How to spend a dwindling greenhouse gas budget

Abstract

The Paris Agreement is based on emission scenarios that move from a sluggish phase-out of fossil fuels to large-scale late-century negative emissions. Alternative pathways of early deployment of negative emission technologies need to be considered to ensure that climate targets are reached safely and sustainably. The historic climate summit in Paris in 2015 galvanized global commitments to an ambitious yet vaguely defined goal of climate stabilization. At the same time some scientists argue that the model-based scenarios with 1.5-and even 2-degree temperature change targets seem unattainable and detached from current political realities 1,2. Here we scrutinize the dominant climate mitigation scenario archetype that projects low global decarbonization rates in the first half of this century followed by large negative emissions in the second half, thanks to Carbon Dioxide Removal (CDR) technologies 3. We call this approach to mitigation timing the "Late Century CDR" scenario archetype (Figure 1a). This archetype is consistent with nearly all of 2-degree scenarios covered by the Fifth Assessment Report (AR5) by the Intergovernmental Panel on Climate Change (IPCC) 4 , 87% of which deploy CDR technologies in the second half of the century 5. Following this predominant archetype might not only turn out to be a risky strategy, but also lead to significant environmental damages and may be economically inefficient. In "Late Century CDR" scenarios, CDR mostly in the form of bioenergy with Carbon Capture and Storage (BECCS) typically removes the equivalent of 20 years of current GHG emissions to reverse the temporary GHG budget overshoot that is tolerated earlier on 6. The challenges and uncertainties associated with CDR are well described in the scientific literature 5,7 , yet the scientific and political debate addressing the consequences of large-scale and late deployment of CDR as a "backstop" strategy is only at an early stage. We argue that a new set of scenarios needs to be generated and analyzed to inform the policy process on robust timing of climate mitigation with the aim of avoiding negative side effects. Essentially, three attributes characterize such budget-constrained scenarios: the timing and magnitude of global peak net emissions and its speed of decline thereafter; the maximum amount of allowable deployment of biomass-based CDRs; and an admissible risk threshold associated with a temperature overshoot.

Contrasting Impact of Future CO2 Emission Scenarios on the Extent of CaCO3 Mineral Undersaturation in the Humboldt Current System

“…With the onset of calcite undersaturation impacting a much wider array of marine organisms than the presence of “simple” aragonite undersaturation, avoiding this consequence of the emissions of CO 2 by anthropogenic activities is of great benefit for the marine organisms of the Humboldt Current System, and ultimately for the entire ecosystem. Thus, this provides an additional argument in engaging in an aggressive mitigation pathway, so that the impact of ocean acidification can be retained within limits (Gattuso et al, ; Steinacher et al, ), even though the RCP2.6 emission pathway looks increasingly outside the realm of possibilities without major employment of negative emissions technologies (Obersteiner et al, ).…”

Negative emissions—Part 1: Research landscape and synthesis

“…Yet, as the ambition to pursue further efforts to limit further warming to below 1.5 • C was added to the Paris Agreement (UNFCCC 2015), NETs have secured a place directly in the spotlight of many climate change mitigation discussions ever since (Hallegatte et al 2016, Hulme 2016, Schleussner et al 2016, Luderer et al 2013, Peters 2016. A series of high level commentaries picked up on the issue , Gasser et al 2015, Lomax et al 2015a, Parson 2017, Peters and Geden 2017, Obersteiner et al 2018, Anderson 2015, Anderson and Peters 2016, Geden 2015, Williamson 2016, Fuss et al 2014. Scenario evidence has been center-stage to this discussion.…”

“…Scenario evidence has been center-stage to this discussion. Authors have highlighted the importance of NETs for achieving the climate targets (Gasser et al 2015, Fuss et al 2014, stressed the limits to global carbon sequestration potentials (Smith et al 2016, Field and, questioned the feasibility of NETs in climate change mitigation (Anderson 2015, Geden 2015, and pointed out the bias in the exploration of alternative NETs futures (Obersteiner et al 2018). At the same time, modelling teams have started with the implementation of larger portfolios of NET options beyond BECCS (e.g.…”

Between Scylla and Charybdis: Delayed mitigation narrows the passage between large-scale CDR and high costs

“…In addition, the large-scale deployment of CDR leads to a peak in carbon removal towards the end of the century. This may cause risks of climate change irreversibility due to temperature overshoot and higher intergenerational imbalance (Obersteiner et al 2018).…”