The limits to global‐warming mitigation by terrestrial carbon removal

Abstract

Massive near-term greenhouse gas emissions reduction is a precondition for staying "well below 2 ∘ C" global warming as envisaged by the Paris Agreement. Furthermore, extensive terrestrial carbon dioxide removal (tCDR) through managed biomass growth and subsequent carbon capture and storage is required to avoid temperature "overshoot" in most pertinent scenarios. Here, we address two major issues: First, we calculate the extent of tCDR required to "repair" delayed or insufficient emissions reduction policies unable to prevent global mean temperature rise of 2.5 ∘ C or even 4.5 ∘ C above pre-industrial level. Our results show that those tCDR measures are unable to counteract "business-as-usual" emissions without eliminating virtually all natural ecosystems. Even if considerable (Representative Concentration Pathway 4.5 [RCP4.5]) emissions reductions are assumed, tCDR with 50% storage efficiency requires >1.1 Gha of the most productive agricultural areas or the elimination of >50% of natural forests. In addition, >100 MtN/yr fertilizers would be needed to remove the roughly 320 GtC foreseen in these scenarios. Such interventions would severely compromise food production and/or biosphere functioning. Second, we reanalyze the requirements for achieving the 160-190 GtC tCDR that would complement strong mitigation action (RCP2.6) in order to avoid 2 ∘ C overshoot anytime. We find that a combination of high irrigation water input and/or more efficient conversion to stored carbon is necessary. In the face of severe trade-offs with society and the biosphere, we conclude that large-scale tCDR is not a viable alternative to aggressive emissions reduction. However, we argue that tCDR might serve as a valuable "supporting actor" for strong mitigation if sustainable schemes are established immediately. Plain Language SummaryIn 2015, parties agreed to limit global warming to "well below" 2 ∘ C above pre-industrial levels. However, this requires not only massive near-term greenhouse gas emissions reductions but also the application of "negative emission" techniques that extract already emitted carbon dioxide from the atmosphere. Specifically, this could refer to the establishment of extensive plantations of fast-growing tree and grass species in combination with biomass conversion to carbon-saving products. Although such deployment is seen as promising, its carbon sequestration potentials and possible side-effects still remain to be studied in depth. In this study, we analyzed two feasibility aspects of such a negative emissions approach using biomass plantations and carbon utilization pathways. First, we show that biomass plantations with subsequent carbon immobilization are likely unable to "repair" insufficient emission reduction policies without compromising food production and biosphere functioning due to its space-consuming properties. Second, the requirements for a strong mitigation scenario staying below the 2 ∘ C target would require a combination of high irrigation water input and development of highly effect...

Large uncertainty in carbon uptake potential of land‐based climate‐change mitigation efforts

“…We conclude that forest maintenance and expansion, as well as large-scale bioenergy production combined with CCS, offer the potential to remove substantial amounts of C from the atmosphere and thus can help to mitigate climate change. However, the size of the removal is highly uncertain and may be much less than previously assumed in IAM/LUM scenarios consistent with the 2°C target (Boysen, Lucht, Gerten, Heck, et al, 2017;Rogelj et al, 2015;Smith, Davis, et al, 2016;Tavoni & Socolow, 2013;Wiltshire & Davies-Barnard, 2015); the C uptake simulated by the LUMs is only crop productivity under commercial production conditions. In addition, the LUMs and some DGVMs need to reconsider their assumptions about soil C sequestration rates following afforestation.…”

“…We conclude that forest maintenance and expansion, as well as large‐scale bioenergy production combined with CCS, offer the potential to remove substantial amounts of C from the atmosphere and thus can help to mitigate climate change. However, the size of the removal is highly uncertain and may be much less than previously assumed in IAM/LUM scenarios consistent with the 2°C target (Boysen, Lucht, Gerten, Heck, et al., ; Rogelj et al., ; Smith, Davis, et al., ; Tavoni & Socolow, ; Wiltshire & Davies‐Barnard, ); the C uptake simulated by the LUMs is only achieved in 1 of 16 combinations of mitigation LUC scenarios and DGVMs. The main reasons for the typically lower C uptake in the DGVMs as initially implemented in the LUMs are slower soil C accumulation (or even losses) following afforestation, different assumptions on potential vegetation C stocks, lower growth rates of forests, and lower bioenergy yields.…”

Agriculture production as a major driver of the Earth system exceeding planetary boundaries

“…But this has the potential of increasing competition with food production and inducing large-scale land-use changes. Thus recent studies suggest that BECCS may only be feasible on more modest scales as a "supporting actor" for strong mitigation actions (Boysen et al 2017, Smith et al 2016. Even with substantial yield increases and intensification, if humanity is to meet future demand for food and biofuels, net area under agriculture will have to expand, putting further pressure on important biomes.…”

The Carbon Dioxide Removal Model Intercomparison Project (CDRMIP): rationale and experimental protocol for CMIP6

“…Idealized Earth system model simulations suggest that CDR does appear to be able to limit or even reverse warming and changes in many other key climate variables (Boucher et al, 2012;Tokarska and Zickfeld, 2015;Wu et al, 2014;Zickfeld et al, 2016). However, less idealized studies, for example when some environmental limitations are accounted for, suggest that many methods have only a limited individual mitigation potential (Boysen et al, , 2017Keller et al, 2014;Sonntag et al, 2016).…”