In Paris last December, the 196 parties to the United Nations Framework Convention on Climate Change (UNFCCC) agreed to balance the human-driven greenhouse-gas budget some time between 2050 and 2100. This commitment is intended to limit the increase in global average temperature above pre-industrial levels to "well below 2 [degrees]C"--and preferably to 1.5 [degrees]C.
A balanced greenhouse-gas budget either requires that industry and agriculture produce zero emissions or necessitates the active removal of greenhouse gases from the atmosphere (in addition to deep and rapid emissions cuts). In most modelled scenarios that limit warming to 2 [degrees]C, several gigatonnes of carbon dioxide have to be extracted and safely stored each year (1). For more ambitious targets, tens of gigatonnes per year must be removed (2).
Many C[O.sub.2]-removal techniques have been proposed. Whether any of them could work at the scale needed to deliver the goal of the Paris agreement depends on three things: feasibility, cost and acceptability. A crucial component of all of these approaches is the non-climatic impacts that large-scale C[O.sub.2]-removal could have on ecosystems and biodiversity.
Until now, the UNFCCC's scientific advisory body, the Intergovernmental Panel on Climate Change (IPCC), has paid relatively little attention to such impacts. It has fallen to other groups to review insights and gaps in our understanding of the influence of C[O.sub.2]-removal techniques on ecology (3-5); to make broad assessments of climate-engineering schemes (6); and to carry out comparative modelling studies (7).
It is time for the IPCC, governments and other research-funding agencies to invest in new, internationally coordinated studies to investigate the viability and relative safety of large-scale C[O.sub.2] removal.
Since its establishment in 1988, the IPCC has predominantly involved physical scientists and modellers, rather than ecologists. This, combined with the only relatively recent evidence that emissions reduction alone is unlikely to avert dangerous climate change, could account for why the IPCC's roughly 5,000-page Fifth Assessment Report, released in 2013 and 2014, leaves out one crucial consideration: the environmental impacts of large-scale C[O.sub.2] removal.
This omission is striking because the set of IPCC emissions scenarios that are likely to limit the increase in global surface temperature to 2 [degrees]C by 2100 (the aim of the RCP2.6 'representative concentration pathway, the IPCC climate-change-response scenario that achieves the lowest emissions) mostly relies on large-scale C[O.sub.2] removal.
These scenarios assume that two techniques could be developed to balance the carbon budget later this century: bioenergy with carbon capture and storage (BECCS), and afforestation. BECCS involves growing bioenergy crops, from grasses to trees; burning them in power stations; stripping the C[O.sub.2] from the resulting waste gases; and compressing it into a liquid for underground storage. Afforestation--planting trees--also relies on photosynthesis to initially remove C[O.sub.2] from the atmosphere. Storage is achieved naturally, in timber and soil.
Limiting the global temperature rise to 2 [degrees]C, with any confidence, would require the removal of some 600 gigatonnes of C[O.sub.2] over this century (the median estimate of what is needed) (8). Using BECCS, this would probably require crops to be...
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