714 11 NOVEMBER 2016 • VOL 354 ISSUE 6313 sciencemag.org SCIENCE
The promise of
IN THEIR PERSPECTIVE “The trouble
with negative emissions” (14 October, p.
182), K. Anderson and G. Peters assert
that negative-emissions technologies are
an “unjust and high-stakes gamble.” This
characterization would sideline negative-emissions technologies and remove
potentially important options from the
portfolio for mitigating and ameliorating
As Anderson and Peters acknowledge,
the remaining carbon budget is pitifully
small; at the current rate, the world will
blow through 600 Gt of CO2 in 15 years.
Dumping this much CO2 in the atmosphere will almost certainly result in more
than 1.5oC warming. Indeed, as advocates
of a 350-ppm target point out, the remaining CO2 budget could be negative.
Anderson and Peters provide no evidence that faith in negative-emissions
technologies is to blame for a delay in
implementing other mitigation plans or
for the failure of countries to cut emissions. This failure is easily explained by
the free-riding behavior of some countries (1), and taking negative-emissions
technologies off the table would not
make collective action any easier. Indeed,
given that negative-emission technologies require financial contributions, not
changes in behavior, their development
and deployment may well be less vulnerable to free riding. Furthermore, we need
a lot of arrows in the quiver to stand a
chance of meeting the Paris targets. This
was a key finding from the integrated
assessment modelers (2).
Rather than dividing mitigation
into competing strategies, an inclusive
approach would focus on stopping climate
change as fast as possible while minimizing risk to vulnerable populations and
to societal stability. Negative-emission
technologies are not unique in facing challenges, risks, and uncertainties. It is true
that negative emissions may fall short of
closing the gap, but to characterize them
as a high-stakes gamble is not consistent with the facts and the plausibility
of meeting the Paris goals without them.
Throwing a life-preserver to a drown-
ing victim may not assure a successful
rescue, but it is not a high-stakes gamble.
Offering the life-preserver is preferable
to withholding it, even though it might
reduce the victim’s incentive for learning
how to swim.
Klaus S. Lackner and 45 additional
School of Sustainable Engineering and the Built
Environment, Arizona State, Univerity, Tempe, AZ
85287, USA. Email: Klaus. Lackner@asu.edu
*The full list of 46 authors and affiliations is
available in the supplementary materials.
1. S. Barrett, R. Stavins, Int. Environ. Agreements3, 349
2. 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, 2014).
Full author list
AS WE WROTE IN OUR Perspective, we
agree with Lackner et al. that negative-emissions technologies should “be the
subject of research, development, and
potentially deployment.” We support
research on the technical, environmental, social, and economic viability of
negative-emissions technologies. However,
we stand by our conclusion that given
the breadth and depth of fundamental
uncertainties associated with negative-emissions technologies (1–6), a program
of timely and deep mitigation in line with
2°C budgets should assume that they will
not be deployed at a large scale.
A mitigation agenda that does not rely
on future large-scale application of
negative-emissions technologies will
require a legislative environment that
delivers profound social and behavioral
change by high-emitters, rapid deploy-
ment of existing low-carbon energy
technologies, and urgent research and
development of new promising energy
technologies, including negative-
emissions technologies. If negative-
emissions technologies do indeed prove
to be successful, then a lower temperature
rise can be subsequently pursued.
Lackner et al. claim that including
negative-emissions technologies in
assessments does not delay other mitiga-
tion tactics. On the contrary, evidence
indicates that an assumption of negative-
emissions success does delay conventional
mitigation. Without negative-emissions
technologies, much more ambitious and
far reaching mitigation is required (2).
The 2°C scenarios assessed by the IPCC
that do not include negative emissions
but do allow afforestation have consider-
ably lower fossil-fuel consumption than
scenarios that include negative emissions
[e.g., Fig. S4 in (7)]. The “emissions gap”
(8, 9) between the necessary level of miti-
gation to deliver on the Paris goals and
the collective proposition of governments
(i.e., the sum of the Intended Nationally
Determined Contributions) would be
much larger if negative emissions were
We stand by our claim that postulating large-scale negative emissions in the
future leads to much less mitigation today.
Negative emissions facilitate the appealing option (10) of exceeding tight carbon
budgets and assuming that the debt will
be paid back later. If we cannot pay back
our carbon debt because the negative-emissions technologies do not deliver as
planned, then we have saddled the vulnerable and future generations with the
Edited by Jennifer Sills
At the current rate of carbon emissions, it will be difficult to meet climate goals.