Tropical coral reefs are increasingly threatened due to global warming. Corals live within a narrow thermal threshold making them one of the most sensitive species to changes in temperature. Recent warming events on the Great Barrier Reef (GBR) (2016, 2017, 2020) have caused mass coral mortality on approximately 30% of the reef (Bozec et al. 2020; Hughes, Kerry et al. 2018). This research focuses on the development and implementation of a 1-D semi-dynamic downscaling method to improve climate projections on the GBR. Coral stress metrics are used to provide detailed projections on the magnitude and frequency of warming for four socio-economic pathways (SSP) under the 6th phase of the Climate Model Intercomparison Project. Following a chapter on methods and model validation, the results in chapter 3 reveal the importance of adhering to the lowest possible emissions trajectory which limits warming to 1.5°C by the end of the century. This scenario keeps projected warming to slightly above current conditions. Under the higher emissions trajectories (~4°C and ~5°C of global average warming) coral stress metrics quadruple present-day warming conditions which would result in annual mass coral mortality events by 2080. In chapter 4, climate refugia have been identified from present-day conditions based on downscaled surface temperature outputs in agreement with observations. The lower emissions trajectories maintain these locations as refugia while the higher emissions trajectories reveal the loss of these increasingly valuable locations. Areas of climate refugia can be attributed to tidal and wind energy fluctuations providing relief from warming. However, this advantage does not persist after global warming exceeds ~3°C. Refugia are more likely to persist in the northern GBR under increased warming even though recent evidence suggests there are fewer refugia in this region. Atmospheric spatial patterns on the GBR under warming above ~3° C reveal a change in wind and shortwave radiation patterns driving a loss in the identified climate refugia locations. Lastly, stratification was tested in chapter 5 to determine if increases in stratification could provide thermal relief to bottom temperature waters from 0-50 m under increased warming into the future using downscaled bottom temperature projections. Chapter 5 results demonstrate that warming influences bottom temperatures of stratified locations, showing little support for deeper reefs to act as a climate refuge. The temporal, spatial, and bottom temperature analysis of downscaled climate projections provides insight into the consequences of a warming planet for the GBR and can be used to inform management and policy decisions to protect coral reefs.

Increases in the magnitude, frequency, and duration of warm seawater temperatures are causing mass coral mortality events across the globe. Although, even during the most extensive bleaching events, some reefs escape exposure to severe stress, constituting potential refugia. Here, we identify present-day climate refugia on the Great Barrier Reef (GBR) and project their persistence into the future. To do this, we apply semi-dynamic downscaling to an ensemble of climate projections released for the IPCC's recent sixth Assessment Report. We find that GBR locations experiencing the least thermal stress over the past 20 years have done so because of their oceanographic circumstance, which implies that longer-term persistence of climate refugia is feasible. Specifically, tidal and wind mixing of warm water away from the sea surface appears to provide relief from warming. However, on average this relative advantage only persists until global warming exceeds ~3°C.

Abstract. The marine impacts of climate change on our societies
will be largely felt through coastal waters and shelf seas. These impacts
involve sectors as diverse as tourism, fisheries and energy production.
Projections of future marine climate change come from global models.
Modelling at the global scale is required to capture the feedbacks and
large-scale transport of physical properties such as heat, which occur
within the climate system, but global models currently cannot provide detail
in the shelf seas. Version 2 of the regional implementation of the Shelf Sea
Physics and Primary Production (S2P3-R v2.0) model bridges the gap between
global projections and local shelf-sea impacts. S2P3-R v2.0 is a highly
simplified coastal shelf model, computationally efficient enough to be run
across the shelf seas of the whole globe. Despite the simplified nature of
the model, it can display regional skill comparable to state-of-the-art
models, and at the scale of the global (excluding high latitudes) shelf seas it
can explain >50 % of the interannual sea surface temperature (SST) variability in
∼60 % of grid cells and >80 % of interannual
variability in ∼20 % of grid cells. The model can be run at
any resolution for which the input data can be supplied, without expert
technical knowledge, and using a modest off-the-shelf computer. The
accessibility of S2P3-R v2.0 places it within reach of an array of coastal
managers and policy makers, allowing it to be run routinely once set up and
evaluated for a region under expert guidance. The computational efficiency
and relative scientific simplicity of the tool make it ideally suited to
educational applications. S2P3-R v2.0 is set up to be driven directly with
output from reanalysis products or daily atmospheric output from climate
models such as those which contribute to the sixth phase of the Climate Model
Intercomparison Project, making it a valuable tool for semi-dynamical
downscaling of climate projections. The updates introduced into version 2.0
of this model are primarily focused around the ability to geographical
relocate the model, model usability and speed but also scientific
improvements. The value of this model comes from its computational
efficiency, which necessitates simplicity. This simplicity leads to several
limitations, which are discussed in the context of evaluation at regional
and global scales.
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