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



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.
opinion & comment
exhibit unusually high density and
growth rates”. However, none of the works
actuallyprovide data on seagrass
Over the last 250,000years, the
maximum abrupt warming experienced by
the Mediterranean Sea was at 1–1.5°C per
, half that of the annual warming
projected by climate models for the current
century (2.8°C per century)
. No evidence
to suggest that P.oceanica was unaected
by these comparatively moderate warming
events in the past was presented by Altaba
We do not argue that vulnerability to
temperature renders conservation eorts
worthless. What we actually claimed
was that “actions to mitigate other local
impacts, although benecial, will have a
modest eect in the seagrass resistance to
warming events.
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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.
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
Here we compare recent trends in carbon
dioxide (CO
) 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
. e IPCC process
has resulted in four generations of
emissions scenarios
: Scientic Assessment
1990 (SA90)
, IPCC Scenarios 1992
, Special Report on Emissions
Scenarios (SRES)
, and the evolving
RepresentativeConcentration Pathways
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
It is important to regularly re-assess the
relevance of emissions scenarios in light
of changing global circumstances
. In
the past, decadal trends in CO
have responded slowly to changes in the
underlying emission drivers because of
inertia and path dependence in technical,
social and political systems
. Inertia and
path dependence are unlikely to be aected
by short-term uctuations
— such as
nancial crises
— and it is probable that
emissions will continue to rise for a period
even aer global mitigation has started
ermal inertia and vertical mixing in the
ocean, also delay the temperature response
to CO
. 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
Global CO
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
, Núria Marbà
* and
Carlos M. Duarte
Department of Ecology and Marine Resources,
IMEDEA (CSIC-UIB),Institut Mediterrani
d’Estudis Avançats, Miquel Marquès 21, 07190
Esporles, Illes Balears, Spain,
of Global Change Research, IMEDEA (CSIC-
UIB),Institut Mediterrani d’Estudis Avançats,
Miquel Marquès 21, 07190 Esporles, Illes
Balears, Spain,
The UWA Oceans Institute,
The University of Western Australia, 35 Stirling
Highway, 6009 - Crawley, Western
Australia, Australia.
© 2013 Macmillan Publishers Limited. All rights reserved
opinion & comment
1980 1990 2000 2010 2020 2030 2040 2050
Global CO
emissions (PgCyr
2012 estimate
Historical uncertainty
SRES scenarios (40)
SRES illustrative scenarios (6)
IS92 scenarios (6)
Figure 1 | Estimated CO
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)
, whereas
the SRES A1B scenario has decreasing
emissions aer 2050 leading to a lower
temperature increase of 3.5°C (range
. Earlier research has noted that
observed emissions have tracked the upper
SRES scenarios
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
. 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)
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
. 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
. For
RCP4.5, RCP6 and RCP8.5, temperatures
will continue to increase aer 2100 due
to on-going emissions
and inertia in the
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
. To follow an emission trend
that can keep the temperature increase
below 2°C (RCP3-PD) requires sustained
global CO
mitigation rates of around 3%
per year, if global emissions peak before
. A delay in starting mitigation
activities will lead to higher mitigation
, higher costs
, and the target
of remaining below 2°C may become
. If participation is low, then
higher rates of mitigation are needed in
individual countries, and this may even
increase mitigation costs for all countries
Many of these rates assume that negative
emissions will be possible and aordable
later this century
. 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
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
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
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
, but well-targeted
technological innovations
are required
to sustain the mitigation rates for longer
. To move below the RCP8.5
scenario — avoiding the worst climate
impacts — requires early action
© 2013 Macmillan Publishers Limited. All rights reserved
opinion & comment
sustained mitigation from the largest
such as China, the United States,
the European Union and India. ese four
regions together account for over half of
global CO
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
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
. e timing
of mitigation eorts needs to account for
delayed responses in both CO
(because of inertia in technical, social
and political systems) and also in global
(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
*, Robbie M.Andrew
Tom Boden
, Josep G. Canadell
, Philippe Ciais
Corinne Le Quéré
, Gregg Marland
Michael R.Raupach
and Charlie Wilson
are at the
Center for International Climate and Environmental
Research – Oslo (CICERO), PO Box 1128, Blindern
0550, Oslo, Norway,
Carbon Dioxide Information
Analysis Center (CDIAC), Oak Ridge National
Laboratory, Oak Ridge, Tennessee 37831-4842,
Global Carbon Project, CSIRO Marine and
Atmospheric Research, GPO Box 3023, Canberra,
Laboratoire des Sciences du Climat
et de l’Environnement, CAE – CNRS – UVSQ,
91191 Gif sur Yvette, France,
Tyndall Centre for
Climate Change Research, University of East Anglia,
Norwich NR4 7TJ, UK, and
Research Institute for
Environment, Energy, and Economics, Appalachian
State University, ASU Box 32067, Boone, North
Carolina 28608-2067, USA.
