By Andy May
Solar radiation penetrates oceans to depths of 10-100 meters (depending on wavelength and water clarity), directly heating the ocean mixed layer. Greenhouse gas (GHG) infrared (IR), being longwave, is absorbed in the top ~10 microns (the thermal or electromagnetic skin layer or “TSL”), where it influences temperature gradients, evaporation, and conduction. The TSL lies on top of the mixed layer and has a different temperature. Below the TSL, especially in the daytime or in the presence of very light winds, there can develop a temperature gradient between it and the “foundation” temperature or the mixed layer temperature (see figure 1). The vertically nearly constant mixed layer temperature is maintained by turbulence and convection and follows overlying air temperature trends (although not the actual air temperature) by a few days to a few weeks, or even longer, depending upon the season and latitude. Higher latitudes respond slower and lower latitudes quicker; wind speed has a large effect on the lag.
Besides changes in atmospheric temperature, the mixed layer also responds to changes in ocean dynamics (currents, etc.), the changes in ocean dynamics also vary by season and latitude and can play an important role in multiyear and multidecadal changes (think ENSO, the AMO, and the PDO, etc.) in mixed layer temperature (Patrizio & Thompson, 2021). Changes in ocean dynamics and the mixed layer’s higher heat capacity cause it to smooth out the more rapid changes in the lower atmosphere and in the TSL. The interaction between the atmosphere and the mixed layer is not just from the atmosphere to the mixed layer, but works in both directions (Patrizio & Thompson, 2021).
The mainstream view
The mainstream view is that GHG IR warms the TSL, which reduces conductive heat loss from the mixed layer, causing total ocean heat content (OHC) to rise. It is clear that both OHC and surface temperatures have been rising in recent decades, but is this warming due to man-made GHGs? Figure 1 is a cartoon from GHRSST (Minnett & Kaiser-Weiss, 2012) showing the electromagnetic skin layer or TSL where all GHG IR is absorbed.
Wong & Minnett (2018) show IR adjusts the thermal skin layer’s curvature, reducing molecular conduction from the mixed layer below. They studied the rate of downwelling IR on clear days versus cloudy days and found that increased downwelling IR (due to clouds) flattens the TSL gradient. This lowered net upward ocean heat flux and caused more absorbed solar heat from deeper in the water column to accumulate in the upper ocean rather than escaping. Their analysis of spectra and profiles supports modulation of upper OHC via this mechanism. But their data is qualitative and not quantitative, we don’t know how much ocean warming is from the warming of the atmosphere in recent decades and how much is from an increase in heat from absorbed man-made GHG IR in the TSL.
Captured GHG IR in the TSL is mostly retained because the only way it can get out is through molecular conduction, as emitted radiation, or evaporation. Energy flux is net upward into the atmosphere. Some TSL emitted radiation will go downward into the mixed layer but in net it will be to the cooler (on average) atmosphere. The atmosphere shares thermal energy with the mixed layer through convection, turbulence, and, to a lesser extent, conduction. This happens through wind and precipitation. It is a much slower process than GHG IR absorption in the TSL, as discussed above.
There are two processes
There are two processes at work. One is energy in the TSL is sent to the atmosphere as discussed above. The other is warming or cooling of the mixed layer by the atmosphere. The latter involves mixing the air with the mixed-layer water through waves and turbulence as they attempt to reach equilibrium. The relative importance of these two mechanisms is unknown and the subject of furious debate within the climate community.
Is there a significant difference between the two? Only in the perception of the cause. We know that changes in solar radiation directly affect the mixed layer energy content and temperature. If additional GHG IR absorbed in the TSL changes the temperature profile of the upper ocean, it can achieve the same result but will be perceived to be an anthropogenic effect. Both processes may be at work, but which is stronger? The IPCC special report on the ocean and cryosphere (IPCC, 2022) asserts that 90% of the extra heat in the climate system has been absorbed by the oceans, but assumes that all the extra heat is anthropogenic and it does not speculate as to the process involved in getting the heat into the ocean, it could be either process or both.
The Critical view
Critics argue IR can’t meaningfully heat the mixed layer because it doesn’t penetrate to its upper surface; instead, it mostly enhances evaporation which cools the ocean surface. Data shows mixed effects. In calm conditions, IR can warm the skin by 0.1-0.3°C, but wind mixing dissipates this quickly. Wind also enhances evaporation (Yu, 2007b). The TSL temperature is little affected by turbulence, but greatly affected by GHG IR.
