[Show abstract][Hide abstract] ABSTRACT: Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth’s orbital configuration, CO2 , additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly under- estimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a sin- gle specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO2 between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of tempera- ture and CO2, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land- use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeo- climate reconstructions.
Climate of the Past 05/2013; 9:1111-1140. · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper summarizes the results of an intercomparison project with Earth System Models of Intermediate Complexity (EMICs) undertaken in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). The focus is on long-term climate projections designed to: (i) quantify the climate change commitment of different radiative forcing trajectories, and (ii) explore the extent to which climate change is reversible on human timescales. All commitment simulations follow the four Representative Concentration Pathways (RCPs) and their extensions to 2300. Most EMICs simulate substantial surface air temperature and thermosteric sea level rise commitment following stabilization of the atmospheric composition at year-2300 levels. The meridional overturning circulation (MOC) is weakened temporarily and recovers to near pre-industrial values in most models for RCPs 2.6–6.0. The MOC weakening is more persistent for RCP 8.5. Elimination of anthropogenic CO2 emissions after 2300 results in slowly decreasing atmospheric CO2 concentrations. At year 3000 atmospheric CO2 is still at more than half its year-2300 level in all EMICs for RCPs 4.5–8.5. Surface air temperature remains constant or decreases slightly and thermosteric sea level rise continues for centuries after elimination of CO2 emissions in all EMICs. Restoration of atmospheric CO2 from RCP to pre-industrial levels over 100–1000 years requires large artificial removal of CO2 from the atmosphere and does not result in the simultaneous return to pre-industrial climate conditions, as surface air temperature and sea level response exhibit a substantial time lag relative to atmospheric CO2.
Journal of Climate 05/2013; 26(16):5782–5809. · 4.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The evolution of the Atlantic Meridional Overturning Circulation (MOC)
in 30 models of varying complexity is examined under four distinct
Representative Concentration Pathways. The models include 25
Atmosphere-Ocean General Circulation Models (AOGCMs) or Earth System
Models (ESMs) that submitted simulations in support of the 5th phase of
the Coupled Model Intercomparison Project (CMIP5) and 5 Earth System
Models of Intermediate Complexity (EMICs). While none of the models
incorporated the additional effects of ice sheet melting, they all
projected very similar behaviour during the 21st century. Over this
period the strength of MOC reduced by a best estimate of 22%
(18%-25% 5%-95% confidence limits) for RCP2.6, 26%
(23%-30%) for RCP4.5, 29% (23%-35%) for RCP6.0 and 40%
(36%-44%) for RCP8.5. Two of the models eventually realized a slow
shutdown of the MOC under RCP8.5, although no model exhibited an abrupt
change of the MOC. Through analysis of the freshwater flux across
30°-32°S into the Atlantic, it was found that 40% of the
CMIP5 models were in a bistable regime of the MOC for the duration of
their RCP integrations. The results support previous assessments that it
is very unlikely that the MOC will undergo an abrupt change to an off
state as a consequence of global warming.
Geophysical Research Letters 10/2012; 39(L20709). · 4.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Both historical and idealized climate model experiments are performed with a variety of Earth System Models of Intermediate Complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO2, additional greenhouse gases, land-use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes seem to be underestimated. It is possible that recent modelled climate trends or climate-carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated.
Several one thousand year long, idealized, 2x and 4x CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate-carbon feedbacks. The values from EMICs generally fall within the range given by General Circulation Models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows considerable synergy between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO2 between the Medieval Climate Anomaly and the Little Ice Age estimated from paleoclimate reconstructions. This in turn could be a result of errors in the reconstructions of volcanic and/or solar radiative forcing used to drive the models or the incomplete representation of certain processes or variability within the models. Given the datasets used in this study, the models calculate significant land-use emissions over the pre-industrial. This implies that land-use emissions might need to be taken into account, when making estimates of climate-carbon feedbacks from paleoclimate reconstructions.
Climate of the Past Discussions 08/2012; 8:4121-4181.
