Guest Post by Willis Eschenbach
There is a more global restatement of Murphy’s Law which says “Nature always sides with the hidden flaw”. Parasitic losses are an example of that law at work.
In any heat engine, either natural or manmade, there are what are called “parasitic losses”. These are losses that tend to reduce the temperature differentials in the heat engine, and thus reduce the overall efficiency of the engine. In general, as a percentage parasitic losses increase rapidly with ∆T, the temperature differences in the engine. In the climate system, two main parasitic losses are the losses from the surface to the atmosphere by way of conduction and convection (sensible heat), and the losses from surface to atmosphere by way of evaporation and transpiration (latent heat). Both of these parasitic losses act to reduce the surface temperature with respect to the overlying atmosphere, by simultaneously cooling the surface and warming the atmosphere … nature siding with the hidden flaw to reduce the overall system efficiency. So I decided to see what the CERES data says about parasitic losses. Figure 1 shows the parasitic losses (the sum of sensible and latent heat losses), as a percentage of the total surface input (downwelling longwave plus shortwave).
Figure 1. Parasitic losses (latent and sensible heat loss) from the surface to the atmosphere. Percentage of parasitic loss is calculated as the sum of sensible and latent loss, divided by the total surface input (downwelling shortwave plus downwelling longwave).
I was most interested in how much the parasitic loss changes when the total surface input increases. Figures 2 to 4 shows that situation:
Figures 2-4. Scatterplots, parasitic loss in watts per square metre (W/m2) versus total surface input (W/m2). Parasitic loss is loss as sensible and latent heat. Gold line shows the loess smooth of the data. Red dots show land gridcells, which are one degree square (1°x1°) in size. Blue dots show ocean gridcells.
I was very encouraged by finding this result. I’ve written before about how at the warm end of the spectrum, parasitic losses would increase to the point where most of each new additional watt striking the surface would be lost as sensible and latent heat, and that little of it would remain to warm the surface. These graphs bear that out entirely. Here’s why.
The slope of the gold line above is the rate of increase in parasitic loss for each additional degree of warming. As you can see, the slope of the line increases from left to right, although the rate of increase goes up and down.
In order to understand the changes, I took the slope (change in parasitic loss divided by the corresponding change in surface input) at each point along the length of the gold line for both the land and the ocean separately. Figure 5 shows that result.￼
Figure 5. Change in parasitic loss (in W/m2) for each additional W/m2 of surface input. “Wobbles”, the looped parts in the two graphed lines reflect subtle changes in the loess smooth, and can be ignored.
Now, what are we looking at here? Well, this is how the parasitic loss changes as more and more energy is input to the surface. Where there is little surface input, the loss is low. In fact, at the South Pole the situation is reversed, and the net flow of energy is from the atmosphere to the surface. This is the result of huge amounts of energy being imported from the tropics.
The key point, however, is that as we add more and more energy to a given gridcell the amount of parasitic losses rises, in perfect accordance with nature siding with the hidden flaw. And at the right hand end of the scale, the warmest end, for every additional watt that is added, you lose a watt …
Is this relationship shown in Figure 5 entirely accurate? Of course not, the vagaries of the smoothing process guarantee that it isn’t a precise measure.
But it clearly establishes what I’ve been saying for a while, which is that parasitic loss is a function of temperature, and that at the top end of the scale, the marginal losses are quite large, close to 100%.
Now, as you can see, nowhere is the parasitic loss more than about 30% … but the important finding is that the marginal loss, the loss due to each additional watt of energy gain, is around 100% at the warm end of the planet. Here is the parasitic loss for the planet as a whole versus total surface input as shown in Figure 2:
Figure 6. Change in parasitic loss (in W/m2) for each additional W/m2 of surface input, as in Figure 5, but for the planet as a whole.Change in parasitic loss (in W/m2) for each additional W/m2 of surface input. “Wobbles”, the looped parts in the two graphed lines reflect subtle changes in the loess smooth, and can be ignored.
Note also that across the main part of the range, which is to say in most of the planet except the tropics and poles, about half of each additional watt of energy increase doesn’t warm the surface … it simply goes into parasitic loss that cools the surface and warms the atmosphere.
Best to all,
PS—If you disagree with what I’ve said please quote my words. That lets all of us know just exactly what you disagree with …