Since April, sea temperatures around the world have been at their highest ever levels, and that’s not good news for the animals and plants that live in the sea.

Prolonged and more frequent extreme temperatures can lead to the extinction of certain species and the invasion of others, which can have devastating effects on the local ecosystem.

Global warming is manifested by a gradual increase in temperatures over time around the world caused by increased greenhouse gas emissions.

However, scientists note that the most significant impacts are due to short-term temperature spikes.

In the ocean, these individual periods of extreme temperatures that last weeks to months are called marine heat waves.

Marine heatwaves can be caused either by the atmosphere or by processes in the ocean.

For example, weather systems such as high pressure systems can lead to low-hanging clouds and increased solar heating, while changes in the oceans can be caused by increased polar currents that transport heat from high to low latitudes.

The probability and intensity of these atmospheric and oceanic causes of marine heat waves can also be influenced by large-scale phenomena such as El Niño or La Niña.

Larger parts of the oceans are likely to experience marine heat waves during El Niño events.

Marine heatwaves can have dramatic impacts on marine organisms and ecosystems that can persist long after temperatures return to normal levels.

The effects range from suppressed growth of microscopic marine plants to mass mortality of fish and marine mammals, the spread of invasive species and the proliferation of toxic algae.

Importantly, marine heatwaves are associated with large-scale die-offs of species such as coral reefs, kelp forests and seagrass meadows, which are home to and breeding grounds for much of the ocean’s biodiversity.

These impacts can have devastating consequences for the fishing, aquaculture and tourism industries, with individual events potentially causing direct damages amounting to hundreds of millions of dollars.

Founded in early 2011, Ningaloo Nino was an extreme event with cult status.

Increased winds along the Pacific equator – combined with an extreme La Niña phenomenon – pushed warm water westward and through the Indonesian archipelago into the Indian Ocean and poleward along the coast of Western Australia.

This flood of warm water triggered a record-breaking marine heatwave that destroyed more than 100 kilometres of endemic kelp forest along Australia’s coast, as well as a third (1,300 square kilometres) of the seagrass meadows of Shark Bay, a UNESCO World Heritage Site.

The Ningaloo Nino caused the death of abalone, scallops and crabs and reduced their reproduction, leading to the closure of the corresponding fisheries for several years.

The Tasman Sea is another hotspot for ocean warming and marine heatwaves.

In the summers of 2015/16 and 2017/18, two consecutive extreme events occurred. Their causes were very different.

The first was mainly due to a strengthening of the warm East Australian Current, while the second was caused by a long-lasting high pressure area over the ocean.

These and earlier events led to an invasion of sea urchins from mainland waters, which resulted in the decimation of kelp forests off the coast of eastern Tasmania.

The 2015/16 event alone resulted in new diseases in farmed oysters, poor salmon performance and high mortality rates in abalone, causing economic losses of more than half a billion dollars.

As marine heatwaves occur against the backdrop of long-term global warming of the oceans, these extreme events are becoming more severe and frequent.

Over the last century, the number of days per year on which heat waves occur at sea has increased by more than 50 percent.

The shorter periods between marine heatwaves mean that many populations have no time to recover between events. This can lead to species shifting their range or becoming extinct.

And this will only get worse in the future.

Many studies show that some coral reefs are losing their hard corals.

And with increasing warming in the future, the very existence of tropical coral reefs is in question.

Since April, the oceans have been warmer than ever before in the instrumental weather record.

It is likely that temperatures were this high at least 100,000 years ago – before the last ice age.

As a result, scientists are observing that larger parts of the ocean are affected by heat waves than ever before – and this is before the El Niño climate phenomenon gains additional momentum.

In the last few weeks alone, extreme marine heatwaves have occurred in all ocean basins, including around the UK and Japan, off Peru, and in the waters off the coast of California, Florida, and the east and west coasts of Canada.

When the warmest season begins in the Northern Hemisphere, marine heat waves are at their most dangerous, pushing marine organisms beyond their thermal limits.

Over the next few months, the first reports of significant damage to the ecosystem are expected to emerge.

Understanding the physical causes of these events and their biological effects allows scientists to predict their likelihood in the future.

This can help marine resource managers make decisions such as relocating aquaculture stocks, reducing catch quotas, or taking direct actions to mitigate warming (such as shading aquaculture sites or moving aquaculture pens out of harm’s way) in small, high-value regions.

Ultimately, to avoid escalating impacts, the only solution is to stop greenhouse gas emissions.

Associate Professor Alex Sen Gupta is a scientist and lecturer at the Climate Change Research Centre and the Centre For Marine Science and Innovation at UNSW. His work focuses on 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 was funded by the Australian Research Council.

Dr Katie Smith is a postdoctoral fellow at the Marine Biological Association in the UK. Her research interests include studying the impacts of climate change on marine species throughout their life history and from the individual to the whole ecosystem level.

Originally published under Creative Commons by 360info.