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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:// 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
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
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
reprints. Correspondence and requests for material
should be addressed to G.P. All data presented in this
paper, including the full global CO
budget for 2011,
can be accessed at
Published online: 2 December 2012
1985–2012 1990–2012 2000–2012 2005–2012
Average growth rates of CO
emissions (% per year)
Figure 2 | Growth rates of historical and scenario CO
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
... The most immediate threats to Antarctic biota are consequences of increased temperature and altered sea ice (Chown et al., 2012). Although the Intergovernmental Panel on Climate Change (IPCC) aims to limit average global warming to 2°C by the end of this century, current trends of global temperature increase, sea level rise, and ice loss are at the upper end of the IPCC's more pessimistic scenarios (Peters et al., 2013;Siegert et al., 2020). The Antarctic Peninsula has already warmed by more than 2°C in the 20th century, with some areas experiencing average annual temperature increases of 3°C or more between 1951 and 2011 (Turner et al., 2014). ...
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Background and Aims. The cultivation of grapevines in England is expected to benefit under climate change. Yet assessments of future wine climates remain undeveloped. Accordingly, this study assesses how climate change might modify frost risk for Chardonnay in the Southeast England viticulture region. Methods and Results. Cold-bias-corrected climate projections from the UKCP18 Regional (12 km) perturbed parameter ensemble (PPE) climate model under RCP8.5 are applied with phenological models to determine how frost risk and the timing of key grapevine phenophases might alter under climate change. Notwithstanding the uncertainties associated with projections of key viticulture-related bioclimate variables, the last spring frost was found to advance at a greater rate than budburst, indicating a general decrease in frost risk. Conclusions. Although projections point to an improving climate for viticulture across Southeast England, frost will remain a risk for viticulture, albeit at a reduced level compared to the present. Furthermore, the strong cold-bias found for temperature simulations used in this study needs to be given careful consideration when using the UKCP18 projections for viticulture impact assessments of climate change. Significance of the Study. This study highlights the present sensitivity of viticulture to climate variability and the inherent uncertainty associated with making future projections of wine climate under climate change.
