Despite evidence to the contrary, there’s one thing climate change alarmists seem to consider a clincher. Well… their stylized account has the seas absorbing heat from our warming atmosphere as human activity forces carbon emissions into the air. That notion seems to be reinforced, at least in the popular imagination, by the fact that the sea is a “carbon sink”, but that is a matter of carbon sequestration and not a mechanism of ocean warming. While ocean temperatures have warmed slightly over the past few decades, it is almost entirely coincidental, rather than a result of slightly warmer air temperatures.

Heat and the Hydrosphere

There is no doubt that the oceans store heat very efficiently, but that heat comes primarily from solar radiation and geothermal sources underseas. In fact, water stores heat far more efficiently than the atmosphere. According to Paul Homewood, a given cross section of sea water to a depth of just 2.6 meters is capable of holding as much heat as a column of air of the same width extending from the ocean surface to the outermost layers of the atmosphere! (See here for an earlier reference.) However, that does not imply that the oceans are very effective at drawing heat from warmer air or particularly carbon back-radiation. Both the air and water draw heat from solar radiation, and how much in any given location depends on the Sun’s angle in the sky.

A solid guide is that air temperatures are heavily influenced by water temperatures, but not as much vice versa. When temperatures in the upper layers of the ocean rise from natural forces, including reduced upward circulation from greater depths, evaporation causes this heat to radiate into the atmosphere along with evaporation of water vapor. Homewood notes that El Niño patterns make the influence of the Pacific Ocean waters on climate pretty obvious. The impact of the Gulf Stream on European climates is also instructive.

The Indian Ocean accounted for about half of the sea warming that occurred within the globe’s top 700 meters of waters over the years 2000 – 2019, though the Indian Ocean represents only about 20% of the world’s sea surface. The authors of that research found that the warming was not caused by trends in surface forcing of any kind, including warmer air temperatures. They said the ocean warming:

“… has been driven by significant changes in oceanic fluxes and not by surface forcing. … the ocean has been driving a rapid increase in Indian Ocean heat content.”

This was consistent with an earlier study of global sea temperatures covering the period 1984 – 2006 that found:

“… diminished ocean cooling due to vertical ocean processes … A conclusion is that natural variability, rather than long-term climate change, dominates the SST [sea surface temperature] and heat flux changes over this 23-yr period.”

It’s a Water World

Heat released by the oceans tends to dominate variations in global temperatures. A 2018 study found that evaporative heat transfer to the atmosphere from the oceans was closely associated with variations in air temperatures:

“When the atmosphere gets extra warm it receives more heat from the ocean, when it is extra cool it receives less heat from the ocean, making it clear that the ocean is the driving force behind these variations. …

The changes in solar radiation received at the Earth’s surface are clearly a trigger for these variations in global mean temperature, but the mechanisms by which these changes occur are a bit more complex and depend on the time-scale of the changes.”

Measurement

Willis Eschenbach reviewed a prominent study of ocean temperature changes and noted that the authors’ estimate of total warming of the oceans was quite small:

“… over the last sixty years, the ocean has warmed a little over a tenth of one measly degree … now you can understand why they put it in zettajoules—it’s far more alarming that way.”

Eschenbach goes on to discuss the massive uncertainty underlying measurements of ocean temperatures, particularly below a depth of 2,000 meters, but even well above that depth given the extremely wide spacing of so-called ARGO floats. However, the relative stability of the point estimates over 60 years is noteworthy, not to mention the “cold water” doused on alarmist claims about ocean overheating.

Sun Engine

Ocean warmth begins with energy from the Sun and from the deep interior of the Earth. The force of solar energy is greatest in the tropics, where sunlight is perpendicular to the surface of the Earth and is least dispersed by the thickness of the atmosphere. The sun’s radiative force is smallest in the polar regions, where the angle of its light is acute. As Anthony Watts says:

“All elements of Earth’s weather, storm fronts, hurricanes, the jet stream, and even ocean currents, are driven to redistribute energy from the tropics to the poles.”

