Climate impacts at coral reefs have been a societal concern and a focus of academic study for over three decades. Climate impacts include the intensification of storms, sea level rise, and ocean acidification. The highest-profile climate impact upon reefs is coral bleaching - the loss single-celled algae symbiotic with coral under very warm ocean temperatures, which can rapidly lead to coral death over large areas of reef. 

As for many other climate impacts on human and natural systems, changes in coral bleaching have been projected under different levels of climate change using global climate models (or general circulation models) - computational models of the heat flux and fluid flow of the atmosphere and ocean. At the turn of the 21st century, the sea-surface temperature values output by global climate models - the necessary data used to estimate coral bleaching - were typically provided as monthly-averages.

To project coral bleaching, monthly "anomalies" were calculated - for each month, if the sea-surface temperature average was warmer than the long-term mean for that month, the amount (in degrees Celsius) that it exceeded the long-term mean by was recorded. If there was more than one monthly anomaly over consecutive months, they were summed together, typically over a three-month or four-month period. If this sum exceeded 1°C then bleaching was predicted to occur over that period, and if it exceeded 2°C then severe bleaching with mortality of many coral colonies was predicted to occur. This approach to projecting coral bleaching using the sea-surface temperatures predicted by global climate models is called "Degree Heating Months".

A related approach is used to predict coral bleaching in the present day, using sea-surface temperatures that are detected by a network of specialised satellites that measure infrared radiation emitted by the ocean's surface. Present-day predictions are highly important for facilitating the coordinated response of coral reef managers, conservationists and scientists to widespread coral bleaching events, such as those that occurred globally in 2014-2017, and 2023-2024. The present-day method, "Degree Heating Weeks" uses the weekly-mean of sea-surface temperatures summed over a 12-week period to predict coral bleaching. 

Degree Heating Weeks and Degree Heating Months are usually considered to be related according to a factor of four: divide any value of Degree Heating Weeks by 4, and the equivalent value in Degree Heating Months will result. This is the basis for the 1°C bleaching and 2°C severe bleaching thresholds used for Degree Heating Months: according to a factor of 4, they are equivalent to Degree Heating Week thresholds of 4°C and 8°C that are used to indicate bleaching and severe bleaching, respectively. Since Degree Heating Weeks is used for present-day predictions, its values can be compared to the actual level of bleaching that occurred on coral reefs. So, the Degree Heating Week thresholds are very well "ground-truthed" in real-life data, whilst the accuracy of Degree Heating Month thresholds is reliant on whether the "factor of 4" relationship between Degree Heating Weeks and Degree Heating Months is sound.

In a study in Nature Geoscience, I and coauthors directly compared Degree Heating Week and Degree Heating Month values for the past two decades to examine the accuracy of Degree Heating Months, for coral reefs on the Great Barrier Reef. This was possible because satellite-detected sea-surface temperatures are available daily for that period, allowing the calculation of both value types. We found that Degree Heating Months substantially over-estimated heat stress during that period, compared to Degree Heating Weeks.

To explore this potential discrepancy further, we took advantage of the increased availability of sea-surface temperature outputs for global climate models to examine the accuracy of Degree Heating Months. In newer generations of global climate models, sea-surface temperature has been provided at a daily interval, rather than as monthly averages. We again compared Degree Heating Week and Degree Heating Month values using global climate models for the past two decades at the Great Barrier Reef. We found that Degree Heating Months substantially over-estimated heat stress, compared to Degree Heating Weeks, in four out of the six global climate models that we examined. 

Degree Heating Months are often used in conjunction with ecosystem modelling to predict the future trajectory of coral reefs this century.  We performed ecosystem modelling of the Great Barrier Reef for the next eight decades, separately for Degree Heating Week values and Degree Heating Month values calculated from the same six global climate models. We found that future projections of Great Barrier Reef coral cover (the percent area of the seafloor that is covered by coral) were approximately double when Degree Heating Weeks was used in projections, compared to Degree Heating Months. In other words, Degree Heating Weeks produced a "more optimistic" prediction of future health of the Great Barrier Reef, compared to Degree Heating Months.

Overall, future projections of coral reefs under climate change this century are still not hopeful - coral cover declined to very low values under the climate change scenarios that are currently considered likely (those scenarios known as RCP 4.5 and RCP 6.0), regardless of which heat stress calculation method was used. However, using Degree Heating Weeks - the method that has been well-validated against observations of coral bleaching - resulted in a slower decline in the Great Barrier Reef as ocean warming intensified. 

Accurate projections of coral reef health will allow better planning for adaptation of communities and countries that rely on coral reefs for their livelihoods and economies, as reefs degrade or change under future warming.  Reef restoration initiatives also require accurate projections of coral ecosystems - for example, to understand the pace of ecosystem change that must be met in order for restoration to be effective. Understanding that Degree Heating Weeks is the best method for future heat stress prediction at coral reefs brings us a step closer to reliable projection of future coral reef health.