What are zettajoules - and what do they tell us about Earths energy imbalance?

When James Prescott Joule lent his name to a unit of energy, he could not have foreseen todays alarming calculationsThe 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.