Rebecca L. Jackson

We present a new regional coupled atmosphere and ocean model of the Great Barrier Reef (GBR), Australia, called ACCESS-EMS-GBR. The model comprises a regional configuration of the Australian Community Climate and Earth System Simulator (ACCESS) atmospheric model (the United Kingdom Met Office Unified Model), coupled to the Commonwealth Scientific and Industrial Research Organisation Environmental Modelling Suite (EMS) hydrodynamic and biogeochemical model of the GBR. The model includes detailed aerosol and cloud microphysics in the atmosphere, as well as hydrodynamics, biogeochemistry, ecology and coral reef processes in the ocean. When coupled, the models form a valuable tool for simulating interactions between the atmosphere and ocean. The ACCESS-EMS-GBR model was developed to assess the plausibility and implications of marine cloud brightening (MCB) in the GBR, as part of the Reef Restoration and Adaptation Program (RRAP) Cooling and Shading Sub-program (RRAP-CS) but can also be utilised to study various processes including sources of marine aerosol and the impact of aerosol deposition on marine biogeochemistry. Here, we describe the model as configured for the GBR region, including the coupling framework, and evaluate the model skill in simulating measured aerosol and seawater temperature and irradiance. We configure the model aerosol scheme and emission sources to capture observed aerosol properties over the GBR region, with boundary layer nucleation and terrestrial biogenic emissions having a large influence on aerosol number concentration, size distribution and composition. The distribution of sea salt emissions is also expanded to include the soluble Aitken mode; a significant improvement in capability for simulating MCB and possibly for improving global climate model biases.

We present a new regional coupled atmosphere and ocean model of the Great Barrier Reef (GBR), Australia, called ACCESS-EMS-GBR. The model comprises a regional configuration of the Australian Community Climate and Earth System Simulator (ACCESS) atmospheric model (the United Kingdom Met Office Unified Model), coupled to the Commonwealth Scientific and Industrial Research Organisation Environmental Modelling Suite (EMS) hydrodynamic and biogeochemical model of the GBR. The model includes detailed aerosol and cloud microphysics in the atmosphere, as well as hydrodynamics, biogeochemistry, ecology and coral reef processes in the ocean. When coupled, the models form a valuable tool for simulating interactions between the atmosphere and ocean. The ACCESS-EMS-GBR model was developed to assess the plausibility and implications of marine cloud brightening (MCB) in the GBR, as part of the Reef Restoration and Adaptation Program (RRAP) Cooling and Shading Sub-program (RRAP-CS) but can also be utilised to study various processes including sources of marine aerosol and the impact of aerosol deposition on marine biogeochemistry. Here, we describe the model as configured for the GBR region, including the coupling framework, and evaluate the model skill in simulating measured aerosol and seawater temperature and irradiance. We configure the model aerosol scheme and emission sources to capture observed aerosol properties over the GBR region, with boundary layer nucleation and terrestrial biogenic emissions having a large influence on aerosol number concentration, size distribution and composition. The distribution of sea salt emissions is also expanded to include the soluble Aitken mode; a significant improvement in capability for simulating MCB and possibly for improving global climate model biases.

We present a new regional coupled atmosphere and ocean model of the Great Barrier Reef (GBR), Australia, called ACCESS-EMS-GBR. The model comprises a regional configuration of the Australian Community Climate and Earth System Simulator (ACCESS) atmospheric model (the United Kingdom Met Office Unified Model), coupled to the Commonwealth Scientific and Industrial Research Organisation Environmental Modelling Suite (EMS) hydrodynamic and biogeochemical model of the GBR. The model includes detailed aerosol and cloud microphysics in the atmosphere, as well as hydrodynamics, biogeochemistry, ecology and coral reef processes in the ocean. When coupled, the models form a valuable tool for simulating interactions between the atmosphere and ocean. The ACCESS-EMS-GBR model was developed to assess the plausibility and implications of marine cloud brightening (MCB) in the GBR, as part of the Reef Restoration and Adaptation Program (RRAP) Cooling and Shading Sub-program (RRAP-CS) but can also be utilised to study various processes including sources of marine aerosol and the impact of aerosol deposition on marine biogeochemistry. Here, we describe the model as configured for the GBR region, including the coupling framework, and evaluate the model skill in simulating measured aerosol and seawater temperature and irradiance. We configure the model aerosol scheme and emission sources to capture observed aerosol properties over the GBR region, with boundary layer nucleation and terrestrial biogenic emissions having a large influence on aerosol number concentration, size distribution and composition. The distribution of sea salt emissions is also expanded to include the soluble Aitken mode; a significant improvement in capability for simulating MCB and possibly for improving global climate model biases.

