The implementation of boundary conditions is a key aspect of climate simulations. We describe here how the Climate Model Intercomparison Project Phase 6 (CMIP6) forcing data sets have been processed ...and implemented in Version 6 of the Institut Pierre‐Simon Laplace (IPSL) climate model (IPSL‐CM6A‐LR) as used for CMIP6. Details peculiar to some of the Model Intercomparison Projects are also described. IPSL‐CM6A‐LR is run without interactive chemistry; thus, tropospheric and stratospheric aerosols as well as ozone have to be prescribed. We improved the aerosol interpolation procedure and highlight a new methodology to adjust the ozone vertical profile in a way that is consistent with the model dynamical state at the time step level. The corresponding instantaneous and effective radiative forcings have been estimated and are being presented where possible.
Plain Language Summary
Climate Model Intercomparison Project Phase 6 is an international project to compare the results from climate model simulations performed according to a common protocol. Such simulations require boundary conditions (called “climate forcings”), which are fed to the models in order to represent, for example, long‐lived greenhouse gases, ozone, atmospheric aerosols, or land surface properties. The same forcing data sets are used by the different modeling groups who carry out the Climate Model Intercomparison Project Phase 6 simulations; however, their implementation may differ as it depends on the model structure. This article gives details of how these forcing data were implemented in the IPSL‐CM6A‐LR model. Some of the forcing data are common to all types all simulations, whereas others depend on the runs considered. Radiative forcings, as estimated in the model, are presented for some of the forcing mechanisms.
Key Points
We present how the CMIP6 forcing data were implemented in the IPSL‐CM6A‐LR climate model for the realization of the CMIP6 set of climate simulations
An improved conservative interpolation procedure for emissions is detailed and illustrated to compute tropospheric aerosols
We present a new methodology to adjust the prescribed ozone vertical profile to match the model atmospheric dynamical state around the tropopause
Aerosols have a dimming and cooling effect and change hydrological regimes, thus affecting carbon fluxes, which are sensitive to climate. Aerosols also scatter sunlight, which increases the fraction ...of diffuse radiation, increasing photosynthesis. There remains no clear conclusion whether the impact of aerosols on land carbon fluxes is larger through diffuse radiation change than through changes in other climate variables. In this study, we quantified the overall physical impacts of anthropogenic aerosols on land C fluxes and explored the contribution from each factor using a set of factorial simulations driven by climate and aerosol data from the IPSL‐CM6A‐LR experiments during 1850–2014. A newly developed land surface model which distinguishes diffuse and direct radiation in canopy radiation transmission, ORCHIDEE_DF, was used. Specifically, a subgrid scheme was developed to distinguish the cloudy and clear sky conditions. We found that anthropogenic aerosol emissions since 1850 cumulatively enhanced the land C sink by 22.6 PgC. Seventy‐eight percent of this C sink enhancement is contributed by aerosol‐induced increase in the diffuse radiation fraction, much larger than the effect of the aerosol‐induced dimming. The cooling of anthropogenic aerosols has different impacts in different latitudes but overall increases the global land C sink. The dominant role of diffuse radiation changes found in this study implies that future aerosol emissions may have a much stronger impacts on the C cycle through changing radiation quality than through changing climate alone. Earth system models need to consider the diffuse radiation fertilization effect to better evaluate the impacts of climate change mitigation scenarios.
Plain Language Summary
The aerosols emitted by human activities can change climate and increase diffuse fraction of sunlight. All these changes can influence the carbon fixation of vegetation on land, further affect the atmospheric CO2 concentration and the climate. Currently, there is still no agreement on whether climate or diffuse light change is more important in affecting land carbon fixation. To solve this problem, we designed a set of experiments and used a newly developed computer code to investigate the impact of anthropogenic aerosols on land carbon sink from each climate factor and diffuse light. We found that since 1850, human‐caused aerosol emissions increased the land carbon sink by about 2 years of present‐day anthropogenic CO2 emissions. Seventy‐eight percent of this large increase in carbon sink is mainly contributed by the increase in diffuse light fraction. The cooling caused by aerosols affected the land carbon sink differently in different latitudes and overall increased the global land carbon sink. The important role of diffuse light found here implies that aerosols emissions may have stronger impacts through changing radiation quality than through changing climate alone in the future, and climate computer codes need to consider diffuse light to better evaluate the impacts of climate change mitigation policies.
