A Balancing Act

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Guest Post by Willis Eschenbach

I’m a visual guy. I understand numbers, but not in tables. I make them into graphs and charts and maps so I can understand what’s going on. I got to thinking again about total absorbed radiation at the surface. Total radiation absorbed by the earth’s surface is a mix of longwave (thermal) and shortwave (solar) radiation. In my last post, Putting It Into Reverse, I looked at the correlation of that absorbed radiation with temperature.

So, being a visual guy, I created a global map of where this total radiation is being absorbed at the surface. But before showing that result, let me digress for a moment about the downwelling shortwave (solar) and downwelling longwave (thermal) radiation. (Note that “downwelling radiation” is radiation headed toward the Earth’s surface and “upwelling radiation” is headed to space.)

Solar radiation starts out as relatively constant at the top of the atmosphere. It’s around 340 watts per square meter (W/m2) as a 24/7 global average. It only varies about ± 0.1 W/m2 over the sunspot cycle.

Next, at any given time and location, somewhere between a little and a lot of the incoming solar is reflected by clouds and aerosols. The amount reflected varies by date, season, temperature, location, altitude, and local weather.

Next, of the remaining solar after reflection at that location, somewhere between a little and a lot of the downwelling solar radiation is absorbed in the atmosphere, mostly by clouds, water vapor, and aerosols (smoke, haze, volcanic aerosols, mineral dust). Again, the amount absorbed varies by date, season, temperature, location, aerosol type, and local weather.

Finally, when the sunshine reaches the surface, somewhere between a little and a lot of it is reflected back into space by the surface itself. Again, the amount reflected varies by date, season, water state (liquid vs ice vs snow), windiness, ground cover, location, altitude, and local weather.

In short, the amount of sunshine absorbed by the ground varies hugely in space and time on all scales.

Downwelling thermal radiation, on the other hand, is radiation emitted by several things in the atmosphere above us—by greenhouse gases such as water vapor and CO2, by aerosols, and by clouds.

The big variations in downwelling radiation are due to varying amounts of clouds, water vapor, and aerosols. CO2 is a fairly well-mixed gas, while on the contrary, water vapor can vary in a short distance from almost none to amounts large enough to condense. Again, the amount of thermal radiation emitted by water vapor, greenhouse gases, aerosols, and clouds varies by date, season, windiness, location, and local weather.

And as with solar radiation, clouds are the big variable. Clouds are almost a perfect blackbody with respect to thermal radiation. On a clear winter night when a cloud comes over, you can instantly feel the warmth. And as above, the amount of thermal radiation emitted by clouds varies by date, season, temperature, location, and local weather.

In short, just as with sunshine, the amount of thermal radiation absorbed by the ground varies hugely in space and time on all scales.

So, with that as prologue, here is Figure 1, showing the total amount of radiation (shortwave + longwave) absorbed by the surface of the earth.

Figure 1. A 1° latitude by 1° longitude map of the total amount of radiation absorbed by the earth’s surface.

I gotta admit, I looked at that graphic when I first made it, scratched my head, and said “How very curious!”. I love surprises in science, and this was one of them.

Here’s what I found odd. The southern hemisphere is mostly water, with a block of ice-covered rock at the bottom. It’s very different from the northern hemisphere, which has much more land, and water instead of icy rock at the top.

From Figure 1, per square meter, the ocean is absorbing about 20% more downwelling radiation than the land. So you’d think that the southern hemisphere, with significantly more ocean, would be absorbing significantly more energy than the northern.

But it’s not. In fact, the two hemispheres are the same to the nearest tenth of a W/m2 … which is why I scratched my head and said “How very curious”.

Naturally, I wanted to know whether this was just a coincidence, or whether this hemispheric equality is an enduring feature of the climate system. So, I looked at the changes over time. Here are annual averages for the period of the CERES satellite data.

Figure 2. Annual averages, total absorbed radiation, shortwave, and longwave.

Curiouser and curiouser. Year after year, the annual northern and southern total energy absorbed are nearly identical—half of the years, the two hemispheres were within a tenth of a percent (~ half a watt per square meter) of each other.

The longwave and shortwave components are equally interesting. Every single year, slightly more longwave radiation than shortwave is absorbed in the northern hemisphere. However, the reverse is true for shortwave radiation. Possibly because of the larger amount of ocean, in the southern hemisphere, more solar energy is absorbed than longwave. In any case, when longwave and shortwave are added, the total radiation absorbed by the two hemispheres are nearly identical.

