In its fifth assessment report (AR5), the Intergovernmental Panel on Climate Change provides a best estimate of the effective radiative forcing (ERF) due to anthropogenic aerosol at −0.9 W m⁻². This value is considerably weaker than the estimate of −1.2 W m⁻² in AR4. A part of the difference can be explained by an offset of +0.2 W m⁻² which AR5 added to all published estimates that only considered the solar spectrum, in order to account for adjustments in the terrestrial spectrum. We find that, in the CMIP5 multimodel median, the ERF in the terrestrial spectrum is small, unless microphysical effects on ice- and mixed-phase clouds are parameterized. In the latter case it is large but accompanied by a very strong ERF in the solar spectrum. The total adjustments can be separated into microphysical adjustments (aerosol “effects”) and thermodynamic adjustments. Using a kernel technique, we quantify the latter and find that the rapid thermodynamic adjustments of water vapor and temperature profiles are small. Observation-based constraints on these model results are urgently needed.

Key Points

The aerosol forcing in the terrestrial spectrum in models is found to be small or accompanied by a strong solar ERF
The adjustments are split into thermodynamic and microphysical adjustments. Thermodynamic adjustments are small in the terrestrial spectrum
Modeled ERF differs depending on complexity of parameterized aerosol-cloud interactions. Less comprehensive models show smaller net forcing

1 Introduction

Anthropogenic emissions perturb the atmospheric aerosol, including the fraction of the aerosol particles that serve as cloud condensation nuclei (CCN) and ice nucleating particles (INP). Aerosols scatter and absorb sunlight; these aerosol-radiation interactions imply a radiative forcing of climate [Charlson et al., 1992]. Serving as CCN and INP, aerosols perturb cloud albedo and cloud emissivity; these aerosol-cloud interactions imply a radiative forcing [“cloud albedo effect” Twomey, 1974]. Aerosol perturbations also result in rapid adjustments. Aerosol-radiation interactions may lead to cloud changes, especially via local heating due to absorption of sunlight [Ackerman et al., 2000]. Aerosol-cloud interactions imply changes in microphysical [Albrecht, 1989; Pincus and Baker, 1994; Lohmann, 2002; Fan et al., 2013] and dynamical [Ackerman et al., 2004; Small et al., 2009] cloud processes. The hypothesis that with suppression of rain formation in the liquid phase in convective clouds, lifting of additional water to the freezing level that might lead to enhanced buoyancy has been called “thermodynamic effect” [Denman et al., 2007] or also “convective invigoration” [Koren et al., 2005] and is considered in our nomenclature part of the cloud microphysical adjustments. The rapid changes of temperature and humidity profiles along with the fast cooling of land surfaces as adjustments to the forcing, occurring at timescales of days to months, impact the radiation budget and also lead to further rapid, thermodynamic adjustments of clouds (in addition to the microphysical adjustments) [Gregory and Webb, 2008]. “Rapid” in this context means faster than ocean surface temperatures adjust. The cloud response to ocean surface warming and implied circulation changes, in turn, is referred to as “cloud feedbacks” in the literature. In the remainder of this paper, we use the term “microphysical adjustments” when referring to the response of clouds to perturbations of cloud microphysical and dynamical processes (“aerosol effects”) and the term “thermodynamic adjustments” when referring to the rapid modifications of atmospheric thermodynamic profiles and land surface temperatures that in turn imply changes in the radiation budget. In its fifth assessment report (AR5) [Boucher et al., 2013], the Intergovernmental Panel on Climate Change (IPCC) introduced the notion of the “effective” radiative forcing, i.e., the sum of the radiative forcing (RF), and perturbations to the radiation budget of the Earth acting at fast timescales (up to months, before ocean surface temperatures respond noticeably), due both to microphysical and thermodynamic adjustments.

AR5 provides a best estimate of the net effective radiative forcing (ERF) due to anthropogenic aerosol at −0.9 W m⁻² (2000 versus 1750). This value is considerably weaker than the estimate of −1.2 W m⁻² in AR4 [Forster and other, 2007]. Since AR4, a value of −1.2 W m⁻² for the ERF (2000 versus 1850, as defined from the Fifth Coupled Model Intercomparison Project, CMIP5, model setup; Taylor et al., 2011) in the solar spectrum was corroborated by evaluating multiple climate models against satellite-derived relationships [Quaas et al., 2009] and the observed trend in surface solar radiation [Cherian et al., 2014]. These emergent constraints [Klein and Hall, 2015] corroborate a more negative forcing estimate (similar to AR4). It is to be noted that the aerosol ERF for reference year 1750 is about 7% stronger than when using the reference year 1850 [Takemura, 2012] and that models in CMIP5 did not use systematically and substantially different emissions compared to previous model assessments [Smith et al., 2011].

