Most of the World Ocean is warm. Unusually hot. Temperatures are likely to break records this year. Since mid-March, the global average sea surface temperature has exceeded 21°C, the highest level since satellite records began. What’s going on? Climate change is the big picture: nine-tenths of all heat retained by greenhouse gases goes to the oceans. But there’s also an immediate reason: the rare trio of La Niña has come to an end. During this cycle, colder water from the depths of the ocean rises to the surface. It’s like the air conditioner is working in the Pacific Ocean. But now the air conditioner is off. We are probably in an El Niño that tends to bring warmer and drier weather to Australia.

When you use the air conditioner, you mask the outside heat. The same goes for our oceans. As global warming continues apace, La Niña has brought with it three years of colder conditions. Now we’re likely to see heat return. If El Niño develops, climate scientists predict it could raise global temperatures by another 0.2℃, and in some areas, could push warming above 1.5℃ for the first time.

What are we seeing?

The wind regime is changing in the eastern part of the Pacific Ocean near Chile. These winds prevented the rise of deeper and colder waters, cooling the surface. For this reason, you can see temperatures well above the average in this region.

This is usually the start of the El Niño cycle, which brings dry, hot air to Australia while damaging fisheries in Ecuador and Peru and bringing torrential rains to parts of South America. But the long-running El Niño-Southern Oscillation cycle is taking place against the backdrop of climate change. This is why some parts of the World Ocean are so hot.

Why are the oceans so important?

Ocean currents, along with atmospheric convection, are the main heat carrier worldwide. The sun does not set at the same speed everywhere. It is colder at the poles as sunlight is more easily reflected. But the equator takes all the power of the sun by heating the air and water. Ocean and air currents carry this heat to the poles. As currents move south, the heat mixes with the surrounding water. The East Australian Current carries warm water south from the tropics and disperses heat throughout southeastern Australia. The current is usually much colder when it reaches Hobart.

Water can hold much more heat than air. In fact, just a few meters above the ocean stores as much heat as Earth’s entire atmosphere. The oceans warm more slowly and cool more slowly. In contrast, the temperature of our atmosphere is much more variable and can change rapidly.

As sunlight heats the water directly and warm winds carry the heat, heat enters the ocean from the surface, as expected. Over time this heat mixes with the rest of the ocean. Most of the additional heat enters the upper two kilometers of seawater, but is heated throughout the water column. The average depth of the oceans is four kilometers.

How much energy? A stunning study shows that between 1971 and 2020, the Earth system trapped an estimated 380 zettajoules of extra heat, 90% of which was captured by the oceans. That’s a really big number, equivalent to 25 billion nuclear bombs. Our study found that warmer currents, where heat is concentrated, are moving south toward Antarctica.

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

Surprisingly, the answer is “not necessarily”. Local dynamics always play a role. As our own expectations. In Sydney, many who went swimming this month were surprised at how warm the water was. The long-term trend of ocean warming plays a role. But even more important is how long the water can retain heat.

This hot Sydney dive is due to the oceans holding back the heat from summer and autumn. The air temperature may drop to 22℃ and the ocean to 21℃. But in fact, this is a fairly common phenomenon in April – cooler air and warmer water. For a swimmer, the contrast makes the ocean feel warm compared to the air, especially if there is a breeze.

This is partly why global warming is so difficult to understand. We experience weather and climate directly through our life experiences. More importantly, the big picture we see. And this is a real concern, given the intense warming near Latin America.