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COMMENTARY: The challenge to keep global warming below 2 degrees C

Authors:

Abstract

The latest carbon dioxide emissions continue to track the high end of emission scenarios, making it even less likely global warming will stay below 2 °C. A shift to a 2 °C pathway requires immediate significant and sustained global mitigation, with a probable reliance on net negative emissions in the longer term.
4 NATURE CLIMATE CHANGE | VOL 3 | JANUARY 2013 | www.nature.com/natureclimatechange
opinion & comment
exhibit unusually high density and
growth rates”. However, none of the works
cited
1
actuallyprovide data on seagrass
populationgrowth.
Over the last 250,000years, the
maximum abrupt warming experienced by
the Mediterranean Sea was at 1–1.5°C per
century
12
, half that of the annual warming
projected by climate models for the current
century (2.8°C per century)
2
. No evidence
to suggest that P.oceanica was unaected
by these comparatively moderate warming
events in the past was presented by Altaba
1
.
We do not argue that vulnerability to
temperature renders conservation eorts
worthless. What we actually claimed
2
was that “actions to mitigate other local
impacts, although benecial, will have a
modest eect in the seagrass resistance to
warming events.
References
1. Altaba, C.R. Nature Clim. Change 3, 2–3 (2013).
2. Jordà, G., Marbà, N. & Duarte, C.M. Nature Clim. Change
2, 821–824 (2012).
3. Duarte, C.M., Borum, J., Short, F.T. & Walker, D.I. in Aquatic
Ecosystems (ed. Polunin, N.V.C.) 281–294 (Cambridge Univ.
Press, 2008).
4. Short, F.T. & Neckles, H.A. Aquat. Bot. 63, 169–196 (1998).
5. Reusch, T.B.H., Ehlers, A., Hammerli, A. & Worm, B.
Proc. Natl Acad. Sci. USA 102, 2826–2831 (2005).
6. Díaz-Almela, E., Marbà, N., Martínez, R., Santiago, R. &
Duarte, C.M. Limnol. Oceanogr. 54, 2170–2182 (2009)
7. Díaz-Almela E. etal. Mar. Poll. Bull. 56, 1332–1342 (2008).
8. Marbà, N. etal. Ecosystems 10, 745–756 (2007).
9. Marbà, N. & Duarte, C.M. Glob. Change Biol.
16, 2366–2375 (2010).
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COMMENTARY:
The challenge to keep global
warming below 2°C
Glen P. Peters, Robbie M. Andrew, Tom Boden, Josep G. Canadell,
Philippe Ciais, Corinne Le Quéré, Gregg Marland, Michael R. Raupach and Charlie Wilson
The latest carbon dioxide emissions continue to track the high end of emission scenarios, making it even
less likely global warming will stay below 2°C. A shift to a 2°C pathway requires immediate significant
and sustained global mitigation, with a probable reliance on net negative emissions in the longer term.
O
n-going climate negotiations have
recognized a “signicant gap
between the current trajectory of
global greenhouse-gas emissions and the
“likely chance of holding the increase in
global average temperature below 2°C
or 1.5°C above pre-industrial levels
1
.
Here we compare recent trends in carbon
dioxide (CO
2
) emissions from fossil-fuel
combustion, cement production and gas
aring with the primary emission scenarios
used by the Intergovernmental Panel on
Climate Change (IPCC). Carbon dioxide
emissions are the largest contributor
to long-term climate change and thus
provide a good baseline to assess progress
and examine consequences. We nd
that current emission trends continue to
track scenarios that lead to the highest
temperature increases. Further delay in
global mitigation makes it increasingly
dicult to stay below2°C.
Long-term emissions scenarios are
designed to represent a range of plausible
emission trajectories as input for climate
change research
2,3
. e IPCC process
has resulted in four generations of
emissions scenarios
2
: Scientic Assessment
1990 (SA90)
4
, IPCC Scenarios 1992
(IS92)
5
, Special Report on Emissions
Scenarios (SRES)
6
, and the evolving
RepresentativeConcentration Pathways
(RCPs)
7
to be used in the upcoming IPCC
Fih Assessment Report. e RCPs were
developed by the research community
as a new, parallel process of scenario
development, whereby climate models are
run using the RCPs while simultaneously
socioeconomic and emission scenarios are
developed that span the range of the RCPs
and beyond
2
.