[Show abstract][Hide abstract] ABSTRACT: The climate response to scenarios of zero future greenhouse-gas
emissions can be interpreted as the committed future warming associated
with past emissions, and represents a critical benchmark against which
to estimate the effect of future emissions. Recent climate-model
simulations have shown that when emissions of carbon dioxide alone are
eliminated, global temperature stabilizes and remains approximately
constant for several centuries. Here, we show that when aerosol and
other greenhouse-gas emissions are also eliminated, global temperature
increases by a few tenths of a degree over about a decade, as a result
of the rapid removal of present-day aerosol forcing. This initial
warming is followed by a gradual cooling that returns global temperature
to present-day levels after several centuries, owing to the decline in
non-carbon dioxide greenhouse-gas concentrations. We show further that
the magnitude of the peak temperature response to zero future emissions
depends strongly on the uncertain strength of present-day aerosol
forcing. Contingent on the climate and carbon-cycle sensitivities of the
model used here, we show that the range of aerosol forcing that produces
historical warming that is consistent with observed data, results in a
warming of between 0.25 and 0.5°C over the decade immediately
following zeroed emissions.
[Show abstract][Hide abstract] ABSTRACT: Recent studies with coupled climate-carbon cycle models suggest that
global mean temperature change is proportional to cumulative
CO2 emissions, independent of the timing of those emissions.
This finding has prompted the suggestion that climate stabilization
targets, such as the 2°C target adopted by the Copenhagen Accord,
can be expressed in terms of cumulative CO2 emissions. Here
we examine the simulated response of a range of global and regional
climate variables to the same cumulative CO2 emissions (2500
PgC) released along different pathways using a complex Earth system
model. We find that the response of most surface climate variables is
largely independent of the emissions pathway once emissions cease, with
the exception of variables with response timescales of centuries, such
as ocean heat content and thermosteric sea level rise. Peak responses of
many climate variables, such as global mean temperature, precipitation
and sea ice, are also largely independent of the emissions pathway,
except for scenarios with cumulative emissions overshoot which require
net removal of CO2 from the atmosphere. By contrast, peak
responses of atmospheric CO2 and surface ocean pH are found
to be dependent on the emissions pathway. We conclude that a
CO2 mitigation framework based on cumulative emissions is
well suited for limiting changes in many impact-relevant climate
variables, but is less effective in avoiding impacts directly associated
with atmospheric CO2, whose peak response is dependent on the
rate of emissions.
Geophysical Research Letters 03/2012; 39(5):5703-. · 4.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Coupled climate-carbon models have shown the potential for large feedbacks between climate change, atmospheric CO(2) concentrations, and global carbon sinks. Standard metrics of this feedback assume that the response of land and ocean carbon uptake to CO(2) (concentration-carbon cycle feedback) and climate change (climate-carbon cycle feedback) combine linearly. This study explores the linearity in the carbon cycle response by analyzing simulations with an earth system model of intermediate complexity [the University of Victoria Earth System Climate Model (UVic ESCM)]. The results indicate that the concentration-carbon and climate-carbon cycle feedbacks do not combine linearly to the overall carbon cycle feedback. In this model, the carbon sinks on land and in the ocean are less efficient when exposed to the combined effect of elevated CO(2) and climate change than to the linear combination of the two. The land accounts for about 80% of the nonlinearity, with the ocean accounting for the remaining 20%. On land, this nonlinearity is associated with the different response of vegetation and soil carbon uptake to climate in the presence or absence of the CO(2) fertilization effect. In the ocean, the nonlinear response is caused by the interaction of changes in physical properties and anthropogenic CO(2). These findings suggest that metrics of carbon cycle feedback that postulate linearity in the system's response may not be adequate.
