| 英文摘要: | Bioenergy with carbon capture and storage could be used to remove carbon dioxide from the atmosphere. However, its credibility as a climate change mitigation option is unproven and its widespread deployment in climate stabilization scenarios might become a dangerous distraction. Future warming will depend strongly on the cumulative CO2 emissions released through to the end of this century1, 2. A finite quota of cumulative CO2 emissions, no more than 1,200 Gt CO2, is needed from 2015 onwards to stabilize climate below a global average of 2 °C above pre-industrial conditions by 2100 with a likelihood of 66%. This corresponds to about 30 years at current emissions levels3. However, during the past decade, emissions from fossil fuel combustion and cement production have increased substantially to 36.1 ± 1.8 Gt CO2 yr−1 in 2013 (refs 4,5), projected to reach 37.0 ± 1.8 Gt CO2 yr−1 in 2014 (ref. 3), 65% above their 1990 level. Staying within the 2 °C limit in a cost-effective way will require strong mitigation action across all sectors, with greater effort needed the longer mitigation is delayed. Actions that could stabilize climate as desired include the deliberate removal of CO2 from the atmosphere by human intervention — called here 'negative emissions'. Along with afforestation, the production of sustainable bioenergy with carbon capture and storage (BECCS) is explicitly being put forth as an important mitigation option by the majority of integrated assessment model (IAM) scenarios aimed at keeping warming below 2 °C in the IPCC's fifth assessment report (AR5)6. Indeed, in these scenarios, IAMs often foresee absorption of CO2 via BECCS up to (and in some cases exceeding) 1,000 Gt CO2 over the course of the century7, effectively doubling the available carbon quota. BECCS is the negative emissions technology most widely selected by IAMs to meet the requirements of temperature limits of 2 °C and below. It is based on assumed carbon-neutral bioenergy (that is, the same amount of CO2 is sequestered at steady state by biomass feedstock growth as is released during energy generation), combined with capture of CO2 produced by combustion and its subsequent storage in geological or ocean repositories. In other words, BECCS is a net transfer of CO2 from the atmosphere, through the biosphere, into geological layers, providing in addition a non-fossil fuel source of energy. Other options include afforestation, direct air capture and increases in soil carbon storage. Afforestation and increased soil carbon storage differ from BECCS in that these land-use and management changes are associated with a saturation of CO2 removal over time, and in that the sequestration is reversible with terrestrial carbon stocks inherently vulnerable to disturbance8.
The IPCC's Working Group 3 (WG3) considered in AR5 over 1,000 emission pathways to 2100 (Fig. 1a). Most scenarios (101 of 116) leading to concentration levels of 430–480 ppm CO2 equivalent (CO2eq), consistent with limiting warming below 2 °C, require global net negative emissions in the second half of this century, as do many scenarios (235 of 653) that reach between 480 and 720 ppm CO2eq in 2100 (Fig. 1b, scenarios below zero). About half of the scenarios feature BECCS exceeding 5% of primary energy supply. Many of those (252 of 581) have net positive emissions in 2100 (Fig. 1b). Thus, BECCS does not ensure net negative emissions (that is, its use need not completely offset all positive emissions). BECCS is an important mitigation technology, especially as the stabilization level is lowered, and if near-term mitigation is delayed. By eventually requiring deeper emissions reductions, BECCS can help reconcile higher interim CO2eq concentrations with low long-term stabilization targets, particularly if overshooting of concentrations is allowed. Taking into account the full scenario range, global net negative emissions would need to set in around 2070 for the most challenging scenarios and progressively later for higher-temperature stabilization levels.
| http://www.nature.com/nclimate/journal/v4/n10/full/nclimate2392.html
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