From Watts Up With That?

Guest Post by Willis Eschenbach (@weschenbach on X, my blog at “Skating Under The Ice“)

If you have a block of steel and you put it outside in the sun, then ceteris paribus, the more sunshine it is absorbing on a constant basis, the warmer it becomes.

However, with the Earth’s climate, things are never that simple. In parts of the world, the more radiation the surface receives, the cooler it becomes. Counterintuitive, I know, but facts is facts. Figure 1 shows where that is happening.

Over most of the world, the correlation is positive, meaning that when absorbed radiation goes up, the surface temperature goes up, just as happens with a block of steel.

However, the outlined blue areas show a negative correlation between temperature and absorbed radiation. In those areas, when total radiation increases, the temperature actually goes down.

Say what? How does this happen?

It happens because the blue areas mark the “Intertropical Convergence Zone” (ITCZ). This the the home to thousands and thousands of thunderstorms. These thunderstorms are mobile refrigeration systems operating on the exact same principle as your home refrigerator, as described below.

Air Conditioning Nairobi, Refrigerating The Planet

Refrigerators operate on a simple cycle. A “working fluid”, which is water in the case of a thunderstorm, evaporates in one location, cooling it down. Then the working fluid is transferred as a gas to a separate location, where it is condensed back into a liquid. Then the liquid is moved back to the first location and the cycle continues. Figure 2 shows this process.

In addition, there are a few things that thunderstorms do that refrigerators can’t do.

First, they are a dual-fuel refrigeration cycle. They’re driven by low-density air rising in a column. Initially, this low-density air is created by the sun, which heats the surface, expanding the air above it and causing it to rise and form the thunderstorm.

However, once the storm is established, it kicks up strong winds around the base. Evaporation rises roughly linearly with wind speed, so this greatly increases evaporation.

Here’s the key. Counterintuitively, water vapor is lighter than air. H₂O has an atomic weight of 18. Air has an atomic weight of 29, being mostly a mix of O₂ with a weight of 32 and N₂ with a weight of 28. So water is only ~ 2/3 the weight of air. As a result, more evaporation gives more low-density air to fuel the thunderstorm, making it stronger. Dual-fuel.

Another factor increasing evaporation is that the thunderstorm strips the water out of the air, so the descending air around the thunderstorm is dry. This dry air can pick up more water, again increasing evaporation.

A further cooling occurs because the rain is falling from several thousand feet up in the air, where it’s much cooler. The falling rain is below the surface temperature, further lowering the surface temperature.

In addition, because the descending dry air between the thunderstorms has little of the main radiatively active gas, water vapor, this allows the surface radiation to cool faster via increased radiation into space.

This dual-fuel multi-modal-cooling nature of the thunderstorm is critical because it means that the thunderstorm can continue to exist despite cooling the surface down below the temperature necessary for the thunderstorm to emerge. It’s not just simple linear feedback reducing warming. Instead, it actively cools the surface to a lower temperature. Thermoregulation.

And that is how in the tropical oceans, the surface temperature can be dropping despite increasing total radiation absorbed by the surface.

Here’s another view of it. This is a scatterplot showing the relationship between total surface absorbed radiation and temperature.

Clearly, the warmer it gets, the less each additional absorbed W/m2 increases the temperature. And at the top right, despite increasing power absorbed by the surface, the temperature is dropping …

We can look at this another way, by comparing how warm the surface would be if there were no surface sensible and latent heat loss to the atmosphere. Figure 4 shows that relationship.

There are a couple things of note in Figure 4.

First, at the bottom left of the graph, the Antarctic plateau is warmer than we’d expect. This is a result of the advection of heat from the tropics to the poles.

Next, the warmer it gets, the larger a percentage of sensible and latent (evaporative) heat is lost from the surface, leaving the surface temperature (yellow line) increasingly cooler than the theoretical S/B temperature (red line). This is strong negative feedback.

Finally, at the far right, to the right of the dotted line, the absorbed power is still increasing at a good clip … but the temperature is decreasing. This is not just negative feedback — it is active thermoregulation.

People keep saying that the climate is just “simple physics”. But as in this example, where absorbed radiation goes up while temperature goes down … in climate, few things are “simple physics”.

My rule of thumb?

In climate, everything is connected to everything else, which in turn is connected to everything else … except when it isn’t.

My best regards to all,

w.

You Know The Drill: I ask that when you comment, you quote the exact words you are discussing. I can defend my words. I can’t defend your understanding of my words.