Now, I started out by saying that because both solar and thermal radiation are functions of a variety of factors, with clouds leading the pack, they constantly vary in time and space. So a priori, we have no reason to assume that the two hemispheres would absorb the same radiation at the surface, and every reason to assume that they would not.

I mean, we have volcanoes and floods and droughts and forest fires and a whole bunch of things that affect downwelling longwave and shortwave radiation … and despite that, each hemisphere receives the same amount of radiation as the other, year after year.

Setting that oddity aside for a moment, the climate can be profitably analyzed as a giant heat engine. It turns incoming solar energy into the endless physical work of driving the motion of the oceans and the atmosphere against turbulence and friction. These oceanic and atmospheric movements carry heat polewards from the tropics, where it is radiated into space.

This unexpected stability over time of the total energy absorbed by the surface clearly indicates that this is a heat engine with a governor. And not only is there a governor. The governor works in part by controlling the climate heat engine’s throttle.

A “throttle” is any mechanism that regulates the amount of energy entering a heat engine. In your car, the throttle is what is controlled by your gas pedal. The clouds perform that function for the climate. They control the amount of energy entering the system by rejecting some of that incoming solar energy back into space. And not just a small amount. Hundreds of watts per square meter. Here’s an example, a day’s record from a moored TAO buoy on the Equator at 110° West (eastern Pacific Ocean).

Figure 3. Downwelling solar energy by the time of day, December 30, 1998.

You can see the clouds changing the amount of downwelling solar energy by several hundred watts per square meter within an hour or so.

And this throttling of the incoming solar energy must be a major part of what is behind the year-after-year stability of the amount of solar energy absorbed by each hemisphere individually and by both hemispheres together.

My hypothesis is that a hierarchy of emergent climate phenomena, mainly in the tropical oceans but elsewhere as well, regulate incoming energy. As can be seen in Figure 3 above, a typical tropical day starts out clear.

Figure 4. Typical tropical ocean early morning conditions. Cloudless sky.

Once a certain temperature threshold is passed, a cumulus cloud field is quickly established. This immediately reduces the amount of solar energy making it to the surface.

Figure 5. Typical tropical ocean late morning conditions. Cumulus field is developing. Cumulus clouds form at the top of the ascending parts of the circulating cells of air.

Then, when a higher temperature threshold is passed, some of the cumulus clouds develop into towering thunderstorms. These cause further reflective losses, as well as directly refrigerating the surface.

Figure 6. Typical tropical ocean afternoon to night conditions. Thunderstorm field develops.

All of these emergent transitions increase the amount of sunlight that is either reflected back to space or absorbed before it gets to the surface. And the timing of emergence, the number, and the strength of those phenomena are all temperature-threshold regulated.

The net result of all of this is that as temperatures go up, clouds form in response and cut down the total energy being absorbed by the surface. The following graph shows a gridcell by gridcell scatter plot of the temperature versus the surface net cloud radiative effect (CRE). The surface net cloud radiative effect (CRE) is the average change in total surface downwelling radiation that results from the presence of clouds.

Figure 7. Scatterplot, gridcell by gridcell temperature versus net cloud radiative effect (CRE). Gridcell size is 1° latitude by 1° longitude. There are a total of 64,800 gridcells shown above.

As you can see, when the temperature gets high, the clouds act strongly to reduce the energy reaching the surface. In many gridcells, clouds are cutting out more than 50 W/m2 of downwelling energy at the surface.

In any case, that’s my explanation for why, despite the hugely variable nature of clouds, water vapor, and aerosols, both in time and space, about the same amount of total radiation is absorbed by the two hemispheres every year. Temperature-threshold-dependent emergent climate phenomena act to cap the possible energy absorbed.

I’m more than happy to hear alternate theories for the unusual stability of the absorbed radiation at the surface. Please don’t say “thermal inertia” unless you can explain how “thermal inertia” is controlling the amount of downwelling solar energy.

Late summer afternoon here in our clearing in the redwood forest. Can’t see the ocean today, foggy at the coast, but no clouds here. My nine-month-old grandson cries in the kitchen, my daughter consoles him. My three-year-old granddaughter explains how she dropped her sock in the cat water. She wants me to play Arlo Guthrie’s “City of New Orleans” on the computer. Done, little lady, done.

The sun is slanting across the house clearing to the tall redwood forest trees visible through my window.

Bedtime for the girlie. She wants to fade out to “Mercury Blues“. I’m not complaining.

My best to each and every one of you, may your lives be full and overflowing.


PS—When you comment, please QUOTE the exact words you are discussing. I can defend my own words. I can’t defend your restatement of them. Thanks.

via Watts Up With That?

August 25, 2022