Journal of Climate 08/2011; 24(16):4255-4275. · 4.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A threat of irreversible damage should prompt action to mitigate climate change, according to the United Nations Framework Convention on Climate Change, which serves as a basis for international climate policy. CO2-induced climate change is known to be largely irreversible on timescales of many centuries, as simulated global mean temperature remains approximately constant for such periods following a complete cessation of carbon dioxide emissions while thermosteric sea level continues to rise. Here we use simulations with the Canadian Earth System Model to show that ongoing regional changes in temperature and precipitation are significant, following a complete cessation of carbon dioxide emissions in 2100, despite almost constant global mean temperatures. Moreover, our projections show warming at intermediate depths in the Southern Ocean that is many times larger by the year 3000 than that realized in 2100. We suggest that a warming of the intermediate-depth ocean around Antarctica at the scale simulated for the year 3000 could lead to the collapse of the West Antarctic Ice Sheet, which would be associated with a rise in sea level of several metres.
[Show abstract][Hide abstract] ABSTRACT: There is uncertainty about the response of the climate system to future trajectories of radiative forcing. To quantify this uncertainty we conducted face-to-face interviews with 14 leading climate scientists, using formal methods of expert elicitation. We structured the interviews around three scenarios of radiative forcing stabilizing at different levels. All experts ranked "cloud radiative feedbacks" as contributing most to their uncertainty about future global mean temperature change, irrespective of the specified level of radiative forcing. The experts disagreed about the relative contribution of other physical processes to their uncertainty about future temperature change. For a forcing trajectory that stabilized at 7 Wm(-2) in 2200, 13 of the 14 experts judged the probability that the climate system would undergo, or be irrevocably committed to, a "basic state change" as > or =0.5. The width and median values of the probability distributions elicited from the different experts for future global mean temperature change under the specified forcing trajectories vary considerably. Even for a moderate increase in forcing by the year 2050, the medians of the elicited distributions of temperature change relative to 2000 range from 0.8-1.8 degrees C, and some of the interquartile ranges do not overlap. Ten of the 14 experts estimated that the probability that equilibrium climate sensitivity exceeds 4.5 degrees C is > 0.17, our interpretation of the upper limit of the "likely" range given by the Intergovernmental Panel on Climate Change. Finally, most experts anticipated that over the next 20 years research will be able to achieve only modest reductions in their degree of uncertainty.
Proceedings of the National Academy of Sciences 07/2010; 107(28):12451-6. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Avoiding "dangerous anthropogenic interference with the climate system" requires stabilization of atmospheric greenhouse gas concentrations and substantial reductions in anthropogenic emissions. Here, we present an inverse approach to coupled climate-carbon cycle modeling, which allows us to estimate the probability that any given level of carbon dioxide (CO2) emissions will exceed specified long-term global mean temperature targets for "dangerous anthropogenic interference," taking into consideration uncertainties in climate sensitivity and the carbon cycle response to climate change. We show that to stabilize global mean temperature increase at 2 degrees C above preindustrial levels with a probability of at least 0.66, cumulative CO2 emissions from 2000 to 2500 must not exceed a median estimate of 590 petagrams of carbon (PgC) (range, 200 to 950 PgC). If the 2 degrees C temperature stabilization target is to be met with a probability of at least 0.9, median total allowable CO2 emissions are 170 PgC (range, -220 to 700 PgC). Furthermore, these estimates of cumulative CO2 emissions, compatible with a specified temperature stabilization target, are independent of the path taken to stabilization. Our analysis therefore supports an international policy framework aimed at avoiding dangerous anthropogenic interference formulated on the basis of total allowable greenhouse gas emissions.
Proceedings of the National Academy of Sciences 09/2009; 106(38):16129-34. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The global temperature response to increasing atmospheric CO(2) is often quantified by metrics such as equilibrium climate sensitivity and transient climate response. These approaches, however, do not account for carbon cycle feedbacks and therefore do not fully represent the net response of the Earth system to anthropogenic CO(2) emissions. Climate-carbon modelling experiments have shown that: (1) the warming per unit CO(2) emitted does not depend on the background CO(2) concentration; (2) the total allowable emissions for climate stabilization do not depend on the timing of those emissions; and (3) the temperature response to a pulse of CO(2) is approximately constant on timescales of decades to centuries. Here we generalize these results and show that the carbon-climate response (CCR), defined as the ratio of temperature change to cumulative carbon emissions, is approximately independent of both the atmospheric CO(2) concentration and its rate of change on these timescales. From observational constraints, we estimate CCR to be in the range 1.0-2.1 degrees C per trillion tonnes of carbon (Tt C) emitted (5th to 95th percentiles), consistent with twenty-first-century CCR values simulated by climate-carbon models. Uncertainty in land-use CO(2) emissions and aerosol forcing, however, means that higher observationally constrained values cannot be excluded. The CCR, when evaluated from climate-carbon models under idealized conditions, represents a simple yet robust metric for comparing models, which aggregates both climate feedbacks and carbon cycle feedbacks. CCR is also likely to be a useful concept for climate change mitigation and policy; by combining the uncertainties associated with climate sensitivity, carbon sinks and climate-carbon feedbacks into a single quantity, the CCR allows CO(2)-induced global mean temperature change to be inferred directly from cumulative carbon emissions.
