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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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

Last updated on 23 October 2016 by gpwayne. View Archives

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Comments

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Comments 51 to 66 out of 66:

  1. RE: My #48, The missing 5 W/m^2 is probably due to Trenberth having greater than 50% of the atmospheric absorption being emitted up out to space.
  2. RW1 @49, the water vapour feedback, ie, the increased GHE from increased water vapour in the atmosphere as temperatures rise is expected to be the dominant feedback mechanism, therefore your assumption that the difference between the power radiated by the surface and the power radiated from the top of the atmosphere to space can only come through changes in albedo is false. What is more, the relative strength of the GHE of CO2 and water vapour changes with increased temperature. At very low temperatures, there is almost no water vapour in the atmosphere, and hence almost all of the GHE comes from CO2. As the temperature climbs, the water vapour concentration climbs logarithmically. This means the water vapour feedback increases approximately linearly with increasing surface temperature. Meanwhile, the CO2 forcing increases by a constant amount with each doubling of concentration.
  3. Sigh, still trying to use the Trenberth diagram for prediction. The logical consequence of your argument would be that 100% cloud cover of venus would give it a cold surface.
  4. Tom Curtis (RE: 52) "RW1 @49, the water vapour feedback, ie, the increased GHE from increased water vapour in the atmosphere as temperatures rise is expected to be the dominant feedback mechanism, therefore your assumption that the difference between the power radiated by the surface and the power radiated from the top of the atmosphere to space can only come through changes in albedo is false." That is not what I'm claiming. I'm simply saying the enhanced warming effect (the 4.5 amplification factor needed for a 3 C rise vs. the system's measured 1.6) can only really come from a reduced albedo. If the water vapor feedback is not already embodied in the system's measured 1.6 amplification, then why have temperatures remained relatively stable for so long? Why also then 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? Yes, there is a slight increase of about 1% in the amplification factor of 1.6 from the additional 3.7 W/m^2 absorbed by the atmosphere, but it's far too small to get the 4.5 needed for a 3 C rise. "What is more, the relative strength of the GHE of CO2 and water vapour changes with increased temperature. At very low temperatures, there is almost no water vapour in the atmosphere, and hence almost all of the GHE comes from CO2. As the temperature climbs, the water vapour concentration climbs logarithmically. This means the water vapour feedback increases approximately linearly with increasing surface temperature. Meanwhile, the CO2 forcing increases by a constant amount with each doubling of concentration." We are only talking about a small 1 C rise from 2xCO2. If, as you say, an additional 3.7 W/m^2 at the surface from 2xCO2 is to become 16.6 W/m^2 largely through water vapor feedback, quantify specifically how the feedback causes this much change while it doesn't for the original 98+% (239 W/m^2) from the Sun.
  5. scaddenp (RE: 53) "Sigh, still trying to use the Trenberth diagram for prediction. The logical consequence of your argument would be that 100% cloud cover of venus would give it a cold surface." No, the logical consequence is that 100% cloud cover on Earth would give it colder surface.
  6. Tom Curtis (RE: 52), "What is more, the relative strength of the GHE of CO2 and water vapour changes with increased temperature. At very low temperatures, there is almost no water vapour in the atmosphere, and hence almost all of the GHE comes from CO2. As the temperature climbs, the water vapour concentration climbs logarithmically. This means the water vapour feedback increases approximately linearly with increasing surface temperature. Meanwhile, the CO2 forcing increases by a constant amount with each doubling of concentration." Furthermore, if water vapor in the system operates in this way - to greatly enhance a small warming through net positive feedback, then why didn't temperatures climb higher and higher during the large El Nino events of 1998 and 2010? In each case, the temperature came back down very quickly:
  7. RW1 - Global warming must be due to decreased albedo? That's your theory? Seriously? Shortwave absorptivity (inverse of albedo) is really not much affected by greenhouse gases. Longwave emissivity is, and the changes there are what drive the temperatures. Note that we can measure albedo; the changes there are fairly minor compared to emission changes. As to your El Nino event issues, keep in mind that a positive feedback is not a runaway feedback (if that is indeed what you are implying, I may have misinterpreted your post) - see the Does positive feedback necessarily mean runaway warming thread. Regarding clouds, I suggest you follow up on the net feedback from clouds thread.
  8. scaddenp (RE: 53), "Sigh, still trying to use the Trenberth diagram for prediction. The logical consequence of your argument would be that 100% cloud cover of venus would give it a cold surface." Also, another logical consequence is that the idea of clouds operating as net positive feedback doesn't make sense. For the feedback to be positive, more clouds would need to block more energy than they reflect away, but as I've shown, that isn't the case. If you want to argue that clouds operate as a positive feedback via reducing clouds, that doesn't fit with the relatively steady (or even slightly increased) albedo.
  9. KR (RE: 57), "RW1 - Global warming must be due to decreased albedo? That's your theory? Seriously?" No, it's not. I mean the 'enhanced' warming outside the system's measured boundary to surface incident energy can only really come from a reduced albedo because COE dictates the atmosphere can't create any energy of its own. You can't simply create the remaining 10.6 W/m^2 out of thin air - it has to come from somewhere. If, as claimed, it's not coming from a reduced albedo (i.e. the Sun) and is within the system's internal boundaries, then it needs to be explained why it doesn't take more like 1075 W/m^2 at the surface for equilibrium (239 W/m^2 in and out). Remember, I agree the physics supports a likelihood of some effect (i.e. some warming) from 2xCO2. I'm mainly disputing the magnitude of 3C predicted by the AGW hypothesis.
  10. KR (RE: 57), "Regarding clouds, I suggest you follow up on the net feedback from clouds thread." Maybe I will.
  11. KR (RE: 57) "As to your El Nino event issues, keep in mind that a positive feedback is not a runaway feedback (if that is indeed what you are implying, I may have misinterpreted your post) - see the Does positive feedback necessarily mean runaway warming thread." Yes, I do understand this; however, positive feedback is also not a temporary effect either. There is no reason why it would not continue to amplify further the remaining amplified change even after the initial forcing subsided. The AGW theory claims an intrinsic rise of 1 C will become 3 C via positive feedback, so most of the change comes from the feedback - not the initial forcing.
  12. RW1 - "Remember, I agree the physics supports a likelihood of some effect (i.e. some warming) from 2xCO2. I'm mainly disputing the magnitude of 3C predicted by the AGW hypothesis." About 1.1C from a doubling of CO2, estimate of about 3C from feedbacks, so this is an issue you have with feedbacks, best addressed on the relevant How sensitive is our climate thread. It's well worth keeping in mind that this is a boundary value issue. Increasing greenhouse gases barely affect incoming radiation at all, but greatly reduce outgoing radiation. Equilibrium isn't reached until the outgoing matches incoming. And that is what drives surface temperature changes.
    Response: Yes, and not just "best" addressed, but "must" be addressed on that other thread or other threads that are more appropriate than this.
  13. I think in general this site does a fantastic job, and fully support it. Though I struggled with this article a little bit, I also thought it was informative, and for the issue of cloud cover albedo, linked to it here

