Workpackage 4

Feedback processes in the climate system

Leader: Risto Makkonen, Ulrike Lohmann (University of Helsinki, Finland/ETH Zurich, Switzerland)

In WP4, the ESMs will be improved by integrating the different findings and developments obtained in WPs 1-3 concerning aerosol emissions (especially organics from the terrestrial and marine biosphere), secondary organic aerosol formation, CCN and IN formation, and cloud system formation. The ESMs will then be applied to i) provide estimates of radiative forcing and effective radiative forcing due to aerosol-cloud interactions, ii) identify and quantify the feedback processes in the biosphere-aerosol-cloud-climate system, iii) investigate the Arctic sea ice feedback related to aerosol-cloud interactions, and iv) provide future climate predictions using state-of-the-art anthropogenic emission scenarios. These objectives are split into five tasks:

Task 4.1: Improved treatment of organics in the ESMs

The aerosol schemes in the ESMs will be developed to explicitly take into account knowledge gained in WP2 with a focus on SOA formation from gas-phase precursor tracers. A volatility basis set approach, equilibrium and kinetic condensation routines, or a combination of these will be used to model the evolution of VOCs and gas-aerosol partitioning. The effect of organic vapours on nucleation and particle growth will be implemented according to findings from process model studies and field measurements in task 2.3. New aerosol source parameterisations developed in task 2.1 will be implemented in the ESMs. This will result in improved aerosol modules that better describe the properties of organic aerosol (size distribution, mixing state, CCN activity). Interactive emissions of BVOC from terrestrial and marine sources will be implemented and coupled with the land-vegetation models of the ESMs to allow the investigation of the feedbacks in the biosphere-aerosol-cloud system.

Task 4.2: Global modelling of IN and IN climatology

The objective of this task is to perform global simulations of IN, to validate them with the dataset generated in WP1 and to construct a global IN climatology. The treatment of heterogeneous ice nucleation in the ESMs will be improved by modifying and updating the schemes currently used in the models. In particular, existing parameterisation schemes of heterogeneous ice nucleation based on e.g. classical nucleation theory (Hoose et al., 2010b) will be validated using the ice nucleation properties obtained at different geographical locations in WP1. If needed, they will be modified in order to match the observed IN data taking into account the findings from process studies in WP2 as well as from CRM studies in WP3. This will enhance the modelsí ability to simulate aerosol-cloud interactions in mixed-phase clouds, which are of particular importance in the Arctic.

Task 4.3: Aerosol climate forcing

Best estimates of pre-industrial and present day aerosol climatologies (including CCN) developed as part of BACCHUS (WP1) will be used as part of a BACCHUS-driven model intercomparison project to provide best estimates of the effective radiative forcing associated with changes in the atmospheric aerosol through the industrial period. In addition we will compare the periods before and after the breakdown of the eastern European economy and the period before the strong rise of the emissions in South East Asia with the present-day. These experiments will build on the aerosol forcing experiments included in CMIP5, but will be based on standardized aerosol climatologies (rather than each model using its own) including climatologies of CCN and IN from task 4.2. The experimental protocol and associated datasets will be explored by the BACCHUS modelling groups, and as part of a developing WCRP initiative on clouds, circulation and climate sensitivity, for eventual incorporation into CMIP6.

Task 4.4: Biosphere-atmosphere-cloud-climate interactions and feedbacks

Using upgraded and evaluated ESMs, key feedbacks (and key links in the feedback-chain) in the biosphere-atmosphere-cloud-climate system will be identified and evaluated, in particular feedbacks associated with: a) Changing emissions from the terrestrial biosphere in a warming climate, and their influence on SOA formation and henceforth CCN (and possibly IN) activity, clouds and climate; b) Shrinking sea ice, leading to enhanced marine emissions of marine organics, DMS and possibly sea salt, with subsequent influence on CCN and IN, and thereby clouds and climate. A suite of simulations will be run with all three ESMs, yielding quantitative estimates of the above interactions in preindustrial, present-day and projected future climates. Sensitivity tests will include uncertainties related to the parameterisation of SOA, VOC emission strength, level of natural background, and wildfire intensity.

Task 4.5: Future scenario simulations

Simulations with different Representative Concentration Pathways (RCPs) will be performed with the NorESM (UiO) and MPI-ESM-HAM (ETHZ, MPI-M, UHEL) improved with advanced aerosol and cloud microphysics schemes. We will use state-of-the-art future emission scenarios of anthropogenic aerosols and aerosol precursors currently being prepared by on-going EU project (such as ECLIPSE and PEGASOS). Because the Arctic is particularly vulnerable to climate change, we will also investigate the extent to which different levels of ship emissions of SO2 and soot over an ice-free Arctic ocean may further impact the local, regional, and global hydrological cycle, clouds, radiative forcing and climate. A starting point is the study by Eckhardt et al. (2013) investigating the impact of cruise ship emissions on air pollution in the Arctic. The effect of enhanced soot deposition on bright surfaces will also be included. This task will result in future climate predictions with substantially reduced uncertainties.

Participating institutions in WP4

Swiss Federal Institute for Technology, Switzerland University of Helsinki, Finland
University of Oslo, Norway Finnish Meteorological Institute, Finland
University of Leeds, United Kingdom Karlsruhe Institute for Technology, Germany
Max Planck Institute for Meteorology and Max Planck Institute for Chemistry, Germany