An exceptionally strong underwater heat wave that has persisted for three years between Alaska and California has led to the death of more than a million seabirds and in addition to massive deaths from fish and marine mammals, say 23 American researchers. Some migratory birds have almost completely disappeared from the radar.

The hot water bubble formed in 2014 and lasted three years, with seawater temperatures that were up to 6 degrees higher than normal. For the ocean temperature - which is more stable than that of the atmosphere - that is a very large temperature difference, which statistically occurs only once or twice a thousand years.

It was also a large area: the exceptionally warm water was spread over a length of more than 3,000 kilometers along the west coast of North America, reaching a depth of 200 meters. The 'warm blob' was still visible last September (see image), says the American KNMI.

Emaciated guillemots did not have a boy for three years in a row

The first sign that something was wrong with the local birds came in the fall of 2015 and the spring of 2016, when a total of 62,000 dead and dying guillemots washed up on the beaches from California to Alaska, the research team writes in science magazine PLOS ONE . The birds were emaciated, a sign that they died due to food shortage.

According to the researchers, most birds must have died far at sea. Because only a small percentage washes up on coasts, the total number of dead guillemots should be around one million. The extent of guillemot mortality is confirmed by research into breeding success. In 2015, 2016 and 2017, 22 breeding colonies had completely failed breeding attempts, with no adult young being produced.

The cause of death lies in the basis of the food chain. The greatly elevated water temperatures have led to the death of a number of important plankton types. As a result, there were fewer small fish, and fish-eating species in turn had nothing to eat - including the guillemots.

The American Geological Service's lead author John Piatt says that a second factor plays a role during the heat: fish get a faster metabolism in warm water. That is why they burn more energy, while less plankton can be found - the main source of calories.

"If you raise the temperature a few degrees, these cold-blooded fish should eat more. And that applies not only to small plankton eaters, but also to larger fish that are dependent on it again." The seabirds also have a very high energy requirement, Piatt explains: "If they find nothing to eat for three to four days, they are dead."

A washed up guillemot (Image: Getty Images).

Greatest heat mortality ever observed

Massive mortality also occurred among small bird species, such as puffins and the Cassins alk. This algae, a small bird that goes far into the sea, only occurs along the west coast of the US and Canada and has been put on the Red List due to the rapid decline in 2015. Taken together, it is the largest heat-related death wave among seabirds that has ever been observed.

In addition to birds and fish, marine mammals were also affected. Large numbers of sea lions died in California, and many whale whales in Alaska in 2015 and 2016.

Thousands of kilometers further on, the ecological consequences of the hot water bubble were signaled. Australian researchers lost most of their population of sparrow-hawk petrels.

These birds migrate back and forth between Alaska and southern Australia where they breed. Instead of the usual 40,000, only a handful of birds returned from Alaska last year. It is possible that we are witnessing their extinction, Australian birders say worriedly.

Piatt says that there are two known death waves that came close in size: one to the west of France and one to the coast of New Zealand. "Great deal: all in the last ten years. And that was the warmest decade ever recorded for the oceans."

2019 brought a new heat record for the oceans. The average temperature of the atmosphere came in position 2, just below 2016.