    But, though I know this is an old post (though I don't think that makes it or comments in it have any less value) I have a question on this article as well.  It reads "Overall, the Earth's albedo has a cooling effect."  

    Has a cooling affect relative to what? Doesn't the earth have to have some level of albedo? So the albedo can't cool or heat, but only cool or heat relative to a higher or lower albedo??

    I also wonder, since the myth is supplied by Anthony Watts, who seems to have gotten a lot of fairly central stuff incorrect, if the end of that statement supplied at the outset "the albedo forcings.. seem to be ...larger than that of all manmade greenhouse gases combined."

    Again, doesn't this have to be relative to some baseline, such as a marked and precise change in albedo over a specific time period? It's also unclear from the myth quote whether he is talking about the allleged "decrease in albedo" over some set of years prior to '97 (followed by a "lull," which presumable means no change from the prior year?) or the alleged increase in albedo after '97. And is the statement of total affect even accurate? And again, relative to what (not Watt) specifically?

    Ned, comment 7 above:   Helpful comment.  It also says "So, a change in the earth's albedo can increase or decrease the amount of energy that is absorbed, without necessarily increasing or decreasing the amount of energy that is emitted." 

    I want to make sure I understand this correctly, as I'm also unclear on this as well.  A higher albedo will reflect more radiation away from the surface, which has not affected the amount of energy the surface is emitting, but is affecting the energy absorbed by increasing (in the case of a higher albedo) the amount simply reflected away (like a white shirt) and thus decreasing the amount not reflected, but simply absorbed. Making high (versus low) albedo huge, because much of it then goes back into space, instead of heating the surface it hit, and then adding to overall thermal emittance from that warmed surface.  I botch anything major there?

    (Maybe this is going too far afield, and also showing whatI need to learn about the changing wavelengths, but if it is simply reflected and the wavelengths stay the same, then it won't be much impeded by atsmopheric gg gas absorption and re radiation - but, if it is not reflected, but aborbed (say into warmer water) not only does it heat the surface (or water) but when some of that energy is released as heat, in longer thermal radiation form, more of it is then trapped, and re radiated in all directions, by the gg gases in the atmosphere, then otherwise would have been had it been bounced back originally in its original shorter wavelength (and thus not, or less? gg gas absorbable) and less goes back out into space, yet again.  ?? )

  14. There is alot more happening with the earth then just some global warming that is the least of the worries i carry around in my head, if we are trying to reflect the suns radiation and solar energy then why are we absorbing it with solar panels more then ever in the last 10 years Solar Panels might be a very bad thing if they are worried about how much we are reflecting MAN worry more about how much we are absobing   YOU THINK  :) 

    Response:

    [PS] Please read the comments policy. no all-caps.

    Note your point is discussed here though I havent checked the figures.

  15. JimmyJames @64, yes I do.  Better than that, I quantify.

    In 2012, global primary energy supply was 71,013 terawatt hours, or 2.56 x 10^20 Joules.  Total insolation after albedo was 153218.46 x 10^20 Joules, so total primary energy supply was just 0.0017% of the energy the Earth recieves from the Sun.  Put another way, at 5% efficiency, we would need to cover just 0.033% of the Earth with solar cells to completely power our civilization.  With a maximum albedo loss of 1, that represents a change in albedo of just 0.033% - ie, completely negligible.

    To put that figure in context, that is significantly less than the area currently covered by roads.

    Of course, if it should ever be a problem, we could just cover a compensating area with a high albedo surface (like concrete, or sand) to prevent any net change in albedo.

  16. My apologies.  @65, I failed to account for the roundness of the Earth, so that total insolation should be divided by 4, and the percentage terms multiplied by 4.  That still requires only 0.132% of the Earth's surface to be covered in solar cells to entirely supply human energy needs at 5% efficiency, and a 0.132% change in albedo.  That represents a maximum forcing of 0.45 W/m^2 easily compensated by other means as noted in my final paragraph.

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