The Science

Scientific Background

The response of the climate system to anthropogenic sulfur emissions and the dependence of this response on natural background levels¹ remain the greatest uncertainties we have in historic climate change. The effect of anthropogenic emissions are weaker if the natural background levels are high, and stronger if the natural background levels are low. As a result, the natural background is a key determinant of the future trajectory of greenhouse gas emissions allowed to avert the climate crisis².

Marine sulfate aerosol produced from biogenic dimethyl sulfide (DMS) is the main component of natural aerosol over many oceanic regions and sets a baseline aerosol concentration against which the magnitude of anthropogenic aerosol radiative forcing is determined¹. The fate of DMS is critical for understanding the formation of new aerosol particles, growth of existing aerosols to cloud condensation nuclei (CCN), and the burden/composition of aerosols in the remote marine environment, and as a result the climate impacts. Therefore, the marine sulfur cycle offers a perfect example of a climatically important biogeochemical cycle within the Earth system – initially described by the CLAW hypothesis³.

The CLAW hypothesis is a keystone of early work in Earth system science. It provides the framework to link increases in emissions of DMS from marine phytoplankton under climate warming, through atmospheric oxidation to form new aerosols that modify clouds and radiation, which in turn reduces surface temperatures. The entire process has been proposed as a feedback loop through which marine biota regulates the Earth’s environment.

After many decades of research, we have gained substantial understanding in certain aspects of CLAW, but the overall hypothesis remains an open question. Even though several studies show that CLAW feedback may be weak in the current atmosphere^4,5 DMS fundamentally determines the baseline aerosol state of the pre-industrial (PI) atmosphere, which causes large uncertainty in anthropogenic aerosol forcing¹.

Adding even more uncertainty, recent discoveries of a new sulfur molecule formed from DMS⁶ – hydroperoxy methylthioformate (HOOCH₂SCHO) – are forcing us to radically re-examine the role of marine sulfur in the climate system. The current understanding in climate models is now significant challenged by new aerosol formation pathways⁷, as well as by observations of marine emissions from other sulfur species that were previously dismissed as important⁸.

There is little consensus about the atmospheric fate and oxidation products of HPMTF. This significant gap in our understanding of the natural sulfur cycle is a major limitation when trying to constrain the pre-industrial climate system, which is itself crucial for determining the amount of allowed greenhouse gas emissions to meet climate stabilisation targets².

The recently discovered species and chemical pathways are not included in any Earth system models that inform global climate change policies through the IPCC, even though aerosols from natural sources are a key driver of uncertainty in radiative forcing. The resulting biases in Earth system models^9,10,11 impede their ability to provide the policy-relevant information needed to address the climate crisis.

How CARES will address these needs

There have been significant discoveries of new species and processes over the past few years, but they have not individually enabled constraint of the climate impacts of natural marine sulfur.

The approach of the CARES project, routed in uncertainty quantification and observational constraints, provides an exciting opportunity for a complete re-examination of the role of marine sulfur within the Earth system through a combination of intensive aircraft and ship observations as well as multi-scale model experiments.

CARES will quantify the atmospheric fate of emissions of natural marine sulfur by performing a series of intensive aircraft and ship observations as well as multi-scale model experiments. CARES will bring together measurements of the broadest range of sulfur compounds to date and will quantify in situ concentrations and fluxes of a large number of exciting, recently discovered sulfur compounds (like HPMTF) as well as cloud and aerosol properties in the Eastern North Atlantic.

CARES brings many ‘firsts’ for the scientific community – airborne fluxes of SO₂ and HPMTF will be made simultaneously for the first time; the FAAM aircraft will detect DMS and CH₃SH (and numerous other VOCs) at high frequency and sensitivity for the first time; and the introduction of CH₃SH emissions into the UKESM-A model will ensure that marine-sulfur emissions encompass DMS and CH₃SH for the first time.

Advancements in models will allow us to better understand contemporary and historical sulfur and climate observations and deliver a substantial revision to our understanding of the fate and impact of natural sulfur emissions. The results will then be used to rectify errors in the representation of sulfur processes in Earth system models, constrain the role of marine sulfur in the Earth System, and improve confidence in simulations that project future change.

References

1. Carslaw et al., Nature, 503(7474), 2013 ↩︎
2. Fyfe et al., PNAS, 118(23), 2021 ↩︎
3. Charlson et al., Nature, 326, 1987 ↩︎
4. Woodhouse et al., Atmos. Chem. Phys., 10(16), 2010 ↩︎
5. Quinn and Bates, Nature, 480(7375), 2011 ↩︎
6. Veres et al., PNAS, 117(9), 2020 ↩︎
7. Zheng et al., Nat. Commun., 12, 2021 ↩︎
8. Lawson et al., Atmos. Chem. Phys, 20(5), 2020 ↩︎
9. Hardacre et al., Atmos. Chem. Phys. Discuss., 2021 ↩︎
10. Sellar, et al., J.A.M.E.S., 11, 2019 ↩︎
11. Zhang et al., Atmos. Chem. Phys. Discuss., 2021 ↩︎

related pages

Objectives

Project Design

The Team