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Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize data sets and methodologies to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly, and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based data products. The terrestrial CO2 sink (SLAND) is estimated with dynamic global vegetation models. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the year 2021, EFOS increased by 5.1 % relative to 2020, with fossil emissions at 10.1 ± 0.5 GtC yr−1 (9.9 ± 0.5 GtC yr−1 when the cement carbonation sink is included), and ELUC was 1.1 ± 0.7 GtC yr−1, for a total anthropogenic CO2 emission (including the cement carbonation sink) of 10.9 ± 0.8 GtC yr−1 (40.0 ± 2.9 GtCO2). Also, for 2021, GATM was 5.2 ± 0.2 GtC yr−1 (2.5 ± 0.1 ppm yr−1), SOCEAN was 2.9 ± 0.4 GtC yr−1, and SLAND was 3.5 ± 0.9 GtC yr−1, with a BIM of −0.6 GtC yr−1 (i.e. the total estimated sources were too low or sinks were too high). The global atmospheric CO2 concentration averaged over 2021 reached 414.71 ± 0.1 ppm. Preliminary data for 2022 suggest an increase in EFOS relative to 2021 of +1.0 % (0.1 % to 1.9 %) globally and atmospheric CO2 concentration reaching 417.2 ppm, more than 50 % above pre-industrial levels (around 278 ppm). Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2021, but discrepancies of up to 1 GtC yr−1 persist for the representation of annual to semi-decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows (1) a persistent large uncertainty in the estimate of land-use change emissions, (2) a low agreement between the different methods on the magnitude of the land CO2 flux in the northern extratropics, and (3) a discrepancy between the different methods on the strength of the ocean sink over the last decade. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding of the global carbon cycle compared with previous publications of this data set. The data presented in this work are available at (Friedlingstein et al., 2022b).
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In this study, Weather Research and Forecasting (WRF) ‐based dynamic downscaling simulation was performed over China at a horizontal resolution of 25 km. To reduce the systematic large‐scale biases from lateral boundary conditions, a dynamic blending (DB) technique was introduced in downscaling, and its performance was compared with downscaling without DB (NO_DB) as well as the driving global climate model (GCM, i.e., HadGEM3). In the present‐day simulation for verification, added values were found in DB, relative to the GCM and NO_DB, in simulating the precipitation extremes, especially over southeastern China. Possible causes responsible for this improvement were further analyzed. In the GCM, excessive moisture and atmospheric heating in the upper troposphere in conjunction with abnormally strong deep convection resulted in excessive extreme precipitation over southeastern China, while in NO_DB, insufficient moisture and atmospheric heating in the whole troposphere in conjunction with abnormally weak convection suppressed extreme precipitation there. In comparison, DB showed a closer representation of observations in terms of vertical velocity and vertical profiles of atmospheric moisture and heating, accounting for the improved simulation of extreme precipitation. In the future projection under the SSP5‐8.5 scenario, both the GCM and DB predicted that extreme precipitation would increase in most parts of China; however, DB indicated a larger increase over southeastern China relative to the GCM. Stronger moisture flux convergence over southeastern China in DB accounted for this larger increase, and in addition, the thermodynamic effect associated with more precipitable water dominated the stronger moisture flux convergence.
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Carbon emissions embodied in anthropogenic activities represent the major cause of global warming. Countries, regions, and cities have implemented comprehensive, multi-level and multi-scale measures to reduce emissions and move towards carbon neutrality. The demand for carbon emission reduction (CER) is made more challenging by different geographical locations, country-owned natural resources, and economic development stages. The main objectives of this paper are to conduct a bibliometric analysis to map the frontiers and directions of CER and to explore the paths and development models of CER from the perspective of spatio-temporal, multi-scale, multi-sectoral, and multi-responsible subjects. This study reveals that carbon emission evaluation and prediction, correlation and causal relationship analysis, and CER-related policy simulation and optimization are the most critical hotspots. Additionally, we point out the shortcomings of and future developments for the three study dimensions above. The bibliometric analysis also highlights the fact that a cooperative global value chain as well as amendable policies and mechanisms for CER will help with climate change mitigation and adaptation through the use of advanced carbon capture and storage technologies. We review the technical measures for and policy responses to CER adopted by different countries and industries at the theoretical and practical levels and provide new recommendations. Our work provides important information for climate actions in different countries and sectors and for developing more effective CER strategies and policies.
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Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental data.
