Nature published an article in the future that many parts of the world will fall into a permanent marine heat wave

On May 5, 2021, Nature published an article saying that marine heat waves will persist and worsen with climate change, which in turn will destroy ecosystems. It is predicted that at some point in the next century, many parts of the world will be trapped in a permanent state of marine heatwave.

Ocean heatwaves are getting worse: Climate change is causing ocean heatwaves to become more frequent and longer-lasting, while heat waves are getting hotter and hotter. Therefore, it is necessary to understand the causes of ocean heat waves and learn how to predict ocean heat waves.

The effects of ocean heatwaves can ripple through the food chain. During the 2013-2016 marine heat wave event known as Blob, water temperatures in the Pacific Northwest rose and nutrients decreased, disrupting phytoplankton growth. Then, the population of the large-scale salmon (Oncorhynchus tshawytscha) plummeted, resulting in the death of as many as 1 million seabirds in the Gulf of Alaska. Over the past few decades, ocean heatwaves have also triggered widespread coral bleaching worldwide.

The summer of 2013-2014 was a tough year for Brazil. The severe drought in São Paulo has wreaked havoc on crops and triggered water shortages. At the same time, ocean temperatures rise and chlorophyll concentrations (a measure of biological productivity) drop dramatically. When the researchers analyzed the data, they found that drought and ocean warming share a common cause: the high-pressure atmospheric system that dominates southeastern Brazil for most of the summer. This long-lasting system of high pressure is associated with atmospheric blocking, one of the most common drivers of ocean heat waves as well as terrestrial heat waves. Atmospheric congestion results in sparse cloud cover and relatively little wind. Due to the lack of clouds, more solar radiation is able to reach the ocean and warm it; at the same time, a windless atmospheric environment can hinder mixing and evaporation. These factors cause heat build-up in the upper ocean, which can change wind direction in a way that exacerbates or prolongs warming times.

In a study published in 2019, researchers found that about 60 percent of ocean heatwave events in the southwestern Atlantic, including those from 2013 to 2014, originate from high-pressure systems thousands of kilometers away over the Indian Ocean. These high-pressure systems then move through the atmosphere in the direction of South America. However, since atmospheric obstruction depends on multiple factors, reconstructing this phenomenon in numerical models is complex.

There is a remote correlation between these seemingly unrelated regions. The researchers found that this process usually begins with the warming and rising of air near the Earth's surface. Convection over the Indian Ocean enters the atmosphere, causing atmospheric waves reaching South America and causing ocean heat waves. This interconnectedness of the climate system complicates forecasting activities. Because researchers not only have to model specific areas in the ocean, but also have to take into account ongoing activities at the global level.

Sometimes, the driver of ocean heat waves is in the ocean itself. For example, during the heat wave event in Western Australia, the Leeuwin Ocean currents flowing southward became more and more powerful. As ocean currents intensified, the amount of warm water transported from the Indian Ocean was higher than normal, covering hundreds of kilometers of coastline in months-long heat waves.

A similar pattern triggered a marine heatwave in the Tasman Sea between Australia and New Zealand in 2015-2016, which set records for both duration and intensity. A 2017 study traced the event back to the strengthening of the Australian Eastern Current, which brought warm tropical waters to the coasts of Australia and New Zealand. Compared to heat wave events triggered by atmospheric activity, such ocean-induced heat wave events can penetrate deep into the ocean.

These deeper events pose a unique challenge for climate scientists. Current understanding of ocean heatwaves is almost always limited to the surface ocean, and researchers can use satellite instruments to map temperatures and track heatwave events in near real time. But beneath the surface ocean is a complex world of ocean currents. The researchers say that while heat waves on the ocean's surface can be observed and defined, this is only the tip of the iceberg of ocean heat waves. Relatively few observation networks are used to track what is going on below the ocean's surface. Buoys provide data in some regions, but in some regions, the data is completely missing. Understanding how abnormal warming below the ocean surface forms, persists and evolves is one of the biggest questions in ocean heatwave research. Since the vast majority of marine life lives below the surface of the ocean, this is an important frontier for scientists to explore. In addition, how marine heat waves below the ocean surface affect ecosystems is not fully understood.

The only thing climate scientists are sure of is that climate change will exacerbate ocean heatwave events. As greenhouse gases continue to be emitted into the atmosphere, global warming will exacerbate aspects of the ocean heatwave, according to climate models. In the context of climate change, extreme events are expected to increase. Some of these changes have already occurred. Between 1925 and 2016, the number of days of marine heat waves increased by more than 50% each year. Beginning with satellite records in 1982, heat waves in nearly two-thirds of the ocean have increased in intensity. Scientists predict that these trends will continue. Multiple studies have shown that even in moderately warming scenarios, almost all ocean heatwaves will increase in frequency and last longer in the coming years.

Over the past few decades, many extreme events have also been exacerbated by climate change. A 2020 study analyzed the 7 ocean heatwave events that have had the greatest impact since 1981 and concluded that in addition to 1 heatwave event, the other 6 heatwave events were at least partially driven by human-induced warming. The researchers compared models of atmospheric CO2 levels before industrialization with models of current levels. The results show that some events are very high in intensity and can be attributed entirely to anthropogenic climate change. It was impossible to happen before industrialization.

The researchers predict that by the next century, much of the ocean will warm and exceed the temperature threshold of heat wave events, leading to permanent ocean heatwaves in many parts of the world. If the warming is so large, then ocean heat waves are no longer extreme events, but normalized events.

Please indicate the source and author of this article: Resource and Environment Dynamic Monitoring Express, Lanzhou Documentation and Information Center, Chinese Academy of Sciences, No. 10, 2021, compiled by Xue Mingmei.