Putting It Into Reverse

Guest Post by Willis Eschenbach

We have an experiential understanding of the effect of radiation on objects. Oh, not nuclear radiation, that’s something different. I’m talking about things like solar radiation, aka sunshine. In the world of climate science, sunshine aka solar radiation is also known as “shortwave radiation”.

This is to distinguish it from thermal “longwave” infrared radiation. Longwave thermal radiation is being given off all the time by everything around us, including the atmosphere. It’s what makes night-vision glasses work. They “see” the longwave radiation. Longwave radiation is also why we can feel the heat from a hot cast-iron stove clear across the room—we can sense the heat on our bodies from the longwave radiation.

Radiation in climate science is distinguished by direction, being either upwelling (headed to space) or downwelling (headed to the earth’s surface).

These types are often referred to by abbreviations. So downwelling shortwave radiation (sunshine) is DSR. Upwelling shortwave radiation (sunshine reflected from the surface) is USR. Similarly, upwelling longwave radiation (that part of the thermal longwave infrared radiation constantly emitted by the surface and atmosphere that is headed to space) is ULR, and downwelling longwave radiation (that part of the longwave radiation emitted by the atmosphere that’s headed toward the earth’s surface) is DLR.

With that as prologue, as I opened by saying, we have an experiential understanding of the effect of radiation on objects. Our experiential understanding of the effect of solar radiation is quite simple.

The more radiation absorbed by some object, the hotter it gets.

Our experiential understanding of longwave radiation is also simple, exemplified by feeling the heat from a cast-iron firewood stove from across the room. That understanding is:

The hotter an object gets, the more longwave radiation it emits.

We experience both of these quite often. Indeed, we have a number of scientific equations that allow us to calculate exactly how much hotter something gets from absorbing a given amount of radiation, and also how much radiation is given off by an object at a certain temperature.

And indeed, our experiential understanding of sunshine is what underlies the fundamental paradigm of climate science:

The more radiation absorbed by the planetary surface, the hotter it gets.

Now, that seems unassailably true, based on both our experiential understanding, as well as the equations that can actually calculate the amount of heating for a given amount of radiation. I mean, we can see every day how the sun comes up and the earth gets warmer … simple physics, right?

So … is it always true that if more radiation is absorbed by some object, it gets warmer?

Well … consider what happens when you walk outdoors during the day. Immediately you are absorbing hundreds and hundreds of watts of additional energy from the sun.

But despite absorbing a large amount of solar radiation, your overall average temperature is unchanged … more radiation has not made you hotter.

Ah, folks will say, but that’s because the human body has systems that regulate our temperatures. We have systems that increase heat loss when absorbed radiation increases, that move the absorbed energy to where it can be lost to the air … and, folks say, that’s very different from the climate.

Hmmm …

Keeping that in mind, let me take a slight detour. There’s a mathematical measure called “correlation”. It measure the similarity of two datasets, and for any pair of datasets, it has a value somewhere between minus one and one. “Correlation” measures whether two sets of data, say temperature and absorbed radiation, move in the same direction. A correlation of 1.0 means the two datasets always move in the same direction—if, for example, when absorbed radiation increases, temperature always goes up.

A negative correlation means the two datasets are generally moving in opposite directions. A correlation of -1.0 means the two datasets always move in opposite directions—when one goes up the other always goes down.

And a correlation of zero means that there is no relationship between the changes in one dataset and the changes in the other.

With that as a prologue, let’s look at the correlation between the earth’s surface temperature and how much radiation the surface is receiving. Per our experiential understanding, the correlation should be strongly positive, meaning that the more radiation that is absorbed by the planetary surface, the hotter it should get, and the less radiation absorbed, the cooler it should get.

Here, using the CERES satellite data, is a gridcell by gridcell display of that correlation. Each gridcell is 1° latitude by 1° longitude.

Figure 1. Gridcell by gridcell correlation of surface absorbed radiation (shortwave + longwave) and surface temperature. Gridcells are 1° latitude by 1° longitude.

Now, this is a most interesting result. Everywhere over the land, with no exceptions, the correlation is just what we’d expect—not only positive, but in general strongly positive. Overall correlation over the land is 0.91, a strong positive correlation, which supports our experiential understanding of absorbed radiation and temperature. Over the land, when absorbed radiation increases, temperatures do in fact go up, and vice versa. Positive correlation.

But over large areas of the tropical ocean, shockingly, there is negative correlation. Contrary to our experiential understanding, contrary to the central paradigm of climate science, contrary to “simple physics”, in those areas more absorbed radiation is NOT making the planetary surface warmer. It’s making the surface cooler … which isn’t possible if absorbed radiation is determining the temperature.

