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DLR Curiosities • Watts Up With That?

Guest Post by Willis Eschenbach.

The CERES dataset continues to yield new insights. My joy is to graph different relationships and then see what I can learn and find out from the graph.

Inter a lot of alia, the CERES dataset allows us to calculate the amount of the very poorly named “greenhouse radiation”. This is the downwelling longwave (thermal) radiation from the atmosphere. It’s commonly called “DLR”, which stands for downwelling longwave radiation.

(NOTE: This is not a thread for disputing the existence of downwelling longwave radiation from the atmosphere. DLR exists, it’s measured both manually and automatically all over the planet every day, and it doesn’t violate the 2nd Law. Get over it.

If you wish to dispute that, fine, I encourage you to do so … but this thread is not the place to do it. There are dozens of threads on the subject, the web is a large place, pick one and go. Please, don’t test me on this. I don’t want to have to snip comments claiming no DLR or that the “greenhouse effect” violates the 2nd Law, but I sure will … END NOTE.]

As first proposed by Ramanathan, the downwelling longwave greenhouse radiation (DLR) can be measured from satellite and ground measurements. It’s the amount of upwelling longwave surface radiation absorbed by the atmosphere. It’s calculated as the upwelling longwave radiation from the surface minus the upwelling longwave radiation measured at the “top of atmosphere” or TOA, meaning in this case at the satellite.

The CERES dataset has values for all-sky and clear-sky radiation. The difference between those is the effect of the clouds.

Now, the amount of downwelling longwave radiation changes over time. And in theory, part of this change is from an increase in CO2. We can calculate the theoretical change in DLR resulting from the change in CO2.

Putting all of that together, we get the following plot of the changes in downwelling “greenhouse” radiation since the turn of the 21st century.

Figure 1. Changes in downwelling “greenhouse” radiation by source.

Now, there are several things of note in this graph.

First, the total change in greenhouse radiation is more than three times the change due to CO2. Presumably, this must be from a combination of changes in water vapor, latent/sensible heat loss, atmospheric solar absorption, and increased surface temperature.

Next, CO2 is the minor player in the greenhouse DLR game. Non-CO2 clear-sky DLR increase over the 2000-2022 period was ~ 3.2 W/m2. Cloud DLR decrease was ~ 1 W/m2. But the CO2 change was only 0.7 W/m2, the smallest of the three.

Next, I theorized a couple of decades ago that thunderstorms, clouds, and other emergent phenomena act to oppose temperature changes and thereby stabilize the temperature. Since then, I’ve provided a variety of evidence to back up that claim. Here, in Fig. 1 you can see that while greenhouse DLR from CO2 and from water vapor are increasing, to the contrary, the greenhouse DLR from the clouds is decreasing. This is evidence in favor of my theory.

In fact, the change in the clouds over the period has more than offset the change due to CO2 … who knew?

Finally, we can compare the change in greenhouse DLR to the change in temperature. This gives us the “TCR”, the transient climate response to a change in DLR.

In this case, the TCR is 0.2°C per W/m2, which would be equivalent to 0.7°C per doubling of CO2. This is markedly smaller than the usual value for the TCR, which is on the order of 1.5W/m2 per 2xCO2 or so. Looking at Figure 1 we can see why that is so. The total change in observed greenhouse DLR is ~ three times the theoretical change from CO2. Since the change is larger, the sensitivity to the change perforce must be smaller.

And since TCR values are typically about 55% of the equilibrium climate sensitivity (ECS), that would make this estimate of the ECS about 1.4°C per 2xCO2.

Finally, there’s been little change in total greenhouse DLR since ~ 2016 … another unsolved mystery of the sea.

Best to all,


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