Biogenic emissions of dimethylsulfide (DMS) are an important source of sulfur to the atmosphere, with implications for aerosol formation and cloud albedo over the ocean. Natural aerosol sources constitute the largest uncertainty in estimates of aerosol radiative forcing and climate and thus, an improved understanding of DMS sources is needed. Coral reefs are strong point sources of DMS; however, this coral source of biogenic sulfur is not explicitly included in climatologies or in model simulations. Consequently, the role of coral reefs in local and regional climate remains uncertain. We aim to improve the representation of tropical coral reefs in DMS databases by calculating a climatology of seawater DMS concentration (DMS_w) and sea‐air flux in the Great Barrier Reef (GBR), Australia. DMS_wis calculated from remotely sensed observations of sea surface temperature and photosynthetically active radiation using a multiple linear regression model derived from field observations of DMS_win the GBR. We estimate that coral reefs and lagoon waters in the GBR (∼347,000 km²) release 0.03–0.05 Tg yr⁻¹of DMS (0.02 Tg yr⁻¹of sulfur). Based on this estimate, global tropical coral reefs (∼600,000 km²) could emit 0.08 Tg yr⁻¹of DMS (0.04 Tg yr⁻¹of sulfur), with the potential to influence the local radiative balance.

We review the evidence for bio-regulation by coral reefs of local climate through stress-induced emissions of aerosol precursors, such as dimethylsulfide. This is an issue that goes to the core of the coral ecosystem’s ability to maintain homeostasis in the face of increasing climate change impacts and other anthropogenic pressures. We examine this through an analysis of data on aerosol emissions by corals of the Great Barrier Reef, Australia. We focus on the relationship with local stressors, such as surface irradiance levels and sea surface temperature, both before and after notable coral bleaching events. We conclude that coral reefs may be able to regulate their exposure to environmental stressors through modification of the optical properties of the atmosphere, however this ability may be impaired as climate change intensifies.

Global climate change and the impacts of ocean warming, ocean acidification and declining water quality are adversely affecting coral-reef ecosystems. This is of great concern, as coral reefs provide numerous ecosystem, economic and social services. Corals are also recognised as being amongst the strongest individual sources of natural atmospheric sulfur, through stress-induced emissions of dimethylsulfide (DMS). In the clean marine boundary layer, biogenic sulfates contribute to new aerosol formation and the growth of existing particles, with important implications for the radiative balance over the ocean. Evidence suggests that DMS is not only directly involved in the coral stress response, alleviating oxidative stress, but also may create an “ocean thermostat” which suppresses sea surface temperature through changes to aerosol and cloud properties. This review provides a summary of the current major threats facing coral reefs and describes the role of dimethylated sulfur compounds in coral ecophysiology and the potential influence on climate. The role of coral reefs as a source of climatically important compounds is an emerging topic of research; however the window of opportunity to understand the complex biogeophysical processes involved is closing with ongoing degradation of the world's coral reefs. The greatest uncertainty in our estimates of radiative forcing and climate change is derived from natural aerosol sources, such as marine DMS, which constitute the largest flux of oceanic reduced sulfur to the atmosphere. Given the increasing frequency of coral bleaching events, it is crucial that we gain a better understanding of the role of DMS in local climate of coral reefs.

