Fellowship Spotlight: Stuart Brown

For this Fellowship Spotlight, we interviewed Stuart Brown who recently completed his Fellowship with Subak Australia. 

Stuart generated a high spatial resolution dataset of sea surface temperatures that characterises trajectories of warming for the world’s coral reefs. The dataset also shows the associated changes in bleaching risk over the time range of 1985 to 2100 using a range of climate models and future scenarios. 

This aggregated data has the capacity to better inform conservation management decisions and highlight areas of extreme concern, particularly with coral reefs around the globe being exposed to increasingly severe and frequent marine heatwaves, causing mass bleaching events that threaten reef ecology. Stuart discusses the variety of datasets being collected to model these projections. He also provides more insights into the severity of rising temperatures for the reefs. 

What was your motivation behind your current postdoctoral research on the ecological impacts of climate change on the world's coral reefs?

I got involved in this project as an offshoot of some other work I was doing. A lot of my recent work has had to use simulations of hindcast and forecasted climate conditions. My colleague and collaborator, Dr Camille Mellin at the University of Adelaide won an Australian Research Council Future Fellowship to look at the impacts of climate change on coral reefs and the knock-on nutritional impacts of those changes. 

And part of her project needed downscaled daily sea surface temperature datasets so she could get an understanding of future warming with regard to coral bleaching events. This data includes looking at how likely corals are to bleach in the future, how long they will be under temperature conditions that expose them to severe bleaching, how frequently it’s meant to occur and to look at the recurrence intervals of potential bleaching conditions. She couldn’t do that data analysis as she had never done it before, and I had a spare day a week, so she invited me on board to complete the temperature analysis.

What datasets have you been generating during your postdoctoral? And how does this differ from existing coral analyses?

The datasets we are using are simulated climate forecasts from a range of CMIP6 climate models. Previously, when people have done this sort of analysis, they have used monthly simulated data sets and assumed that you can take a monthly average sea surface temperature and apply those values to daily average temperatures for the month, which can bias your calculations as this won’t pick up extreme or rapid changes in sea surface temperature. 

Whereas this time we have taken daily simulated sea surface temperatures and statistically downscaled them to a more relevant spatial scale. The CMIP6 models are typical ~100km scale, and we’ve reduced that to ~50km scale. So now we’re working with daily estimates of sea surface temperatures at a 50km scale from 1985-2100 under three different climate change scenarios. The data has been statistically validated against observational datasets and we do have quite a bit of confidence in the projections that we have.

Using these daily downscaled and bias-corrected sea surface temperatures we’ve generated a range of climate change metrics that are relevant to corals. These metrics include the number of degree heating weeks (DHW), which is a measure of accumulated heat stress and a strong predictor of coral bleaching. We have estimates of the timing of when the DHW goes past certain thresholds that are indicative of severe to catastrophic bleaching, and annual summaries of coral stress under different climate change scenarios and different models. Using these datasets, we’ve been able to calculate the maximum degree heating weeks that we can expect to see and how that relates to coral stress and future bleaching events.

We also calculated estimates of the average recurrence interval of bleaching conditions. Typically, corals will bleach during the warmer months and tend to get a bit of a reprieve during the cooler seasons when the sea surface temperature drops. We have calculated the change in that reprieve period and how long that reprieve period will be relative to the current period. This gives us an idea of how long-lasting the bleaching events will last, and how quickly they’re likely to re-occur.

What have been your major findings on the extinction and biodiversity status of coral reefs?

We haven’t looked at this specifically yet, but we have looked at threats on coral reefs with regards to possible bleaching events. Obviously, under the more extreme SSP-5 climate change scenario we see much more extreme impacts compared to the lower SSP2 warming scenario. Even the SSP2 is a lot higher than baseline conditions so we are finding the time of occurrence (when that coral bleaching event will start to happen) is becoming earlier and earlier in the year. 

For instance, you might have a coral reef that usually starts having bleaching events in January that could for example get pushed back to November under some of the future scenarios. That obviously means that bleaching events will start occurring earlier, so that reprieve period gets smaller and smaller. For example, a reef might get 6 months of reprieve from bleaching conditions now, but in the future that could reduce to less than 2 months. We’ve found that for a lot of reefs the conditions that lead to bleaching will be starting sooner, lasting longer, and reoccurring more rapidly than they currently do.

What discoveries have you made on coral species or geographical landscapes? Specifically on their resilience to climate change?

There are distinct latitudinal gradients in both sea surface temperatures and predicted degree heating weeks. Near the equator water is getting much warmer and will continue to get warmer in the future. We do see some latitudinal variation as we go away from the equator. These below images clearly demonstrate what locations will have the most extreme consequences under the most extreme SSP5-8.5 climate change scenario.

Graph 1

The graph shows predictions for sea surface temperatures under condition SSP5-8.5 for 2020, 2050 and 2080.

Graph 2

This graph shows the number of days above 8-degree heating weeks which are the conditions that will lead to catastrophic coral bleaching under the predicted SSP5-8.5 scenario for 2020, 2050 and 2080.

Finally, what are your hopes for the data you are aggregating, and how would you like it to be used in the future by researchers and agencies?

We have generated these extensive daily sea surface temperature datasets and multiple datasets of coral bleaching risk. We're hoping that scientists, managers, NGOs, etc. will be able to incorporate them into management plans, analyses of future species distributions, threat assessments, etc.

Particularly with corals, a lot of the degree heating weeks datasets we’re generating can be used to potentially inform conservation management and identify regions where any intervention might, unfortunately, be futile, regardless of the scenario the coral/ecosystem will be severely affected either way. 

But it can also inform us that if we do go down one path we might be in trouble. Still, if we get our act together and go down a lower emissions climate path, we may be able to identify areas where we might be able to manage and protect the corals and the communities living on the coral reef as it begins to warm.