The amount of additional future temperature change following a complete
cessation of CO2 emissions is a measure of the unrealized warming to
which we are committed due to CO2 already emitted to the atmosphere.
This “zero emissions commitment” (ZEC) is also an important quantity when
estimating the remaining carbon budget – a limit on the total amount of
CO2 emissions consistent with limiting global mean temperature at a
particular level. In the recent IPCC Special Report on Global Warming of
1.5 ∘C, the carbon budget framework used to calculate the
remaining carbon budget for 1.5 ∘C included the assumption that
the ZEC due to CO2 emissions is negligible and close to zero. Previous
research has shown significant uncertainty even in the sign of the ZEC. To
close this knowledge gap, we propose the Zero Emissions Commitment Model
Intercomparison Project (ZECMIP), which will quantify the amount of
unrealized temperature change that occurs after CO2 emissions cease and
investigate the geophysical drivers behind this climate response.
Quantitative information on ZEC is a key gap in our knowledge, and one that
will not be addressed by currently planned CMIP6 simulations, yet it is
crucial for verifying whether carbon budgets need to be adjusted to account
for any unrealized temperature change resulting from past CO2
emissions. We request only one top-priority simulation from comprehensive
general circulation Earth system models (ESMs) and Earth system models of
intermediate complexity (EMICs) – a branch from the 1 % CO2 run with
CO2 emissions set to zero at the point of 1000 PgC of total CO2
emissions in the simulation – with the possibility for additional
simulations, if resources allow. ZECMIP is part of CMIP6, under joint
sponsorship by C4MIP and CDRMIP, with associated experiment names to enable
data submissions to the Earth System Grid Federation. All data will be published
and made freely available.
Introduction
The zero emissions commitment (ZEC), or the amount of global mean
temperature change that is still expected to occur after a complete
cessation of CO2 emissions, is a key component of estimating the
remaining carbon budget to stay within global warming targets as well as an
important metric to understand impacts and reversibility of climate change
(Matthews and Solomon, 2013). Much effort is put into measuring
and constraining the TCRE – the Transient Climate Response to cumulative
CO2 Emissions (Allen
et al., 2009; Matthews et al., 2009; Zickfeld et al., 2009; Raupach et al.,
2011; Gillett et al., 2013; Tachiiri et al., 2015; Goodwin et al., 2015;
Steinacher and Joos, 2016; MacDougall, 2016; Ehlert et al., 2017; Millar and
Friedlingstein, 2018). The TCRE describes the ratio between CO2-induced
warming and cumulative CO2 emissions up to the same point in time, but
it does not capture any delayed warming response to CO2 emissions
beyond the point that emissions reach zero. When using the TCRE to derive
the carbon budget consistent with a specific temperature limit, the ZEC is
often assumed to be negligible and close to zero (Matthews
et al., 2017; Rogelj et al., 2011, 2018). Constraints on ZEC have not been
systematically researched so far, although both TCRE and ZEC are required to
relate carbon emissions to the eventual equilibrium warming (Rogelj et al., 2018).
It has been shown that continued CO2 removal by natural sinks following
cessation of emissions offsets the continued warming that would result from
stabilized CO2 concentration (Matthews
and Caldeira, 2008; Solomon et al., 2009; Frölicher and Joos, 2010;
Matthews and Weaver, 2010; Joos et al., 2013). This is partly due to the
ocean uptake of both heat and carbon sharing some similar processes and
timescales, and it is therefore expected to lead to ZEC being small (Allen
et al., 2018; Ehlert and Zickfeld, 2017; Gillett et al., 2011; Matthews and
Zickfeld, 2012). This has been shown to be a general result across a range
of models (Gillett
et al., 2011; Lowe et al., 2009; Matthews and Zickfeld, 2012; Zickfeld et
al., 2013). Most such literature focused on long timescales (up to and
beyond a century). This led IPCC SR15 (Rogelj et al., 2018)
to make the assumption for the estimation of carbon budgets that for
timescales up to a century ZEC was uncertain, yet centred around zero. More
detailed studies, however, have shown that ZEC can be (a) non-zero, possibly
of either positive or negative sign that may change in time during the
period following emissions ceasing (Frölicher et al., 2014;
Frölicher and Paynter, 2015), and (b) it is both state and rate
dependent – i.e. it varies depending on the amount of carbon emitted and
taken up by the natural carbon sinks, and the CO2 emissions pathway of
its emissions prior to cessation (Ehlert
and Zickfeld, 2017; Krasting et al., 2014; MacDougall, 2019).