It is important to regularly re-assess the
relevance of emissions scenarios in light
of changing global circumstances
3,8
. In
the past, decadal trends in CO
2
emissions
have responded slowly to changes in the
underlying emission drivers because of
inertia and path dependence in technical,
social and political systems
9
. Inertia and
path dependence are unlikely to be aected
by short-term uctuations
2,3,9
— such as
nancial crises
10
— and it is probable that
emissions will continue to rise for a period
even aer global mitigation has started
11
.
ermal inertia and vertical mixing in the
ocean, also delay the temperature response
to CO
2
emissions
12
. Because of inertia,
path dependence and changing global
circumstances, there is value in comparing
observed decadal emission trends with
emission scenarios to help inform the
prospect of dierent futures being realized,
explore the feasibility of desired changes
in the current emission trajectory and help
to identify whether new scenarios may
beneeded.
Global CO
2
emissions have increased
from 6.1±0.3PgC in 1990 to 9.5±0.5PgC
in 2011 (3% over 2010), with average
annual growth rates of 1.9% per year in
the 1980s, 1.0% per year in the 1990s, and
3.1% per year since 2000. We estimate that
emissions in 2012 will be 9.7±0.5PgC or
2.6% above 2011 (range of 1.9–3.5%) and
58% greater than 1990 (Supplementary
Information and ref. 13). e observed
growth rates are at the top end of all
four generations of emissions scenarios
11. Nykaer, L. Clim. Res. 39, 11–17 (2009).
12. Martrat, B. etal. Science 306, 1762–1765 (2004).
Gabriel Jordà
1
, Núria Marbà
2
* and
Carlos M. Duarte
2,3
1
Department of Ecology and Marine Resources,
IMEDEA (CSIC-UIB),Institut Mediterrani
d’Estudis Avançats, Miquel Marquès 21, 07190
Esporles, Illes Balears, Spain,
2
Department
of Global Change Research, IMEDEA (CSIC-
UIB),Institut Mediterrani d’Estudis Avançats,
Miquel Marquès 21, 07190 Esporles, Illes
Balears, Spain,
3
The UWA Oceans Institute,
The University of Western Australia, 35 Stirling
Highway, 6009 - Crawley, Western
Australia, Australia.
*e-mail: nmarba@imedea.uib-csic.es
© 2013 Macmillan Publishers Limited. All rights reserved
NATURE CLIMATE CHANGE | VOL 3 | JANUARY 2013 | www.nature.com/natureclimatechange 5
opinion & comment
16
14
12
10
8
6
4
2
0
1980 1990 2000 2010 2020 2030 2040 2050
Year
Global CO
2
emissions (PgCyr
–1
)
Historical
2012 estimate
RCP8.5
RCP6
RCP4.5
RCP3-PD
Historical uncertainty
SRES scenarios (40)
SRES illustrative scenarios (6)
IS92 scenarios (6)
A1FI
A2
A1B
A1T
B1
B2
IS92-C
IS92-D
IS92-E
IS92-F
IS92-B
IS92-A
Figure 1 | Estimated CO
2
emissions over the past three decades compared with the IS92, SRES and the
RCPs. The SA90 data are not shown, but the most relevant (SA90-A) is similar to IS92-A and IS92-F. The
uncertainty in historical emissions is ±5% (one standard deviation). Scenario data is generally reported at
decadal intervals and we use linear interpolation for intermediate years.
(Figs 1 and 2). Of the previous illustrative
IPCC scenarios, only IS92-E, IS92-F and
SRES A1B exceed the observed emissions
(Fig.1) or their rates of growth (Fig.2),
with RCP8.5 lower but within uncertainty
bounds of observed emissions.
Observed emission trends are in line
with SA90-A, IS92-E and IS92-F, SRES
A1FI, A1B and A2, and RCP8.5 (Fig.2).
e SRES scenarios A1FI and A2 and
RCP8.5 lead to the highest temperature
projections among the scenarios, with a
mean temperature increase of 4.2–5.0°C
in 2100 (range of 3.5–6.2°C)
14
, whereas
the SRES A1B scenario has decreasing
emissions aer 2050 leading to a lower
temperature increase of 3.5°C (range
2.9–4.4°C)
14
. Earlier research has noted that
observed emissions have tracked the upper
SRES scenarios
15,16
and Fig.1 conrms
this for all four scenario generations.
is indicates that the space of possible
pathways could be extended above the
top-end scenarios to accommodate the
possibility of even higher emission rates in
the future.
