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The albedo effect and global warming

What the science says...

Select a level... Basic Intermediate

The long term trend from albedo is of cooling. Recent satellite measurements of albedo show little to no trend.  

Climate Myth...

It's albedo

"Earth’s Albedo has risen in the past few years, and by doing reconstructions of the past albedo, it appears that there was a significant reduction in Earth’s albedo leading up to a lull in 1997. The most interesting thing here is that the albedo forcings, in watts/sq meter seem to be fairly large. Larger than that of all manmade greenhouse gases combined." (Anthony Watts)

The Unsettled Science of Albedo

“Clouds are very pesky for climate scientists…”

Karen M. Shell, Associate Professor,  College of Earth, Ocean, and Atmospheric Sciences,  Oregon State University, writing about cloud feedback for RealClimate

Albedo is a measure of the reflectivity of a surface. The albedo effect when applied to the Earth is a measure of how much of the Sun's energy is reflected back into space. Overall, the Earth's albedo has a cooling effect. (The term ‘albedo’ is derived from the Latin for ‘whiteness’).

The basic principle is analogous to strategies employed by people who live in hot places. Building are finished with white exteriors to keep them cool, because white surfaces reflect the sun’s energy. Black surfaces reflect much less. People wear light colours in summer rather than dark ones for the same reason.

The Earth’s surface is a vast patchwork of colours, ranging from the dazzling white of ice and snow, to the dark surfaces of oceans and forests. Each surface has a specific effect on the Earth’s temperature. Snow and ice reflect a lot of the sun’s energy back into space. The darker oceans absorb energy, which warms the water. Oceans help keep the Earth warm because they absorb a lot of heat (approximately 90%). This warming increases water vapour, which acts as a greenhouse gas and helps to keep temperatures within ranges humans have largely taken for granted for millennia.

A Cloudy Outlook

It isn’t just the Earth’s surface that has a reflective quality. Clouds also reflect sunlight, contributing to the cooling effect of albedo. They also contribute to warming at the same time, because they consist of condensed water vapour, which retains heat.

And if clouds complicate matters, so too do the seasons. Every year, albedo peaks twice. The first peak occurs when the Antarctic sea-ice is at its winter maximum. The second peak, which is larger, occurs when there is snow cover over much of the Northern Hemisphere.

Albedo also changes due to human interaction. Forests have lower albedo than topsoil; deforestation increases albedo. Burning wood and fossil fuels adds black carbon to the atmosphere. Some black carbon settles on the surface of the ice, which reduces albedo.

Albedo and Global Warming

The most significant projected impact on albedo is through future global warming. With the exception of Antarctic sea-ice, recently increasing by 1% a year, nearly all the ice on the planet is melting. As the white surfaces decrease in area, less energy is reflected into space, and the Earth will warm up even more.

The loss of Arctic ice is of particular concern. The ice is disappearing quite fast; not only is albedo decreasing, but the loss triggers a positive feedback. By exposing the ocean surface to sunlight, the water warms up. This melts the ice from underneath, while man-made CO2 in the atmosphere warms the surface. Humidity also increases; water vapour is a powerful greenhouse gas.  More ice therefore melts, which exposes more water, which melts more ice from underneath…

This loop fuels itself, the effect getting more and more pronounced. This is a good example of a positive feedback. Increased water vapour also has another effect, which is to increase the amount of cloud. As mentioned already, clouds can increase albedo (a negative feedback), but also warming (a positive feedback).

Measuring Albedo

The albedo of a surface is measured on a scale from 0 to 1, where 0 is a idealised black surface with no reflection, and 1 represents a white surface that has perfect reflection. 

Taking measurements of something with so many variables and influences is clearly going to be a challenge. Satellite data is constrained by the orbit of the satellite. Clouds can be hard to distinguish from white surfaces.

Indirect measurement may also be problematic. The Earthshine project investigated a phenomenon where light reflected by Earth illuminates the dark side of the moon. By measuring the brightness, the amount of albedo - reflectivity - could be estimated.

The project reported a counter-intuitive finding. The Earth’s albedo was rising, even as the planet was warming. This seems contradictory, as Anthony Watts was quick to note when he voiced his sceptical argument in 2007. If higher albedo was having a cooling effect, how could global warming be taking place?

Tricky Business

Science constantly seeks to improve itself. The first Earthshine paper, Palle (2004), claimed to have discovered a very significant cooling effect through a big increase in global albedo.