When we consider stringent climate targets, such as limiting global mean
warming to 1.5 or 2 ∘C, and in light of approximately 1 ∘C warming to date and potential future warming from non-CO2 greenhouse
gases, an uncertainty in ZEC of 0±0.1∘C already leads to a
substantial uncertainty in the remaining carbon budget. Given the current
central estimate of the TCRE of 1.6 ∘C per 1000 PgC
(Collins et al., 2013), each 0.1 ∘C of warming equates
to approximately 60 PgC of CO2 emissions, or approximately 6 years of
current fossil fuel emission rates (Le Quéré et
al., 2018). It has therefore emerged that quantitative information on ZEC is
a key gap in our knowledge, and one that is not filled by currently planned
simulations for the sixth phase of the Coupled Model Intercomparison Project (CMIP6).
ZECMIP aims to fill this gap as efficiently as possible. Thereby, ZECMIP
will support the assessment of remaining carbon budgets based on the CMIP6
simulations and supersede the current practice of applying a single model
estimate of ZEC or an estimate from a limited number of studies from the
literature. Much more preferable is to coordinate parallel studies, with
Earth system general circulation models (ESMs) and Earth system models of
intermediate complexity (EMICs), to measure both TCRE and ZEC in a common
scenario. Hence, we proposed using the 1 % per annum increase in CO2
concentration experiment (1pctCO2) from the CMIP6 Diagnostic Evaluation and
Characterization of Klima (DECK) simulations (Eyring et al., 2016) as a
common baseline simulation for estimating both the TCRE and the ZEC.
ZECMIP simulations and priorities for ESMs and EMICs.
ZECMIPCMIP6ESM priorityEMIC priorityexperimentexperiment IDDescription(at least 100 years)(1000 years)A0esm-1pctCO2An emissions-driven simulation (fully interactive CO2), initiated from the esm-piControl using CO2 emissions diagnosed from the 1pctCO2 experiment so that the emissions-driven run replicates as closely as possible the 1pctCO2 concentration profile. It may be required to create start conditions for A1–3 (see Sect. 2.1) and not required if model can use DECK 1pctCO2.If requiredIf requiredA1esm-1pct-brch-1000PgCA zero-emissions simulation (fully interactive CO2), branched from the point in the 1pctCO2 experiment (or A0 above) when the cumulative carbon emissions reach 1000 PgC.11A2esm-1pct-brch-750PgCA zero-emissions simulation (fully interactive CO2), branched from the point in the 1pctCO2 experiment (or A0 above) when the cumulative carbon emissions reach 750 PgC.21A3esm-1pct-brch-2000PgCA zero-emissions simulation (fully interactive CO2), branched from the point in the 1pctCO2 experiment (or A0 above) when the cumulative carbon emissions reach 2000 PgC.2B1esm-bell-1000PgCAn emissions-driven simulation (fully interactive CO2), initiated from esm-piControl using CO2 emissions, amounting to 1000 PgC, following a bell-shaped curve for 100 years followed by zero emissions for at least 100 years.1B2esm-bell-750PgCAn emissions-driven simulation (fully interactive CO2), initiated from esm-piControl using CO2 emissions, amounting to 750 PgC, following a bell-shaped curve for 100 years followed by zero emissions for at least 100 years.2B3esm-bell-2000PgCAn emissions-driven simulation (fully interactive CO2), initiated from esm-piControl using CO2 emissions, amounting to 2000 PgC, following a bell-shaped curve for 100 years followed by zero emissions for at least 100 years.2
As a late addition to CMIP6, ZECMIP has been designed to address this
important question with only one high-priority simulation
– A1: “a zero-emission experiment following 1000 PgC emissions” – implemented as a branching off from the 1pctCO2 simulation from the point at which
1000 PgC in diagnosed cumulative emissions is reached. Additional
simulations of lower priority are also suggested, which will aid further
analysis. Branching from this idealized simulation avoids complications of
non-CO2 forcing and land-use or nitrogen deposition impacts on the
carbon cycle, and also makes the quantified ZEC consistent with the TCRE
values also derived from this simulation.
