In the autumn of last year, an ecological catastrophe occurred in the coastal waters of Kamchatka, accompanied by the mass death of marine animals. Scientists have proved that the reason was not man-made pollution, as they immediately thought, but the bloom of microalgae associated with the arrival of abnormally warm water to the shores of the peninsula. This happens regularly in the oceans, and it's not just about global warming.
In the winter of 2010-2011, the waves threw tons of dead fish on the beaches of Western Australia for two months.
Then several natural factors coincided. The main one is the record strong phase of the Southern Oscillation - La Niña, during which the surface layer of water in the equatorial part of the Pacific Ocean heats up. The Leeuwyn Current off the southern coast of Australia, carrying warm water from the Indian Ocean, also sharply increased, and the blocking anticyclone caused an anomalous flow of heat from the atmosphere into the ocean.
All this is against the background of the annual maximum air temperatures in the Southern Hemisphere. As a result, by February 2011, the ocean along the coast of Western Australia was three, and on some days even five degrees warmer than usual. Marine ecosystems have been severely affected.
Since then, dozens of similar cataclysms have been recorded. The most significant was in 2014-2015 off the western coast of North America. It was named "The Blob". A huge patch of warm water has formed in the northwestern Pacific.
Having approached the coast, the "Drop" blocked the cold waters, rich in nutrients, rising from the depths. The phytoplankton disappeared, and the entire food pyramid fell. As a result, the population of coho salmon and chinook salmon sharply decreased, and about a million seabirds died in Alaska. In addition, for the first time in history, scientists have observed a massive bleaching of Hawaiian coral reefs.
The sea surface temperature map from September 1, 2014 shows three areas of warm water located off the coast of Mexico, Canada and the Bering Sea
Dead zones in the oceans of the past
A new study by American and Japanese scientists showed that this happened regularly in the recent geological past. In the cores obtained while drilling the bottom of the Bering Sea, over the past 1, 2 million years, 27 dead zones have been counted without traces of the vital activity of organisms.
Everything points to hypoxia - a sharp depletion of water in oxygen. Moreover, a clear relationship was established between dead zones and climate warming.
"Such harsh hypoxic events are common in the geological record. They almost always occur during warm interglacial periods, such as the one that is now," notes Professor Ana Cristina Ravelo.
Ocean hypoxia occurs after intensive growth, or, as biologists say, the bloom of tiny algae, phytoplankton, in warm surface waters. Algae block sunlight and actively consume oxygen dissolved in water. During the decomposition of dead phytoplankton, toxic substances are released. As a result, an oxygen-free dead zone is formed, where neither fish, nor marine animals, nor plants survive. This is exactly what happened off the coast of Kamchatka last fall.
Dead zones have expanded significantly in recent decades, including in water bodies on land. Deoxygenation of freshwater lakes is now several times faster than sea water. This conclusion was reached by scientists from 16 countries who analyzed data over the past 80 years on the oxygen content in 393 lakes in Europe and the United States. The article was published by the journal Nature.
Since 1980, the oxygen level in the surveyed lakes has decreased by 5.5 percent at the surface and 18.6 percent at depth. The reason is trivial - an increase in water temperature associated with general warming in the temperate zone. According to the laws of physics, the amount of oxygen that water can hold decreases with increasing temperature. Since surface water warms an average of 0.38 degrees Celsius per decade, oxygen concentration declines by 0.11 milligrams per liter over the same period.
The problem is that in many lakes of the middle zone, the water temperature has reached values favorable for the mass reproduction of cyanobacteria - blue-green algae that release toxins. This greatly affects the ecosystem and the quality of drinking water.
Closer to the bottom, where the temperature is generally stable, oxygen also disappears. Warming of surface waters enhances stratification - immiscibility of layers with different densities. And oxygen simply ceases to penetrate to depth. The same thing happens in the oceans.
Negative effects of sea heat waves
The critical role of man
Sea heat waves cause many negative effects, so scientists are trying to predict them. To do this, you need to understand how they arise. Experts agree that anomalous warming in certain parts of the World Ocean usually occurs due to the superposition of several, mainly natural, factors.
One of the most common are blocking anticyclones in the atmosphere. They stay in place for a long time, and the temperature rises steadily. This was the case in the winter of 2013-2014 in the South Atlantic. And in 2019, sea heat waves in the southwestern part of the Atlantic Ocean were formed due to a blocking anticyclone that arose thousands of kilometers over the Indian Ocean and then moved towards South America.
Not the least role is played by sea currents and long-term periodic fluctuations in ocean surface temperature, such as El Niño - the South Oscillation, or the Indian Ocean Dipole. So, in 2015-2016, a heat wave in the Tasman Sea between Australia and New Zealand was caused by an intensification of the East Australian Current, which carries warm waters from the equator. And the most powerful heat waves in the Pacific Ocean in 2011 and 2014-2015 coincided with the warm phase of the Southern Oscillation.
Add global warming to this. In a recent study published in the journal Science, scientists at the University of Bern estimate that the likelihood of heat waves has increased 20 times over the past decades. The authors associate this with human activity.
Experts predict that warming of seawater, which occurred once every hundreds or thousands of years in the pre-industrial era, will soon become common. Simulations show that if the global average temperature rises by one and a half degrees, extreme sea heat waves will occur several times a decade, and if by three degrees - almost annually.