e new RCPs are particularly relevant
because, in contrast to the earlier scenarios,
mitigation eorts consistent with long-
term policy objectives are included
among the pathways
2,
. RCP3-PD (peak
and decline in concentration) leads to a
mean temperature increase of 1.5°C in
2100 (range of 1.3–1.9°C)
14
. RCP3–PD
requires net negative emissions (for
example, bioenergy with carbon capture
and storage) from 2070, but some scenarios
suggest it is possible to stay below 2°C
without negative emissions
17–19
. RCP4.5
and RCP6 — which lie between RCP3–PD
and RCP8.5 in the longer term— lead
to a mean temperature increase of 2.4°C
(range of 1.0–3.0°C) and 3.0°C (range
of 2.6–3.7°C) in 2100, respectively
14
. For
RCP4.5, RCP6 and RCP8.5, temperatures
will continue to increase aer 2100 due
to on-going emissions
14
and inertia in the
climatesystem
12
.
Current emissions are tracking slightly
above RCP8.5, and given the growing
gap between the other RCPs (Fig.1),
signicant emission reductions are
needed by 2020 to keep 2°C as a feasible
goal
18–20
. To follow an emission trend
that can keep the temperature increase
below 2°C (RCP3-PD) requires sustained
global CO
2
mitigation rates of around 3%
per year, if global emissions peak before
2020
11,19
. A delay in starting mitigation
activities will lead to higher mitigation
rates
11
, higher costs
21,22
, and the target
of remaining below 2°C may become
unfeasible
18,20
. If participation is low, then
higher rates of mitigation are needed in
individual countries, and this may even
increase mitigation costs for all countries
22
.
Many of these rates assume that negative
emissions will be possible and aordable
later this century
11,17,18,20
. Reliance on
negative emissions has high risks because
of potential delays or failure in the
development and large-scale deployment
of emerging technologies such as carbon
capture and storage, particularly those
connected tobioenergy
17,18
.
Although current emissions are tracking
the higher scenarios, it is still possible to
transition towards pathways consistent
with keeping temperatures below 2°C
(refs 17,19,20). e historical record shows
that some countries have reduced CO
2
emissions over 10-year periods, through
a combination of (non-climate) policy
intervention and economic adjustments
to changing resource availability. e
oil crisis of 1973 led to new policies
on energy supply and energy savings,
which produced a decrease in the share
of fossil fuels (oil shied to nuclear) in
the energy supply of Belgium, France
and Sweden, with emission reductions of
4–5% per year sustained over 10 or more
years (Supplementary Figs S17–19). A
continuous shi to natural gas — partially
substituting coal and oil — led to sustained
mitigation rates of 1–2% per year in the
UK in the 1970s and again in the 2000s, 2%
per year in Denmark in the 1990–2000s,
and 1.4% per year since 2005 in the USA
(Supplementary Figs S10–12). ese
examples highlight the practical feasibility
of emission reductions through fuel
substitution and eciency improvements,
but additional factors such as carbon
leakage
23
need to be considered. ese
types of emission reduction can help
initiate a transition towards trajectories
consistent with keeping temperatures
below 2°C, but further mitigation
measures are needed to complete and
sustain thereductions.
Similar energy transitions could be
encouraged and co-ordinated across
countries in the next 10years using
available technologies
19
, but well-targeted
technological innovations
24
are required
to sustain the mitigation rates for longer
periods
17
. To move below the RCP8.5
scenario — avoiding the worst climate
impacts — requires early action
17,18,21
and
© 2013 Macmillan Publishers Limited. All rights reserved
6 NATURE CLIMATE CHANGE | VOL 3 | JANUARY 2013 | www.nature.com/natureclimatechange
opinion & comment
sustained mitigation from the largest
emitters
22
such as China, the United States,
the European Union and India. ese four
regions together account for over half of
global CO
2
emissions, and have strong
and centralized governing bodies capable
of co-ordinating such actions. If similar
energy transitions are repeated over many
decades in a broader range of developed
and emerging economies, the current
emission trend could be pulled down to
make RCP3-PD, RCP4.5 and RCP6 all
feasiblefutures.
A shi to a pathway with the highest
likelihood to remain below 2°C above
pre-industrial levels (for example, RCP3-
PD), requires high levels of technological,
social and political innovations, and an
increasing need to rely on net negative
emissions in the future
11,17,18
. e timing
of mitigation eorts needs to account for
delayed responses in both CO
2
emissions
9
(because of inertia in technical, social
and political systems) and also in global
temperature
12
(because of inertia in the
climate system). Unless large and concerted
global mitigation eorts are initiated soon,
the goal of remaining below 2°C will very
soon become unachievable.