This paper documents the ZECMIP simulations with a focus on the details
needed for ESMs and EMICs to contribute the top-priority simulation of a ZEC
run from the point of 1000 PgC emissions following 1 % per year growth in CO2.
ZECMIP analysis will draw on carbon cycle feedbacks and process
understanding from C4MIP (Coupled Climate Carbon Cycle
Model Intercomparison Project; Jones et al., 2016) and aims to complement
analysis on reversibility and CO2 removal under CDRMIP (Carbon Dioxide Removal Model
Intercomparison Project; Keller et al., 2018). Both C4MIP and CDRMIP
encourage participation in the ZECMIP top-priority simulation. For
simplicity, the data request is a replica of that for the CMIP6
emission-driven historical simulation (esm-hist). No new variables have been
added. For EMICs the request is to output the same model variables as from
the 1 % run, which forms the basis of ZECMIP, with the one addition of also
providing atmospheric CO2 concentration. Data can be published via the
Earth System Grid Federation (ESGF) (for ESMs contributing to CMIP6). An
equivalent data repository will be available for EMICs and likely based at
the University of Victoria – details will be communicated during summer 2019
via C4MIP and CDRMIP websites.
Simulation protocol
Due to time pressures and a limit to computational resources for modelling
groups, ZECMIP has just one high-priority simulation, with a second lower-priority
simulation suggested (See Table 1). Other lower-priority simulations
are also detailed and welcomed. For EMIC model groups, there is an extended
protocol with longer and additional experiments. We welcome ESM groups to
also perform these additional simulations, but this is not required. Given
that the overall CMIP6 protocol (Eyring et al., 2016) has been
years in development, it is not possible to initiate a new MIP nor allocate
new CMIP tier-1 simulations during 2019. Instead, ZECMIP simulations are
being included under C4MIP and CDRMIP and included in CMIP as tier-2 and
tier-3 simulations so that they do not become mandatory “entry card”
requirements for C4MIP or CDRMIP. Hence, our top-priority simulation, A1, is
classed as a CMIP tier-2 simulation; all others are classified as tier-3
simulations. However, Table 1 lists the simulations prioritized by ZECMIP to
guide groups who have limited resources to perform the simulations. We hope
as many groups as possible perform as many of the simulations as possible,
and participating model groups will be offered co-authorship on the
article containing the analysis to be submitted this year (by December 2019).
Simulation set A: abrupt zero emissions
All ZECMIP simulations are required to be in “emissions-driven mode”.
Experiments under set A require branching off from a simulation where
CO2 concentration follows a 1 % per annum increase from
pre-industrial levels. This presents model groups with a choice of how to
initialize experiments A1 to A3. Some models may have the capability to
switch from concentration-driven to emissions-driven configurations but some
models may not or model groups may not have confidence that they can do so
without a shock to the model system. In the case of the former, the
concentration-driven DECK 1pctCO2 simulation can be used to initiate
experiments A1 to A3. Otherwise, models should perform simulation A0 to
generate initial conditions for A1 to A3.
We do not specify a precise definition of how to make this choice but
suggest that when an emissions-driven control run is initiated from a
concentration-driven control run, any subsequent change in atmospheric
CO2, major carbon stores, or global temperature should all be
approximately within the expected interannual variability of the control
run. We note that if simulation A0 is required to initialize the A1
simulation, then it should be treated as equal priority to A1 and data
submission to the ESGF is required.
A0: “esm-1pctCO2”. Run an emissions-driven version of 1pctCO2 to
get to the branch-off point for A1 to A3. The requirement to run this is a
model-by-model decision. The compatible emissions time series for this
simulation should be calculated from the 1pctCO2 and used to branch
esm-1pctCO2 from esm-piControl to replicate the 1 % profile as closely as
possible up to the desired cumulative emission before setting emissions to
zero from this point.
