The Paris Agreement calls for holding global average temperature increase to “well below” 2°C, and to “pursue efforts” to limit the temperature increase to 1.5°C. Given the slow pace of action to date, there is increasing interest in “negative emissions” – measures that remove carbon from the atmosphere – to help make up for lost time.
The most widely discussed options are large-scale afforestation, and bioenergy in combination with carbon capture and storage (BECCS). Landscape restoration – both restoration of closed canopy forests, and “mosaic” restoration of more intensively used landscapes – could also contribute to climate goals. (Other ideas, such as direct air capture or ocean fertilization, remain in the realm of speculation.)
The analysis finds three significant risks in counting on these: (i) that negative emission options will not ultimately prove feasible; (ii) that their large-scale deployment involves unacceptable ecological and social impacts; and, (iii) that negative emissions activities prove less effective than hoped, either because they are subsequently reversed by human or natural forces, or because climate change impacts prove irreversible.
Although many models anticipate significant negative emissions from land-based measures, especially BECCS, the analysis suggests that only 370–480 Gt CO2 of negative emissions could be achieved without exceeding biophysical constraints, through forest restoration and reforestation. Even that would be extremely challenging to achieve, however, and would require large amounts of land that might otherwise be used for food production.
A review of modelled mitigation pathways suggests, however, that the goal of limiting warming to 2°C or 1.5°C can still be achieved without excessive reliance on negative emissions. This would require immediate global mitigation on a scale that greatly exceeds that which was pledged by nations under the Paris Agreement, as well as protecting land carbon stocks by reversing forest loss and allowing degraded forests to recover.
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