From that, we can only conclude that in those areas, the causation is going in reverse. Instead of total absorbed radiation determining temperature, the temperature is determining total absorbed radiation.

A primary mechanism that explains this apparent impossibility is the temperature-controlled emergence of cumulus fields and thunderstorms. These increase with increasing temperature, and they greatly reduce the amount of solar radiation absorbed by the surface. And so the temperature is regulating the amount of absorbed solar radiation, via clouds and thunderstorms.

And this is a very strong regulation. Here’s a scatterplot of the net effect of clouds on the downwelling radiation versus the surface temperature.

Figure 2. Scatterplot, ocean temperature versus total downwelling radiation change due to clouds (cloud radiative effect, “CRE”).

Note that at the warmest temperatures, the clouds are reducing total downwelling radiation (shortwave + longwave) by up to 60 W/m2 … by comparison, a doubling of CO2 is said to increase radiation by 3.7 W/m2.

Next, I need to show that the phenomenon of reversed causation/negative correlation is in fact temperature-related. I mean, it could just be some peculiarity of the tropical ocean that isn’t particularly related to the temperature.

The first way I investigated that question was by making a scatterplot of the relationship between temperature and the correlation shown in Figure 1. Here is that result.

Figure 3. Scatterplot. Horizontal axis shows the temperature of each 1° x 1° gridcell. Vertical axis is the correlation of absorption and temperature in that gridcell. The box at bottom right encompasses all of the gridcells which have a negative correlation between the absorbed radiation and the temperature.

A couple of things are clear here. First, the reversal of cause and effect leading to the negative correlation of absorption and temperature only occurs at ocean temperatures over ~ 23°C.

And second, in that area in the lower right showing all gridcells with negative correlation, the warmer the temperature, the greater the maximum observed negative correlation.

So this is evidence strongly supporting the idea that the emergence of negative correlation is indeed temperature-based.

However, while this shows the conditions on average over the period of the satellite record, this is only a long-term calculation. We still need to investigate what happens in the gridcells as temperatures warm and cool over time.

Now, my hypothesis is that the surface temperature is regulated by emergent phenomena including tropical cumulus fields and thunderstorms. If that is the case, then as the temperature increases, the strength of this negative correlation should wax and wane.

More specifically, a corollary of my hypothesis is that the area of the ocean surface where the correlation is negative should be larger in the summer when the ocean is warmer, and the area of negative correlation should be smaller in the winter when the ocean is cooler. So I did the calculations and graphed it up. Of course, to do this I had to split the data into northern and southern hemisphere gridcells, since the seasons are reversed in the two hemispheres.

Figure 4. Monthly variations in the area of the ocean surface where the temperature and the absorbed radiation are negatively correlated.

As would be expected if my hypothesis is correct, in the northern hemisphere (red line) the area of negative correlation is largest in the summer. In fact, it peaks out at about 50% larger in summer than the winter minimum.

And at the same time, the area in the southern hemisphere (blue line) is at a minimum, because it is the southern hemisphere winter. It is an even larger swing in the southern hemisphere, with the maximum area of negative correlation being almost twice the minimum area.

So both of these methods show that indeed, the negative correlation is a function of temperature.

Summary: When the ocean temperature gets high enough, the normal everyday “simple physics” positive correlation between absorbed radiation and a resulting temperature increase breaks down, and the correlation between radiation and temperature goes negative. This acts to reduce the ocean surface temperature. It is another of the many emergent phenomena which act in concert to thermoregulate the planet.

How good is this planetary thermal regulation? Well, although we inhabit a world that is balanced at a temperature on the order of 50°C warmer than it would be without greenhouse gases, a world regulated by clouds, winds, and waves, a world where the land temperature varies by up to ± 30°C (± 10%) from summer to winter, and the ocean varies by up to ± 8°C (± 3%) from summer to winter … despite being in the midst of all of that daily and monthly fluctuation, the global average temperature only varied by ± 0.4°C (± 0.1%) over the entire 20th century …

To me, this is the big unanswered question in climate science—not why the temperature varies, but why it varies so little. And the existence of the negative correlation discussed above is a testament to how “simple physics” is completely inadequate to explain the unbelievably complex, chaotic climate system.

My best regards to all,

w.

The Usual: When you comment please quote the exact words you are discussing. I can defend my own words. I cannot defend your restatement of my words.

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August 7, 2022 at 12:40PM

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