In an attempt to account for forcing and fast adjustments in the terrestrial spectrum, AR5 adds an offset of +0.2 W m⁻² to all published estimates that considered the forcing only in the solar spectrum irrespective of their magnitude. The basis of this approach is the fact that for cloud perturbations, a negative radiative effect in the solar spectrum tends to be accompanied by a positive effect in the terrestrial spectrum and vice versa [e.g., Harrison et al., 1990; Cess et al., 1996]. The choice of a fixed offset in AR5, although applied only to some of the studies examined by IPCC, is a considerable fraction of the difference between the AR4 and AR5 estimates.

The question arises whether the expert judgment estimate of +0.2 W m⁻² effective forcing in the terrestrial spectrum is justified. To this end, AR5 explicitly cites studies by two groups (their section 7.5.3, p. 618). Ghan et al. [2012] provide values of +0.46 W m⁻² and +0.27 W m⁻², which they attribute to an increased greenhouse effect via homogeneous freezing of additional haze particles. The studies by Storelvmo et al. [2008] and Storelvmo et al. [2010], in turn, find a negative effective forcing in the terrestrial spectrum of −0.32 W m⁻², attributed to a more efficient dissolution of mixed-phase clouds via the Bergeron–Findeisen effect due to additional INP. The AR5 estimate of −0.9 m⁻² was supported with other lines of evidence, such as the smaller magnitude of satellite-based estimates of the global forcing when compared to model estimates. However, new evidence suggests that the satellite-based estimates were biased low [e.g., Costantino and Bréon, 2010; Penner et al., 2011; Stier, 2016] and new studies show stronger forcings [Chen et al., 2014]. The final forcing estimate provided by AR5 is unfortunately only derived as “expert judgment” and thus cannot be easily falsified. As such, the aim of the present study is to assess the quantified value of +0.2 W m⁻² in the terrestrial spectrum. It does so based on models as available in CMIP5 and as published in the literature.

2 Microphysical Adjustments

The effective aerosol forcing can be diagnosed from the CMIP5 simulations [Zelinka et al., 2014]. Here the total ERF is diagnosed as the difference between the sstClimAerosol and sstClim simulations [Taylor et al., 2011], namely, simulations with prescribed climatological sea surface temperature and sea ice extent, with preindustrial (1850) boundary conditions for all species in sstClim, but year 2000 aerosol emissions for sstClimAerosol. The values for the models for which the relevant diagnostics are available are listed in Table 1. In Figure 1, the ERF values are shown separately for the solar and terrestrial spectra for all models and for four groups of models with increasingly complex representations of parameterized microphysical adjustments of clouds to aerosol perturbations.

Table 1. Global, Multi-Annual Mean Effective Radiative Forcing (2000 Versus 1850) in the Solar and Terrestrial Spectra, and Thermodynamic Rapid Adjustments in the Terrestrial Spectrum, for the Individual Modelsa

Model Acronym	BCC	CanESM	CSIRO	FGOALS	GFDL	HadGEM	IPSL	MIROC	MPI	MRI	NorESM
Aerosol effects	0	1	2	0	2	2	1	3	0	3	2
Terrestrial ERF	+0.014	+0.070	−0.228	+0.033	+0.132	+0.094	−0.210	+0.488	−0.160	+0.953	−0.042
Solar ERF	−0.392	−0.937	−1.180	−0.412	−1.732	−1.327	−0.506	−1.764	−0.318	−2.051	−0.953
Total ERF	−0.378	−0.867	−1.408	−0.379	−1.600	−1.233	−0.716	−1.276	−0.478	−1.098	−0.995
Planck adjustment	−	+0.118	−	+0.097	−	+0.154	+0.129	+0.216	−	−0.053	−
Lapse rate adjustment	−	−0.047	−	−0.058	−	−0.073	−0.173	−0.028	−	−0.169	−
Water vapor adjustment	−	+0.016	−	+0.007	−	+0.006	+0.011	−0.023	−	+0.141	−
Total adjustment w/o clouds	−	+0.087	−	+0.047	−	+0.088	−0.033	+0.165	−	−0.081	−
Cloud adjustments	−	−0.017	−	−0.014	−	+0.006	−0.177	+0.323	−	+1.036	−

^a Not all models reported the necessary diagnostics to compute the rapid adjustments. For the aerosol effects, “0” indicates the parameterization of only aerosol-radiation interactions, “1” the additional inclusion of the cloud albedo effect for liquid-water clouds, “2” in addition to this also the parameterization of microphysical adjustments for liquid-water clouds, and “3” the further inclusion of microphysical adjustments for ice- and mixed-phase clouds. For all quantities, the units are W m⁻².

