The primary unit of climate collapse is the zettajoule. If you have never heard of this term, you are not alone. Even scientists who work on a planetary scale struggle to relate the immensity of the change measured by this titanic unit of energy.
What is a zettajoule?
A zettajoule is a billion trillion joules. Typed out on a calculator or computer screen, the row of 21 zeros looks absurdly long – a train of seven carriages, each with three empty windows. Experts often have to resort to abstract terms like “unfathomable”, “almost beyond comprehension” and “really big” to ensure our tiny human minds are sufficiently blown away by what these numbers convey.
Why are zettajoules in the news (again)?
When used to calculate the heat on our planet, that train is accelerating and running out of track. The World Meteorological Organization (WMO) warned this week in its latest annual State of the Global Climate report that our world has a huge and growing energy imbalance, which is warming the oceans, the land and the air to dangerous levels.
The new report says Earth’s energy imbalance increased by about 11 zettajoules a year between 2005 and 2025, which is equivalent to about 18 times total human energy use.
As most of us are now well aware, the burning of oil, gas, coal and forests is releasing heat-trapping greenhouse gases such as carbon dioxide, methane and nitrous oxide into the atmosphere. This constipates the Earth system’s ability to excrete heat out into space. And the more energy gets stuck in our home, the more we – and all other forms of life – feel hot, uncomfortable and are vulnerable to heatwaves, storms, floods, droughts or fire.
How quickly is the planet’s energy imbalance growing?
Last year, the energy imbalance in the world’s oceans, which absorb more than 90% of solar radiation, hit a record 23 zettajoules, more than double the average of the previous two decades.
Scientists have used various comparisons to put that in context. John Kennedy, the lead author of the WMO report, said the 2025 imbalance in the oceans was about 39 times the annual human energy use for the whole globe.
The scientist John Abraham has previously calculated the accumulation of energy in terms of the explosive power of the Little Boy atomic bomb that devastated Hiroshima in 1945. In 2020, he observed that the amount of heat being added to the oceans was equivalent to about five Hiroshima bombs of energy every second. By 2022, this had gone up to seven Hiroshimas every second. Last year, the WMO figures suggest, it was closer to 11 Hiroshima explosions per second.
The Chinese Academy of Sciences has used an Olympian comparison. Three years ago, it noted that the then 15 zettajoule accumulation of energy in the oceans was enough to boil away the water of 2.3bn Olympic-sized swimming pools (50 metres in length, 25 metres in width and 2 metres in depth). By that calculation, the updated 2025 figure would be enough to vaporise about 3.4bn Olympic pools.
However you dress this up, the number is terrifying and moving in a horrifying direction.
How did the humble joule hit zetta levels?
That humanity allowed this to happen would surely have dismayed James Prescott Joule, the Victorian-era physicist after whom the basic unit of energy is named. In his day, not long after the Industrial Revolution, a joule was a handy way to describe the amount of effort required to produce one watt of power for one second, equivalent to the work required to pick an apple off the floor and put it on a table.
Since then, humans have developed ever more sophisticated ways to harness ever greater amounts of energy – kilojoules (150kJ to boil the water for a cup of tea), megajoules (15MJ to heat a large bathtub), gigajoules (the energy discharged by a car using half a tank of petrol), terajoules (63TJ for the Hiroshima bomb), petajoules (3PJ for the daily energy consumption of London) and exajoules (more than 600EJ for the yearly energy use of all 8 billion people on Earth).
But we have put far less effort into the question of what happens to all that energy after we have finished using it. Or how this consumption affects the far greater quantity of solar radiation coming from the sun. None of it just disappears into thin air, though it could seep into space if left unblocked.
Mr Joule would have considered this a negligent reckoning. His work on heat conservation contributed to the development of the first law of thermodynamics, which states that energy cannot be created or destroyed, only transferred or converted from one form to another. That 175-year-old principle remains valid today and is the fundamental explanation of climate breakdown, even if those most responsible for the energy imbalance try to deny it. Their folly too can be measured in zettajoules.
