Mapping the dependence of black carbon radiative forcing on emission region and season

Peer reviewe

Evaluating stratospheric ozone and water vapour changes in CMIP6 models from 1850 to 2100

Stratospheric ozone and water vapour are key components of the Earth system, and past and future changes to both have important impacts on global and regional climate. Here, we evaluate long-term changes in these species from the pre-industrial period (1850) to the end of the 21st century in Coupled Model Intercomparison Project phase 6 (CMIP6) models under a range of future emissions scenarios. There is good agreement between the CMIP multi-model mean and observations for total column ozone (TCO), although there is substantial variation between the individual CMIP6 models. For the CMIP6 multi-model mean, global mean TCO has increased from ∼300 DU in 1850 to ∼ 305 DU in 1960, before rapidly declining in the 1970s and 1980s following the use and emission of halogenated ozone-depleting substances (ODSs). TCO is projected to return to 1960s values by the middle of the 21st century under the SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0, and SSP5-8.5 scenarios, and under the SSP3-7.0 and SSP5-8.5 scenarios TCO values are projected to be ∼ 10 DU higher than the 1960s values by 2100. However, under the SSP1-1.9 and SSP1-1.6 scenarios, TCO is not projected to return to the 1960s values despite reductions in halogenated ODSs due to decreases in tropospheric ozone mixing ratios. This global pattern is similar to regional patterns, except in the tropics where TCO under most scenarios is not projected to return to 1960s values, either through reductions in tropospheric ozone under SSP1-1.9 and SSP1-2.6, or through reductions in lower stratospheric ozone resulting from an acceleration of the Brewer-Dobson circulation under other Shared Socioeconomic Pathways (SSPs). In contrast to TCO, there is poorer agreement between the CMIP6 multi-model mean and observed lower stratospheric water vapour mixing ratios, with the CMIP6 multi-model mean underestimating observed water vapour mixing ratios by ∼ 0.5 ppmv at 70 hPa. CMIP6 multi-model mean stratospheric water vapour mixing ratios in the tropical lower stratosphere have increased by ∼ 0.5 ppmv from the pre-industrial to the present-day period and are projected to increase further by the end of the 21st century. The largest increases (∼ 2 ppmv) are simulated under the future scenarios with the highest assumed forcing pathway (e.g. SSP5-8.5). Tropical lower stratospheric water vapour, and to a lesser extent TCO, shows large variations following explosive volcanic eruptions

Mapping the dependence of black carbon radiative forcing on emission region and season

Peer reviewe

Extratropical–Tropical Interaction Model Intercomparison Project (Etin-Mip): Protocol and Initial Results

International audienceThis article introduces the Extratropical–Tropical Interaction Model Intercomparison Project (ETIN-MIP), where a set of fully coupled model experiments are designed to examine the sources of longstanding tropical precipitation biases in climate models. In particular, we reduce insolation over three targeted latitudinal bands of persistent model biases: the southern extratropics, the southern tropics, and the northern extratropics. To address the effect of regional energy bias corrections on the mean distribution of tropical precipitation, such as the double intertropical convergence zone problem, we evaluate the quasi-equilibrium response of the climate system corresponding to a 50-yr period after the 100 years of prescribed energy perturbation. Initial results show that, despite a large intermodel spread in each perturbation experiment due to differences in ocean heat uptake response and climate feedbacks across models, the southern tropics is most efficient at driving a meridional shift of tropical precipitation. In contrast, the extratropical energy perturbations are effectively damped by anomalous heat uptake over the subpolar oceans, thereby inducing a smaller meridional shift of tropical precipitation compared with the tropical energy perturbations. The ETIN-MIP experiments allow us to investigate the global implications of regional energy bias corrections, providing a route to guide the practice of model development, with implications for understanding dynamical responses to anthropogenic climate change and geoengineering

Aerosol indirect effects: General circulation model intercomparison and evaluation with satellite data

