Marine Heatwaves
Marine heatwaves (MHWs) are extreme rises in ocean temperature for extended periods of time – at least 5 consecutive days. They can occur at different locations in the ocean, and at any given time of the year. It can last for days, months and even years. Marine heatwaves have been detected at the ocean surface, and at more than 1000 meters deep.
In the past decades, marine heatwaves have become more frequent, more intense and widespread.
What are the impacts of marine heatwaves?
Marine heatwaves cause environmental and socio-economic impacts. Extreme temperatures affect marine life, having a direct toll on ecosystem functioning, food availability, and fisheries and aquaculture. Long periods of extremely high temperatures have been the reason for the development of harmful algae blooms, contributing to the decrease in oxygen levels in the water and to the release of toxins impacting marine ecosystems and coastal populations.
The impact on key species can have profound ecosystem consequences, such as the disappearance of seagrass meadows and the decrease in CO₂ sequestration, or the destruction of coral reefs and the weakening of natural coastal protection against waves and storms.
Credit: The Ocean Agency / Ocean Image Bank
Why monitor marine heatwaves?
The ocean has been warming consistently since the late 19th century, although in recent years, the rate of warming has accelerated. According to the Intergovernmental Panel on Climate Change (IPCC), at the current global warming rates, marine heatwaves will likely become more frequent and more intense.
Besides the impact sea temperatures have on marine life, they also play a crucial role in shaping weather patterns, ocean circulation, and the global water cycle. Being able to predict the occurrence of extreme events can help mitigate impacts on economic activities and coastal populations.
How are marine heatwaves calculated?
The main characteristics of marine heatwaves are their duration and intensity. During a marine heatwave event at a given location, seawater temperature is significantly higher than the “normal” average temperature for that area, at that time of the year.
Using a long period of observations (over the last 30 years), we use statistical descriptions to detect when and how much a temperature threshold is surpassed for at least 5 consecutive days.
A marine heatwave is defined as one where the measured temperature is within 10% of the maximum values observed (i.e., above the Threshold (90th quantile) see diagram adapted from Hobday et al. (2018) above), for at least 5 consecutive days. For this, the intensity is compared to the difference between the climatological mean and the 90th percentile value (threshold). A marine heatwave intensity between 1 and 2 times this difference corresponds to a heatwave of moderate category; between 2 and 3 times, to a strong category; between 3 and 4 times, to a severe category; and a difference greater than 4 times corresponds to an extreme category.
Categorisation of marine heatwaves according to its intensity: I – moderate, II- strong, III- Severe and IV- extreme. Source: Hobday et al. (2018). Image credit: Marine Heatwaves International Working Group.
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Notes and references
Oliver, E. C. J., Benthuysen, J. A., Darmaraki, S., Donat, M. G., Hobday, A. J., Holbrook, N. J., Schlegel, R. W., and Gupta, A. S. (2021): Marine Heatwaves, Annual Review of Marine Science, 13, 313–342, https://doi.org/10.1146/annurev-marine-032720-095144
Pearce, A., Lenanton, R., Jackson, G., Moore, J., Feng, M. and Gaughan, D. (2011). The “marine heat wave” off Western Australia during the summer of 2010/11. Fisheries Research Report No. 222. Department of Fisheries, Western Australia. 40pp.
IPCC, 2023: Sections. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 35-115, doi: 10.59327/IPCC/AR6-9789291691647