Ocean heat content is the number you would want if you wanted to understand what the climate has actually been doing over the past century—not what the atmosphere has been doing, not what the land surface temperatures suggest, but the complete accounting of the energy that greenhouse gasses have been trapping in the Earth system. Climate discussions frequently center on surface air temperature records, which are crucial. However, they only record the small layer of the atmosphere that humans occupy. The heat is transferred to the ocean. For more than a century of industrial carbon emissions, it has been quietly traveling there in massive amounts, and the record it has created is more comprehensive and reliable than anything the atmosphere can provide.
The physics underlying this begins with a straightforward characteristic of water that is simple to describe but difficult to completely understand in terms of its implications. For the kind of approximate comparison that helps explain why oceans dominate the climate’s thermal budget, water retains heat much better than air—roughly 3,500 times more per unit of volume. The ocean reaches an average depth of over four kilometers and makes up around 71% of the planet’s surface.
The world’s oceans contain a total of about 1.335 billion cubic kilometers of water. Over 90% of the energy trapped in the Earth system by excess greenhouse gas forcing is absorbed by the ocean. A tiny portion is absorbed by the atmosphere. A portion is absorbed by the earth surface. The remainder is used to thaw ice. The great majority is taken by the ocean.
Since industrialization started raising atmospheric CO2 concentrations above pre-industrial levels, the ocean has been continuously absorbing heat—not at a steady pace, but in the growing amounts that have coincided with rising emissions. As a result, a thermal record of the planet’s energy imbalance is stored in the water itself and can be read by carefully measuring the temperature at various depths. This record is less noisy than surface temperature records and less susceptible to the year-to-year variability that atmospheric measurements encounter. Surface temperature records reveal variations in both warm El Niño and chilly La Niña years. The underlying trend is much cleaner in the ocean heat content data.
In recent years, there has been a notable shift in the depth at which this heat is now observed. The upper 700 meters of the ocean, which interacts with the atmosphere most directly through surface mixing and wind-driven circulation, were the focus of the signal for a large portion of the time that ocean warming was first being routinely monitored. That layer’s warming was well-documented and in line with model predictions. The warming signal is now detectable below 2,000 meters in basins that oceanographers previously believed to remain thermally stable on any timescale relevant to human climate change, because to advancements in monitoring and the growth of the Argo float network. The abyssal plains are warming at a rate of 0.02 to 0.04 degrees Celsius per ten years, which, when compounded by the volume of water involved, constitute significant amounts of heat.
Although it is inextricably linked to the heat storage narrative, the sea level impact of this deep warming is frequently debated independently. As water warms, it expands. Sea level rise is a result of thermal expansion across a vast volume caused by warming the entire water column, not just the surface layer but also the deep water beneath it. This expansion accumulates and persists on timescales far longer than weather events or even the interannual variability that sea level monitoring tends to focus on. Because the heat stored in the deep ocean is already there and will continue to expand the water as it distributes throughout the system, the thermal expansion contribution to sea level rise is committed and will continue to be realized even if the surface warming that created it stopped today.
One of the more depressing parts of what the ocean’s thermal record shows is the idea of committed warming, or the rise in surface temperature that is locked in by heat already stored in the ocean. Even a total cessation of greenhouse gas production would not immediately stop warming, according to climate models that estimate future temperature trajectories and account for this committed warming. Over decades and centuries, the heat stored in the deep ocean will continue to migrate onto the surface system, causing surface effects that have not yet fully manifested. For more than a century, the ocean has physically postponed the full manifestation of human pollutants. It is not a cancelation. It’s postponed.
