The average rise in sea level is about three millimetres a year. That is thinner than a coin, which is precisely why the number is so easy to wave away. But three millimetres is a rate, not a total. A rate multiplied by decades and by inertia that cannot be reversed quickly produces a result of an entirely different scale.
A slope that does not flatten
Cumulative change in mean sea level. Watch not the value but the persistence of the slope.
The line does not oscillate around a horizontal — it climbs steadily, and the slope shows no sign of levelling off. The rise has two main causes, and both are about heat. The first is thermal expansion: warm water occupies more volume than cold. The second is melting glaciers and ice sheets, adding new mass of water to the ocean.
Heat content: an indicator without seasonal noise
Surface temperature jumps from season to season and misleads easily. The heat content of the upper ocean is a far calmer and more honest indicator: it integrates heat over volume and is almost immune to weather noise.
Accumulated heat content of the upper ocean. Units are illustrative (10²² J).
The heat-content series is nearly monotonic: the bars grow year after year with minimal wobble. It is this smoothness that makes it decisive: where air temperature invites arguments about “one warm year”, ocean heat content leaves no room for seasonal sophistry.
How to read data like this
Three rules. First: don’t confuse rate with total — “three millimetres a year” and “how much over thirty years” are different questions, and the second matters more. Second: for slow processes with inertia, the outcome is set not by today’s inflow but by the sum of the entire past. Third: if you want one honest warming indicator free of seasonal arguments, look at ocean heat content, not at air temperature for a single month.