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Mitigating climate change requires directed innovation efforts to develop and deploy energy technologies. Innovation activities are directed towards the outcome of climate protection by public institutions, policies and resources that in turn shape market behaviour. We analyse diverse indicators of activity throughout the innovation system to assess these efforts. We find efficient end-use technologies contribute large potential emission reductions and provide higher social returns on investment than energy-supply technologies. Yet public institutions, policies and financial resources pervasively privilege energy-supply technologies. Directed innovation efforts are strikingly misaligned with the needs of an emissions-constrained world. Significantly greater effort is needed to develop the full potential of efficient end-use technologies.
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Climate projections for the fourth assessment report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) were based on scenarios from the Special Report on Emissions Scenarios (SRES) and simulations of the third phase of the Coupled Model Intercomparison Project (CMIP3). Since then, a new set of four scenarios (the representative concentration pathways or RCPs) was designed. Climate projections in the IPCC fifth assessment report (AR5) will be based on the fifth phase of the Coupled Model Intercomparison Project (CMIP5), which incorporates the latest versions of climate models and focuses on RCPs. This implies that by AR5 both models and scenarios will have changed, making a comparison with earlier literature challenging. To facilitate this comparison, we provide probabilistic climate projections of both SRES scenarios and RCPs in a single consistent framework. These estimates are based on a model set-up that probabilistically takes into account the overall consensus understanding of climate sensitivity uncertainty, synthesizes the understanding of climate system and carbon-cycle behaviour, and is at the same time constrained by the observed historical warming.
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In recent years, international climate policy has increasingly focused on limiting temperature rise, as opposed to achieving greenhouse-gas-concentration- related objectives. The agreements reached at the United Nations Framework Convention on Climate Change conference in Cancun in 2010 recognize that countries should take urgent action to limit the increase in global average temperature to less than 2C relative to pre-industrial levels. If this is to be achieved, policymakers need robust information about the amounts of future greenhouse-gas emissions that are consistent with such temperature limits. This, in turn, requires an understanding of both the technical and economic implications of reducing emissions and the processes that link emissions to temperature. Here we consider both of these aspects by reanalysing a large set of published emission scenarios from integrated assessment models in a risk-based climate modelling framework. We find that in the set of scenarios with a 'likely' (greater than 66%) chance of staying below 2C, emissions peak between 2010 and 2020 and fall to a median level of 44 Gt of CO 2 equivalent in 2020 (compared with estimated median emissions across the scenario set of 48 Gt of CO 2 equivalent in 2010). Our analysis confirms that if the mechanisms needed to enable an early peak in global emissions followed by steep reductions are not put in place, there is a significant risk that the 2C target will not be achieved.
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This paper summarizes the development process and main characteristics of the Representative Concentration Pathways (RCPs), a set of four new scenarios developed for the climate modeling community as a basis for long-term and near-term modeling experiments. The four RCPs together span the range of year 2100 radiative forcing values found in the open literature, i.e. from 2.6 to 8.5 W/m2. The RCPs are the product of an innovative collaboration between integrated assessment modelers, climate modelers, terrestrial ecosystem modelers and emission inventory experts. The resulting product forms a comprehensive data set with high spatial and sectoral resolutions for the period extending to 2100. Land use and emissions of air pollutants and greenhouse gases are reported mostly at a 0.5 x 0.5 degree spatial resolution, with air pollutants also provided per sector (for well-mixed gases, a coarser resolution is used). The underlying integrated assessment model outputs for land use, atmospheric emissions and concentration data were harmonized across models and scenarios to ensure consistency with historical observations while preserving individual scenario trends. For most variables, the RCPs cover a wide range of the existing literature. The RCPs are supplemented with extensions (Extended Concentration Pathways, ECPs), which allow climate modeling experiments through the year 2300. The RCPs are an important development in climate research and provide a potential foundation for further research and assessment, including emissions mitigation and impact analysis.