Key Points
A set of factorial simulations are set up to investigate the anthropogenic aerosol impacts on land C fluxes during 1850–2014
Anthropogenic aerosols cumulatively enhanced land C sink by 22.6 PgC since 1850
The large C sink increase is mainly attributed to aerosol‐induced diffuse radiation changes, followed by the cooling effect
This study presents the global climate model IPSL‐CM6A‐LR developed at Institut Pierre‐Simon Laplace (IPSL) to study natural climate variability and climate response to natural and anthropogenic ...forcings as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This article describes the different model components, their coupling, and the simulated climate in comparison to previous model versions. We focus here on the representation of the physical climate along with the main characteristics of the global carbon cycle. The model's climatology, as assessed from a range of metrics (related in particular to radiation, temperature, precipitation, and wind), is strongly improved in comparison to previous model versions. Although they are reduced, a number of known biases and shortcomings (e.g., double Intertropical Convergence Zone ITCZ, frequency of midlatitude wintertime blockings, and El Niño–Southern Oscillation ENSO dynamics) persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. A large ensemble of more than 30 members for the historical period (1850–2018) and a smaller ensemble for a range of emissions scenarios (until 2100 and 2300) are also presented and discussed.
Plain Language Summary
Climate models are unique tools to investigate the characteristics and behavior of the climate system. While climate models and their components are developed gradually over the years, the sixth phase of the Coupled Model Intercomparison Project (CMIP6) has been the opportunity for the Institut Pierre‐Simon Laplace to develop, test, and evaluate a new configuration of its climate model called IPSL‐CM6A‐LR. The characteristics and emerging properties of this new model are presented in this study. The model climatology, as assessed from a range of metrics, is strongly improved, although a number of biases common to many models do persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5.
Key Points
The IPSL‐CM6A‐LR model climatology is much improved over the previous version, although some systematic biases and shortcomings persist
A long preindustrial control and a large number of historical and scenario simulations have been performed as part of CMIP6
The effective climate sensitivity of the IPSL model increases from 4.1 to 4.8 K between IPSL‐CM5A‐LR and IPSL‐CM6A‐LR
In the companion (Part I) paper, we have described and evaluated a new versatile optical particle counter/sizer named LOAC (Light Optical Aerosol Counter), based on scattering measurements at angles ...of 12 and 60°. That allows for some typology identification of particles (droplets, carbonaceous, salts, and mineral dust) in addition to size-segregated counting in a large diameter range from 0.2 µm up to possibly more than 100 µm depending on sampling conditions (Renard et al., 2016). Its capabilities overpass those of preceding optical particle counters (OPCs) allowing the characterization of all kind of aerosols from submicronic-sized absorbing carbonaceous particles in polluted air to very coarse particles (> 10–20 µm in diameter) in desert dust plumes or fog and clouds. LOAC's light and compact design allows measurements under all kinds of balloons, on-board unmanned aerial vehicles (UAVs) and at ground level. We illustrate here the first LOAC airborne results obtained from a UAV and a variety of scientific balloons. The UAV was deployed in a peri-urban environment near Bordeaux in France. Balloon operations include (i) tethered balloons deployed in urban environments in Vienna (Austria) and Paris (France), (ii) pressurized balloons drifting in the lower troposphere over the western Mediterranean (during the Chemistry-Aerosol Mediterranean Experiment – ChArMEx campaigns), (iii) meteorological sounding balloons launched in the western Mediterranean region (ChArMEx) and from Aire-sur-l'Adour in south-western France (VOLTAIRE-LOAC campaign). More focus is put on measurements performed in the Mediterranean during (ChArMEx) and especially during African dust transport events to illustrate the original capability of balloon-borne LOAC to monitor in situ coarse mineral dust particles. In particular, LOAC has detected unexpected large particles in desert sand plumes.
The northern-high-latitude permafrost contains almost twice the carbon
content of the atmosphere, and it is widely considered to be a non-linear and
tipping element in the earth's climate system ...under global warming. Solar
geoengineering is a means of mitigating temperature rise and reduces some of
the associated climate impacts by increasing the planetary albedo; the
permafrost thaw is expected to be moderated under slower temperature rise.
We analyze the permafrost response as simulated by five fully coupled earth
system models (ESMs) and one offline land surface model under four future
scenarios; two solar geoengineering scenarios (G6solar and G6sulfur) based
on the high-emission scenario (ssp585) restore the global temperature from
the ssp585 levels to the moderate-mitigation scenario (ssp245) levels via
solar dimming and stratospheric aerosol injection. G6solar and G6sulfur can
slow the northern-high-latitude permafrost degradation but cannot restore
the permafrost states from ssp585 to those under ssp245. G6solar and
G6sulfur tend to produce a deeper active layer than ssp245 and expose more
thawed soil organic carbon (SOC) due to robust residual high-latitude
warming, especially over northern Eurasia. G6solar and G6sulfur preserve
more SOC of 4.6 ± 4.6 and 3.4 ± 4.8 Pg C (coupled ESM simulations) or
16.4 ± 4.7 and 12.3 ± 7.9 Pg C (offline land surface model
simulations), respectively, than ssp585 in the northern near-surface
permafrost region. The turnover times of SOC decline slower under G6solar
and G6sulfur than ssp585 but faster than ssp245. The permafrost
carbon–climate feedback is expected to be weaker under solar geoengineering.
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