Squeezing The Sponge

Over in the Tweetosphere, where I’m @WEschenbach, I read that we’re in for rainy times:

The atmosphere cools and shrinks when the Sun gets sleepy. Rain is wrung out of it like a sponge. We have been entering a solar grand minimum since 2008. The bottom of it will be around 2035.

There are two parts to that claim. One is that in times of low solar activity (signified by low sunspots) the atmosphere, in particular the troposphere where the weather occurs, will cool down. The other is that when the troposphere cools down, we’ll get significantly more rain as the water is “wrung out of” the troposphere. So let me look at the parts separately.

First, does the troposphere cool down during times when low sunspots signify low solar activity? If so, nobody told the troposphere. If temperatures actually dropped when sunspot numbers dropped, then temperatures and sunspot numbers would be positively correlated … but here’s the reality:

Figure 1. Correlation between UAH MSU monthly lower tropospheric temperature anomaly in various areas of the planet and monthly sunspots, Dec 1979 to June 2020. Blue is positive correlation, red is negative. Latitude bands as follows: Global -85 to +85 latitude
Hemispheric 0 to +/- 85 latitude
Extratropics +/- 20 to +/- 85 latitude
Polar +/- 60 to +/- 85 latitude

Note that there are no negative correlations between tropospheric temperatures in different parts of the world. When the world warms or cools, it seems the motions of the troposphere and ocean must move the heat around the planet fairly rapidly. The only area in Figure 1 where the troposphere is relatively uncoupled from the rest of the planet is the South Pole.

But not one part of the troposphere is positively correlated with sunspots as the claim would require.

Now, the absence of evidence is not evidence of absence. So all I can say is that once again, I find no evidence that sunspots and atmospheric temperatures are significantly positively correlated as the theory requires. This agrees with my previous research on the subject as put forth in the 24 or so posts listed here …

Next, let’s examine the claim that we’ll have lots more rain because it would get “wrung out of” the cooler troposphere. I’ve not run the numbers yet, but that seems highly improbable. The amount of rainfall is a function of the amount of water leaving the surface, passing through the clouds, and returning to the surface. It’s not so much a function of the amount that the atmosphere can hold at a given instant.

Here’s a way to envision it. If you think of the hydrological cycle as a giant waterwheel lifting water from the surface to the clouds and then returning the water back to the surface as rain, the amount of rain is a function of how fast the waterwheel is turning, not just the size of the buckets.

So, having considered what I might expect to find, I ran the numbers. Virtually all atmospheric water is in the troposphere, the lowest level of the atmosphere. The amount of water in the troposphere is called “total precipitable water” or “TPW”, with units of kilograms of water per square metre (kg/m2) of surface area. Globally, the average TPW is about 28 kg/m2.

Figure 2. Distribution of total precipitable water.

(Unfortunately, I can’t find numbers for global TPW. However, TPW above the ocean is bound to be greater than TPW above the desert or in the mountains. So the values above represent a maximum possible value for the global TPW.)

Now, the metric system is lovely. One liter of water weighs one kilo. And one millimetre of rainfall over one square metre is one liter of rainfall. So if every drop of the 28 kg/m2 of precipitable water were squeezed out of the sky, we’d get 28 mm (about an inch) of rainfall. And since the global average rainfall is about 1 metre (39 inches) per year, the atmosphere only holds water to the amount of about 2.8% of average annual rainfall. A small amount. As I said, the amount of rainfall is not a function of atmospheric capacity.

But wait, that’s converting all 28 kg/m2 of the precipitable water to rain. The amount squeezed out by a temperature change is far less than that. Per the discussion here, the change in TPW at the global mean temperature of about fifteen degrees C is on the order of one kg per degree.

Figure 3. Scatterplot, Remote Sensing Systems (RSS) total precipitable water (TPW) versus the ReynoldsOI sea surface temperature data.

So if the troposphere were to cool by say 2°C, it might squeeze 2 kilos of water per square metre, which is 2 mm of rain, out of the atmospheric sponge … and that’s a one-time 2 mm increase spread out over the entire 15-year period of projected cooling. So it would be much less than a millimetre per year.

And that’s a change in annual rainfall of much less than a tenth of one measly percent—not even detectable.

Conclusions? The claims that decreasing solar activity

• will bring tropospheric cooling, and that
• the cooling will “wring” a significant amount of water out of the troposphere,

both fail to find any observational or theoretical support in the tropospheric temperature and TPW datasets considered above.

Best to all, stay safe, stay well,

w.

PS—When you comment, please quote the exact words you are discussing, so we can all understand just exactly what and who you are referring to.