A crucial year for understanding how ocean warming affects marine life

What's happening: Global sea surface temperatures jumped to unprecedented levels over the past several months.

• The Pacific Ocean is transitioning out of La Niña conditions that have been in place for more than two years. As El Niño sets in, less colder water is being carried up from the depths of the ocean and mixed at the surface, leading the temperature of the sea surface water to rise.
• But the sea surface temperature rises observed this year can't be ascribed to an El Niño event that is just emerging, says Kim Cobb, an earth sciences professor at Brown University. "It takes a full year to see the full impact."
• Other explanations — including less dust from the Sahara Desert moving across the Atlantic Ocean that typically checks ocean temperatures there — are being put forth.

On top of that, marine heatwaves — once-rare ocean weather events where water temperatures rise and persist for days or months — are becoming more common. The number of marine heatwave days doubled between 1982 and 2016, and is projected to increase even more.

• They're also occurring on the seafloor around North America.
• The lack of long-term observational records of marine heatwaves limits scientists' knowledge about their frequency, says Curtis Deutsch, a geoscience professor at Princeton University.

The impact: Warming ocean waters from climate change are driving a drop in water pH and a decrease in oxygen in the water, or hypoxia. Ocean warming also amps up the metabolism of marine animals that then need more food. At the same time, a dearth of oxygen can reduce that food supply.

• Modeling and direct observations indicate marine heatwaves are commonly associated with low oxygen extremes as well, Deutsch says.
• Populations of marine species will move or go locally extinct if hit with a heatwaves, Deutsch says. The big question is, "How frequent can these extreme events happen and still have a viable population?"
• Most of the 32 coral reefs analyzed in a recent study already have at least weak hypoxic conditions — and it is expected to worsen as the oceans warm.

Zoom in: A brittle star species was absent from a coral reef on the Caribbean coast of Panama with an average temperature the species can tolerate, according to a study Deutsch and his colleagues published this year.

• Deutsch says that absence can be explained by the frequency of ocean weather extremes occurring just a couple of weeks a year.

The big picture: The impact of warming ocean water and hypoxia on different species is being documented around the world — from sea urchins that have a harder time flipping over in oxygen-depleted water to long-term warming diminishing the nutritional value of kelp to fish migrating to cooler waters.

• In India's Gulf of Mannar, annual blooms of algae cause significant coral mortality through asphyxiation, says K. Diraviya Raj, a professor at Suganthi Devadason Marine Research Institute in India, who studies corals in the gulf. The consistent timing of the blooms each year indicates the role of climate change, he says.

• One recent study of reef fish fossils from 50 million to 60 million years ago suggest the fish developed faster rates of growth in the warm oceans of that time. Scientists are concerned warming from climate change could push some species to live fast and die young.

But, but, but... There are some species thriving. On some isolated reefs, there are corals more resistant to changes in heat than others.

• Still, "it just means they are more resistant not that they are free from the impacts," says Thomás Banha, a Ph.D. student studying coral reef ecology and conservation at the University of São Paulo.
• Coral bleaching can happen due to climate change but can also occur seasonally. Marine "heat waves are becoming more common and hindering the capacity of corals to recover from natural bleaching," Banha says.
• "There is a threshold," he says. "If the environment is kept always above the threshold they at one time won’t be able to resist the conditions."

What to watch: "This is an important year for monitoring and identifying ways to assist species through these next couple of decades," Cobb says.

• The combination of El Niño and global warming will bring "a stepwise decline in marine ecosystem capacity," she says, recalling the mass coral mortality events she witnessed in 2016, also an El Niño year. "It's not steady and gradual, it's a cliff that species and ecosystems fall off."
• "It is very alarming that there are such large portions of the ocean that are so warm ahead of an El Niño event," she says. "It does not bode well for ecosystems around the world."