Glen P.Peters
1
*, Robbie M.Andrew
1
,
Tom Boden
2
, Josep G. Canadell
3
, Philippe Ciais
4
,
Corinne Le Quéré
5
, Gregg Marland
6
,
Michael R.Raupach
3
and Charlie Wilson
5
are at the
1
Center for International Climate and Environmental
Research – Oslo (CICERO), PO Box 1128, Blindern
0550, Oslo, Norway,
2
Carbon Dioxide Information
Analysis Center (CDIAC), Oak Ridge National
Laboratory, Oak Ridge, Tennessee 37831-4842,
USA,
3
Global Carbon Project, CSIRO Marine and
Atmospheric Research, GPO Box 3023, Canberra,
Australia,
4
Laboratoire des Sciences du Climat
et de l’Environnement, CAE – CNRS – UVSQ,
91191 Gif sur Yvette, France,
5
Tyndall Centre for
Climate Change Research, University of East Anglia,
Norwich NR4 7TJ, UK, and
6
Research Institute for
Environment, Energy, and Economics, Appalachian
State University, ASU Box 32067, Boone, North
Carolina 28608-2067, USA.
*e-mail: glen.peters@cicero.uio.no
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Acad. Sci.108, 8903–8908 (2011).
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Acknowledgements
is work is a collaborative eort of the Global
Carbon Project, a joint project of the Earth System
Science Partnership, to provide regular analyses of
the main global carbon sources and sinks (http://
www.globalcarbonproject.org/). G.P.P. and R.M.A
were supported by the Norwegian Research Council
(project 221355/E10). T.B. and the Carbon Dioxide
Information Analysis Center (CDIAC) are supported
by the US Department of Energy, Oce of Science,
Biological and Environmental Research. C.L.Q. thanks
the UK NaturalEnvironment Research Council (project
NE/103002X/1) and the European Commission
(projectFP7-283080) for support. J.G.C. and M.R.R.
thank the Australian Climate Change Science Program
forsupport.
Author contributions
All authors contributed to the planning of the paper.
G.P.P. led the work. G.M. and T.B. contributed the
updated CO
2
emission data. R.M.A. prepared the gures
and associated analysis. G.P.P. did the 2012 emission
estimate and the analysis of the historical reduction rates.
All authors contributed to data interpretation and to the
writing of thepaper.
Additional information
Supplementary information is available in the online
version of the paper. Reprints and permissions
information is available online at www.nature.com/
reprints. Correspondence and requests for material
should be addressed to G.P. All data presented in this
paper, including the full global CO
2
budget for 2011,
can be accessed at http://www.globalcarbonproject.org/
carbonbudget/
Published online: 2 December 2012
SA90 IS92 SRES RCPs
−0.5
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
SA90–A
SA90–B
SA90–C
SA90–D
IS92–A
IS92–B
IS92–C
IS92–D
IS92–E
IS92–F
A1B
A1FI
A2
B1
A1T
B2
RCP8.5
RCP6
RCP4.5
RCP3–PD
1985–2012 1990–2012 2000–2012 2005–2012
Average growth rates of CO
2
emissions (% per year)
Figure 2 | Growth rates of historical and scenario CO
2
emissions. The average annual growth rates of
the historical emission estimates (black crosses) and the emission scenarios for the time periods of
overlaps (shown on the horizontal axis). The growth rates are more comparable for the longer time
intervals considered (in order: SA90, 27years; IS92, 22years; SRES, 12years; and RCPs, 7years).
The short-term growth rates of the scenarios do not necessarily reflect the long-term emission
pathway (for example, A1B has a high initial growth rate compared with its long-term behaviour and
RCP3PD has a higher growth rate until 2010 compared with RCP4.5 and RCP6). For the SRES, we
represent the illustrative scenario for each family (filled circles) and each of the contributing model
scenarios (open circles). The scenarios generally report emissions at intervals of 10years or more
and we interpolated linearly to 2012; a sensitivity analysis shows a linear interpolation is robust
(Supplementary Fig. S14).
© 2013 Macmillan Publishers Limited. All rights reserved
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... An example is restaurants and the hospitality industry [41], which, similar to other service industrial enterprises, has overcome these difficulties and survived through resiliency and methods of recovery. At this critical moment, a method for modest learning from global warming lessons [42] and the COVID-19 Omicron of SARS-CoV-2 variant [43] is required, and accordingly, an important roadmap is thus proposed for ESD for 2030. ...
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