The devastating impact of extreme weather is etched into our collective memory. In 2021 alone, the world saw record-shattering heat in western North America, deadly floods in Europe, China and west Africa, and wildfires in the Mediterranean.
But alongside these events happening on land, the oceans experience extreme conditions too. In 2015, the North Pacific saw the largest marine heatwave ever recorded, known simply as the “Blob”.
As Earth warms, marine heatwaves are likely to become more intense, happen more often and last longer. But as our recent paper in Nature explains, we are also set to see more occasions – as with the “Blob” – where two or more types of extreme events occur simultaneously.
These “compound” events likely have more devastating impacts than a single heatwave, affecting marine life such as tiny floating plants, seabirds and sea lions.
The ‘Blob’ in the Pacific
In 2015-16, much of the tropical Pacific was experiencing record high sea surface temperatures associated with a strong El Niño event, causing widespread bleaching and death of coral reefs.
At the same time, the North Pacific was seeing extreme warmth, in an event that became known as the “Blob”. At its peak in 2015, the Blob covered about 4m square kilometres (km2) of ocean, stretching from Alaska to Baja California.
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Unsurprisingly, the Blob caught the headlines at the time. National Geographic, for example, described it as, “The blob that cooked the Pacific”. Some of the ecological consequences of the Blob are still lingering today, with several marine animal populations not having fully recovered.
A ‘triple-compound’ extreme event
Our research used simulations from a regional ocean biogeochemical model to explore not only the extreme heat that characterised the Blob, but also whether the upper water column in the northeast Pacific at the time was unusual in other ways too. In particular, with respect to its oxygen levels and acidity (measured by the pH of the seawater).
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The figure below shows a timeseries for the part of the northeast Pacific where the Blob occurred. Light orange shading denotes when a marine heatwave occurred (a single event). Dark orange and purple denote dual-compound events involving a marine heatwave and either low oxygen or extremely low pH, respectively. Grey signifies times when all three types of extreme occurred simultaneously.
The chart shows grey shading for several months in 2015 in the northeast Pacific. This indicates that the heatwave coincided with extremes in acidity and oxygen, making at least part of the Blob a triple-compound event. The chart also shows several dual extremes (dark orange and purple) prior to the peak in 2015, making the Blob period from 2013 until 2015 highly unusual in the record extending back to the 1980s.
The triple-compound event covered a large area of ocean, as shown by the grey areas in the map on the left-hand side below. The darker the grey, the deeper the extreme conditions penetrated the ocean.
A future with more extremes
As part of our research, we carried out a global model analysis of extremes in ocean temperature, acidity and oxygen, which allowed us to put the results from the Blob into a global perspective.
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Our results show that between pre-industrial times and the present, marine heatwaves have become 10 times more common and low oxygen extremes have become about five times more common. Ocean acidity extremes have become essentially near permanent, meaning they have increased nearly 100-fold.
These striking increases are a direct consequence of the upward trends in ocean warming, oxygen depletion – known as “deoxygenation” – and ocean acidification, which themselves are caused primarily by emissions of CO2 into the atmosphere. We can, therefore, say with high confidence that the increase in marine heatwaves and ocean acidification extremes are primarily caused by human activities.
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Reference: Gruber, N. et al. (2021) Biogeochemical extremes and compound events in the ocean, Nature, doi: 10.1038/s41586-021-03981-7
Carbon Brief, 24 January 2022. Press release.
