Guest Post by Willis Eschenbach

If aliens in spaceships saw our world, they wouldn’t name it “Earth”.

They’d name it “Water” because that’s what makes up more than 70% of the surface. And it’s also what controls the climate.

A couple days ago I stumbled across something I’d been trying to find for a while, a longer-term gridded global rainfall dataset. I finally located one at the Copernicus website. It runs from 1979 to December 2021. Here’s the global average rainfall from that site.

Figure 1. Annual average rainfall, 1979-2021

This shows some interesting aspects. The endless rainstorms of the intertropical convergence zone (ITCZ) are seen as the blue band above the Equator. The Pacific Warm Pool is marked by the blue blob of heavy rain north of Australia.

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A short digression. The current central paradigm of mainstream climate science is that the change in global temperature is a lagged linear function of the change in total downwelling solar and longwave (thermal) radiation. In other words, forcing (which in the climate world generally means changes in downwelling radiation, hey, don’t blame me, I didn’t invent the term) rules temperature, and everything else averages out.

I hold a different view. I hold that a variety of emergent climate phenomena act in various ways, places, and times to thermoregulate the climate. One of the strongest of these phenomena is the daily emergence of tropical thunderstorms. When ocean temperatures exceed some local limit, thunderstorms form, rain falls, and the surface is cooled. As a result, in the current generally warming climate, we should expect an increase in tropical thunderstorms.

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To investigate this prediction, I took a look at the rainfall trends. Figure 2 shows those, in millimeters per decade. Blue is getting wetter, and red is drying.

Figure 2. Rainfall trends, 1° latitude x 1° longitude gridcells.

As my theory would predict, the warming is leading to increased rainfall over the Pacific Warm Pool and around the Intertropical Convergence Zone.

However, the reason that I was looking for the rainfall dataset was not for the rain per se. It was because the rain is a measure of the evaporative cooling of the surface. The global average rainfall is about one meter per year. It takes ~ 80 watts per square meter (W/m2) of radiation over a one year period to evaporate one meter of seawater. In addition, there’s a cooling of another ~ 2.5 W/m2 due to the cold rain falling on the surface.

This means that globally, rainfall directly cools the surface by ~ 82 W/m2.

What I didn’t know until I got the Copernicus rainfall dataset was how that cooling is distributed around the planet. Here’s a view of that. Of course, it looks like Figure 1, only with different units.

Figure 3. Average ongoing cooling from rainfall, 1979-2021

The cooling is centered over the Pacific Warm Pool in the western Pacific. This is the warmest open ocean area. It’s well known that the average temperature of the Pacific Warm Pool never gets warmer than about ~ 30°C … and clearly, the 250+ W/m2 cooling of the warm pool due to rain is among the reasons.

Now, I wanted this information because it is a main part of the effect of clouds on temperature. The other main part is the separate and independent effect of the clouds on total radiation hitting the surface. Clouds warm some parts of the planet and cool others, by a combination of cooling by reflecting sunshine and warming by increased downwelling longwave radiation. Overall, these changes in radiation due to clouds cool the planet by about ~ 20 W/m2.  Here’s how that is distributed around the planet.

Figure 3. Average ongoing cooling and warming from the cloud radiative effect, 2000-2023

Note the spatial similarity between the surface cloud radiative effect and the cooling due to the rainfall. No surprise there.

Note also the system’s efficiency—the clouds’ cooling effects (rainfall + radiation) are focused on the warmest areas. And this is true at both the local and the global scale—thunderstorms form preferentially over local surface hot spots. This gives the most cooling for the smallest effort.

Adding these two different cloud effects together gives us a measure of most of the effect of clouds on the surface temperature. I say “most of” because there are some other cooling effects. These include:

• Snow, sleet, hail, and graupel. Since these are frozen, there’s additional surface cooling from the melting of the ice.

• Clear dry descending air around thunderstorms. Because most of the water as well as most of the aerosols have been stripped out of the air by rainfall, there’s far less water vapor and aerosols to absorb radiation leaving the surface. This allows for greater surface radiation making it to space, cooling the surface.

• A cold wind from the condensation level of the atmosphere is entrained by the falling rain and hits the surface vertically. This wind then spreads out when it hits the surface, cooling a much larger surface area around each rain cloud.

Setting those other cooling effects aside for the moment, here’s the distribution of total cloud cooling (radiative plus rainfall) around the planet.

Figure 4. Full effect of clouds on the surface temperature

Note that rather than the ~ -20 W/m2 cooling from just the radiative effects of the clouds, the true effect of the clouds is about 100 W/m2, and there are large areas where the cooling exceeds -300 W/m2.

Next, we can take a look at the relationship between total cloud cooling (radiative plus rainfall) and surface temperature. This is clearest over the 70% of the surface that is water. Here’s that relationship.

Figure 5. Scatterplot, total cloud cooling (rainfall plus radiation) versus sea surface temperature. Each blue dot is a 1° latitude by 1° longitude area of the ocean surface.

This is exactly the shape we’d expect to see in a thermoregulatory system. As the sea surface temperature increases, the total cloud cooling reduces … but only up to about 26°C. Above that, the cloud cooling increases very rapidly, quickly becoming around -300 to -400 W/m2 of cooling as the sea surface temperature gets up close to ~ 30°C.

Now, the yellow line in Figure 3 shows the slope of the cloud cooling/temperature relationship, which is how much the cloud cooling changes for every 1°C of surface warming. And over at the right of Figure 3, that slope is ~ -100 to ~ -150 W/m2 of increased cooling for every 1°C of surface warming.

In closing, let me note that since 1950, CO2 has theoretically increased downwelling radiation by something on the order of 1.4 W/m2 … and that would be totally undone by a mere 1.4% increase in cloud cooling. In that context, bear in mind that global cloud cooling changes by up to 9% from one month to the next, and we never even notice …

[CODA] The important thing about cloud cooling is that it is temperature-controlled. It has nothing to do with forcing. When tropical sea surface temperatures go above about 26°C, it rains, regardless of the forcing. Period. See below.

Figure 6. Pacific equatorial rainfall, 5° north to 5° south.

My regards to all, and remember—rather than cursing the storm, learn to dance in the rain …

w.

As Always: Please quote the exact words you are discussing. It avoids endless arguments. And me, I’m going to be in town today when this publishes, so play fair, no eye gouging, and may the best wo/man win …