Marine heatwave: what's that?

We’ve all personally experienced heat waves before. A short period of time where the warm temperatures take us by surprise and next minute we’re wearing shorts and complaining about the ice tray not being refilled. We look at each other, raise our eyebrows and say “Well, that’s climate change for you!”

But what most people don’t know is that these heatwaves are happening in our oceans too. 

Known as Marine Heat Waves (MHW), scientists have been investigating the causes of these periodic water temperature increases.

It’s important to note that when studying ocean processes, time and spatial scales are important as short-term local trends can completely contrast with long-term global ones.

Research has found that the length, severity and frequency of MHW’s are concentrated in areas such as the equatorial Pacific, and they are progressively worsening with a 50% increase in annual MHW’s over the past century (Oliver et al., 2018). This is expected to be due to increase in sea surface temperatures related to global climate change.

Whilst we land dwellers may not feel the direct affects of these heatwaves, they are wreaking havoc on marine ecosystems.

In 2016 a mass bleaching event of the Great Barrier Reef saw a 50% reduction in live coral within one year (Stuart-Smith et al., 2018). Coral is the foundation of a barrier reef, providing complexity for other marine species to co-exist in and therefore populations of fish which eat algae from coral declined on the northern reefs. Note also that other herbivorous fish species in the south increased. 

We are still seeing the impacts of this event years down the track and won’t know the full impacts until the bleached coral has eroded. However, it is clear that once ocean temperatures increase beyond the survival range for a species it is difficult to recover and biodiversity decreases. 

Looking to the future, scientists are continuing to collect data on ocean temperature fluctuations in order to observe long term patterns and therefore increase MHW prediction accuracy. With accurate predictions we can investigate how to protect ecosystems from further degradation.

HOLBROOK, N. J., SCANNELL, H. A., SEN GUPTA, A., BENTHUYSEN, J. A., FENG, M., OLIVER, E. C. J., ALEXANDER, L. V., BURROWS, M. T., DONAT, M. G., HOBDAY, A. J., MOORE, P. J., PERKINS-KIRKPATRICK, S. E., SMALE, D. A., STRAUB, S. C. & WERNBERG, T. 2019. A global assessment of marine heatwaves and their drivers. Nature Communications,10,2624.

JACOX, M. G. 2019. Marine heatwaves in a changing climate. Nature 485-487.

OLIVER, E. C. J., DONAT, M. G., BURROWS, M. T., MOORE, P. J., SMALE, D. A., ALEXANDER, L. V., BENTHUYSEN, J. A., FENG, M., SEN GUPTA, A., HOBDAY, A. J., HOLBROOK, N. J., PERKINS-KIRKPATRICK, S. E., SCANNELL, H. A., STRAUB, S. C. & WERNBERG, T. 2018. Longer and more frequent marine heatwaves over the past century. Nature Communications,9,1324.

STUART-SMITH, R. D., BROWN, C. J., CECCARELLI, D. M. & EDGAR, G. J. 2018. Ecosystem restructuring along the Great Barrier Reef following mass coral bleaching. Nature,560,92-96.