Since April, the world has seen a record rise in sea temperatures and that’s bad news for the plants and animals that call the ocean home.
Long and persistent periods of extreme temperatures can cause some species to migrate and others to invade, having devastating effects on local ecosystems.
Global warming appears as a gradual increase in temperatures around the world over time, caused by increasing greenhouse gas emissions.
However, scientists believe that the most significant effects come from short-term increases in temperature.
In the ocean, these distinct periods of extreme temperatures, lasting from several weeks to several months, are called marine heat waves.
Marine heat waves can be generated by atmospheric or oceanic processes.
For example, weather systems such as high pressure systems can cause lower clouds and more solar heating, while ocean changes can be driven by stronger poleward currents, which move heat from higher to lower latitudes.
The likelihood and intensity of these atmospheric and oceanic drivers of marine heat waves can also be influenced by large-scale events such as El Niño or La Niña.
During an El Niño event, marine heat waves are likely to occur across large parts of the oceans.
Marine heatwaves can have dramatic impacts on marine organisms and ecosystems that may persist long after temperatures return to normal.
Its effects range from reduced growth of microscopic marine plants to mass mortality of fish and marine mammals, encroachment of invasive species, and outbreaks of toxic algae.
Importantly, marine heatwaves are linked to widespread destruction of species such as coral reefs, seagrass forests and seagrass beds, which provide habitats and breeding grounds for a large portion of ocean biodiversity.
These impacts can have devastating effects on the fisheries, aquaculture and tourism industries, with individual events causing direct losses amounting to hundreds of millions of dollars.
The Ningaloo Nino, which was created in early 2011, was an iconic extreme event.
Strong winds over the equatorial Pacific Ocean – associated with an extreme La Niña – pushed warm water westward and poleward across the Indonesian archipelago into the Indian Ocean and along the coast of western Australia.
This influx of warm water created record-breaking marine heat, which destroyed endemic seagrass forests across more than 100 kilometres along the Australian coast, including a third (1,300 square kilometres) of the Shark Bay seagrass meadow, a UNESCO World Heritage Site.
The Ningaloo Niño caused mortality and reduced reproduction of abalone, scallops and crabs, resulting in the closure of related fisheries for several years.
The Tasman Sea is another centre of ocean warming and marine heat waves.
Two consecutive extreme events occurred in the summers of 2015/16 and 2017/18. Their causes were very different.
The first was caused mainly by the intensification of the East Australian Warm Current, while the second was caused by a prolonged high pressure system over the ocean.
These and previous events led to an invasion of sea urchins from mainland waters, which led to the destruction of seagrass forests along the coast of eastern Tasmania.
The 2015/16 event alone led to new diseases in cultured oysters, poor salmon performance and high mortality of abalone. All of this led to economic losses of over half a billion dollars.
As marine heatwaves exist against a backdrop of long-term global ocean temperature rise, these extreme events are becoming more intense and more frequent.
The number of days with marine heat wave conditions each year has increased by more than 50 percent over the past century.
The short time between marine heatwaves means that many species do not have time to recover between events, which may cause species to shift their ranges or become extinct.
And this situation will become even worse in the future.
Several studies show that some coral reefs are losing their hard corals.
And the survival of tropical coral reefs is also in doubt because of future increases in temperatures.
Since April, the oceans have been warmer than at any time during the instrumental record.
Temperatures may have been this warm at least 100,000 years ago—before the last ice age.
As a result, scientists are seeing more marine heatwaves than ever before. And that’s before the added pressure of a developing El Niño.
In just the past few weeks, extreme marine heat waves have been observed across all ocean basins, including the waters around Britain and Japan, off the coast of Peru, and off the coasts of California, Florida, and eastern and western Canada.
As the Northern Hemisphere enters its hottest season, marine heatwaves are at their most dangerous, pushing marine organisms past their thermal limits.
In the next few months, reports of significant damage to the ecosystem will begin to emerge.
Understanding the physical drivers of these events and their biological implications gives scientists some ability to predict their likelihood in the future.
This can help marine resource managers make decisions, such as relocating aquaculture stocks, reducing fishing quotas, or taking direct action to limit temperature increases in small, high-value areas (such as shading aquaculture farming areas, or moving aquaculture impoundments out of harm’s way).
But ultimately, the only solution to avoid increasing impacts is to curb 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 affects 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 is funded by the Australian Research Council.
Dr. Katie Smith is a Postdoctoral Research Assistant at the Marine Biological Association in the U.K. Her research interests include understanding the effects of climate change on marine species, throughout their life histories and from the individual to the whole ecosystem level.
Originally published by 360info under Creative Commons.
