biophysical, technical and political limits on large scale dependence on NETs. Therefore, to avoid these dangers, they argue that ramping up national mitigation efforts is urgently needed with the addition of near-term investment focused toward quickly reaching modest levels of negative emissions:

“considerations of both intergenerational equity and climate/ environment safety motivate early and moderate — rather than extreme — deployment of negative emission technologies as well as a timely peak in net carbon emissions as early as 2020. As a consequence all of the near-term and mid-century net emission reduction, targets should be reformulated to include targets of early action on CDR technology portfolios. Furthermore, our calculations point to significant indirect land use effects and other cascading impacts of delayed actions in phasing out fossil fuel emissions.” Obersteiner et al. 2018

In the IE-NETS project we have recently estimated (see Chapter 8 in our literature review) a total remaining Irish CO₂-only equity carbon quota share of the remaining “well below 2ºC” global carbon budget at about 500 MtCO2 as of end-2017. Inequitably allowing a global carbon budget share based on Ireland’s current share of emissions gives an inertia CO2 budget of 900 MtCO2 as of end-2017.  However, a quota based on responsibility (including historic emissions) and on capacity (the ability to pay for mitigation) would result in a quota much smaller even than the equity quota,

With current emissions now over 42 MtCO₂ per year and rising (including nett land use emissions), Ireland’s very limited remaining CO2 quota is currently being exhausted at an increasing rate. Turning the current upward trend around toward echoing any one of the Obersteiner et al. pathways will require major efforts in the short-term to decide on regulation, incentives and investment to enable overall energy demand reduction, low carbon energy supply and potentially rapid development of land-based carbon sequestration and carbon capture and storage that could underpin some level of BECCS and direct air capture (DAC) of CO₂ in future.

Increasingly, there are no easy or inexpensive choices in climate action: a portfolio of clearly communicated, strongly monitored and well funded efforts are needed to achieve actual decarbonisation. The analysis by Obersteiner et al. suggests that doing nothing is by far the most expensive, and highest risk, choice and that early and sustained action to cut emissions rapidly and deploy negative emissions at modest scale has significantly lower cost and less danger. Aligning global and national climate action with meeting the Paris targets requires immediate and sustained effort by all nations, especially those with high per person emissions and with the relative wealth to act fastest.

In societal reality, planning a national mitigation pathway and ensuring sufficient investment and oversight to achieve emission reductions and verifiable negative emissions both depend on early and sustained political decision-making. Research can outline the risks of inaction but it is policymakers globally and nationally who need to make difficult choices that determine whatever pathway is actually taken. Making those choices politically acceptable may not be easy but the physical reality of climate impacts from failing to make them is rapidly becoming clear, the imminent loss of almost all coral reefs, rapid Arctic ice loss and extreme heat episodes being the most strongly evidenced signs of escalating climate change impacts.

As the IE-NETs Literature Review has shown, climate research strongly shows that the often-easiest immediate political choice – doing little or nothing to deliver emission cuts or, secondarily, to enable early negative emissions – is the least cost-effective option simply because it gives no hope at all of delivering pathways achieving sufficient climate mitigation.


Larkin, A. et al., 2017. What if negative emission technologies fail at scale? Implications of the Paris Agreement for big emitting nations. Climate Policy, pp.1–25.

Obersteiner, M. et al., 2018. How to spend a dwindling greenhouse gas budget. Nature Climate Change, 8(1), pp.7–10.