Zettajoules (ZJ) are an extremely large unit of energy—equal to 10²¹ joules—used by climate scientists to quantify changes in the Earth system’s heat content. Recent measurements indicate that Earth’s energy imbalance is on the order of ~0.5 to 1 watt per square metre, which may sound small but, when integrated across the entire planet and over time, translates into hundreds of zettajoules of excess heat accumulating per decade. This imbalance occurs because greenhouse gases trap outgoing infrared radiation more efficiently than the planet can radiate heat back into space.
Most of this trapped energy is absorbed by the oceans—estimated at over 90% of total excess heat, making the ocean the primary buffer of global warming. The upper layers of the ocean are warming, while deeper layers are also increasingly storing heat, contributing to thermal expansion and rising sea levels. The remaining energy goes into melting ice sheets in regions like Antarctica and Greenland, warming the atmosphere, and heating land surfaces. Scientists use zettajoules because they provide a direct, physically grounded way to track how much energy is accumulating in the climate system—offering a clearer picture of long-term warming trends than temperature alone. More