The compatible emission rate E (PgC yr-1) can be calculated from the
1pctCO2 concentration-driven simulation, as described in Jones et al. (2013; see their Sect. 2b). In summary,
changes in atmospheric CO2 concentration (CA) are balanced by
anthropogenic emissions, E, and changes in the natural land and ocean carbon
reservoirs (CL and CO, respectively). Therefore, the compatible
emissions can be calculated simply as
E=ddtCTot=ddtCA+ddt(CL+CO),
where units of all quantities are in petagrams of carbon (PgC). Changes in atmospheric CO2
can be converted from concentration (ppm) to mass (PgC) by a simple scaling
of 2.12. Typically, the time derivative, d/dt, is taken to imply changes per year
– i.e. annual changes in the carbon stores are used in order to calculate
annual emission, E. The calculation is done using global total amounts.
Emissions should be prescribed as globally uniform at the surface. Models
that have run multiple ensemble members for the concentration-driven 1pctCO2
experiment should use ensemble-mean values of CL and CO from those
runs to derive the emissions for forcing the esm-1pctCO2 simulation. This
will minimize the effect of interannual variability of carbon sinks on the
diagnosed compatible emissions. If desired, numerical smoothing of the
global mean time series of emissions may also be applied as long as the
cumulative total is not affected.
Example results from simulation A1 from the UVic ESCM (Weaver et al.,
2001; MacDougall and Knutti, 2016; blue) and GFDL-ESM2M (Dunne et al., 2012, 2013; red)
models. (a)CO2 concentration prescribed (black line) in the 1pctCO2
simulation and simulated (red, blue lines) by the two models; (b) simulated
global mean surface air temperature for the same period; (c) global mean
temperature response from the branch point off the 1 % simulation with
zero subsequent emissions.
ZECMIP simulation set A is based on CO2-only 1 % run (either
concentration-driven DECK “1pctCO2” or the above described A.0
“esm-1pctCO2”), with all the other external forcing held at pre-industrial
conditions (i.e. non-CO2 greenhouse gases, aerosols, volcanoes,
land-use changes, solar irradiance). After following the CO2
concentration up to the level described below, branch off with prognostic
CO2 (a.k.a. “emissions driven”) but with carbon emissions set to zero
(E=0). Simulate the subsequent reduction in atmospheric CO2 and
change in climate for at least 100 years.
Branch off at the following given cumulative emissions.
A1: “esm-1pct-brch-1000PgC”, 1000 PgC. This is the ZECMIP top-priority
simulation. This corresponds to approximately 2 ∘C CO2-induced
warming above pre-industrial levels (with the year 1850 here taken as proxy for
pre-industrial levels). Figure 1 shows example results from two models.
A2: “esm-1pct-brch-750PgC”, 750 PgC. This is a simulation
corresponding to approximately 1.5 ∘C CO2-induced warming above
1850 and is optional.
A3: “esm-1pct-brch-2000PgC”, 2000 PgC.
This simulation will give insights into ZEC for a possible higher
CO2-induced warming and is optional.
The experimental design is for all models to branch off at a common
cumulative carbon emission level, acknowledging that this will mean a
different year for ceasing emissions and thus a slightly different
atmospheric CO2 concentration and departure of global mean temperature
from 1850 for each model at the beginning of the ZECMIP simulations. EMICs
should run the simulations for at least 1000 years. We anticipate that the
small signal-to-noise ratio of the ZEC versus the internal climate
variability may require an ensemble of simulations. However, acknowledging ESM
time pressure and limits to computational resources, only one ensemble member
is required.
Time series of global CO2 emissions for bell-shaped curve pathways
B1 to B3. The numbers in the legend indicate the cumulative amount of
CO2 emissions for each simulation.
Experiment A1 aims to quantify ZEC at 1000 PgC (cumulative emissions) at
which point TCRE will be calculated. A2 and A3 explore the
state dependence of ZEC at approximately 1.5 ∘C
CO2-induced warming above 1850 and at significantly higher cumulative
emissions, respectively.
Example results from simulation B1 from the UVic ESCM (Weaver et
al., 2001; MacDougall and Knutti, 2016; blue) and GFDL-ESM2M (Dunne et al.,
2012, 2013; red) models. (a)CO2 concentration simulated by the two
models; (b) simulated global mean surface air temperature for the same
period; (c) global mean temperature response from year 100 onwards with zero
subsequent emissions.
