Abstract

Coral reefs produce atmospheric dimethylsulfide (DMSa) which oxidises to non-sea-salt (nss) sulfate aerosols, precursors of cloud condensation nuclei (CCN) and low level cloud (LLC), reducing solar radiation and regulating sea surface temperatures (SSTs). Here we report measurements of solar radiation, SST, LLC, DMS flux, , and rainfall before, during and after a major coral bleaching event at Magnetic Island in the central Great Barrier Reef (GBR). Measurements are compared with those made at the nearby fringing reef of Or-pheus Island where coral bleaching did not occur. Extreme solar radiation levels occurred from November to late January and could have reflected cloud radiative effects that increased downwelling of solar radiation. High levels of LLC often coincided with high periodic fluxes of DMS from the unbleached coral reef at Orpheus Island (e.g. 14 - 20 μmol·m-2·d-1), in direct contrast to the very low fluxes of DMS that were emitted from the bleached, human-impacted Magnetic Island fringing reef (nd-0.8 μmol·m-2·d-1) when SSTs were >30°C. Continuous SSTs measurements at the Magnetic Island reef revealed various heating and cooling periods, interspersed with stable SSTs. Cooling periods (negative climate feedback) ranged from -1°C to -3°C (7 day mean -1.6°C), and often seemed to occur during low tides, periodic pulses of DMS flux and LLC, keeping SSTs °C. In contrast warming periods of +1°C to +3°C (positive climate feedback, 7 day mean +1.52°C), seemed to occur during increasing tides, decreasing DMS flux and low to medium levels of LLC which increased solar radiation and caused SSTs over 30°C and corals to bleach. Alternation between these two states or types of feedback is indicated in this research and may be a function of enhanced scattering of solar radiation from nss-sulfate aerosols that originate from oxidation of DMSa produced from the coral reefs in the GBR. The net radiative forcing from clouds can be as high as four times as large as the radiative forcing from a doubling of CO2 levels in the atmosphere, which needs to be taken into account when ascribing coral bleaching events in the GBR solely to GHG warming. Further studies are needed to more critically assess the importance of this GBR coral reef-cloud feedback to the climate of northern Australia and the western Pacific, where the greatest biomass of coral reefs occurs.

Highlights

  • Reef-building corals contain high concentrations of dimethylsulfoniopropionate (DMSP) [1]-[6] that can be enzymatically converted to dimethylsulfide (DMS), a trace sulfur gas that oxidizes in the atmosphere to form nss-sulfate aerosol particles and cloud condensation nuclei (CCN) (Figure 1)

  • From 4-9th January (DOY 65-70), daily air temperatures averaged over 30 ̊C, and included a series of exceptionally high maximum daytime temperatures in excess of 36 ̊C recorded by the Townsville Bureau of Meteorology (TBM)

  • Our research has provided preliminary evidence suggesting that variations in the flux of DMSa from coral reefs in the Great Barrier Reef (GBR) may enhance the scattering of solar radiation from nss-sulfate aerosols derived from oxidation of DMSa, which could be responsible for the extreme solar radiation and sea surface temperatures (SSTs) observed during mass coral bleaching events

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Summary

Introduction

Reef-building corals contain high concentrations of dimethylsulfoniopropionate (DMSP) [1]-[6] that can be enzymatically converted to dimethylsulfide (DMS), a trace sulfur gas that oxidizes in the atmosphere to form nss-sulfate aerosol particles and cloud condensation nuclei (CCN) (Figure 1). Atmospheric DMS can be oxidized to nss-sulfate aerosol by hydroxyl (OH) and nitrate (NO3) radicals [12], O3, HO2, and H2O2 can be involved [13]. The products of this oxidation produce SO2, methanesulfonate acid (MSA) and gas-phase sulfuric acid (H2SO4) which can condense onto aerosol particles [14], or nucleate to form new sulfuric acid particles [15] (Figure 1). The climate impact of atmospheric DMS (DMSa) depends on the ability of its oxidation products to increase CCN, cloud droplet concentration [13], LLC and rainfall [16] [17]

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