Ocean temperatures are off the charts right now, and scientists are alarmed

What's behind this rapid acceleration in heat isn't totally clear yet, but researchers are concerned it could be the "start of an even more worrying trend."

(CNN) -- Ocean surface heat is at record-breaking levels. Temperatures began climbing in mid-March and skyrocketed over the course of several weeks, leaving scientists scrambling to figure out exactly why.

Temperatures have fallen since their peak in April -- as they naturally do in the spring -- but they are still higher than they have ever been on record for this time of year.

"It is remarkable," said Gregory C. Johnson, an oceanographer at the National Oceanic and Atmospheric Administration, which calculates the ocean surface temperature using a network of ships, buoys, satellites and floats.

Although it's still preliminary data, if it holds up, he said, "this is another milestone."

The record may not seem huge -- it's nearly two-tenths of a degree higher than the previous record in 2016 -- but given how much heat is needed to warm up this huge body of water, "it's a massive amount of energy," Matthew England, professor of ocean and climate dynamics at the University of New South Wales, Australia, told CNN.

What's behind this rapid increase isn't totally clear yet. "These temperatures just rocketed up, people haven't had a chance to puzzle it all out," Johnson said.

Some scientists are concerned the scale of these new records could mark the start of an alarming trend. Others say record-breaking temperatures like these are always concerning but to be expected given the human-caused climate crisis.

All agree the consequences are likely to be significant. Warmer oceans bleach coral, kill marine life, increase sea level rise and make the ocean less efficient at absorbing planet-warming pollution -- the warmer oceans get, the more the planet will heat.

The return of El Niño

One major driver of the heat is believed to be an approaching -- and potentially strong -- El Niño, a natural climate fluctuation associated with warming in the central and eastern tropical Pacific Ocean, which has a global heating effect.

The world has just emerged from a 3-year La Niña, El Niño's cooler counterpart, which has helped mask the full impact of global warming. Since La Niña ended in March, ocean temperatures seem to be on a rebound, scientists say.

"It's a little bit like we've had the freezer door open for a while and it's helped to cool the planet," Johnson said. But even while that freezer has been open, background temperatures have continued to rise. Now the freezer is closed, everything is hotter than before.

The World Meteorological Organization said on Wednesday there is about an 80% chance El Niño will develop between July and September. But part of what has puzzled scientists is temperatures have risen so much before it has even arrived.

Some are concerned this suggests climate change might be progressing in ways climate models have not predicted.

"An El Niño event is brewing, but it's probably too early to blame that as the cause," professor Mike Meredith, science leader at the British Antarctic Survey, told CNN. "It's vital that we find out what is causing [the peak in ocean surface temperature] though, and understand whether this is an isolated extreme high or the start of an even more worrying trend."

Deeper ocean heat

Surface temperatures tell one story about what's happening to the world's oceans.

But to understand where the world is heading in the long term, it's important to also look at the deeper ocean temperatures, said Sarah Purkey, an assistant professor at the Scripps Institution of Oceanography.

Thousands of floats in the world's oceans measure its heat content, analyzing temperature from the surface down to the deep waters, feeding back data.

"Ocean heat content has been on a very steady, sometimes accelerating, rise," Purkey told CNN, because of human-caused global warming. The oceans absorb around 90% of the excess heat the world produces, as well as around 25% of carbon pollution.

In 2022, the oceans were the warmest on record for the fourth year in a row.

A study published in April found the heat in the climate system was accelerating, spelling bad news for the oceans.

It found the rate of change in how much heat the Earth has accumulated has more than doubled over the last two decades -- and most of that is going into the ocean.

"There's a really urgent need to understand this because if it's part of a long term trend, this is really highly concerning," said Karina von Schuckmann, an oceanographer at Mercator Ocean International in France and a co-author on the study.

One surprising reason could be the reduction of aerosols in the atmosphere. In 2020, regulations were introduced to limit the amount of sulfur in the fuel ships used -- a policy aimed at addressing air pollution.

Though air pollution has a significant impact on human health, it also acts as an artificial sunscreen and reflects sunlight away from the Earth. One theory is the absence of aerosols may have turned up the heat, von Schuckmann said.

Worrying impacts of ocean warming

Whatever the reasons behind the increase in ocean heat, the impacts are potentially catastrophic if temperatures continue to head off the charts.

The oceans shield us from the full impacts of the climate crisis. "We should thank the ocean for taking up most of what we've done to the climate system, otherwise we would be seeing effects that are really 100 times what we're seeing right now," Purkey said.

But this buffering role comes at a high cost.

Hotter oceans cause coral reef bleaching and are linked to toxic algae blooms, which can suck oxygen from the water and choke marine life, sometimes forcing fisheries to close. Warmer waters are also less effective at absorbing carbon, which means more is left in the atmosphere, which in turn fuels more global warming.

Sea level rises as water warms -- not only through the melting of ice sheets, but water also expands as it heats up.

Surface warming supercharges cyclones and hurricanes.

Scientists are particularly concerned about the impact of warming on meridional overturning currents, ocean "conveyor belts" that push surface water to the deeper ocean and play a key role in regulating the planet's energy balance. "It's probably the most important thing to monitor," said Purkey.

The strength of the currents will determine how efficient the ocean is at taking up the excess heat humans are producing, she said. And, for example, if the overturning current in the Atlantic Ocean weakens, or even collapses, the consequences could be dire -- including very cold winters in Western Europe, rapid sea level rise and disruption to tropical monsoons.

For now, ocean surface temperatures have started to fall, even if they remain high for this time of year.

As scientists continue to analyze the reasons for record ocean warming, they are clear records will continue to be smashed as the climate crisis intensifies.

"This is a bit of a wake-up call, I hope, for everybody globally that this trajectory of warming that we're on is not going to stop until we bring our emissions right back down to zero," England said.

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