‘7.6 billion mugs of tea a second’: a new way to make sense of the heat pouring into our oceans

Imagine almost every person on Earth doing nothing but making cups of tea, day and night, one every second – and pouring every single one into the sea.

It sounds absurd. Yet it helps us picture one of the biggest changes happening to our planet.

Although we experience global warming through hotter air and record-breaking heatwaves, the atmosphere is only a sideshow, which we happen to especially notice because we live within it. The more important feature is that the ocean is absorbing some 90% of the extra heat now being trapped at the Earth’s surface.

This heat is being measured year by year, with increasing precision, by a network of satellites and ocean-based instruments. A few years ago, Chinese climate scientist Lijing Cheng and colleagues put together all the data for 2021 from this ocean-wide panoply of devices. The oceans, they said, absorbed some 15 zettajoules of energy that year. That sounds impressive – but what does it mean?

A zettajoule is 1 followed by 21 noughts: unintelligible in itself, and even more confusing when placed in stark contrast to the humble joule, which is the energy needed to raise 0.239 grams of water by 1°C. To try to convey the hugeness of 15 zettajoules, one needs to find some suitable analogy. One common comparison is that it’s the energy equivalent of seven Hiroshima-scale atom bombs exploding every second. It’s a dramatic and oft-used image – but (luckily) outside most people’s direct experience.

That’s why we wanted to bring things a little closer to home. Let’s use a mug of tea.

What’s brewing in the oceans?

Heating a mug of tea needs about 100,000 joules. Divide 15 zettajoules by that figure, and then divide that by the number of seconds in year, gives 4.8 billion mugs of hot tea entering the ocean each second.

We can even try to be properly scientific about this, and invent the measure of a billion mugs of hot tea per second. Following the scientific convention of “giga” for billion, you could even use gigamugs per second: or GMug/s. In 2021, ocean heat input was running at 4.8 GMug/s.

These are large numbers, and it is sobering that the energy directly expended for global human use each year (from all sources) is much smaller: a bit more than half a zettajoule – or some 0.2 billion mugs of hot tea per second in our new measure. Make a mug of tea with fossil-fuelled energy, and the carbon dioxide emitted in that simple process means that you are also doing the equivalent of pouring a dozen or more mugs of hot tea into the sea.

The oceans are absorbing even more heat

But global warming has not been standing still since 2021. It has been accelerating, partly because greenhouse gas levels in the atmosphere continue to rise, partly because the air is getting cleaner and letting more sunlight in, and perhaps partly because warming is beginning to reduce reflective cloud cover, allowing even more heat to be absorbed. This would be an example of a positive feedback: warming causing changes that lead to yet more warming. If so, that could be a slippery slope for the global climate – and for us.

Global average ocean temperature anomaly from the surface to nearly 2000 m depth over the past 20 years:

That acceleration has now been measured in the oceans too. An updated analysis estimated that, in 2025, the oceans absorbed about 23 zettajoules of extra energy. That’s roughly half as much again as the 2021 figure, although the precise figure is subject to natural variation and measurement uncertainty.

Translating that into our new measure means the oceans are now absorbing the equivalent of some 7.6 billion cups of hot tea every second. That’s almost one cup every second for every person on the planet – just as we asked you to imagine at the start.

That extra heat is the physical effect of the trillion or more tonnes of carbon dioxide, plus other greenhouse gases, that we have added to the air since we began burning fossil fuels in earnest, some two centuries ago. It’s raising sea levels, driving marine heat waves and melting polar ice.

The tea cup analogy is one way of getting the point across – that the Earth is absorbing ever more heat. It’s something to remember as national promises of pathways to net zero become increasingly diluted, delayed, mocked or forgotten, and as investments in fossil fuels rise steeply: a path that might briefly enrich oil company shareholders, but that will permanently impoverish and endanger everyone else, for countless generations to come.

Better by far to invest instead in energy sources that are not carbon-based. That’s something to ponder on, over a cup of tea.