In 25 years, the oceans have absorbed heat equivalent to the energy of 3.6 billion Hiroshima-size atom bomb explosions, the study's lead author said.
Ocean Warming Is Speeding Up, with Devastating Consequences, Study Shows.

The world's oceans are warming at a rapidly increasing pace, new research shows, and the heat is having devastating effects on marine life and intensifying extreme weather.

Last year, the oceans were warmer than any time since measurements began over 60 years ago, according to a study published Monday in the journal Advances in Atmospheric Sciences. 

While global surface temperature measurements go back farther in time, the measurement of ocean heat content is considered one of the most effective ways to show how fast Earth is warming because more than 90 percent of the heat trapped by greenhouse gases goes into the oceans.

The new study, the first to analyze ocean temperatures for 2019, was based on two independent data sets and used a new way of filling data gaps to measure ocean temperatures going back to the 1950s.

When the scientists compared ocean temperature data from the last three decades (1987-2019) to the three decades before that (1955-1986), they found the rate of warming had increased 450 percent, "reflecting a major increase in the rate of global climate change."

Measured by a common energy unit used in physics, the oceans absorbed 228 sextillion joules of heat in the past 25 years. That's equivalent to adding the energy of 3.6 billion Hiroshima-size atom bomb explosions to the oceans, said the study's lead author, Lijing Cheng, with the International Center for Climate and Environmental Sciences at the Institute of Atmospheric Physics.

It's "irrefutable proof of global warming" that leaves no other explanation aside from the effects of human-caused heat-trapping greenhouse gas pollution, Cheng said.

The warming of the oceans has widespread effects. It causes marine heat waves that kill fish and coral reefs, fuels hurricanes and coastal downpours, spawns harmful toxin-producing algal blooms and also contributes to heat waves on land, said study co-author Kevin Trenberth, with the National Center for Atmospheric Research.

He linked the rising ocean heat content with ocean heat waves like the 2013 to 2015 "warm blob" in the Northeastern Pacific that resulted in a major loss of marine life, including a crash in cod populations.

"The ocean heat content changes are the primary memory of global warming," he said. "This manifestation of global warming has major consequences."
Warmer Oceans Fuel Hurricanes

Trenberth, with other scientists, has documented how rising ocean heat contributes to more intense tropical storms, including hurricanes that affect the United States.

"Hurricanes pump the ocean heat content into the atmosphere in the form of moisture. That results in extreme and record rainfall from storms like Harvey and Florence. It makes for stronger and longer-lasting storms," Trenberth said.

From 1960 to through 2019, the average temperature for the upper 2000 meters of the oceans increased by 0.12 degree Celsius, Cheng said. However, the ocean surface, where hurricanes draw their energy, and the air just above it have warmed almost 1 degree Celsius from the pre-industrial era.

The heat buildup will affect global transfer of heat via deep ocean currents.

It's nearly irreversible, at least on a human timescale, Trenberth said.

"Imagine mixing a pot of hot and cold water in the sink. It gets warm, and you can never get the hot or the cold back," he said.

The new study is based in part on data since 2005 from ARGO, a network of 3,000 free-floating sensors that record temperature and salinity in the upper 2,000 meters (6,561 feet) of the oceans.

Before 2005, scientists measured ocean temperatures with different devices, including expendable sinking thermometers dropped from research ships. Those reading weren't as widespread, so models were used to extrapolate temperatures over wider areas.

The new evaluation of ocean heat content reinforces other recent signs of global warming. This past decade was the warmest on record since measurements started, and 2019 ended up the second-warmest year on record, though it was the warmest in the oceans.
Ocean Warming Also Affects Land

In addition to increasing precipitation from tropical storms, rising ocean heat has consequences for sea level rise and for El Niño, Trenberth said.

"In the Pacific, a consequence is El Niños being bigger, and with stronger droughts and floods around the world," he said. "Even more modest things in the tropical Indian Ocean, called the Indian Ocean Dipole, can lead to patterns of weather that contribute to the heat waves and bushfires in Australia." In late in 2019, these anomalies changed radically and it became very warm around Indonesia, creating major flooding in Jakarta and continuing the dry spell over Australia, he said.

Recent scientific research also shows that the marine heat waves caused by rising ocean heat content can contribute to increasing outbreaks of toxin-producing algal blooms, in association with pollution. And rising ocean temperatures are likely contributing to the spread of seaweed in the Caribbean and the Gulf of Mexico, Trenberth said. The ocean warms from the top down, which increases stratification and reduces mixing and aeration, breeding conditions favorable for seaweed.

"It is critical to understand how fast things are changing," said John Abraham, a co-author of the study and a climate researcher at the University of St. Thomas School of Engineering in Minnesota.

"This problem is not going to go away, it is getting worse. We are already seeing the impacts of warming on society, from rising sea levels to hotter waters to more intense storms and to more wild weather.

"But this problem is solvable," he said. "The first thing we need to do is use energy more wisely. Let's not waste energy for no reason. Let's make our cars, homes and workplaces more efficient. In the end, we will reduce greenhouse gas emissions and save money."

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