Such events also can provide a natural pulse of heat released from the ocean to the atmosphere, boosting the odds that 2018 and possibly 2019 could be among the top five warmest years on record.

The big picture: The last El Niño event took place in 2015 and 2016, and it was one of the strongest on record. Michelle L’Heureux, a meteorologist at NOAA's Climate Prediction Center in Maryland, tells Axios that the upcoming event — which has about a 70% likelihood of occurring by the upcoming winter — is unlikely to be as potent.

"If something forms it’s likely to be on the weaker side of things," she said. "In general, weaker events tend to be a bit tougher to predict than stronger events.”

For signs of El Niño, scientists like L'Heureux look at sea surface temperatures in specific parts of the tropical Pacific, known as El Niño regions. These are the boxed areas on the sea surface temperature chart. In recent weeks, ocean temperatures have increased in parts of these areas.

But the formation of El Niño is a complicated dance between air and sea, and now, L'Heureux says, it's the atmosphere's turn to alter trade winds in a way that reinforces the changes in the water. These air and ocean feedbacks are what really get an El Niño going.

Why it matters: Marine species that already live close to the upper end of their heat tolerance could perish in prolonged and more severe marine heatwaves in the near future — much as portions of the Great Barrier Reef did during the 2014 to 2017 global coral bleaching event.

• Recent studies have shown that the frequency of marine heat waves has increased in the past several decades, but a lot has remained unknown about these events, until now.

How they did it: The new study uses satellite observations of ocean temperatures during the period from 1982 to 2016, combined with a suite of Earth system computer models to provide more insight into the likelihood and severity of such phenomena. The researchers identified an event as a marine heat wave when the sea surface temperature exceeded its local 99th percentile.

What they found: The study's findings are grim, as they indicate that global warming is already sharply raising the risk of marine heat waves, and this is likely to get significantly worse.

According to Thomas Frölicher, lead author of the study and a researcher at the University of Bern in Switzerland, if greenhouse gas emissions continue on their present course and temperatures were to rise by 6.3°F (3.5°C) by 2100, "the number of marine heatwave days will be 41 times higher than in preindustrial times," he told Axios.

• "On average, the spatial extent of the heatwaves would be 21 times larger, their duration would increase to 112 days and their maximum intensity would rise to 2.5°C," or 4.5°F.
• "The probability of marine heat waves is projected to increase almost everywhere," Frölicher said.
• However, these numbers would decrease significantly if emissions are cut, and global warming were to be limited to just 1.5°C, or 2.4°F, above preindustrial levels, the study found.

Interestingly, the study found that the number of marine heat waves is increasing faster than the number of heat waves on land.

How it works: Marine heat waves can trigger large-scale changes in marine ecosystems, killing some species or forcing them to move.

"Recent observed marine heatwaves, such as the marine heatwave in Western Australia in 2011 or the blob in the North Pacific from 2013 to 2015, demonstrated the high vulnerability of marine ecosystems to such events," Frölicher said.

What we're watching: The new study goes a long way toward attributing trends in marine heat waves and projecting what may lie ahead. However, according to Frölicher, scientists currently lack a full understanding of the processes behind the formation of a marine heat wave, as well as the decay of such events. Researchers and scientists alike will need to do more work to understand how marine ecosystems will respond to more of these events.

Eric Oliver, a physical oceanographer at Dalhousie University in Nova Scotia who led a prior study on marine heat waves, said the new research is noteworthy for "quantifying the future projected changes.”

“It’s a very important issue in part because it’s so easy to ignore. When we see heat waves on land it’s obvious that people are dying," Oliver said. "It’s just much more obvious. Whereas unless you spend your life underwater, which very few people do… it’s really hard to see the impacts.”

The bottom line: The finding that oceans are facing a similarly scorching future as land areas is yet another warning that without sharp cuts in greenhouse gas emissions, we're in for a far more volatile and damaging future than we've been accustomed to.

"Together with the projected increase in marine heatwaves with future warming, this provides a worrying picture for the global oceans," Fischer says.

The big picture: July's historic temperatures brought heat waves to a large portion of the northern hemisphere and set the stage for deadly and devastating wildfires in California and Sweden, and massive fires have been burning in Siberia as well. All-time national heat records fell in Japan, North and South Korea, Algeria and Taiwan, along with numerous all-time records for individual cities.

The unusually hot conditions in the western U.S., along with the U.K., Scandinavia, parts of Eurasia and southeast Asia are clear from NASA's July temperature anomaly map. Interestingly, much of the Arctic — which is the fastest-warming area on Earth — has been cooler than average this summer, including in July.

What we're watching: Other global temperature monitoring agencies have yet to weigh in with their rankings for July, and they may differ slightly from NASA's. However, the month, and 2018 overall, are likely to rank in the top 5 warmest such periods on record.

Go deeper:

• Heat records fall in the Arctic as fires erupt in Sweden and Siberia
• The big picture: Climate change worsened deadly Greek wildfires
• 2018's global heat wave is so pervasive it's surprising scientists