Simulation set B: bell-shaped zero emissions
This second set of experiments, B1 to B3, aims to explore the dependence of
ZEC on CO2 emissions rate by following a pathway
emitting the same cumulative emissions as A1 to A3 but with a smooth
transition to zero emissions, followed by 100 years of E=0 (EMICs for at
least 1000 years). The main purpose of this experiment is to quantify the
dependency of ZEC on emission pathways and the emission rate prior to the
point when TCRE is evaluated as the Earth system is subject to
comparatively low emissions, occurring just before the TCRE evaluation point
of zero emissions after 100 years of simulation – compared to the sudden
cessation of high emissions in experiments A1, A2, and A3.
The conventional way of estimating TCRE is using 1 % CO2 model
simulations. The tier-1 A1 simulation thus provides the most complementary
and internally consistent quantification of the ZEC, which is why we consider
this to be the top priority. However, additional ZECMIP experiments with
more gradually phased out emissions enable us to determine how the ZEC is
expected to materialize over the timescales of more societally relevant CO2
emissions reduction rates. Analysis of pairs of A and B experiments
will allow us to generalize the findings for other emission reduction
pathways, allowing us to answer the question of whether temperature will continue to
increase following a more realistic cessation of CO2 emissions.
These B experiments are run in emissions-driven configuration
(CO2-only: following 1pctCO2 and piControl, all other external forcing
is fixed at pre-industrial levels), assuming a bell-shaped emissions profile
(Fig. 2), for which we have chosen an arbitrary
Gaussian distribution (see Appendix A). At the end of 100 years emissions
profile, simulations should continue with zero emissions for at least 100 years (for ESMs) or 1000 years (EMICs).
The bell-shaped curve is designed to give the following cumulative emissions.
B1: “esm-bell-1000PgC”, 1000 PgC (Fig. 3 shows example results from two models);
B2: “esm-bell-750PgC”, 750 PgC;
B3: “esm-bell-2000PgC”, 2000 PgC.
By design, this set B utilizes the same cumulative emissions
as the respective simulations in set A experience up to their branch
point. These emissions are applied over 100 years, followed by zero
emissions for 100 years (ESMs) or 1000 years (EMICs). These additional
simulations allow for a direct comparison of the two ZEC experiment sets,
given the same amount of cumulative emissions. A model decision is required
on the spatial pattern of emissions – we suggest globally uniform at
surface. The time series of global CO2 emissions for the above curves is
listed in Appendix A and is hosted on the C4MIP (http://www.c4mip.net/index.php?id=3387, last access: 6 September 2019)
and CDRMIP (https://www.kiel-earth-institute.de/CDR_Model_Intercomparison_Project.html, last access: 6 September 2019) websites.
ZECMIP outlook and conclusions
The experiments outlined above will lay the foundation for coordinated
multi-model analysis of the zero emissions commitment. The absence of a
dedicated experiment to quantify ZEC across CMIP models was identified and
is addressed by our top-priority experiment, A1. Investigations into the
state, rate, and pathway dependence of the ZEC are aided by further
experiments with sudden and gradual cessation of emissions. ZECMIP was
motivated to keep the experiment design both lightweight and simple to
follow; in future, further simulations could be defined to explore
additional issues such as cessation of emissions of non-CO2 greenhouse
gases, aerosols, or from land-use activities. The complexity of defining
such experiments precluded an exhaustive inclusion in this first generation
of ZECMIP but we acknowledge the importance of rate and pathway dependency,
as well non-CO2 aspects in determining ZEC and the remaining carbon
budget overall (MacDougall
et al., 2015; Rogelj et al., 2015; Mengis et al., 2018; Tokarska et al., 2018).
The requirement for specific information regarding ZEC to assess remaining
carbon budgets was identified in the IPCC Special Report on Global Warming
of 1.5 ∘C (Rogelj et al.,
2018). An initial paper exploring ZEC in this context, explicitly on
timescales of relevance to 21st century carbon budgets, is planned on a
timeline that could support an improved assessment of the ZEC and its
influence on carbon budgets in the IPCC Sixth Assessment Report. All participating
model groups who are able to complete and provide data for simulation A1 in
time will be invited to join this analysis.
ZECMIP welcomes community engagement in the participation of simulations and
their analysis, as well as input to future analysis and experimental design. We
hope to bring together ESMs and EMICs to enable analysis across timescales
from decadal through centennial to millennial.
