By Alex Sen Gupta and Katie Smith
Since April, the world has seen record high ocean temperatures and that’s bad news for the marine life like plants and animals that call the ocean home.
Longer and more frequent bouts of extreme temperatures can cause the exodus of some species and the invasion of others, with potentially devastating impacts on the resident ecosystem.
This year the oceans have been warmer than at any time during the instrumental record. It’s probably at least 100,000 years — before the last ice age — since temperatures could have been this warm. As a consequence, scientists are seeing more of the ocean experiencing marine heatwave conditions than ever.
And that’s before the added push from the developing El Niño. Over the past few weeks alone, there have been extreme marine heatwaves in all of the ocean basins, including around the UK and Japan, off Peru and in waters extending off of the Californian, Florida and east and west Canadian coasts.
As the Northern Hemisphere enters its warmest season, marine heatwaves are at their most dangerous, pushing marine organisms above their thermal limits.
Over the next few months, expect reports of significant ecosystem harm to start to emerge. An understanding of the physical drivers of these events and their biological impacts provides scientists with some ability to forecast their likelihood in the future. This can help marine resource managers make decisions, like moving aquaculture stocks, reducing fishing quotas or taking direct action to suppress warming (like shading of aquaculture cultivation areas, or moving aquaculture pens out of harm’s way) in small, high-value regions.
Global warming manifests as a gradual increase in temperatures over time around the world, caused by increased greenhouse gas emissions.
However, scientists are finding that the most important impacts come from short-term spikes in temperature. In the ocean, these discrete periods of extreme temperatures, lasting weeks to months, are called marine heatwaves and they can be can be generated by either the atmosphere or by ocean processes.
Marine heatwaves can have dramatic impacts on marine organisms and ecosystems that may extend for long periods after temperatures have returned to normal. Impacts range from the suppressed growth of microscopic marine plants to mass deaths in fish and marine mammals, encroachment of invasive species and toxic algal outbreaks.
Importantly, marine heatwaves have been associated with extensive dieback of species like coral reefs, kelp forests and seagrass beds that form the homes and breeding grounds for a large amount of the ocean’s biodiversity.
These impacts can have devastating knock-on effects for fisheries, aquaculture and tourism industries, with individual events linked to direct losses of up to hundreds of millions of dollars.
The Ningaloo Niño that formed early in 2011 was an iconic extreme event. Intensified winds along the Pacific equator — associated with an extreme La Niña — forced warm water westwards and through the Indonesian Archipelago, into the Indian Ocean and poleward along the coast of Western Australia.
This flood of warm water caused a record-breaking marine heatwave that wiped out the endemic kelp forest for over 100 km along the Australian coast along with one-third (1,300 square kilometres) of the Shark Bay seagrass meadow, a UNESCO world heritage site.
The Ningaloo Niño caused deaths and reduced reproduction in abalone, scallops and crabs that led to the closure of associated fisheries for several years.
The Tasman Sea is another hotspot for ocean warming and marine heatwaves. Two consecutive extreme events occurred the summer of 2015/16 and 2017/18. These and previous events brought an invasion of sea urchins from mainland waters that led to the decimation of kelp forests off eastern Tasmania. The 2015/16 event alone led to new diseases in cultured oysters, poor salmon performance and high mortalities of abalone.
Together this led to economic losses of more than half a billion dollars.
The shorter time between marine heatwaves means that many populations no longer have the time to recover between events, which can lead to species moving their range or being wiped out.
But ultimately, to avoid escalating impacts the only solution is to stop greenhouse gas emissions.
Associate Professor Alex Sen Gupta is a research scientist and lecturer at the Climate Change Research Centre and the Centre For Marine Science and Innovation at UNSW. His work revolves around the role of the ocean in the climate system, how the ocean influences regional climate and what global climate models tell us about the future of the ocean, with a recent focus on marine heatwaves.Associate Professor Sen Gupta’s research has been funded by the Australian Research Council.Dr Katie Smith is a postdoctoral research assistant at the Marine Biological Association in the UK. Her research interests include understanding the impacts of climate change on marine species, throughout their life history and from individual to whole-ecosystem level.
The Ultramarine project – focussing on research and innovation in our marine environments – is supported by Minderoo Foundation's Flourishing Oceans initiative.