Aerosol indirect effects continue to constitute one of the most important uncertaintiesfor anthropogenic climate perturbations. Within the international AEROCOM initiative,the representation of aerosol-cloud-radiation interactions in ten different general cir-culation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects,and none of the models explicitly parameterizes aerosol effects on convective clouds.We compute statistical relationships between aerosol optical depth (τa) and variouscloud and radiation quantities in a manner that is consistent between the models andthe satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (Nd) compares relatively well to the satellite data at leastover the ocean. The relationship betweenτaand liquid water path is simulated muchtoo strongly by the models. It is shown that this is partly related to the representationof the second aerosol indirect effect in terms of autoconversion. A positive relationshipbetween total cloud fraction (fcld) andτaas found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them.In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strongfcld–τarelationship, our results indicate that none can be identified asunique explanation. Relationships similar to the ones found in satellite data betweenτaand cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR –τarelationship show a strongpositive correlation betweenτaandfcld. The short-wave total aerosol radiative forc-ing as simulated by the GCMs is strongly influenced by the simulated anthropogenicfraction ofτa, and parameterisation assumptions such as a lower bound onNd. Never-theless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred fromthe combination of these predictors for the modelled forcings with the satellite-derivedstatistical relationships yields a global annual mean value of−1.5±0.5 Wm−2. An alternative estimate obtained by scaling the simulated clear- and cloudy-sky forcingswith estimates of anthropogenicτaand satellite-retrievedNd–τaregression slopes,respectively, yields a global annual mean clear-sky (aerosol direct effect) estimate of−0.4±0.2 Wm−2and a cloudy-sky (aerosol indirect effect) estimate of−0.7±0.5 Wm−2, with a total estimate of−1.2±0.4 Wm−2.ISSN:1680-7375ISSN:1680-736

Aerosol indirect effects ? general circulation model intercomparison and evaluation with satellite data

Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth ({tau}{sub a}) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (N{sub d}) compares relatively well to the satellite data at least over the ocean. The relationship between {tau}{sub a} and liquid water path is simulated much too strongly by the models. This suggests that the implementation of the second aerosol indirect effect mainly in terms of an autoconversion parameterisation has to be revisited in the GCMs. A positive relationship between total cloud fraction (f{sub cld}) and {tau}{sub a} as found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them. In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strong f{sub cld} - {tau}{sub a} relationship, our results indicate that none can be identified as a unique explanation. Relationships similar to the ones found in satellite data between {tau}{sub a} and cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR - {tau}{sub a} relationship show a strong positive correlation between {tau}{sub a} and f{sub cld} The short-wave total aerosol radiative forcing as simulated by the GCMs is strongly influenced by the simulated anthropogenic fraction of {tau}{sub a}, and parameterization assumptions such as a lower bound on N{sub d}. Nevertheless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred from the combination of these predictors for the modelled forcings with the satellite-derived statistical relationships yields a global annual mean value of -1.5 {+-} 0.5 Wm{sup -2}. In an alternative approach, the radiative flux perturbation due to anthropogenic aerosols can be broken down into a component over the cloud-free portion of the globe (approximately the aerosol direct effect) and a component over the cloudy portion of the globe (approximately the aerosol indirect effect). An estimate obtained by scaling these simulated clear- and cloudy-sky forcings with estimates of anthropogenic {tau}{sub a} and satellite-retrieved Nd - {tau}{sub a} regression slopes, respectively, yields a global, annual-mean aerosol direct effect estimate of -0.4 {+-} 0.2 Wm{sup -2} and a cloudy-sky (aerosol indirect effect) estimate of -0.7 {+-} 0.5 Wm{sup -2}, with a total estimate of -1.2 {+-} 0.4 Wm{sup -2}

Interactions between the atmosphere, cryosphere, and ecosystems at northern high latitudes

