Conscious Climate: Ocean Heat Capacity

 
 
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The Climate-Regulating Role of the Global Ocean

(adapted from Mahlman 2001, The Timing of Climate Change Policies)

Perhaps you’ve experienced the mild shock of jumping into a body of water in on a warm spring day, only to find that the water temperature is much colder, while the opposite can be experienced in the cooler days of early fall. This “seasonal lag” of the water temperature changes relative to the march of the seasons is mostly due to the much higher heat capacity of water relative to that of air.

Similarly, lakes and swimming pools tend to cool off more efficiently than they warm. In the fall, as the weather cools, the water surface cools rather quickly, becomes denser and sinks to lower levels, mixing the cooling effect through the entire depth. But as spring warming heats the upper layer of the water, it becomes less dense, and tends to remain at the surface without appreciable downward mixing. The unwary swimmer often experiences much colder water a short distance below the surface.

Similar processes act as climate changes over long time scales. A cooling climate can efficiently propagate the cooling signal to the depths of the ocean, while a warming climate tends to concentrate the heat in the upper layers and thereby impede downward mixing of the warmth. Resulting stratification can enhance the early surface temperature response to radiative forcing from increased greenhouse gases, even as it delays the ultimate fully-mixed equilibrium warming of the entire system.

Given an ocean heat-carrying capacity over 1000 times greater than that of the global atmosphere, this key observation carries many implications for how the atmosphere and earth’s surface responds over time to a build-up of atmospheric greenhouse gases.

Suppose, for example, that only the top meter of the ocean could respond to the atmosphere’s heating and cooling effects, while the rest of the world ocean remained totally isolated (no circulation, no mixing, no exchange with the top layer). The atmosphere/one-meter ocean system would respond within a year to an added pulse of atmospheric CO2: the upper meter of the ocean would soon be in equilibrium with the new atmospheric CO2 concentration and the time lag to top-of-atmosphere (TOA) radiative equilibrium would be short.

Now, suppose that our “one-meter lid” ocean is replaced with the real ocean (average depth 3800 meters), but with rapid mixing all the way to the bottom. Now the rate of atmospheric warming due to added CO2 would be sharply suppressed because the modest-heat-capacity atmosphere would be losing added heat to the cooler ocean with its more than 1000 times greater heat capacity. One would have to wait for some time to even be able to measure the small amount of net atmospheric warming, which would be dwarfed by natural variability in the global-mean surface temperature for quite a while. (In fact, Trenberth 2007 suggests an ocean thermal inertia of 230 years for a rapidly-mixed ocean.)

The real world lies between these hypothetical extremes. Temperature measurements show a lower atmosphere that is warming considerably slower than an “almost-ocean-free” planet would, but noticeably faster than it would in the “fast-deep-mixing” ocean case, with an actual mixing layer of roughly 90 meters contributing an ocean thermal inertia of 6 years (again, Trenberth 2007).

Note as well: the new climate cannot be in final equilibrium until the ocean has finished warming all the way to the bottom. The last degree or two of warming could take well over a hundred years beyond the time at which atmospheric greenhouse gas concentrations stabilize; the last few tenths of a degree could require over a thousand years.

Most state-of-the-art climate models forecast a slowing down of the ocean’s overturning circulation given future warming, and thus less mixing of excess CO2 into the ocean and therefore a slower penetration of heat into the deeper layers of the ocean. This will produce an earlier surface-temperature hike but a delayed whole-system equilibrium. Levitus et al. (2001) show how most of the heat from twentieth century warming is stored in the ocean, and not in the atmosphere, soil, or land glaciers - consistent with theoretical and modeling expectations - but the rate of the oceanic mixing that will eventually fill up the huge ocean heat reservoir remains uncertain.

In summary, the thermal inertia of the ocean will play a key role in how global warming unfolds. At a minimum, it guarantees that further surface warming is “in the pipeline.” And to the degree that ocean mixing becomes suppressed, the atmospheric warming rate will be temporally enhanced, and final TOA radiative equilibrium correspondingly postponed.