The results were problematic.  They flatly contradicted the NASA CERES satellite observations, and the discrepancy became the subject of investigation. In 2004, a new telescope was installed at the Big Bear observatory, where the project was located. It became evident that the original analysis was in inaccurate. Once corrected, the Earthshine project and the satellite measurements were more consistent.

Figure 1: Earth albedo anomalies as measured by earthshine. In black are the albedo anomalies published in 2004 (Palle 2004). In blue are the updated albedo anomalies after improved data analysis, which also include more years of data (Palle 2008).

Over a five-year period, scientists found that albedo did increase slightly. Since 2003 the CERES satellite records shows a very slight reduction.

 

Figure 2: CERES Terra SW TOA flux and MODIS cloud fraction for 30S–30N between March 2000 and February 2010 (Loeb et al. (2012) - PDF)

Global versus Local

There are contradictory assessments of current trends in global albedo, possibly because the changes and effects are small. Research is being conducted into the role of clouds, both as forcings and feedbacks, and the role of albedo in cloud formation.

Recent research indicates that global albedo is fairly constant, and having no material effect on global temperatures. Local effects may be more pronounced. Loss of albedo in the Arctic could heat the water sufficiently to release methane stored in ice crystals called clathrates. (Methane is a greenhouse gas far more potent than CO2).

Loss of albedo in the Arctic will accelerate warming across adjacent permafrost, releasing methane. Melting permafrost may reduce its albedo, another positive feedback that will accelerate warming. Ocean warming from reduced Arctic albedo will also accelerate melting at the edges of the Greenland ice cap, speeding up sea level rise.

Conclusions

Albedo is a subject needing a lot more research. It’s an important feature of our climate, and a complex one. It is not yet possible to make definitive statements about what the future may hold. In fact, it is a good example of the ‘unsettled’ nature of climate change science.

We know the planet is warming, and that human agency is causing it. What we cannot say yet is how climate change is affecting albedo, how it might be affected in the future, and what contribution to climate change - positive or negative - it may make.

Basic rebuttal written by GPWayne

This rebuttal was updated by Kyle Pressler in September 2021 to replace broken links. The updates are a result of our call for help published in May 2021.

Last updated on 23 October 2016 by gpwayne. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 26 to 50 out of 94:

  1. Here's my question: If the albedo doesn't appear to be shrinking, where is all the energy coming from that is supposed to be causing the warming - specifically the 3 C rise in temperature from 2xCO2?
  2. Read further down articles - it may be shrinking though CERES says stable. However, Flaner discussed elsewhere here indicates albedo decreasing faster than expected. However, to the primary point of your question. Albedo (unless otherwise stated) refers to energy reflected back in visible spectrum. Obviously, this is an important feedback but the source of the energy for warming by GHG is the increase in DLR. The reduction in OLR due to increased GHG is not captured by measurement of albedo.
  3. scaddenp, "Read further down articles - it may be shrinking though CERES says stable. However, Flaner discussed elsewhere here indicates albedo decreasing faster than expected." Do you have a free link to the paper? I'm not paying $18 to read it, and the summary is too vague. "However, to the primary point of your question. Albedo (unless otherwise stated) refers to energy reflected back in visible spectrum. Obviously, this is an important feedback but the source of the energy for warming by GHG is the increase in DLR. The reduction in OLR due to increased GHG is not captured by measurement of albedo." I understand, but it takes over 16 W/m^2 of additional power at the surface for a 3 C rise in temperature. The intrinsic absorption of 2xCO2 is only 3.7 W/m^2, so even assuming all of this is directed toward the surface, it needs to be amplified greater than 4 times over. The average gain of each W/m^2 from the Sun at the surface is about 1.6 - only about one third of that required for a 16 W/m^2 rise. 3.7 W/m^2 x 1.6 = 5.9 W/m^2 - leaving a deficit of over 10 W/m^2. The amount of the albedo from the surface is only about 23 W/m^2 according to Trenberth's diagram. That means the surface albedo would need to decrease by nearly half to get 16+ W/m^2 for a 3 C rise. That doesn't seem possible from just a 1 C global average intrinsic rise from 2xCO2 given we seem to be relatively close to minimum ice.
  4. First, 3.7W/m2 is "effective top of troposphere forcing", so this is effectively the same as 3.7W/m2 of downward. Talking "even if all directed at surface" is misunderstanding how the forcing is calculated. Second, sensitivity would be much lower as you suggest if there were no feedbacks. Albedo plays big role when ice sheets large, now, not so much. The other big feedback is GHG effect of water vapour.
  5. scaddenp, "First, 3.7W/m2 is "effective top of troposphere forcing", so this is effectively the same as 3.7W/m2 of downward. Talking "even if all directed at surface" is misunderstanding how the forcing is calculated." For the purposes of my question, I'm willing to accept this. "Second, sensitivity would be much lower as you suggest if there were no feedbacks. Albedo plays big role when ice sheets large, now, not so much. The other big feedback is GHG effect of water vapour." Why is the water vapor 'feedback' not embodied in the gain of about 1.6?
  6. Its not clear to me where you get your 1.6 from? Geometric correction? Why do you assume the solar number includes water vapour feedback when the CO2 value explicitly does not. There is little value in talking about TOA forcings if there is a feedback value included. The extent of feedback for a given forcing is the key to calculating climate sensitivity. (how many degrees of temp rise for a doubling of CO2). There is no back of the envelope way to do this - its an output from full GCM - and, yes the greatest uncertainty in the system. However, if you look at Realclimate's latest model/data comparison , you will see that AR4 values of about 3 fit well with data.
  7. scaddenp, "Its not clear to me where you get your 1.6 from? Geometric correction? Why do you assume the solar number includes water vapour feedback when the CO2 value explicitly does not." How could the roughly 239 W/m^2 of post albedo power from the Sun (amplified to about 390 W/m^2 at the surface) not include the effects of water vapor 'feedback'? In other words, how could the effects of water vapor not have fully manifested over decades or even centuries? Even over a hundred years ago, the gain was still about 1.6. For what physical or logical reason would the water vapor response be radically different from each W/m^2 of power from the Sun? "There is little value in talking about TOA forcings if there is a feedback value included. The extent of feedback for a given forcing is the key to calculating climate sensitivity. (how many degrees of temp rise for a doubling of CO2)." I'm not referencing just "TOA forcings" - but the intrinsic forcing of 3.7 W/m^2 plus gain, which is about 5.9 W/m^2. "There is no back of the envelope way to do this - its an output from full GCM - and, yes the greatest uncertainty in the system. However, if you look at Realclimate's latest model/data comparison , you will see that AR4 values of about 3 fit well with data." I know what the models are outputting, but fit well with what data, specifically? I don't see anything in that link about water vapor, which you claim is a big 'feedback' I'm not accounting for.