The critics note that the TSL on the ocean surface captures all GHG radiation. The mechanism for heat transport within this layer is molecular conduction and not turbulence (Soloviev & Lukas, 2014) & (Wong & Minnett, 2018). The direction of heat flow is normally from the ocean to the atmosphere, so heat from absorbed longwave radiation is conducted upward to the sea surface and then eventually into the atmosphere. However, as shown by Wong and Minnett, IR warms the ocean surface which should slow mixed layer cooling somewhat by reducing the rate of mixed layer heat content loss.
We don’t really have enough data to be sure which of the two processes dominates. The key assumption feeding the consensus opinion that increasing ocean heat content (OHC) is due to additional greenhouse gas warming is that solar input into the climate system is constant. Thus, they reason, if OHC is increasing, it must be additional greenhouse gases via the process of elimination. This opinion is based on models and not demonstrated fact.
It is reasonable to assume that if the additional GHG IR is trapped in a ~10-micron layer at the top of the ocean, it will be quickly returned to the atmosphere. Is this more or less than the thermal energy, of solar origin, that a warmer TSL partially traps in the mixed layer? I don’t know and I’ve seen no measurements that quantify the relative amounts.
How accurately is OHC known?
Analyzing the Wong and Minnett study is complicated by the fact that OHC trends are not known very accurately. This is true even today with ARGO floats and accurate buoys. Even less is known about OHC before 2005, which was when the ARGO floats became widespread enough to provide good data on the upper 2,000 meters of the oceans. Below 2,000 meters there is very little data. Prior to 2005, our information is mostly just about a small part of the mixed layer, just a few centimeters to a few meters below the surface. It may be many decades before we have enough data to determine reasonable trends in OHC to a reasonable depth. Trends in upper ocean temperatures can change in cycles of over 60 years, as in the case of the AMO (May & Crok, 2024). Other multidecadal ocean oscillations are discussed here.
Solar Variability
We have just come out of the longest solar grand maximum (SGMx) in over 5,000 years, could it be the cause of the additional ocean heat content, or part of it? The consensus claims the change in TSI during the 20th century SGMx is small and we are on the downside of it anyway. However, changes in TSI are not the only way solar activity affects our climate (Lean, 2017), (Scafetta, 2023), & (Haigh, 2011). The world is warmer than 100 years ago, and the mixed layer tends to stay in tune with atmospheric temperature, but should we assume that atmospheric temperature and the mixed layer are warmer due only to additional greenhouse gases? I think not.
The temperature difference across the TSL is between 0.1K during high wind speeds (>7 m/sec) to 0.6K during low wind speeds (<2.5 m/sec) (Wong & Minnett, 2018). The TSL is always present except for momentary breaks caused by breaking waves and rainfall. Restoring the TSL after disruptions takes only a few seconds. There seems to be no mechanism for the IR thermal energy to get into the mixed layer, except in small amounts due to molecular conduction. The strong temperature gradient through the TSL exists due to the poor efficiency of heat transfer by molecular conduction (Wong & Minnett, 2018).
Discussion
Heat losses from the ocean surface through evaporation and radiation originate within the TSL. There will be an increase in retained OHC due to a higher TSL temperature, as Wong and Minnett show in their paper. But their study does not preclude an increase in retained OHC if the atmosphere is warmer, regardless of why the atmosphere is warmer. We don’t know the ratio of the two. The consensus and the IPCC hold that 90% of the excess OHC is due to GHGs (IPCC, 2022, pp. 9,83), but I’ve seen no data to support this proportion or how much of the 90% is due to each process described above. Just because GHGs are increasing and surface temperatures are increasing does not mean all the warming is due to GHGs, solar variability must be considered as well.
The argument that 90% of the OHC increase observed recently is due to additional man-made greenhouse gas IR warming the TSL seems flimsy in my opinion. Wong and Minnett’s implication that the observed increase in upper ocean heat content is due to more being trapped beneath the TSL is also weak. It is just as likely that a warming atmosphere, due to increased evaporation and radiation from the TSL, and a variable sun, is causing the mixed layer to warm. I’m not skeptical of the Wong and Minnett data or their analysis, I just don’t think their conclusions or the IPCC’s conclusions follow from them.
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This post benefited from discussions with Grok and Dave Burton, but I don’t think either of them agree with me. Any mistakes are mine alone.
