The Thermostatic Throttle

Guest Post by Willis Eschenbach

I have theorized that the reflective nature of the tropical clouds, in particular those of the inter-tropical convergence zone (ITCZ) just above the equator, functions as the “throttle” on the global climate engine. We’re all familiar with what a throttle does, because the gas pedal on your car controls the throttle. The throttle on any heat engine controls the running conditions by limiting (throttling) the amount of incoming energy.

Similarly, in the climate heat engine, the throttle is the tropical albedo (reflectivity). The tropical albedo controls how much incoming solar energy is rejected back to space at the hot end of the heat engine. In other words, the albedo throttles the incoming energy to control the entire system.

I have further said that the tropical albedo is a threshold-based and extremely non-linear function of the temperature. So I thought I’d use the CERES satellite data to take a look at how strong this climate throttle is in watts per square metre (W/m2), and exactly where the throttle is located. If such a throttle exists, one of its characteristic features would be that the amount of solar energy reflected must increase with increasing temperature. Figure 1 shows the results of that analysis.

Figure 1. Average change in reflected solar from a 1° increase in surface temperature. Red areas show greater reflection with increasing temperature. The change in reflected energy is calculated on a per-gridcell basis as the change in albedo per 1° temperature increase for that gridcell, times the average solar radiation for that gridcell. Gray line shows zero change in albedo with temperature. Dotted lines show the tropics (23.45°N/S) and the Arctic/Antarctic circles (66.55°N/S).

Clearly, then, such a throttle mechanism exists. It is also where we would expect to find it, located near the Equator where the maximum energy is entering the system. On average, the throttle operates in the areas enclosed by the gray line. I was surprised by the strength of the mechanism, however. There are large areas (red) where a one degree C warming in temperature increases the solar reflection by 10 W/m2 or more. Obviously, this thermostatically controlled throttle would be a factor in explaining the observations of a hard upper open ocean temperature of about 30°C.

The throttle mechanism is operating over much of the tropical oceans and even some parts of the tropical land. It is strongest in the ITCZ, which runs below the Equator in the Indian Ocean and over Africa, and above the Equator in the Pacific and Atlantic.

Next, it is worth noting that overall the effect of temperature on solar reflections is about zero (global area-weighted average is -1.5 W/m2 per degree, which is smaller than the uncertainty in the data). In addition, large areas of both the land and the ocean in the extra-tropics are quite similar, in that they are all just slightly negative (light orange). This is another indication that we have a thermoregulatory system at work. Since over much of the planetary surface the albedo is relatively insensitive to changes in temperature, small changes in temperature in the tropics can have a large effect on the amount of energy that is entering the system. Figure 2 shows the relationship (land only) between absolute temperature in °C, and the change in reflected energy per degree of warming.

Figure 2. Change in reflected solar (W/m2 per °C) versus absolute surface temperature (°C) over the land. Note that where the annual temperature averages below freezing (0°C), there is little variation in surface reflection with temperature. From freezing to about 20°C, the amount reflected is generally dropping as temperatures increase. Above about 20°C, there are two kinds of responses—sizeable increases or sizeable decreases in reflected solar with temperature.

Next, over the oceans the areas near the poles show the reverse of the behavior in the tropics. While the tropical albedo changes cool the tropics, near the poles as the surface warms, the albedo and the reflected sunshine decreases with increasing temperatures.

Figure 3. Change in reflected solar (W/m2 per °C) versus absolute surface temperature (°C) over the ocean, annual averages. Where the annual temperature averages near freezing, there is strong negative variation in surface reflection with temperature. From freezing to about 20°C, the variation is stable and slightly negative. Above about 20°C, there are two kinds of responses—sizeable increases or sizeable decreases in reflected solar with temperature, up to the hard limit at 30°C

What this means is that in addition to limiting overall energy input to the entire system, the temperature-related albedo-mediated changes in reflected sunlight tend to make the tropics cooler, and the poles warmer, than they would be otherwise. Clearly this would tend to limit the overall temperature swings of the planet.

Finally, the use of monthly averages obscures an important point, which is that the changes in tropical albedo occur on the time scale of minutes, not months. And on a daily scale, there is no overall 10 W/m2 per degree of temperature change. Instead, up to a certain time of day there are no clouds, and the full energy of the sun is entering the system. During that time, there is basically no change in tropical albedo with increasing temperature.

Then, on average around 11 am, within a half hour or so the albedo takes a huge jump as the cumulus clouds emerge and form a fully-developed cumulus regime. This makes a step change in the albedo, and can even drive the temperature down despite increasing solar forcing, as I showed here, here,  here, here, and here

From this we see that the thermal regulation of tropical albedo is occurring via changes in the time of the daily onset and the strength of the cumulus/cumulonimbus regime. The hotter the surface on that day, the earlier the cumulus and cumulonimbus clouds will form, and the more of them there will be. This reduces the amount of energy entering the system by hundreds of watts per square metre. And on the other hand, during cooler days, cumulus form later in the day, cumulonimbus may not form at all, and there are fewer clouds. This increases the energy entering the system by hundreds of W/m2.

I bring this up to emphasize that the system is not applying an average throttle of e.g. 10 W/m2 over the average area where the throttle operates.

Instead, it is applying a much larger throttle, of a couple hundred watts/square metre, but it is only applying the throttle as and where it is needed in order to cool down local hot-spots, or to warm up local cold spots. As a result, the averages are misleading.

The final reason that it is important to understand that the albedo changes are HOURLY changes, not monthly average changes, is that what rules the system are instantaneous conditions controlling cloud emergence, not average conditions. Clouds do not form based on how much forcing there is, whether the forcing is from solar or CO2 or volcanoes. They form only when the temperatures are high enough.

And this means that things won’t change much if the forcing changes … because the cloud emergence thresholds are temperature-based, and not forcing-based.

I hold that this immediate response is the main reason that it is so hard to find e.g. a solar signal in the temperature record—because the thermoregulation is temperature based, not forcing based, and thus operates regardless of changes in forcing.

This is also the reason that volcanoes make so little difference in the global temperature—because the system responds immediately to cooling temperatures by reducing albedo, opening the thermostatically controlled-throttle to allow the entry of hundreds of extra W/m2 to counteract the drop in temperature.

There is plenty more to mine from the CERES dataset, and although I’ve mined some of it, I still haven’t done lots of things with it—an analysis of the efficiency of the climate heat engine, for example. However, I think this clear demonstration of the existence of a temperature-regulated throttle controlling the amount of energy entering the climate system is important enough to merit a post on its own.

Best regards to all on a sunny December day,

w.