INSIGHTS | PERSPECTIVES
19 MAY 2017 • VOL 356 ISSUE 6339 707 SCIENCE sciencemag.org
climate change impacts, large mitigation
costs, and unacceptable trade-offs.
Some CDR approaches already combine
technological readiness, low costs, and
clear co-benefits. Ecosystem stewardship
and building with biomass are examples,
Rightsizing CDR involves
taking full advantage of the ap-
proaches that are available now
while simultaneously investing in research
and early-stage deployment, driving down
the costs of the immature options, and evalu-
ating side effects. Most important, rightsizing
avoids reckless assumptions that massive-
scale CDR with low costs and limited side ef-
fects will quickly materialize (3, 5).
Much of the recent discussion about CDR
concerns deployments at vast scales. Many
candidate technologies have the potential
to be the foundation for strong enterprises,
capturing many millions of tons of carbon
dioxide per year, in locations where each
approach makes sense technically and economically, with a favorable mix of co-benefits and side effects. It is at much larger
scales (billions of tons of CO2 per year) that
the technologies warrant concern.
For its latest report, the Intergovernmental Panel on Climate Change (IPCC) analyzed
about 900 scenarios from about 30 integrated assessment models (6). These models
determine a cost-effective mix of technologies, based on estimated technology costs
and on climate policy, including carbon pricing. Of the 116 scenarios with a 66% or better
chance of limiting global warming to 2°C by
2100, 101 include CDR, mostly BECCS, in the
technology mix for the second half of the 21st
century (7). Across these scenarios, the me-
dian commitment to carbon dioxide removal
from BECCS in 2100 is about 12 billion tons
of CO2 per year, equivalent to more than 25%
of current CO2 emissions (8).
This is truly massive use of a technology
with little real-world experience and poorly
known economics. The requirements for
land and water are large but uncertain.
Based on relatively optimistic assumptions
about future yields, this BECCS commitment corresponds to 0.4 to 0.7 billion ha
of productive land (7); more conservative
assumptions yield a land requirement of
1.2 billion ha (9). This range is about 25 to
80% of total current global cropland or up
to 8% of Earth’s land area. Converting land
on this staggering scale would pit climate
change responses against food security and
biodiversity protection. Massively expanding managed land for CDR could crash
through the planetary boundary for sustainable land use (10–12).
Compared with BECCS, afforestation
and reforestation would require even more
land and water, and the carbon sinks might
saturate or reverse (7). Direct air capture
might require much less land but entail
much higher costs and consumption of a
large fraction of global energy production
(7). The required land would operate as an
immense array of industrial facilities. The
other CDR technologies have questionable
potential for reaching deployment at the
scale of several billion tons per year.
THE PROBLEMS WITH PEAK AND DECLINE
Scenarios with massive CDR deployments
often involve temperatures peaking and then
declining. If the goal is to avoid the worst
effects of climate change, such peak and de-
cline scenarios imply substantial risks. High
temperatures reached transiently may lock
in permanent effects—for example, by trig-
gering ice-sheet instabilities that cause sub-
stantial irreversible sea level rise (13). High
peak temperatures could stimulate signifi-
cant releases of greenhouse gases from the
Arctic or the Amazon, further exacerbating
climate change (14). Species, communities, or
economic activities that have adapted to peak
temperatures may struggle to adjust back as
temperatures fall. Similarly, land and ocean
sinks have absorbed substantial emissions to
date. But the ramifications of reversing gears
through substantial negative emissions re-
main poorly understood (1, 15).
The meteoric rise of CDR technologies in
planning for climate solutions has stirred
up discomfort and debate. The dynamic
is familiar. Early work on climate change
adaptation controversially implied that climate change mitigation might fall to the side.
Similarly, the question now is
not whether to either reduce
emissions or deploy CDR.
Again, the answer is both, staying focused on not only climate effects but also broader
planetary sustainability. Opportunities for ecosystem
can be tapped now. BECCS can
be tested, developed, and deployed while keeping expectations in check. Research and
development on direct air capture and other emerging options can help
clarify their future relevance.
Across direct emissions reductions and
CDR, a transparent and balanced approach
is necessary. By avoiding cavalier assumptions of future technological breakthroughs
and embracing whole-hearted near-term
ambition, we can protect societies and the
planet on which we depend. j
1. S. Fuss et al., Climate Change 4, 850 (2014).
2. National Research Council, Climate Intervention: Carbon
Dioxide Removal and Reliable Sequestration (The National
Academies, Washington, DC, 2015).
3. P. Williamson, Nature 530,153(2016).
4. D. W.Keith, Science 325,1654(2009).
5. K. Anderson, G. Peters, Science 354, 182 (2016).
6. IPCC, Climate Change 2014: Mitigation of Climate Change,
Contribution of Working Group III to the Fifth Assessment
Report of the Intergovernmental Panel on Climate
Change, O. Edenhofer et al., Eds. (Cambridge Univ. Press,
Cambridge/New York, 2014).
7. P. Smith et al ., Nat. Clim. Change 6, 42 (2016).
8. C. Le Quéré et al ., Earth Syst. Sci. Data 8, 605 (2016).
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Biogeochem. Cycles 22, 10.1029/2007GB002947 (2008).
10. J. Rockström etal., Ecol.Soc. 14, 32 (2009).
11. W. Steffen et al ., Science 347, 1259855 (2015).
12. E.O. Wilson, Half-Earth: Our Planet’s Fight for Life (Norton,
13. R. M. DeConto, D. Pollard, Nature 531, 591 (2016).
14. J. Settele et al ., in Climate Change 2014: Impacts,
Adaptation, and Vulnerability, Part A: Global and Sectoral
Aspects, Contribution of Working Group II to the Fifth
Assessment Report of the Intergovernmental Panel of
Climate Change, C. B. Field et al., Eds. (Cambridge Univ.
Press, Cambridge/New York, 2014), pp. 271–359.
15. C.D.Jones et al., Environ. Res. Lett. 11,095012(2016).
Careful stewardship of forests can help to store
carbon while maintaining biodiversity. The image
shows Killiecrankie Gorge in Perthshire, Scotland.
A sampling of CDR technologies
Comparative features of three widely discussed, potentially large-scale
strategies for carbon dioxide removal (2, 7).
FOREST AND SOIL
Level of engineering complexity Low Medium High
Environmental cobenefits High Low Low
Land area required for
High High Low
Risk of later carbon dioxide release High Low Low
Energy status ~Neutral Production Consumption