  8. RW1 - I am very concerned about this use of "gain" and "amplification". It suggests you are thinking about this via a very inappropriate electronic analogy. There is no "fixed gain" controlling how incoming flux translates to surface heat flux. That depends on GHG gas composition and surface temperature and a host of other elements. Your translation of 3.7 to 5.9 is plain wrong. Science of Doom has a series of lengthy articles with a lot of discussion on the actual mechanisms. I can only suggest a detailed study and throw out the "gain" concept. As to model output. The real physics, not simplistic analogy, including the role of water vapour as a feedback are calculated in the models. The output from the model is surface temperature through time given the actual forcings of solar, GHG concentration, volcanoes etc. From the output, you work backwards from temperature to determine the value of sensitivity - which comes to about 3. The validity of the model is tested by comparing forecast surface temperature actual observed surface temperature. If the sensitivity - which involves all those feedbacks is wrong- then temperature prediction would be too. I thought this was plain in the article.
  9. RW1 - let me expand here a bit. To see why idea of fixed gain doesnt work, consider what happens if there is no CO2. If it gets cold enough from loss of DLR and increasing albedo, then all water vapour is condensed out of atmosphere and there is no GHG effect. There is then no "gain". Likewise increasing the GHG increases your "gain". It seems to be that you are trying to use some heuristics and the Trenberth diagram to predict what the Trenberth diagram would look like with 2xCO2 from pre-industrial. You have to use the models to do this. Assuming models are correct then the changes would be like this: No change to TOA inputs. The 3.7W/m2 for increased is CO2 is "effective" not a real change to TOA flux. You could get change in cloud and surface albedo from model results but for simplicity assume increases in one are cancelled by decrease in other. Evaporation etc also change but are minor players. Surface OLR changes to from 390 to 406 and DLR increases from 324 to 340. Your "gain" as you have defined it, increases from 1.63 to 1.69. What is your "gain" when you put two blankets on your bed at night instead of one?
  10. "To see why idea of fixed gain doesnt work" I don't claim the gain is fixed. I know it isn't.
  11. A rise of about 1 C or 5.9 W/m^2 results in a new gain of only about 1.66 from 1.63 (396/239 = 1.66), which is a negligible increase. More importantly, it is still much less than the over 4x needed to get 16+ W/m^2 for a 3 C rise. More importantly If the effects of the 'feedbacks'(including specifically water vapor) are not embodied in the gain, then explain why it doesn't take over 975 W/m^2 at the surface to offset the 239 W/m^2 coming in from the Sun? Then also explain why the response of 'feedbacks' on next 3.7 W/m^2 at the surface will all of the sudden be nearly 3 times greater than the response of 'feedbacks' acting on the original 98+%?
  12. RW1 - The "gain" isn't the correct way to treat the issue, since the relative value is an output of the models, not a simplification you can use for input purposes. A 3.7 W/m^2 imbalance at the TOA results in about 1C of surface warming (5.9 or so W/m^2 higher IR at the surface, although backradiation also increases with atmospheric warming, so that's not a direct imbalance). And then feedbacks occur, changing levels of water vapor, long term albedo from ice melt, CO2 balance with the ocean, etc., each of which induce additional TOA imbalances and subsequent warming. Once feedbacks kick in their TOA imbalances are in addition to the original 3.7 W/m^2 forcing from doubling CO2. As I recall, we had a ~450 post discussion, primarily on these issues with you and George White (who apparently originated this "gain" idea) - I don't believe a single person on the thread agreed with you two, for a lot of very good reasons. You might want to take that into consideration...
  13. RW1 - the increase in "gain" (which certainly does include feedback) is 406/239 from model results. Again it seems you are trying to predict feedback (the increase in "gain"). Its a bogus procedure to say that "gain" * increase in CO2 will be the increase backradiation. You have to calculate it properly.
  14. As for sudden increase in gain... Gain for 0ppm of CO2 = 1 (no amplification) " 360ppm " = 1.63 (1997 value of CO2) " 580ppm " = 1.69 (doubling of CO2) using your definition of "gain" as surface OLR/(solar-albedo) and my calculation of Trenberth using values for models with sensitivity of 3 from above.
  15. Of course, the importance of water vapour is common (misunderstand) skeptic argument. See Water vapour for more detail.
  16. scaddenp, "RW1 - the increase in "gain" (which certainly does include feedback) is 406/239 from model results. Again it seems you are trying to predict feedback (the increase in "gain"). Its a bogus procedure to say that "gain" * increase in CO2 will be the increase backradiation. You have to calculate it properly." The 406 W/m^2 you quote isn't from any measurement but from model predictions from numerous assumptions that only exist in a computer. The 3.7 W/m^2 from 2xCO2 is from empirical measurement, so is the gain of about 1.6, which represents the amplification at the surface for each 1 W/m^2 of energy from the Sun. If 3.7 W/m^2 of additional infrared from 2xCO2 is amplified to 16+ W/m^2 at the surface, why isn't the 239 W/m^2 from the Sun amplified by proportionally the same amount to over 1000 W/m^2?
  17. KR, "A 3.7 W/m^2 imbalance at the TOA results in about 1C of surface warming (5.9 or so W/m^2 higher IR at the surface, although backradiation also increases with atmospheric warming, so that's not a direct imbalance). And then feedbacks occur, changing levels of water vapor, long term albedo from ice melt, CO2 balance with the ocean, etc., each of which induce additional TOA imbalances and subsequent warming. Once feedbacks kick in their TOA imbalances are in addition to the original 3.7 W/m^2 forcing from doubling CO2." Why don't the same feedbacks occur (excluding the surface albedo) on the 239 W/m^2 from the Sun?
