Negative greenhouse effect over the high and cold Antarctic plateau

It was Schmithusen and colleagues who in 2015 published research showing that the radiative greenhouse effect went into reverse over Antarctica’s high and cold plateaus. This is because the elevated ground level is colder than the stratosphere. This made sense in terms of the “emission height” based explanation of the CO2 greenhouse effect. Normally the temperature at the lower stratosphere where IR photons can “see the sky” and escape the atmosphere, is lower than the surface, ergo decreased IR emission to space and climate warming. According to this narrative, increasing CO2 increases the altitude of the emission height at which it becomes colder still.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL066749

This striking assertion of a negative greenhouse effect over Antarctica was of course tested by the scientific community. Thus in 2018 Sergio Sejas and colleagues from NASA in the USA published a combined theoretical and experimental (measurement based) confirmation that the radiative greenhouse effect did indeed go into reverse around the South Pole.

https://www.nature.com/articles/s41612-018-0031-y

But there was an important twist to the story. The reverse Antarctic greenhouse effect was due to water vapour, not CO2. The analysis and reasoning are complex and multi factorial; however the essence is that the radiative effects of water vapour are simply much stronger than those of CO2. But in the unusual frigid and elevated environment of Antarctica, the outcome is the same – a negative radiative warming, meaning cooling, for all of the year except a short summer interval.

But there’s more to this paper than just the Antarctic situation. Reading between the lines, there appears to be some controversy over what the “greenhouse effect” really is. There is talk of a new paradigm based on the new concept of “saturation flux” challenging an existing paradigm based on temperatures at different heights including the emission or radiative height. The new element of IR “emissivity” – also changing with height – adds a new dimension.

Sergio and colleagues of course argue that their study is orthodox, not challenging the narrative of anthropogenic warming. But their research nonetheless puts if not a cat among the pidgeons, then at least a playful kitten or two. They say this about how emissivity compromises the “traditional” temperature with heigh greenhouse explanation:

“The conventional radiating layer explanation incorrectly attributes the negative GHE in the CO₂ band solely to the warmer stratospheric temperatures relative to the surface.¹³ Located approximately between 1 and 5 hPa, the CO₂ band radiating layer is warmer than the surface, but a positive GHE is observed in the CO₂ band wings (Fig. 5). The radiating layer concept breaks down due to the neglect of the radiating layer emissivity and the variations of vertical emissivity and temperature below it, which dictate the saturation curve and how the upward flux approaches it.”

The closest that the authors swing to controversy is their conclusion that the amount of the CO2 may not be the dominant factor in the strength or even the sign (positive or negative) of the greenhouse effect:

“… the sign of the GHE strongly depends on the vertical temperature gradient. This dependence on the vertical temperature profile is important, since it implies an increase (decrease) of greenhouse gases does not necessarily enhance (suppress) the GHE, as indicated by the negative radiative forcing produced by increasing the CO₂mixing ratio over the Antarctic Plateau.”

This is a complex yet excellent paper well worth a deep read – and re-read – by all those with an interest in the profound and far from trivial question of the actual roles of CO2, water vapour, temperature and IR radiation in earth’s atmosphere.

Figure 1 from Sergio Sejas et al 2018: blue regions denote negative greenhouse effect cooling rather than warming.