Sunday, May 8, 2011

“Therefore, water absorbs almost no sunlight. ”

I hear that a lot on the climate change blogs. Does it really make any sense?

The chart above is the water absorption spectrum. A lot of people look at that and assume that the ocean, made of water, doesn't absorb solar radiation. There just is not very much curiosity now a days. The chart says water doesn't absorb much in the visible light spectrum, but if you think about it you know that it has to. That is if you have ever been diving or looked down in clear water.

If you look down in clear water you can see stuff under the surface through the glare with polarized glasses. There is some reflection, so you may need shades or a shadow, but you can see down. Just like above the surface, the things illuminated under the surface are absorbing and reflecting light at different frequencies. Light sand reflects a lot of light because it has a higher albedo. That is a fancy term for reflectivity. Dark things don't reflect much, they mainly absorb light which turns into heat just like a blacktop road in the summer sun. Only in the water, it doesn't get as hot as the road.

In the deep blue water of the ocean where I fish, you can't see bottom. You can see the rays of light penetrating the cobalt blue waters, sometimes seeing neat things like fish. That is why sport fishing boats have tuna towers, not only to see farther, but to see deeper.

Sometimes you see a color change. That is where the water is not that cobalt blue. It may be powder blue, heavy powder blue, powder powder green, nasty green or milk. Those are all water visibility conditions that fishermen use. Divers describe visibility in feet or meters, how far they can see. I have seen over two hundred feet of visibility and been where I could barely see my hand in front of my mask. Two hundred feet of visibility is pretty close to the maximum. Why?

Because the water absorbs the visible light. According to that chart it can't. But it does. So what's the deal?

The color changes should be the first clue for all the wanna be climate scientists. Water is not pure water, it has stuff in it. Different wavelengths of light penetrate so far and eventually are absorbed. The chart is perfectly correct, red light is absorbed first, any diver knows that. Every diver knows that low visibility is due to junk in the water, it can be silt, algae, plankton, sand fish jiss, any number of things. What a lot of divers may not know is why about two hundred feet is the maximum visibility. That is because there is stuff in the water you can't see. Microscopic critters and salt.

Depth % absorbed wavelengths absorbed
1 m 60 infrared (heats surface waters)
10 m 80 longer visible
150 m 99 [only short, green-blue-violet light remains]

I haven't verified that link, but the data for salt water absorption sounds about right. If you dive below two hundred feet you can see a little bit up. Around three hundred feet you can't see much from what I hear, but algae and other photosynthesis creatures can survive to about 300 to 350 feet. So there is some light still being absorbed to 150 meters or 450 something feet. None of that comes back, so it is pretty much absorbed.

So the next time I hear, "Therefore, water absorbs almost no sunlight." I have this link to explain things.

Update: Maxwell and others may think I am missing the point or we may be talking past each other, I don't know. The way the ocean absorbs and releases heat is fascinating. The depth of warming changes with season and conditions. Cold air masses from the continents increase cooling dramatically while warm air has only a small warming effect. Life increases in layers with optimal conditions that has a feedback on the layers. There is a lot of stuff going on. This chart is better for seeing the impact of the changes of absorption with water conditions.

This link to Ocean World at Texas A&M university has a lot of fairly easy to read information.


maxwell said...


Now that I realize you've taken my comment out of context, I can properly put you in your place.

First, as I already pointed out on Judy's blog, you need a physical model to account for light absorption. We typically use the Beer-Lambert Law to account for linear absorption processes. That equates the optical density, absorbance of a material system, with the concentration of the absorbers times the path length through the absorbers times the molar extinction coefficient, which we can derive from the absorption coefficient that you've plotted.

Now, look at your numbers. What is the concentration of absorbers (water molecules) in the ocean. 1000 M? 10,000 M? So even with a HUGE concentration of absorbers, you still need 100 m of path length to absorb 99 of a 100 photons in at the peak of the solar spectrum.

If that seems like a significant absorption to you, you're a lost cause.

Second, the context of the discussion dealt accounting for the fact that surface of the earth is warmer than would be with no GHG's in the atmosphere. Someone claimed that water molecules in the atmosphere could absorb sunlight and that is why air is warmer than would otherwise be. I pointed out that water molecules in the atmosphere absorb basically no sunlight. I found a graph of the water absorption spectrum to support this claim, which you have reproduced.

So I never made the claim that the ocean didn't absorb light. Though, without any significant physical understanding of the situation, I can understand why you would grasp at that statement. With nothing else to contribute, you want to feel like you're holding someone on a claim he/she made, even if it means taking that comment completely out of context.

So let's recap. The fact that it takes 100 m of 100% water to reach an OD of 2 at the peak of the solar spectrum implies that water is a terrible absorber of sunlight. The scale on that picture isn't logarithmic for no reason.

On top of that fact, which you cannot grasp for some reason, you've taken my original comment completely out of context for no other reason than to inflate your own ego.

Thanks for letting me know to ignore your correspondence in the future.

Dallas said...

That is interesting, about 70% of the Earth's surface is ocean, the majority of the ocean is deeper than 100 meters. Visible light makes up a significant percentage of solar energy. Nearly 94% of that solar energy is absorbed by that poor absorber, the ocean, either through direct absorption of heat transfer from other organisms or trace elements which amount to a huge concentration of absorbers. This amounts to a huge amount of energy, even though the absorber is "poor". That poor absorber also happens to be a fantastic thermal mass which make a major contribution to the average temperature of our planet.

You are right! I am a friggin' idiot. Your meaningful explanation of why, "Therefore, water absorbs almost no sunlight" could never be construed as water absorbs an insignificant amount of sunlight, even though it absorbs the vast majority of the sunlight, that is absorb by Earth.

Dude, that path length is meaningful it you don't have it. Your analysis is correct but misleading.