27 JUNE 2014 bull VOL 344 Issue 6191 1431SCIENCE sciencemagorg

In its April 2014 report the Intergovernmental

Panel on Climate Change (IPCC) recognized that

reducing greenhouse gas (GHG) emissions by 40

to 70 by mid-century will require more than

just implementing emission-free solutions Many

scenarios for stabilizing GHG concentrations that

were evaluated by the panel included removing

carbon dioxide (CO2) from the atmosphere so-called

ldquonegative emissionsrdquo or carbon dioxide removal (CDR)

Among the most promising CDR methods are refores-

tation afforestation (planting new forests) and bioen-

ergy with carbon capture and storage (BECCS) How-

ever for BECCS strategies to

succeed major hurdles must be

overcome

All BECCS approaches rely

on removal of CO2 from the

atmosphere by plants during

photosynthesis In nature as

a plant decays it releases CO2

back into the atmosphere But

with BECCS CO2 is captured

and permanently stored under-

ground resulting in a net nega-

tive reduction in atmospheric

carbon At least three BECCS

technologies are being investi-

gated today CO2

released dur-

ing the microbial fermentation

of plant sugars to ethanol can

be captured CO2 can also be

captured during the gasifica-

tion of biomass to synthetic gas

for conversion to transportation

fuels chemicals or electricity

generation And CO2 from the

combustion of biomass either

with or without coal can be

captured

Since 2009 a consortium supported by the US De-

partment of Energy has successfully operated a BECCS

test facility in Illinois CO2 emitted during the fermen-

tation of corn is captured and stored in a sandstone

formation about 7000 feet underground The project

removes 300000 metric tons of CO2 per year from the

atmospheremdashthe equivalent of removing about 70000

cars from the road annually But thatrsquos just a fraction of

the amount of CO2 that will have to be removed to curb

global warming and of the estimated 2 to 10 gigatons

(Gt) of CO2 per year that could be removed from the

atmosphere with BECCS by 2050

For either conventional CCS or BECCS the cost must

come down Right now CCS costs range from about

$30 to $140 per ton of CO2 depending on the source

from which it is captured the capture technology and

the form of storagedagger Improving energy-conversion effi-

ciency would address the cost hurdles but this requires

further research and development Increased confi-

dence in long-term geological

storage security is also needed

to better understand the risks

of BECCS strategies Capturing

and storing 1 Gt of CO2 from

the atmosphere using BECCS

would require about 05 to 1

Gt of biomass (equivalent to 10

to 20 exajoules of primary en-

ergy) Concerns about whether

this much biomass could be

practically and sustainably har-

vested dried and collected at

this scale without interfering

with food production or nega-

tively affecting other ecosystem

services must be examined

Combined with sustainably

managed reforestation and af-

forestation the potential co-

benefits of habitat creation

carbon mitigation and renew-

able energy make BECCS an

attractive choice Rigorous re-

search and development are

needed so that the potential

of BECCS is clear to scientists

policy-makers and the public

Negative-emissions technologies such as BECCS

can be thought of as part of an insurance policy for

climate change mitigation This approach still leaves

unanswered questions but to not consider it carefully

would be too risky

Negative-emissions insurance

Sally M Benson

is director of the

Precourt Institute

for Energy and

the Global Climate

and Energy

Project and a

professor in the

department of

Energy Resources

Engineering

at Stanford

University

Stanford

CA E-mail

smbenson

stanfordedu

EDITORIAL

ndash Sally M Benson

101126science1257423

httpreportmitigation2014orgdraftsfinal-draft-postplenaryipcc_wg3_ar5_final-draft_postplenary_techni-cal-summarypdf daggerhttpswwwipccchpdfspecial-reportssrccssrccs_chapter8pdf and httpdecarbonisesitesdefaultfilespublications24202costs-ccs-and-other-low-carbon-technologiespdf

ldquoThis approach still leavesunanswered questions butto not consider it carefully

would be too riskyrdquo

Ethanol refinery

P H O T O 983080 R I G H T 983081 S T A N F O R D U N I V E R S I T Y 983080 I N S E T 983081 T A D P H O T O I S T O C K P H O T O

C O M

Published by AAAS

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2 0 1 4

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D o w n l o a d e d f r o m