  18. "Why don't the same feedbacks occur (excluding the surface albedo) on the 239 W/m^2 from the Sun?" What makes you think they don't? There is some water vapor in the air already, is there not? Why is it there in the first place?
  19. They do. Water vapour amplifies any surface temperature increase (and vice versa). However, your "Gain" analogy is inadequate to quantify those feedbacks.
  20. "so is the gain of about 1.6, which represents the amplification at the surface for each 1 W/m^2 of energy from the Sun." Again, this is bogus way to do it. You need a certain amount of energy to get past the threshold of having any water vapour at all. With zero CO2, you would still have same sun, but snowball earth and no water vapour. Above a certain point, the solar minus albedo is strong enough to give water vapour and get that feedback. Note also that albedo feedback becomes more important at lower temperatures too. The 3.7W/m2 is calculation by the way too, but you can the verify the RTEs used to calculate it empirically. I repeat, you have to calculate feedback with a model, not some half-baked "gain" idea. And the actual response of surface temperature to increasing CO2 gives a way to empirically estimate sensitivity (or test the sensitivity of model). 3 looks pretty good, but see the IPCC WG1 for variety of other empirical estimates.
  21. Continuing from another thread RW1 - I am lost at what you are trying to do here but pretty obviously, you dont lose 48W/m2 for each m2 of cloud! You seem be trying to predict something about change in albedo associated with clouds but what about calculating the +ve change in DLR too? Clouds do both.
  22. scaddenp (RE: 46), (Sorry I'm late on this) "RW1 - I am lost at what you are trying to do here but pretty obviously, you don't lose 48W/m2 for each m2 of cloud!" According to Trenberth's numbers, you do: Clouds cover about 2/3rds of the surface, so 341 W/m^2*0.67 = 228 W/m^2 average incident on the clouds. 79 W/m^2 divided by 228 W/m^2 = 0.34 average reflectivity of clouds. 1/3rd of the surface is cloudless, so 341 W/m^2*0.33 = 113 W/m^2 average incident on the cloudless surface. 23 W/m^2 divided by 113 W/m^2 = 0.20 average reflectivity of the cloudless surface. 0.34-0.20 = 0.14. 341 W/m^2*0.14 = 48 W/m^2 loss for each additional m^2 of cloud cover. "You seem be trying to predict something about change in albedo associated with clouds but what about calculating the change in DLR too? Clouds do both." I'm well aware clouds do both. The whole point is incrementally more clouds reflect away more energy than they re-direct back to the surface; thus, the energy needed to get the 16+ W/m^2 for a 3 C rise can only come from a reduced albedo.
  23. scaddenp (RE: 46), Let's run the numbers on how much energy incrementally more clouds trap: If, according to Trenberth, the cloudy sky has a transmittance of 30 W/m^2, and the surface emitted through the cloudy sky is about 265 W/m^2 (396 x 0.67 = 265). 265 W/m^2 - 30 W/m^2 = 235 W/m^2 absorbed by the cloudy sky. The clear sky has a transmittance of 40 W/m^2, and the surface emitted through the clear sky is 131 W/m^2 (396 x 0.33 = 131). 131 W/m^2 - 40 W/m^2 = 91 W/m^2 absorbed by the clear sky. 91 W/m^2 divided by 131 W/m^2 = 0.69; 235 W/m^2 divided by 265 W/m^2 = 0.89. 0.89 - 0.69 = 0.20 difference between the cloudy and clear sky. 0.20 x 396 W/m^2 = 79 W/m^2 additional absorbed for each additional m^2 of cloud cover. If we assume that roughly half of the absorption and re-emission is back toward the surface (Trenberth actually has this being less than half), that comes to about 39 W/m^2, or about 10 W/m^2 less than the 48 W/m^2 reflected away.
  24. The main point I'm getting at here is if the albedo is NOT decreasing (or has even slightly increased), where is the energy coming from that is supposed to be causing the warming? If, as you claim, an additional 3.7 W/m^2 at the surface is to become 16.6 W/m^2 then why doesn't it take more like 1075 W/m^2 at the surface to offset the 239 W/m^2 coming in from the Sun (16.6/3.7 = 4.5; 239 x 4.5 = 1075)?? Looked at from another angle: In energy balance terms, it takes about 390 W/m^2 at the surface to allow 239 W/m^2 to leave the system, offsetting the 239 W/m^2 coming in from the Sun (power in = power out). If, as you claim, it will take an additional 16.6 W/m^2 at the surface to allow an additional 3.7 W/m^2 to leave the system to restore equilibrium, then why doesn't it take 1075 W/m^2 emitted at the surface to allow 239 W/m^2 to leave the system to achieve equilibrium? What is so special about the next few watts at the surface that the system is all of the sudden going to respond to them nearly 3 times a powerful as the original 98+%? Furthermore, since the atmosphere cannot create any energy of its own, the remaining difference of about 10.6 W/m^2 (3.7 x 1.6 = 6 W/m^2; 16.6 - 6 = 10.6 W/m^2) can only come from a reduced albedo. So again, where is all the energy coming from that is supposed to be causing the warming?
  25. RE: My #48, If anyone doubts my calculations, I have backed check them by assuming that if half of the absorption is directed up out to space, then the weighted average totals should correspond to a temperature of about 255K. 0.69/2 (absorbed clear sky) + 0.31 passing through the clear sky = 0.66 and 0.89/2 (absorbed cloudy sky) + 0.11 passing through the cloudy sky = 0.55; 0.66 x 0.33 (clear sky) = 0.22 and 0.55 x 0.67 (cloudy sky) = 0.37; 0.22 + 0.37 = 0.59 emitted to space from the surface; 396 W/m^2 x 0.59 = 234 W/m^2 (about 254K), which is pretty close.

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