All things Equal

By Andy May

In an interesting linkedin debate between Tinus Pulles and me, two subjects came up that are related to one another and too complicated for a comment. First is Tinus’ question, “Will more atmospheric CO₂, all other variables being equal, lead to a higher surface temperature.”

The second question arose when I quoted the following from the IPCC AR6 report:

“As a result, non-condensing GHGs with much longer residence times serve as ‘control knobs’, regulating planetary temperature, with water vapour concentrations as a feedback effect (Lacis et al., 2010, 2013).” (IPCC, 2021, p. 179).

And this comment from AR5:

“Currently, water vapour has the largest greenhouse effect in the Earth’s atmosphere. However, other greenhouse gases, primarily CO₂, are necessary to sustain the presence of water vapour in the atmosphere. … So greenhouse gases other than water vapour provide the temperature structure that sustains current levels of atmospheric water vapour. Therefore, although CO₂ is the main anthropogenic control knob on climate, water vapour is a strong and fast feedback that amplifies any initial forcing by a typical factor between two and three. Water vapour is not a significant initial forcing, but is nevertheless a fundamental agent of climate change.” (IPCC, 2013, p. 667).

Oddly, Tinus doesn’t think the IPCC is serious about Lacis, et al.’s idea that CO₂ is a control knob for surface temperature and he apparently disagrees with it. His excuse is that in the first quote “control knob” is in quotes and in the second is in a colored box, which identifies it as an answer to a “frequently asked question” or FAQ 8.1 (chapter 8, pp 666-667).

But both reports clearly cite Lacis, et al. (2010 & 2013) and use Lacis et al.’s language and agree with them. We can comfortably assume that the IPCC AR5 and AR6 reports agree with Lacis et al., regardless of Tinus’ objections.

The interesting thing is that these two points are intimately related to one another in an interesting way. Tinus’ first question is a leading question with the underlying assumption that CO₂ controls the climate. He knows, as everyone does, that if infrared radiation is shined on pure CO₂ in a laboratory, it will absorb some of it and warm up. The laboratory experiment is the “all things equal” he is talking about.

Outside the laboratory and in the real world there are a number of other factors that need to be dealt with that can change the result, this is why the question Tinus is asking is leading, the question is framed to get at a particular answer. Often, the way a question is framed can result in an answer that is incorrect, so we need to avoid answering leading questions.

Now we come to the second issue, calling CO₂ the climate “control knob.” Lacis et al. explains this idea, which the IPCC clearly supports:

“Ample physical evidence shows that carbon dioxide (CO₂) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO₂, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO₂ and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.” (Lacis, Schmidt, Rind, & Ruedy, 2010)

Lacis et al. estimate that water vapor supplies about 75% of the overall greenhouse effect, which is in the ballpark of other estimates, but no one knows for sure because the effect of clouds is unknown. Clouds have a large positive (warming) greenhouse effect at night, keeping heat in and a large negative (cooling) albedo effect during the day because they are bright white and reflect a lot of incoming sunlight. Further, clouds vary over time and with location (more on clouds here and here).

This means that the greenhouse effect changes both temporally and areally. In the tropics where it is humid all the time the greenhouse effect is very large and in deserts and in the polar regions in the winter it is very small, even negative at the poles over much of the winter. In the polar regions in winter and in deserts the skies are usually cloud free.

As Lacis, et al. say, water vapor condenses, and is unevenly distributed over Earth’s surface. That is the crux of their argument that CO₂ and other non-condensing GHGs are the “control knob” for climate and water vapor is a significant, but relatively unimportant, “feedback” that does what the superior GHGs tell it to do.

Does that argument hold water? The pun is fully intended. As Wim Röst has said (in a very good June, 2023 presentation in Hillegom, The Netherlands) water, snow, and water vapor dominate the greenhouse effect, cool the surface through evaporation, release much of their latent heat to space, and determine all weather. The various water-driven processes in the troposphere control the amount of incoming and outgoing radiation by varying both the location and movement of clouds and latent heat.

Wyatt and Curry [ (Wyatt & Curry, 2014), (Wyatt M. G., 2012c), (Wyatt M., 2014)] have shown that numerous ocean and atmospheric oscillations move across Earth’s surface in a coordinated manner, that they call the “stadium wave,” that forms a roughly 65-70-year climate cycle or oscillation. This oscillation can be seen in global average temperature as shown in (May & Crok, 2024) in figure 1 below.

Figure 1. The Atlantic Multidecadal Oscillation (AMO) plotted in its raw form (top) and as a detrended index (bottom plot). The HadCRUT4 global temperature average record has also been detrended and overlain, as a gray dashed line, on the detrended AMO. Data from NOAA. Plot from (May & Crok, 2024), abstract here

As figure 1 shows the detrended global average surface temperature (HadCRUT4) conforms very well to the detrended AMO index. This correspondence is better, at least visually, than the correspondence between CO₂ and temperature. The AMO is not the leading oscillation in the stadium wave, but it is an important component of it. The correspondence of the AMO to the global average surface temperature opens the possibility that water and water vapor are not a “feedback,” but a driver of climate change. CO₂ and other noncondensing greenhouse gases probably have some effect on climate change, but by all accounts, it is small, and it is doubtful that they are in the driver’s seat.

Download the bibliography here.