Climate Change: The Oceanic Thermostat

by Kevin Birdwell

The current debate over climate change has raised many questions about what is and isn’t currently understood about the global climate system.

While most of the debate focuses on atmospheric conditions, research shows that attention should be paid to the impact Earth’s oceans have on climate change. 

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Earth’s climate represents a complex balance of incoming solar energy, reflective surface properties (albedo), the hydrological cycle, and storage of heat (atmosphere and oceans). The amount of heat in the climate system is affected by a large number of factors—including greenhouse gases, aerosols (carbon, mineral dust, indirect aerosol effects), land use, cloud cover, and other influences.¹ Of the retained heat, the vast majority ends up in the planet’s oceans; the atmosphere holds an insignificant proportion by comparison (three orders of magnitude less in terms of total Joules).

Consequently, if one wants to understand atmospheric climate, one must first understand oceanic systems. Unfortunately, researchers do not have a complete understanding of the ocean’s storage and transportation of heat. A paper published in the Journal of Climate (2011) sheds some light on the role oceans might play in regulating atmospheric temperatures.² Authors Barreiro, Cherchi, and Masina suggest that the current rate of ocean heat transport is near its optimum with regard to its effect on the global distribution of heat.

Solar energy is largely absorbed by the tropical oceans where solar radiation and cold water upwells to the surface. Globally distributed ocean currents move most of the excess heat from the tropics toward the poles, where it is removed by atmospheric processes (cold and dry winds, especially in winter). Heat transport via ocean currents is significant in the tropics, but becomes dominated by the atmospheric circulation beyond about 45 degrees north and south latitude.³ 

Researchers largely attribute modifications of oceanic circulations and heat transport to changes in incoming solar radiation, greenhouse gases, and continental drift. Although changes associated with continental configuration occur on geologic time-scales, changes in incoming solar radiation can occur on very short time-scales, especially with regard to those effects that change surface albedo (cloud cover, land cover, aerosols, etc.).

Barreiro, Cherchi, and Masina modeled the effects of ocean heat transport at rates ranging from 0 to 200 percent of current levels using current continental configurations. As expected, they found that cooling tended to occur in high latitudes for decreased rates of ocean heat transport. This effect resulted partially from increased sea ice and low clouds in those areas. However, when modeled ocean heat transport was increased beyond 15 to 25 percent above present-day values, the results became highly dependent on radiative feedbacks between tropical low cloud formation and sea surface temperatures. In these cases, increased tropical low clouds enhanced surface albedo and resulted in significant cooling of the tropical oceans. Thus, when the ocean heat transport was increased too much above present-day levels, the cooling of the tropics acted as a brake on global heat transport. This resulted in a decrease in the global mean temperature related to ocean heat transport.

The authors also found that if modeled ocean heat transport was increased more than 25 percent beyond present-day values, then the climate transitioned abruptly (often within a decade or less) to the braking or cooling mode. A warming climate continued as ocean heat transport was increased to 115 percent of present-day values but began to cool when transport exceeded the 120 percent threshold. Essentially, this effect occurred because the deep tropical atmosphere tended to dry out when ocean heat transport increased too much. As a result, the tropical atmosphere became more stable (that is, less likely to mix vertically) and atmospheric temperature inversions that tended to produce extensive low clouds occurred. 

These low clouds greatly reduce the amount of solar radiation reaching the surface in the tropical regions. Barreiro, Cherchi, and Masina found that this effect’s significance began to increase when ocean heat transport exceeded 110 percent of present-day values. Atmospheric drying also occurred when ocean heat transport declined below the present-day values. However, for decreased ocean heat transport, high-latitude regions became drier. 

In both cases, changes to ocean heat transport reduce the atmospheric water vapor feedback (though in different ways). Reduced atmospheric water vapor helps slow the warming effects from other greenhouse gases. The planet’s tropical zones were also found to be largest (widest) when warming from ocean heat transport was at a maximum.

The net cooling effect resulting from ocean heat transport beyond 115 percent of present-day values was surprising. Past research suggested a more significant warming effect due to ocean heat transport. For example, in 1991, Rind and Chandler proposed that 46 percent more ocean heat transport may have warmed the Jurassic-era climate by 6°C over the present day.⁴ However, much higher levels of greenhouse gases (particularly CO₂) and a significantly different configuration of the continents also contributed to that period’s warming effects by changing ocean circulation patterns. 

Conversely, research published in 1993 by Barron et al. broadly confirms Barreiro, Cherchi, and Masina’s findings, suggesting that ocean heat transport is presently close to its maximum warming effect given the present continental configuration.⁵ Barron’s study found that increasing ocean heat transport from zero to present-day rates warmed the global climate by about 2°C, but that further increasing heat transport to 200 percent of present-day values resulted in only 0.6°C of additional warming.

Current understanding of ocean heat transport is still lacking in some respects, especially with regard to the nature of oceanic circulation pattern changes. However, Barreiro, Cherchi, and Masina’s research shows how Earth’s oceans may regulate the global climate through changes in ocean heat transport rates, atmospheric humidity, cloud cover, and sea ice. 

Although this oceanic “thermostat” does not fully compensate for the effects of both natural and anthropogenic causes, the existence of such an ocean-climate regulator suggests that our world may possess a built-in tendency to maintain climate equilibriums. The distribution of the oceans and the present continental configuration suggest design climate stability. Without this thermostat, climate changes that occur on human time scales would be more abrupt and more disruptive to society than those that have been observed.