Variability in atmospheric dimethylsulfide (DMS_a) and the potential influence on atmospheric aerosols was investigated at Heron Island in the southern Great Barrier Reef (GBR), Australia. This work compiles previously published DMS_a data (reported in Swan, Jones, Deschaseaux, & Eyre, 2017, https://doi.org/10.5194/bg‐14‐229‐2017), with additional surveys of DMS_a, atmospheric particle number concentration, and other oceanic and atmospheric data sets. DMS_a was higher in summer (mean 3.2 nmol m⁻³/78 ppt) than winter (mean 1.3 nmol m⁻³/32 ppt), reflective of seasonal shifts in phytoplankton biomass and emissions from corals in the southern GBR. Seasonally extreme spikes in DMS_a were detected during low tide and low wind speed, supporting findings that the coral reef can be an important source of DMS_a above background oceanic emissions. A significant link was present between DMS_a and aerosol concentration (ranging from 0.5 to 2.5 μm) during calm, daylight hours, when conditions were optimal for the local oxidation of DMS_a to sulfate aerosol precursors. This link may reflect condensational growth of existing fine particles (< 0.5 μm), which is the dominant pathway by which biogenic trace gases influence aerosols in the marine boundary layer. Aerosol concentration significantly correlated with reduced surface solar irradiance and sea surface temperature, which is potential evidence of a local negative feedback mitigating coral physiological stress. These findings provide a step toward a better understanding of the processes influencing DMS_a and aerosol concentrations and of the consequences for the local radiative balance over coral reefs; an increasingly important topic with ongoing ocean warming and coral bleaching.

Coral reefs are being threatened by global climate change, with ocean warming and acidification, compounded by declining water quality in many coastal systems, adversely affecting coral health and cover. This is of great concern as coral reefs provide numerous ecosystem, economic and social services. Corals are also recognized as being amongst the strongest individual sources of natural atmospheric sulfur, through stress-induced emissions of dimethylsulfide (DMS). In the clean marine boundary layer, biogenic sulfates contribute to new aerosol formation and the growth of existing particles, with important implications for the radiative balance. Evidence suggests that DMS is not only directly involved in the coral stress response, alleviating oxidative stress, but may create an ocean thermostat which suppresses sea surface temperature (SST) through changes to aerosol and cloud properties. This review provides a summary of the current major threats facing coral reefs and describes the role of dimethylated sulfur compounds in coral physiology and climate. The role of coral reefs as a source of climatically important compounds is an emerging topic of research however, the window of opportunity to understand the complex biogeophysical processes involved is closing with ongoing degradation of the world's coral reefs. The greatest uncertainty in our estimates of radiative forcing and climate change are derived from natural aerosol sources, such as marine DMS, which constitutes the largest flux of oceanic reduced sulfur to the atmosphere. Gaining a better understanding of the role of coral reef DMS emissions is crucial to predicting the future climate of our planet.

It is proposed that emissions of volatile sulfur compounds by coral reefs contribute to the formation of a biologically-derived feedback on sea surface temperature (SST) through the formation of marine biogenic aerosol (MBA). The direction and strength of this feedback remains uncertain and constitutes a fundamental constraint on predicting the ability of corals to cope with future ocean warming. We investigate the effects of elevated SST and irradiance on satellite-derived fine-mode aerosol optical depth (AOD) throughout the Great Barrier Reef, Australia (GBR) over an 18-year time period. AOD is positively correlated with SST and irradiance and increases two-fold during spring and summer with high frequency variability. As the influence of non-biogenic and distant aerosol sources are found to be negligible, the results support recent findings that the 2,300 km stretch of coral reefs can be a substantial source of biogenic aerosol and thus, influence local ocean albedo. Importantly however, a tipping point in the coral stress response is identified, whereby thermal stress reaches a point that exceeds the capacity of corals to influence local atmospheric properties. Beyond this point, corals may become more susceptible to permanent damage with increasing stress, with potential implications for mass coral bleaching events.