Details are in the caption following the image — **Figure 1**
Open in figure viewer PowerPoint

Effective radiative forcing (2000 versus 1850) diagnosed from CMIP5. Blue: solar; red: terrestrial spectrum. Boxes show minimum, median, and maximum of the multimodel range. (top) The median values are also given as numbers, and (bottom) the number of models in each category. Categories are models that only include aerosol-radiation interactions (ERF_ari), models that also parameterize the cloud albedo effect for liquid-water clouds (radiative forcing, RF, due to aerosol-cloud interactions, aci, +RF_aci), models that also parameterize other effects on liquid-water clouds (+ERF_aci,liq), and models that also parameterize aerosol effects on ice clouds (+ERF_aci,ice).

The multimodel median suggests an effective forcing in the terrestrial spectrum of +0.033 W m⁻², almost an order of magnitude smaller than IPCC AR5 suggested. The median ERF in the terrestrial spectrum is also small for the models that do not parameterize aerosol-cloud interactions or that only consider effects on liquid-water clouds. However, the effective forcing in the solar spectrum becomes increasingly stronger as more effects are parameterized. A substantially larger effective forcing is found for the two models that parameterize aerosol effects on ice clouds, both in the solar and terrestrial spectra. These two models, like the other models that parameterize second indirect effects, suggest a net effective forcing of about −1.2 W m⁻².

The analysis of the CMIP5 models has the advantage that simulations are carried out consistently across models. It might be beneficial also to assess the published estimates of the ERF (including those not based on CMIP5 simulations). In Table S1 in the supporting information we list the values reported in the literature for the ERF split where possible into the contributions from the solar and terrestrial spectrum and separated in Table 1 by the interactions included in each simulation. Across these models, indeed a median terrestrial ERF of +0.2 W m⁻² is found (Figure S1 in the supporting information). However, as for CMIP5, this is accompanied by a larger, negative ERF in the solar spectrum for a net ERF of −1.6 W m⁻².

It is interesting to note that two models (CSIRO and IPSL) show a substantial negative ERF in the terrestrial spectrum (Table 1, of −0.23 and −0.21 W m⁻², respectively). These two models show a substantial decrease in upper tropospheric cloud cover (Figure S2 in the supporting information) which is not seen to the same degree in the other models. This decrease is widespread in CSIRO, but largest in the Tropics, and is concentrated on the inner-tropical convective regions in the IPSL model.

3 Thermodynamic Adjustments

Most models did not parameterize microphysical adjustments (“effects”) for ice- and mixed-phase clouds. Most of the contribution to the ERF in the terrestrial spectrum may thus be expected from thermodynamic adjustments. Rapid adjustments to changes in land surface temperature and thermodynamic profiles have been studied extensively in the context of CO₂ perturbations [Andrews et al., 2012; Block and Mauritsen, 2013; Kamae and Watanabe, 2013; Zelinka et al., 2013]. An increase in atmospheric CO₂ concentration leads to increases in land surface temperatures at timescales of days to months and alters the temperature and humidity profiles in the troposphere [Andrews et al., 2011]. It also changes vegetation respiration [Doutriaux-Boucher et al., 2009], although this is specific to the CO₂ perturbation and not of interest in the context of aerosol perturbations.

Among the five physical processes that respond to thermodynamic adjustments (Planck, lapse rate, water vapor, surface albedo, and clouds, please see below for more details), the surface albedo (snow cover extent) change only affects the solar spectrum and only reacts to a negligible extent to the aerosol perturbation (not shown). The temperature change can be split into the “Planck” response, due to rapid land surface warming and the change in the temperature lapse rate in the troposphere. Note that the land surface temperatures show at least some response at short timescales, and the resulting radiative effects are included in the adjustment analysis. In turn, ocean temperatures respond on much slower timescales, and the resulting radiative effects are to be considered feedback processes and so are not included in the effective forcing analysis. Specific humidity profiles may change at fast timescales, impacting the water vapor greenhouse effect. The response of clouds to thermodynamic adjustments cannot be disentangled from the microphysical adjustments. Here the total cloud response is diagnosed as a residual. We compute the thermodynamic adjustments using the radiative kernel technique following Block and Mauritsen [2013]. The results are shown in Figure 2 and listed in Table 1 for six models from the CMIP5 for which the relevant model output was available. All models except for one show a positive Planck response, since continental surfaces cool due to the negative aerosol effective forcing (implying a reduction in outgoing terrestrial radiation). The lapse rate on average is reduced in all models (less temperature decline with height; more cooling at the surface than aloft) such that the greenhouse effect is reduced. This constitutes a negative adjustment. This reduction in lapse rate is expected due to the change in moist adiabat with land surface cooling. Water vapor changes only slightly and mostly increases, resulting in a very slightly increased greenhouse effect. The response of clouds (combined thermodynamic and microphysical adjustment) also is small, except for the two models that parameterize aerosol-ice cloud interactions for which it is large and positive.

