We've put the heat equivalent of 3.6 billion Hiroshima bomb explosions into the oceans. The water was hotter last year than ever in recorded history.

• Last year, ocean temperatures were the highest they've been since scientists started keeping track in the 1950s.
• According to a new study, the oceans have absorbed the heat equivalent of 3.6 billion Hiroshima bomb explosions over the last 25 years.
• Warmer oceans can kill coral, fish, and seabirds.
• Higher ocean temperatures also cause sea levels to rise, leading to coastal flooding and more severe hurricanes.
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Last year, Earth's oceans were the warmest they've been since scientists started keeping track in the 1950s.

According to a study published in the journal Advances in Atmospheric Sciences, that recording-breaking average concludes the warmest-ever decade for global oceans. The average ocean temperature for 2019 broke the 2018 record, which in turn smashed 2017's.

The study found that average 2019 ocean temperatures were about 0.18 degrees Fahrenheit (0.075 degrees Celsius) above the average between 1981 and 2010. While that might not seem like a lot, it means the ocean has taken in 228 sextillion (that's 21 zeros) joules of heat.

"That's a lot of zeros indeed," Lijing Cheng, the lead author of the study, said in a press release, adding, "the amount of heat we have put in the world's oceans in the past 25 years equals to 3.6 billion Hiroshima atom-bomb explosions."

Cheng's team compared ocean temperatures between 1987 and 2019 to an earlier period: 1955 to 1986. The results showed that the rate of warming increased by a factor of 4.5.

The consequences of a warming ocean are far-reaching: We can expect more severe storms, unprecedented sea-level rise, and the death of marine animals.

Warm waters bleach coral and kill fish and seabirds

Oceans absorb 93% of the extra heat that greenhouse gases trap in the atmosphere. A January 2019 study revealed that the world's oceans are heating up 40% faster (on average) than the prior estimate.

That's devastating for coral reefs, since higher temperatures prompt coral bleaching - a phenomenon in which coral expels its algal food sources and turns ghostly white, increasing its risk of death. When coral dies, that in turn threatens the survival of the fish that rely on a reef; a quarter of all fish species spend some part of their life cycle in reefs, despite the fact that reefs cover less than 1% of the ocean floor.

At present rates, it's expected that 60% of all coral reefs will be highly or critically threatened by 2030, and 98% of reefs will be exposed to potentially fatal conditions every year. By 2018, more than half of Australia's Great Barrier Reef had already died as a result of bleaching.

As water heats up, it expands

Rising ocean temperatures also lead sea levels to rise, since water (like most things) expands when it's heated. Even small changes in temperature results in a large increase in water volume.

In a scenario in which we continue emitting greenhouse gasses as projected, rising ocean temperatures could lead sea levels to get 3 feet higher by the year 2100. That would displace hundreds of millions of people along coastlines around the world.

Warmer oceans also cause ice in the Arctic and Antarctic to melt more quickly - at both poles, ice is melting six times faster now than it was 40 years ago.

According to an April 2019 study, Greenland's ice sheet is sloughing off an average of 286 billion tons of ice per year. Two decades ago, the annual average was just 50 billion.

If both Antarctica and Greenland's ice sheets were to melt completely, that would lead sea levels to rise more than 200 feet.

Ocean warming makes marine heatwaves worse and more frequent

According to Cheng, global ocean warming is causing marine heat waves - when sea-surface temperatures spike precipitously - in the Tasman Sea and other regions in the south Pacific.

These marine heat waves typically occur when weather causes the ocean to absorb more heat than usual, or if certain conditions prevent the ocean from releasing heat.

In late December, a 386,000-square-mile (1,000,000-square-kilometer) chunk of ocean near New Zealand experienced an unprecedented heat wave: Ocean temperatures spiked to almost 10 degrees Fahrenheit (6 degrees Celsius) higher than average.

Between 2013 and 2015, the US West Coast experienced a similar heat wave, which scientists dubbed "the blob." It led seals and sea-birds to die, algal blooms to spread, and coral to bleach.

According to Kevin Trenberth, a co-author of the new study, that blob caused a major loss of sea life on all levels of the food chain, "from phytoplankton to zooplankton to fish - including 100 million cod - to marine animals, such as whales."

Warmer ocean water makes hurricanes more severe

As ocean temperatures continue to increase, we can also expect more supercharged hurricanes, since a hurricane's wind speed is influenced by the temperature of the water below. A 1-degree Fahrenheit rise in ocean temperature can increase a storm's wind speed by 15 to 20 miles per hour, according to Yale Climate Connections.

Trenberth said a marine hot spot in the Gulf of Mexico spawned Hurricane Harvey three years ago. The following year, a hot spot in the Atlantic Ocean near the Carolinas contributed to Hurricane Florence's development.

Collectively, these two storms caused 135 deaths and up to $158 billion in damages.