The basic unit for measuring climate breakdown is the zettajoule. If you have never heard the term, you are not alone: even scientists working on a planetary scale struggle to grasp the magnitude of the change measured by this gigantic unit of energy.
One zettajoule equals a billion trillion joules. Typed on a screen, the string of 21 zeros looks absurdly long. Experts often turn to terms such as “inconceivable” and “almost beyond comprehension” to describe what the numbers show.
The World Meteorological Organization (WMO) warned this week, in its latest annual State of the Global Climate report, that our planet has a vast and growing energy imbalance that is heating the oceans, land and air to dangerous levels. According to the report, Earth's energy imbalance increased by about 11 zettajoules annually between 2005 and 2025—an amount equivalent to about 18 times total human energy consumption.
The cause is well known: burning oil, gas, coal and forests releases greenhouse gases such as carbon dioxide, methane and nitrous oxide into the atmosphere. This “clogs”, as the article puts it, the Earth system's ability to release heat into space. The more energy becomes trapped in our home, the more exposed we all are—humans and other forms of life—to heatwaves, storms, floods, droughts and fires.
Last year, the energy imbalance in the oceans, which absorb more than 90% of solar radiation, reached a record 23 zettajoules, more than twice the average of the previous two decades. John Kennedy, the lead author of the WMO report, said the 2025 ocean imbalance was about 39 times the annual human energy use of the entire planet.
Scientists have used various comparisons to make the scale understandable. In 2020, scientist John Abraham calculated that the heat being added to the oceans was equivalent to about five Hiroshima atomic bombs every second. By 2022, that figure had risen to seven. Based on last year's WMO data, we were now approaching 11 Hiroshima explosions per second.
The Chinese Academy of Sciences used an Olympic comparison: it noted that the accumulation of 15 zettajoules of energy in the oceans was enough to boil and evaporate the water in 2.3 billion Olympic swimming pools. By that calculation, the updated 2025 figure would be enough to evaporate about 3.4 billion Olympic swimming pools. However you present it, the number is terrifying and heading in a horrifying direction.
The fact that humanity allowed this to happen would disappoint James Prescott Joule, the Victorian physicist after whom the basic unit of energy was named. In his time, shortly after the Industrial Revolution, a joule was a convenient way to describe the effort required to produce one watt of power for one second—the work needed to lift an apple off the floor and put it on a table.
Since then, humanity has developed increasingly sophisticated ways to harness ever larger amounts of energy: kilojoules to boil the water for a cup of tea, megajoules to heat a bathtub, gigajoules for half a tank of petrol, terajoules (63 terajoules for the Hiroshima bomb), petajoules (3 petajoules for London's daily energy consumption) and exajoules (more than 600 exajoules for the annual energy use of all 8 billion people on Earth).
But we have devoted far less effort to asking what happens to all that energy once we are finished with it, or how it affects the much larger amount of solar radiation arriving from the sun.
Joule himself would have considered this careless accounting: his research on the conservation of heat contributed to the first law of thermodynamics, which states that energy is neither created nor destroyed, only transferred or transformed.
This 175-year-old principle remains valid today and provides the fundamental explanation for climate breakdown—even if those most responsible for the energy imbalance try to deny it. And their foolishness is now measured in zettajoules.
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