The Nordic Centre of Excellence CRAICC (Cryosphere–Atmosphere Interactions in a Changing Arctic Climate), funded by NordForsk in the years 2011–2016, is the largest joint Nordic research and innovation initiative to date, aiming to strengthen research and innovation regarding climate change issues in the Nordic region. CRAICC gathered more than 100 scientists from all Nordic countries in a virtual centre with the objectives of identifying and quantifying the major processes controlling Arctic warming and related feedback mechanisms, outlining strategies to mitigate Arctic warming, and developing Nordic Earth system modelling with a focus on short-lived climate forcers (SLCFs), including natural and anthropogenic aerosols. The outcome of CRAICC is reflected in more than 150 peer-reviewed scientific publications, most of which are in the CRAICC special issue of the journal Atmospheric Chemistry and Physics. This paper presents an overview of the main scientific topics investigated in the centre and provides the reader with a state-of-the-art comprehensive summary of what has been achieved in CRAICC with links to the particular publications for further detail. Faced with a vast amount of scientific discovery, we do not claim to completely summarize the results from CRAICC within this paper, but rather concentrate here on the main results which are related to feedback loops in climate change–cryosphere interactions that affect Arctic amplification.The CRAICC team acknowledges the following institutions for financial support: the Finnish Cultural Foundation grant, Markku Kulmala “International Working Groups”; Russian mega-grant no. 11.G34.31.0048 (University of Nizhny Novgorod); Academy of Finland contracts 259537, 257411, and 254195; Beautiful Beijing (Finland–China collaboration project) funded by TEKES; Nordforsk CRAICC-PEEX (amendment to contract 26060); CRAICC-CRUCIAL (project no. 81257); Icelandic Research Fund (Rannis) grant no. 152248- 051; Danish Environmental Protection Agency with means from the Dancea fund for environmental support to the Arctic region (M 112 002700); the Villum Foundation; the Carlsberg Foundation (project 009_1_0515); COST1303 (TOPROF); COST ES1404 (HarmoSnow); and the Pan-Eurasian Experiment (PEEX). The development and use of NorESM1 was supported by the Norwegian Research Council through the projects Earth-Clim (207711/E10), EVA (grant no. 229771), NOTUR (nn2345k), and NorStore (ns2345k) and through the Nordic Centre of Excellence eSTICC (57001) and the EU H2020 project CRESCENDO (grant no. 641816). The CRAICC team also thanks Rogier Floors for providing Fig. 8 and Christoph Münkel for Fig. 9. The authors and entire CRAICC community would like to thank and acknowledge the work and inspiration of Jon Egill Kristjansson, whose life was cut short during these collaborations. Jon Egill Kristjansson is deeply missed, but his scientific legacy continues.Peer Reviewe

Interactions between the atmosphere, cryosphere, and ecosystems at northern high latitudes

Abstract The Nordic Centre of Excellence CRAICC (Cryosphere–Atmosphere Interactions in a Changing Arctic Climate), funded by NordForsk in the years 2011–2016, is the largest joint Nordic research and innovation initiative to date, aiming to strengthen research and innovation regarding climate change issues in the Nordic region. CRAICC gathered more than 100 scientists from all Nordic countries in a virtual centre with the objectives of identifying and quantifying the major processes controlling Arctic warming and related feedback mechanisms, outlining strategies to mitigate Arctic warming, and developing Nordic Earth system modelling with a focus on short-lived climate forcers (SLCFs), including natural and anthropogenic aerosols. The outcome of CRAICC is reflected in more than 150 peer-reviewed scientific publications, most of which are in the CRAICC special issue of the journal Atmospheric Chemistry and Physics. This paper presents an overview of the main scientific topics investigated in the centre and provides the reader with a state-of-the-art comprehensive summary of what has been achieved in CRAICC with links to the particular publications for further detail. Faced with a vast amount of scientific discovery, we do not claim to completely summarize the results from CRAICC within this paper, but rather concentrate here on the main results which are related to feedback loops in climate change–cryosphere interactions that affect Arctic amplification