Preliminary studies suggest that the thermal inertia of the upper layers of the oceans, combined with vertical mixing of deeper oceanic waters, could delay the response of the globally averaged surface temperature to an increasing atmospheric COâ concentration by a decade or so relative to equilibrium calculations. This study extends the global analysis of the transient response to zonal averages, using a hierarchy of simple energy balance models and vertical mixing assumptions for water exchange between upper and deeper oceanic layers. It is found that because of the latitudinal dependence of both thermal inertia and radiative and dynamic energy exchange mechanisms, the approach toward equilibrium of the surface temperature of various regions of the earth will be significantly different from the global average approach. This suggests that the actual time evolution of the horizontal surface temperature gradients--and any associated regional climatic anomalies-may well be significantly different from that suggested by equilibrium climatic modeling simulations (or those computed with a highly unrealistic geographic distribution of ocean thermal capacity). Also, the transient response as a function of latitude is significantly different between globally equivalent COâ and solar constant focusing runs. It is suggested that the nature of the transient response is a major uncertainty in characterizing the COâ problem and that study of this topic should become a major priority for future research. An appendix puts this issue in the context of the overall COâ problem.
Global carbon dioxide emissions from fossil-fuel combustion and cement production grew 5.9% in 2010, surpassed 9 Pg of carbon (Pg C) for the first time, and more than offset the 1.4% decrease in 2009. The impact of the 2008 2009 global financial crisis (GFC) on emissions has been short-lived owing to strong emissions growth in emerging economies, a return to emissions growth in developed economies, and an increase in the fossil-fuel intensity of the world economy.
This study compares emission pathways aimed at limiting temperature increase to 2°C under varying constraints. In a first set of pathways, the timing of emission reductions is such that over the 2010–2100 period, assuming full participation from 2013 onwards, mitigation costs are minimized. In a second set of pathways, we set emissions in 2020 at a level based on the pledges of the Copenhagen Accord. In the ‘Copenhagen Potential’ scenario, climate talks result in satisfying conditions linked by countries to their ‘most ambitious’ proposals. Contrasting, in the ‘Copenhagen Current’ scenario, climate talks fall short of satisfying the conditions to move beyond current unilateral pledges. We include scenarios with and without the availability of bio-energy in combination with carbon capture and storage. We find that for a ‘Copenhagen Potential’ scenario, emissions by 2020 are higher (47 GtCO 2 eq/yr) than for a least-cost pathway for 2°C (43 GtCO 2 eq/yr with a 40–46 GtCO 2 eq/yr literature range). In the ‘Copenhagen Potential’ scenario the 2°C target can still be met with a likely chance, although discounted mitigation costs over 2010–2100 could be 10 to 15 % higher, and up to 60 % in the 2040–2050s, than for least-cost pathways. For the ‘Current Copenhagen’ scenario, maintaining an equally low probability of exceeding 2°C becomes infeasible in our model, implying higher costs due to higher climate risks. We conclude that there is some flexibility in terms of 2020 emissions compared to the optimal pathways but this is limited. The 2020 emission level represents a trade-off between short-term emission reductions and long-term dependence on rapid reductions through specific technologies (like negative emission reductions). Higher 2020 emissions lead to higher overall costs and reduced long-term flexibility, both leading to a higher risk of failing to hold warming below 2°C.
Long-term scenarios developed by integrated assessment models are used in climate research to provide an indication of plausible long-term emissions of greenhouse gases and other radiatively active substances based on developments in the global energy system, land-use and the emissions associated with these systems. The phenomena that determine these long-term developments (several decades or even centuries) are very different than those that operate on a shorter time-scales (a few years). Nevertheless, in the literature, we still often find direct comparisons between short-term observations and long-term developments that do not take into account the differing dynamics over these time scales. In this letter, we discuss some of the differences between the factors that operate in the short term and those that operate in the long term. We use long-term historical emissions trends to show that short-term observations are very poor indicators of long-term future emissions developments. Based on this, we conclude that the performance of long-term scenarios should be evaluated against the appropriate, corresponding long-term variables and trends. The research community may facilitate this by developing appropriate data sets and protocols that can be used to test the performance of long-term scenarios and the models that produce them.