Both land and sea absorb heat from the Sun and from volcanic activity, though the heat is moderated by the sea. That moderation is especially impactful in the Southern Hemisphere, which has far less land area, greater exposure of sea surface to the Sun, and about half of the average ocean temperature variation experienced in the North.

Ultimately, the importance of natural sunlight on air and sea temperatures can’t be overemphasized. Henrik Svensmark and some co-authors have estimated that a cosmic ray flux of 15% from a coronal mass ejection leads to a reduction in cloud cover within roughly 9 – 12 days. The ultimate increase in the Earth’s “energy budget” over about a week’s time is about the same size as a doubling of CO2, which certainly puts things in perspective. However, the oceans, and hence cloud cover, moderate the impact of the Sun, with or without the presence of additional greenhouse gases forced by human activity.

Vapors

The importance of evaporation from bodies of water also deserves great emphasis. No one doubts the massive influence of greenhouse gases (GHGs) on the climate. Water vapor accounts for about 90% of GHGs, and it originates predominantly from oceans. Meanwhile, carbon dioxide accounts for less than 4% of GHGs, and it appears that only a small part is from anthropogenic sources (and see here and below).

The impact of changing levels of water vapor dominates GHG levels. They are also a critical input to cloud formation, a phenomenon that climate models are generally ill-equipped to explain. Clouds reflect solar radiation back into space, reducing the Sun’s net contribution to the Earth’s energy budget. On the other hand, clouds can trap heat in the lower layers of the atmosphere. The globe has an average of 60 – 70% cloud cover, and most of that is over the oceans. Increased cloud cover generally leads to declines in temperature.

A 2015 study identified a process through which the sea surface has an unexpectedly large impact on climate. This was from the formation of isoprene, a film on the ocean surface, which leads to more cloud formation. In addition to biological sources, isoprene was found to originate, surprisingly, from the effect of sunlight.

The Big Sink

Man-made emissions of CO2 constitute only about 5% of naturally discharged CO2, which is roughly matched by natural removal. CO2 is absorbed, dissolved, or transformed in a variety of ways on both land and sea, but the oceans collectively represent the world’s largest carbon sink. They hold about 50 times more CO2 than the atmosphere. Carbon is stored in sea water at great depths, and it enhances undersea vegetation just as it does on land. It is sequestered in a variety of sea organisms as calcium carbonate and is locked in sediments as well. A longstanding question is whether there is some limit on the capacity of the oceans and other sinks to store carbon, but apparently the uptake over time has remained roughly constant at just under 50% of all natural and man-made CO2 emissions (also see here). So far, we don’t appear to be approaching any sort of “saturation point”.

One claim about the rising carbon stored undersea is that it will drive down the oceans’ pH levels. In other words, it will lead to “ocean acidification” and harm a variety of marine life. Rud István has ridiculed that term (quite rightly) because slightly less alkaline sea water does not make it “acidic”. More substantively, he notes the huge natural variations in ocean pH levels across different marine environments, the exaggeration inherent in some estimates of pH changes that do not account for physical buffering, and the fact that the impact on many organisms is inconsistent with the presumed harms of reduced pH. In fact, errors in some of the research pointing to those harms has been acknowledged. In addition, the much feared “coral crisis” seems to have been a myth.

Conclusion

The upper layers of the oceans have warmed somewhat over the past 60 years, but the warming had natural causes. Heat transfer from the atmosphere to the hydrosphere is relatively minor compared to the absorption of heat by oceans via solar forcings. It is also minor compared to the transfer of temperature from oceans to surface air. As Jim Steele has explained it:

“Greenhouse longwave energy penetrates only a few microns into the ocean surface and even less into most soils, but the sun’s shortwave energy passes much more deeply into the ocean.”

It’s reasonable to concede that warmer air temperatures via man-made GHGs might be a minor reinforcement to natural sources of ocean warming, or it might slightly moderate ocean cooling. However, measuring that contribution would be difficult against the massive background of natural forcings on ocean temperatures.

Oceans are dominant in terms of heat storage from natural forcings and in terms of carbon sequestration. In fact, the oceans have thoroughly outperformed alarmist projections as a carbon sink. Dire prognostications of the effect of carbon dioxide on marine life have been drastically over-emphasized as well.