║ SWP-B
╙ Global and Regional Climate Impacts
Background
Natural composition-climate feedbacks, like CLAW1, are important in determining how the climate system responds to anthropogenic forcing, and therefore the allowed emissions to meet climate targets (such as the Paris Agreement).
There is, however, considerable uncertainty about the sign and magnitude of changes in emissions of DMS under climate change. Some models predict a decrease in emissions, whereas others predict a significant increase2. There is further uncertainty over the fate of DMS that affects the radiative response to a change in emissions3,4.
The current generation of ESMs lacked the new insights around the oxidation of DMS and formation of HPMTF, and all work to date has focused exclusively on DMS and neglected the potential role of CH3SH.
CARES aims to determine the feedback parameters for the marine-sulfur system (DMS and CH3SH) under pre-industrial, present day and future conditions using state-of-the-art Earth System Models (ESMs) that include our latest insights into marine sulfur chemistry.
Hypotheses
H3.1: The climate feedback parameter related to marine sulfur emissions is state dependent and improving the representation of marine sulfur chemistry will increase the climate feedback parameter.
H3.2: Uncertainty ingas phase chemistry contributes more to the multi-model spread in DMS climate feedback parameter than model resolution and structural differences.
H3.3: The effects of improved sulfur chemistry on effective radiative forcing are regionally dependent, with a bigger impact in the mid latitudes than higher latitudes.
Workplan
CARES will address H3.1 through simulations that extend the AerChemMIP protocol5 to explore state-dependence. We will use information from SWP-A to introduce CH3SH emissions into the UKESM-A model, scaled to the modelled DMS emissions and ensure that marine-sulfur emissions encompass DMS and CH3SH for the first time.
Control and scaled emission runs will be formed to calculate the Effective Radiative Forcing (ERF) and radiative efficiency for DMS and CH3SH. We will determine the climate feedback parameter by combining these radiative efficiencies with changes in DMS and CH3SH emissions per unit temperature calculated from previous UKESM-1 simulations. These additional experiments will enable us to quantify the state dependence of the feedback factor to background physical conditions and atmospheric oxidants and aerosol distributions.
To address H3.2, we will perform a multi-model intercomparison using the same optimised and updated DMS and CH3SH chemistry scheme and compare these against archived AerChemMIP simulations.
We will address the role of model resolution, H3.2., by repeating the standard AerChemMIP experiments with UKESM-A but at N216 resolution.
To address H3.3, we will analyse the simulations described above to evaluate the regional climate impacts.
To quantify the effects of uncertainty in marine sulfur emissions under climate change on its climate feedback parameter, we will use our results for radiative efficiency that address H3.2 combined with climate change derived from other models to calculate a range of feedback parameter values to compare against existing literature.
Associated Deliverables
D3.1: New model simulations and archived data to support science studies.
D3.2: Paper(s) addressing the state-dependence of marine sulfur ERF and climate feedback parameter.
D3.3: Paper(s) addressing the role of uncertainty in the chemistry and emissions of DMS on its ERF and climate feedback parameter.
D3.4: Paper(s) addressing the role of changes in DMS and CH3SH chemistry on regional ERF.
D3.5: Paper(s) addressing the role of model resolution on marine sulfur ERF and climate feedback parameter.
References
SWP-C is led by Prof Alex Archibald and Prof Ken Carslaw
CARES 