Robust evidence for reversal in the aerosol effective climate forcing trend

Atmospheric Chemistry and Physics Discussions European Geosciences Union (2022)

Authors:

Johannes Quaas, Hailing Jia, Chris Smith, Anna Lea Albright, Wenche Aas, Nicolas Bellouin, Oliver Boucher, Marie Doutriaux-Boucher, Piers M Forster, Daniel Grosvenor, Stuart Jenkins, Zig Klimont, Norman G Loeb, Xiaoyan Ma, Vaishali Naik, Fabien Paulot, Philip Stier, Martin Wild, Gunnar Myhre, Michael Schulz

Abstract:

Anthropogenic aerosols exert a cooling influence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only up to several days, the aerosol forcing responds quickly to emissions. Here we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow to robustly conclude that the anthropogenic aerosol effective radiative forcing – both aerosol-radiation and aerosol-cloud interactions – has become globally less negative, i.e. that the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the fine-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends consistent in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3 W m-2, up to 12 % of the total climate forcing in 2019 compared to 1750 according to IPCC.

Scientific data from precipitation driver response model intercomparison project.

Scientific data 9:1 (2022) 123

Authors:

Gunnar Myhre, Bjørn Samset, Piers M Forster, Øivind Hodnebrog, Marit Sandstad, Christian W Mohr, Jana Sillmann, Camilla W Stjern, Timothy Andrews, Olivier Boucher, Gregory Faluvegi, Trond Iversen, Jean-Francois Lamarque, Matthew Kasoar, Alf Kirkevåg, Ryan Kramer, Longbo Liu, Johannes Mülmenstädt, Dirk Olivié, Johannes Quaas, Thomas B Richardson, Dilshad Shawki, Drew Shindell, Chris Smith, Philip Stier, Tao Tang, Toshihiko Takemura, Apostolos Voulgarakis, Duncan Watson-Parris

Abstract:

This data descriptor reports the main scientific values from General Circulation Models (GCMs) in the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). The purpose of the GCM simulations has been to enhance the scientific understanding of how changes in greenhouse gases, aerosols, and incoming solar radiation perturb the Earth's radiation balance and its climate response in terms of changes in temperature and precipitation. Here we provide global and annual mean results for a large set of coupled atmospheric-ocean GCM simulations and a description of how to easily extract files from the dataset. The simulations consist of single idealized perturbations to the climate system and have been shown to achieve important insight in complex climate simulations. We therefore expect this data set to be valuable and highly used to understand simulations from complex GCMs and Earth System Models for various phases of the Coupled Model Intercomparison Project.

Boundary conditions representation can determine simulated aerosol effects on convective cloud fields

Communications Earth and Environment Springer Nature 3:1 (2022) 71

Authors:

Guy Dagan, Philip Stier, George Spill, Ross Herbert, Max Heikenfeld, Susan C van den Heever, Peter J Marinescu

Abstract:

Anthropogenic aerosols effect on clouds remains a persistent source of uncertainty in future climate predictions. The evolution of the environmental conditions controlling cloud properties is affected by the clouds themselves. Hence, aerosol-driven modifications of cloud properties can affect the evolution of the environmental thermodynamic conditions, which in turn could feed back to the cloud development. Here, by comparing many different cloud resolving simulations conducted with different models and under different environmental condition, we show that this feedback loop is strongly affected by the representation of the boundary conditions in the model. Specifically, we show that the representation of boundary conditions strongly impacts the magnitude of the simulated response of the environment to aerosol perturbations, both in shallow and deep convective clouds. Our results raise doubts about the significance of previous conclusions of aerosol-cloud feedbacks made based on simulations with idealised boundary conditions.

Tropical and boreal forest – atmosphere interactions: A review

Tellus B: Chemical and Physical Meteorology Stockholm University Press 74 (2022) 24-163

Authors:

Paulo Artaxo, Hans-Christen Hansson, Eliane Gomes Alves, Frida Bender, Jinshu Chi, Stefano Decesari, Florian Ditas, Sandro Fuzzi, Alex Guenther, Veli-Matti Kerminen, Radovan Krejci, Markku Kulmala, Jost V Lavric, Marco Aurelio M Franco, Maria Praß, Christopher Pöhlker, Mira L Pöhlker, Ulrich Pöschl, Ilona Riipinen, Janne Rinner, Luciana V Rizzo, Daniel Rosenfeld, Maria AF Silva Dias, Larisa Sogecheva, Philip Stier

Abstract:

This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiäla in Finland. The review is complemented by short-term observations from networks and large experiments. The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction. Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink. It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.

The global atmosphere‐aerosol model ICON‐A‐HAM2.3 - initial model evaluation and effects of radiation balance tuning on aerosol optical thickness

Journal of Advances in Modeling Earth Systems American Geophysical Union 14:4 (2022) e2021MS002699

Authors:

M Salzmann, S Ferrachat, C Tully, S Münch, D Watson‐Parris, D Neubauer, C Siegenthaler‐Le Drian, S Rast, B Heinold, T Crueger, R Brokopf, J Mülmenstädt, J Quaas, H Wan, K Zhang, U Lohmann, Philip Stier, I Tegen

Abstract:

The Hamburg Aerosol Module version 2.3 (HAM2.3) from the ECHAM6.3-HAM2.3 global atmosphere-aerosol model is coupled to the recently developed icosahedral nonhydrostatic ICON-A (icon-aes-1.3.00) global atmosphere model to yield the new ICON-A-HAM2.3 atmosphere-aerosol model. The ICON-A and ECHAM6.3 host models use different dynamical cores, parameterizations of vertical mixing due to sub-grid scale turbulence, and parameter settings for radiation balance tuning. Here, we study the role of the different host models for simulated aerosol optical thickness (AOT) and evaluate impacts of using HAM2.3 and the ECHAM6-HAM2.3 two-moment cloud microphysics scheme on several meteorological variables. Sensitivity runs show that a positive AOT bias over the subtropical oceans is remedied in ICON-A-HAM2.3 because of a different default setting of a parameter in the moist convection parameterization of the host models. The global mean AOT is biased low compared to MODIS satellite instrument retrievals in ICON-A-HAM2.3 and ECHAM6.3-HAM2.3, but the bias is larger in ICON-A-HAM2.3 because negative AOT biases over the Amazon, the African rain forest, and the northern Indian Ocean are no longer compensated by high biases over the sub-tropical oceans. ICON-A-HAM2.3 shows a moderate improvement with respect to AOT observations at AERONET sites. A multivariable bias score combining biases of several meteorological variables into a single number is larger in ICON-A-HAM2.3 compared to standard ICON-A and standard ECHAM6.3. In the tropics, this multivariable bias is of similar magnitude in ICON-A-HAM2.3 and in ECHAM6.3-HAM2.3. In the extra-tropics, a smaller multivariable bias is found for ICON-A-HAM2.3 than for ECHAM6.3-HAM2.3.