[Show abstract][Hide abstract] ABSTRACT: Climate sensitivity and transient climate response characterise climate
feedbacks on the response to equilibrium and transient changes in
radiative forcing, but do not relate directly to emissions of carbon
dioxide, and do not account for carbon cycle feedbacks. Previous
experiments with climate-carbon models have shown that the
time-integrated radiative forcing per unit CO2 emission is approximately
independent of the background CO2 concentration, due to reduced
effectiveness of carbon sinks at higher CO2 concentration cancelling the
logarithmic dependence of radiative forcing on CO2 concentration; that
the allowable cumulative emissions for climate stabilisation are
independent of the emissions pathway; and that global mean temperature
remains approximately constant on multi-centennial timescales in
simulations in which CO2 emissions cease completely. Here we generalise
these results to show that Carbon Climate Sensitivity (CCS), defined as
the ratio of temperature change to cumulative emissions, is
approximately independent of both the atmospheric concentration of CO2
and its rate of change, and is well-constrained by observations to be in
the range 1.0 - 2.0 K/EgC, consistent with estimates based on
climate-carbon models of 1.0 - 2.1 K/EgC. CCS therefore aggregates
information about climate feedbacks and carbon cycle feedbacks, and
represents a simple yet robust metric for comparing models. CCS may also
have more general applications in the fields of climate change
mitigation and policy, since it allows CO2-induced global mean
temperature change to be inferred directly from emissions.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we present the integrated assessment model dimrise (dynamic integrated model of regular climate change impacts and singular events). This model is designed to investigate the stability of the Atlantic thermohaline circulation (THC) and to derive related
climate policy recommendations. It is written in GAMS and comprises a dynamic model of the THC coupled to a climate model
and a global economy model for assessing the monetary cost of climate protection. The THC model is a dynamic four-box interhemispheric
extension of the classic Stommel model calibrated against results obtained using the CLIMBER-2 climate model. The reduced-form
climate model used to drive the THC model is the ICLIPS multi-gas climate model, which is a computationally efficient, globally
aggregated model able to mimic the response of more sophisticated carbon cycle and atmosphere-ocean general circulation models.
The THC and climate modules are coupled to a globally aggregated Ramsey-type optimal growth model of the world economy derived
from the Nordhaus DICE model. Together, these components create a novel dynamic fully coupled computationally efficient integrated
assessment model. Illustrative applications demonstrate that dimrise is able to derive (constrained) economically optimal emissions paths that comply with prescribed bounds on admissible THC
weakening imposed in order to avoid an irrevocable breakdown. In addition, emissions corridors are presented which contain
all possible emissions paths that do not endanger the stability of the THC and that simultaneously obey restrictions on welfare
loss arising from mitigation efforts. The presented results show that, under worst-case conditions, the stability of the THC
may be threatened within two decades if global emissions would not deviate from the business-as-usual trajectory.