Furthermore, as a set of numerical simulations, ZECMIP is intended to
complement existing CMIP activity, especially on carbon cycle feedbacks,
CO2 removal, and reversibility of the climate system. C4MIP simulations
aim to address model evaluation during the historical period from 1850 to
present day, along with process-level feedback analysis. CDRMIP adds to this
with exploration of the processes controlling the response of the climate
and carbon cycle to negative emissions and reversibility of components of
the Earth system. ZECMIP will contribute additional simulations and analysis
to aid understanding of the mechanisms of the climate response to CO2
emissions and relationships between transient and equilibrium climate
sensitivities. We hope that ZECMIP analysis will address the crucial
knowledge gap surrounding committed warming following ceasing emissions and
will provide valuable support for assessment of carbon budgets to achieve climate
targets.
Data availability
As with all CMIP6-endorsed MIPs, the model output from the ZECMIP
simulations described in this paper will be distributed through the Earth
System Grid Federation (ESGF) with version control and digital object
identifiers (DOIs) assigned. No additional model forcings are required
beyond those already used for piControl and 1pctCO2 simulations apart from
the emission inputs for the proposed B experiments, which are described in
Appendix A of this paper and are hosted on the C4MIP and CDRMIP websites.
CO2 emissions for bell-shaped curve simulations B1–3
This table lists the global CO2 emissions (PgC yr-1) to be
applied for the first 100 years of simulations B1–3. This period should be
followed by at least 100 years of zero emissions for ESMs or 1000 years for
EMICs (see Fig. 2). These emissions should be
prescribed as globally uniform at the surface.
The data were calculated from a Gaussian curve according to
E=k12πσ2e-x-μ22σ2,
where emissions, E, are scaled by a constant, k, so that the
cumulative total matches the required amount for each scenario (1000 PgC for
B1, 750 PgC for B2, 2000 PgC for B3). The parameters were set as μ=50
as the centre of the 100-year period and σ=100/6 so that the
distribution spans 3 standard deviations about the centre.
These data in .csv file format are available from the C4MIP (http://www.c4mip.net/index.php?id=3387, last access: 6 September 2019) and CDRMIP (https://www.kiel-earth-institute.de/CDR_Model_Intercomparison_Project.html, last access: 6 September 2019) websites.
Global CO2 emissions (PgC yr-1) to be applied during each year for the first 100 years of simulations B1–3.
CDJ, TLF, CK, AHM, HDM, KZ, JR, KBT, NPG, TI, MM, NM, and RS participated in workshop discussions to identify research needs towards better understanding of remaining carbon budgets. ZECMIP was the direct outcome of this workshop and the participants were all active in breakout discussions to design the experimental protocol described here. ME was instrumental in providing support and data storage for EMIC simulations and provided valuable guidance around the data request detailed in the article. FAB performed simulations with GFDL-ESM2M to specifically test the experimental design and provide data for Figs. 1 and 3. All authors contributed to the development of the article, provided text, and responded to review comments and revisions.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This protocol was devised at a Global Carbon Project workshop supported by
H2020 EU project CRESCENDO under grant agreement no. 641816. Chris D. Jones was
supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate
Programme (GA01101). Joeri Rogelj, Katarzyna Tokarska and Roland Séférian were supported by H2020 EU project CONSTRAIN
under grant agreement no. 820829. Tatiana Ilyina and Thomas L. Frölicher were supported by H2020 EU
project CCICC under grant agreement no. 821003. Thomas L. Frölicher acknowledges support from
the Swiss National Science Foundation under grant PP00P2_170687. GFDL-ESM2M simulations were performed at the Swiss National
Supercomputing Centre (CSCS). Kirsten Zickfeld and Andrew H. MacDougall acknowledge support from the
National Sciences and Engineering Research Council of Canada's Discovery
Grants Program. Charles Koven acknowledges support from the US DOE BER Regional &
Global Model Analysis programme through the Early Career Research Program and
the RUBISCO SFA projects. Katarzyna B. Tokarska was also supported by the UK NERC-funded SMURPHs
project (NE/N006143/1).
Financial support
This research has been supported by the European Commission (grant no. CRESCENDO (641816)).
Review statement
This paper was edited by Carlos Sierra and reviewed by two anonymous referees.
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