What’s Absorbing The Sun?
Guest Post by Willis Eschenbach
In considering how the energy flows around the planet, I got to thinking about the amount of solar energy that is absorbed rather than transmitted by the atmosphere and the clouds. As with many other such questions, I turned to the wonderful CERES satellite data. Figure 1 shows what CERES has to say about the average amount of solar energy absorbed by the atmosphere.
Figure 1. Total amount of solar energy absorbed by the atmosphere on average on a 1° latitude x 1° longitude basis. CERES Data, Mar 2000 – Feb 2017.
As you can see, just under eighty watts per square metre of incoming solar energy doesn’t make it to the ground. Instead, it is absorbed in the atmosphere. This is a bit more than a fifth (22%) of incoming solar energy. It is also about the same amount of sunlight that is reflected by the clouds.
And Figure 2 below shows the same data, but this time showing the absorption as a percentage of incoming solar energy. Obviously, where there is more solar energy, more energy will be absorbed in the atmosphere. Showing the atmospheric absorption as a percentage of incoming solar energy removes that bias.
Figure 2. Total amount of solar energy absorbed by the atmosphere, as a percentage of incoming solar energy, on average on a 1° latitude x 1° longitude basis. CERES Data, Mar 2000 – Feb 2017.
As I’ve mentioned before, I love the surprises that come from turning a huge mass of numbers into a picture. Here is what is the surprise of the 64,800 individual 1°x1° gridcell calculations was for me. See the red areas? Those are the areas where the largest percentage of incoming solar energy is being absorbed.
Now, the absorption of solar energy in the atmosphere is due to “aerosols”. In the most general sense, this is a term for a variety of chemicals and elements which are held aloft in the atmosphere. Aerosols include things like sulfur dioxide from volcanic eruptions, salt crystals and molecules from sea spray, a variety of bacteria, and black carbon and hydrocarbons from fossil fuels and forest fires. A number of aerosols are human-generated. I’d kind of expected to see increased absorption near cities and industrialized areas of the northern hemisphere.
But none of that was the case. The surprise to me was, it looks like the red areas are from plant-generated aerosols. The Amazon rainforest, the tropical forest areas of Africa and Asia, the forested tropical islands of Indonesia and Papua New Guinea, those were the main sources of aerosols.
And on the other hand, there is little vegetation in the arid areas of northern Mexico, the Sahara and Atacama deserts, Southern Australia, and southern Africa; or in the mountainous areas of the Rockies, the Andes and Himalayas; or in the polar areas of Greenland and Antarctica. These areas in greens and blues have clearer air, with less solar energy absorbed in the atmosphere.
Huh. Plant-based aerosols are the major player in terms of solar absorption. Go figure That would certainly not have been my first guess.
And this brings up another of those curious evolutions over time. Warmer surface temperatures generally mean more plants. More plants mean more plant aerosols. More plant aerosols mean more atmospheric absorption of incoming solar. More atmospheric absorption of solar means less solar energy making it to the ground. And finally … less solar energy hitting the ground means cooler surface temperatures.
And vice-versa, of course.
So the plants are affecting the amount of sunlight making it to the ground, with more sunlight making it through the atmosphere when and where plants are scarce and less sunlight making it through the atmosphere when and where plants are abundant …
Who knew? Likely somebody, but certainly not me …
Next, here’s the evolution over time of the amount of solar energy absorbed by the atmosphere:
Figure 3. Change over time of the absorption of solar energy by the atmosphere. Top panel is the raw data. Middle panel shows the repeating monthly changes. Bottom panel shows the residual signal after the seasonal component is removed. CERES Data.
There is a very slight drop over time in the absorption (a tenth of a watt per decade) which is not statistically significant (p-value 0.08). Overall, the data is surprisingly stable.
I also note that the El Nino/La Nina pump is clearly visible in the 2015-16 data. I showed in a paper called The La Nina Pump that there is an oddity about the La Nina pumping action. The La Nina pumping action is wind-driven, and it moves huge amounts of warm water first westward across the Pacific and from there towards the poles. The oddity is that it begins in November, lasts one year, and ends in the following November.
This same change is visible in the bottom panel above. In November 2015, the atmospheric absorption of solar energy peaked and began to drop. This drop ended in November of 2016, in parallel with the La Nina pumping action of that Nino/Nina episode.
Overall? I’d say what stands out is the stability, plus or minus half of a watt over the period.
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