Mitigation and Adaptation Strategies for Global Change 01/2009; 14(1):61-83. · 1.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper discusses the risks of a shutdown of the thermohaline circulation (THC) for the climate system, for ecosystems in and around the North Atlantic as well as for fisheries and agriculture by way of an Integrated Assessment. The climate model simulations are based on greenhouse gas scenarios for the 21st century and beyond. A shutdown of the THC, complete by 2150, is triggered if increased freshwater input from inland ice melt or enhanced runoff is assumed. The shutdown retards the greenhouse gas-induced atmospheric warming trend in the Northern Hemisphere, but does not lead to a persistent net cooling. Due to the simulated THC shutdown the sea level at the North Atlantic shores rises by up to 80 cm by 2150, in addition to the global sea level rise. This could potentially be a serious impact that requires expensive coastal protection measures. A reduction of marine net primary productivity is associated with the impacts of warming rather than a THC shutdown. Regional shifts in the currents in the Nordic Seas could strongly deteriorate survival chances for cod larvae and juveniles. This could lead to cod fisheries becoming unprofitable by the end of the 21st century. While regional socioeconomic impacts might be large, damages would be probably small in relation to the respective gross national products. Terrestrial ecosystem productivity is affected much more by the fertilization from the increasing CO2 concentration than by a THC shutdown. In addition, the level of warming in the 22nd to 24th century favours crop production in northern Europe a lot, no matter whether the THC shuts down or not. CO2 emissions corridors aimed at limiting the risk of a THC breakdown to 10% or less are narrow, requiring departure from business-as-usual in the next few decades. The uncertainty about THC risks is still high. This is seen in model analyses as well as in the experts’ views that were elicited. The overview of results presented here is the outcome of the Integrated Assessment project INTEGRATION.
[Show abstract][Hide abstract] ABSTRACT: We present emissions corridors for the 21st century reducing the risk of collapse of the Atlantic thermohaline circulation (THC) while considering expectations about
the socio-economically acceptable pace of emissions reductions. Emissions corridors embrace the range of CO2 emissions that are compatible with normatively defined policy goals or ‘guardrails’. They are calculated along the conceptual
and methodological lines of the tolerable windows approach. We investigate the sensitivity of the emissions corridors to key
uncertain physical quantities (i.e. climate sensitivity and North Atlantic hydrological sensitivity, emissions of non-CO2 greenhouse gases and sulfate aerosols) as well as to the guardrails. Results indicate a large dependency of the width of
the emissions corridor on climate and hydrological sensitivity: for low values of the climate and/or hydrological sensitivity,
the corridor boundaries are far from being transgressed by business-as-usual emissions scenarios for the 21st century. In contrast, for high values of both quantities already low non-intervention scenarios leave the corridor in the
early decades of this century. The width of the CO2 emissions corridor is also affected by the emissions pathway of non-CO2 greenhouse gases and sulfate aerosols, but to a lesser extent. We further find that the choice of the policy goal strongly
influences the shape of the emissions corridor. Pursuit of a more ambitious THC target, for instance, tightens the corridor
considerably. More strict expectations concerning the socio-economically admissible pace of emissions reduction (expressed
in terms of a maximum emissions reduction rate and a transition time towards a de-carbonizing economy) act in the same direction.
This indicates that a trade-off between THC and socio-economic guardrails may be unavoidable in the case of very tight emissions
[Show abstract][Hide abstract] ABSTRACT: Preventing "dangerous anthropogenic interference with the climate
system" requires stabilization of atmospheric greenhouse gas
concentrations and substantial reductions in anthropogenic emissions.
Here we present a novel approach to coupled climate-carbon cycle
modelling which allows one to estimate the probability that any given
level of greenhouse gas emissions will exceed specified long-term global
mean temperature targets for "dangerous anthropogenic interference",
taking into consideration uncertainties in climate sensitivity and the
carbon cycle response to climate change. Results obtained within this
framework can serve as a basis for selecting a greenhouse gas emissions
level given a global mean temperature target and an overshoot
probability that society is willing to accept. For instance, we show
that in order to stabilize global mean temperature at 2°C above
pre-industrial levels with a probability of 0.66, cumulative
CO2-equivalent emissions after 2000 must not exceed a best estimate of
about 640 PgC (uncertainty range 280-930 PgC), independently of the path
taken to stabilization.
Proceedings of the National Academy of Sciences 11/2008; -1:07. · 9.81 Impact Factor