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In hot water: here’s why ocean temperatures are the hottest on record


Large parts of the world’s oceans are warm. Unusually warm. The heat this year probably will breaking records. Since mid-March, the average global sea surface temperature has exceeded 21℃ – the highest since satellite records began.

What is happening? Climate change is the big picture: nine-tenths of all heat is trapped by greenhouse gases goes in the oceans. But there is also a direct cause: the rare triple-dip La Niña is over. During this cycle, cooler water from deep in the ocean rises to the surface. It’s like the Pacific Ocean’s air conditioning is running. But now the air conditioner is off. It’s likely we’re set for an El Niño, which tends to bring warmer, drier weather to Australia.

When you run your air conditioner, you mask the heat outside. The same goes for our oceans. La Niña brought three years of cooler conditions as global warming continued at a rapid pace.

Now we will probably see the heat roar back. If El Niño develops, climatologists estimate it could add an extra 0.2 ℃ to global temperatures, which would see some areas warm more than 1.5 ℃ for the first time.

Almost all of the extra heat from the sun, trapped by greenhouse gases, has ended up in the oceans.

What do we see?

Wind patterns change over the eastern Pacific near Chile. These winds have stopped the upwelling of deep colder water to cool the surface. Therefore you can see temperatures much higher than average in that area.

This is often the start of an El Niño cycle, which usually brings fire weather in Australia – dry and hot – while hurting fisheries in Ecuador and Peru and bringing torrential rain to parts of South America.

But the age-old El Niño southern oscillation cycle is happening amid climate change. That is why it is so hot in large parts of the world’s oceans.

Read more: Changing ocean currents are pushing more and more heat into the southern hemisphere’s cooler waters

Why are the oceans so important?

Ocean currents, in addition to atmospheric convection, are an important carrier of heat around the world. The sun does not fall at the same rate everywhere. At the poles it is easier for sunlight to bounce off, which is why they are colder. But the equator gets the full power of the sun, warming up air and water.

Ocean and air currents move this heat to the poles. As the currents move south, the heat mixes with the surrounding water. The East Australian Current carries warm water from the tropics south and spreads heat along South East Australia. By the time the current reaches Hobart, it is usually much cooler.

Water can hold much more heat than air. In fact, only the top few feet of the ocean save the same amount of heat like the entire atmosphere of the earth. The oceans warm up more slowly and cool down more slowly. The temperature of our atmosphere, on the other hand, is much more mercury and can change quickly.

Heat ends up in the ocean at the surface, as you might expect, because that’s where sunlight directly heats the water, as do warm winds that transfer heat. Over time, this heat is mixed with the rest of the ocean. Most of the extra heat goes to the top two kilometers of seawater, but there is warming throughout the water column. On average, the oceans are four kilometers deep.

How much energy? A startling one study suggests the Earth system retained about 380 zettajoules of extra heat from 1971-2020 – 90% of which the oceans stored. That is a really huge number, the equivalent of 25 billion nuclear bombs.

Our research has shown that warmer currents – where heat is concentrated – move further south, towards Antarctica.

Is that why my ocean swims are so hot this month?

Surprisingly, the answer is “not necessarily”. Local dynamics always play a role. And the same goes for our own expectations.

bondi beach
Swimming at Bondi beach in April isn’t necessarily due to climate change – cool air and warmer water make it feel like the water is unusually hot.

In Sydney, many people were surprised by how warm the water felt when they dared to take a dip this month. The long-term trend of ocean warming plays a role. But more important is how long water can retain heat.

That warm dip in Sydney is due to the oceans retaining their heat from summer and autumn. The air temperature can drop to 22℃ while the ocean is 21℃. But that’s actually very common in April: cooler air and warmer water. To a person swimming, the contrast makes the ocean feel warm compared to the sky, especially when a breeze is blowing.

Partly because of this, global warming is difficult to grasp. We experience the weather and climate directly, through our lived experience. What is more important is the big picture we see. And that, based on Latin America’s intense warming, is a real concern.

Read more: El Niño is coming and ocean temperatures are already at record highs – that could spell disaster for fish and corals

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