This analysis shows that the fast Planck response in all models leads to a dampening of the negative aerosol effective forcing. It is, however, substantially compensated by lapse rate adjustments. Water vapor adjustments are mostly small. So overall in the models the thermodynamic adjustments are small (on average less than 0.05 W m⁻²). Also, for most models the cloud adjustments are small. Only for one of the two models (IPSL) with a substantial negative ERF in the terrestrial spectrum was the analysis of the thermodynamic adjustments possible. From this analysis it is clear that this negative ERF is due to cloud adjustments, attributable to the upper tropospheric cloud fraction decrease (Figure S2 in the supporting information). Only in the two models that parameterize microphysical adjustments of ice- and mixed-phase clouds to aerosol perturbations is the (combined thermodynamic and microphysical) cloud adjustment large and positive.

4 Conclusions

The effective radiative forcing by anthropogenic aerosols, as simulated by the CMIP5 multimodel ensemble, was analyzed separately for the solar and terrestrial spectra. It was found that the model-median effective radiative forcing in the terrestrial spectrum is small. The exception is for two models that explicitly parameterize aerosol-ice cloud interactions. The effective radiative forcing in the solar spectrum is stronger as more microphysical adjustments are parameterized in the models.

A detailed analysis of the thermodynamic adjustments shows that the surface temperature adjustment and tropospheric lapse rate adjustment tend to offset each other, and that water vapor adjustments are small, so that the net effect of thermodynamic adjustments in the terrestrial spectrum is small. The exception is for the two models that parameterize aerosol-ice cloud interactions for which the effective forcing in the terrestrial spectrum due to cloud perturbations on average is about +0.7 W m⁻², which is the sum of microphysical and thermodynamic cloud adjustments. These two models also exhibit a much larger negative ERF in the solar spectrum compared to the other models. In another model study including aerosol effects on ice, Salzmann et al. [2010] found a terrestrial aerosol ERF of +0.31 W m⁻², but again together with a large solar aerosol ERF of −1.99 W m⁻² which yields a total aerosol ERF of −1.68 W m⁻². Other published studies support these findings (shown here as supporting information). In any case, it seems unjustified to assume an offset of +0.2 W m⁻² to the aerosol ERF from adjustments in the terrestrial spectrum irrespective of the estimated magnitude in the solar spectrum in IPCC AR5.

Our previous research constrained the effective radiative forcing as simulated by climate models using statistical relationships derived from satellite data [Quaas et al., 2009] and from decadal trends over continental Europe [Cherian et al., 2014], suggesting −1.2 ± 0.4 W m⁻² and −1.3 ± 0.4 W m⁻², respectively. This is closest to the median forcing in the solar spectrum for the class of models that include microphysical adjustments for liquid clouds but substantially smaller than the one for the two models that also include the microphysical adjustments for ice clouds. In contrast, Rotstayn et al. [2015] use the nearly linear relationship between aerosol effective forcing and temperature change over certain periods of the twentieth century to identify an effective radiative forcing of about −0.9 W m⁻². Ekman [2014] found in comparison to temperature trends that models with a comprehensive treatment of the cloud droplet activation are closer to the observations but did not reach a conclusion with respect to the inclusion of microphysical adjustments.

The analysis in this paper relies on global climate models. Beyond climate model results, very little is known quantitatively about the global forcing due to the reaction of ice-phase, mixed-phase, and deep convective clouds to aerosol perturbations. A truly realistic estimate, or even just reliable uncertainty interval, thus requires substantial further research, especially for effects in the terrestrial spectrum. Observational estimates are urgently needed. The current state of the art, however, suggests that the effective forcing in the terrestrial spectrum is either small, or, for models where it is large, is accompanied by a large negative effective forcing in the solar spectrum.

Acknowledgments

The study was funded by the European Research Council, Starting grant “QUAERERE”, grant agreement no 306284. The climate modeling community, PCMDI, and the ESGF are acknowledged for providing the climate model results. All data analyzed in this study are available via ESGF. The authors would like to thank Robert Pincus and two anonymous reviewers for constructive comments on this study.

Supporting Information

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Volume44, Issue2

28 January 2017

Pages 1001-1007

Assessment of simulated aerosol effective radiative forcings in the terrestrial spectrum

Abstract

Key Points

1 Introduction

2 Microphysical Adjustments

3 Thermodynamic Adjustments

4 Conclusions

Acknowledgments

Supporting Information

References

Citing Literature

Figures

References

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Assessment of simulated aerosol effective radiative forcings in the terrestrial spectrum

Abstract

Key Points

1 Introduction

2 Microphysical Adjustments

3 Thermodynamic Adjustments

4 Conclusions

Acknowledgments

Supporting Information

References

Citing Literature

Figures

References

Related

Information

RESOURCES

PUBLICATION INFO