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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)

At a glance

What is albedo? It is an expression of how much sunshine is reflected by a surface. The word stems from the Latin for 'whiteness'. Albedo is expressed on a scale from 0 to 1, zero being a surface that absorbs everything and 1 being a surface that reflects everything. Most everyday surfaces lie somewhere in between.

An easy way to think about albedo is the difference between wearing a white or a black shirt on a cloudless summer's day. The white shirt makes you feel more comfortable, whereas in the black one you'll cook. That difference is because paler surfaces reflect more sunshine whereas darker ones absorb a lot of it, heating you up.

Solar energy reaching the top of our atmosphere hardly varies at all. How that energy interacts with the planet, though, does vary. This is because the reflectivity of surfaces can change.

Arctic sea-ice provides an example of albedo-change. A late spring snowstorm covers the ice with a sparkly carpet of new snow. That pristine snow can reflect up to 90% of inbound sunshine. But during the summer it warms up and the new snow melts away. The remaining sea-ice has a tired, mucky look to it and can only reflect some 50% of incoming sunshine. It absorbs the rest and that absorbed energy helps the sea-ice to melt even more. If it melts totally, you are left with the dark surface of the ocean. That can only reflect around 6% of the incoming sunshine.

That example shows that albedo-change is not a forcing. That's the first big mistake in this myth. Instead it is a very good example of a climate feedback process. It is occurring in response to an external climate forcing - the increased greenhouse effect caused by our carbon emissions. Due to that forcing, the Arctic is warming quickly and snow/ice coverage shows a long-term decrease. Less reflective surfaces become uncovered, leading to more absorption of sunshine and more energy goes into the system. It's a self-reinforcing process.

If you look at satellite images of the planet, you will notice the clouds in weather-systems appear bright. Cloud-tops have a high albedo but it varies depending on the type of cloud. Wispy high clouds do not reflect as much incoming sunshine as do dense low-level cloud-decks.

Since the early 2000s we have been able to measure the amount of energy reflected back to space through sophisticated instruments aboard satellites. Recently published data (2021) indicate planetary albedo, although highly variable, is showing an overall slow decrease. The main cause is thought to be warming of parts of the Pacific Ocean leading to less coverage of those reflective low-level cloud-decks, but it's early days yet.

Albedo is an important cog in the climate gearbox. It appears to be in a long-term slow decline but varies a lot over shorter periods. That 'noise' makes it unscientific to cite shorter observation-periods. Conclusive climatological trend-statements are generally based on at least 30 years of observations, not the last half-decade.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

"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.

Earth's albedo is the fraction of shortwave solar radiation that the planet reflects back out to space. It is one of three key factors that determine Earth's climate, alongside the evolution of both solar irradiance and the greenhouse effect. Back in the 1990's, the evolution of Earth's albedo was by far the least understood of the three key factors. To address that uncertainty, it was proposed to measure Earth's albedo continuously over at least one full solar cycle. The long data series thereby obtained also helped scientists to explore potential correlations between varying solar activity and albedo change.

Thus was born the Earthshine project. It began in the Big Bear Solar Observatory (BBSO) in California in the mid-1990's. Measuring Earth's albedo was done by making observations of the illumination of the dark side of the Moon at night by light reflected off the dayside Earth. This method was pioneered in 1928 by French astronomer Andre-Louis Danjon (1890-1967).

Trial Earthshine observations were made in 1994–1995 and regular, sustained data-collection commenced in 1998. Data-collection continued until the end of 2017, representing some 1,500 nights spread over two decades.

 Illustration of Earthshine.

Fig. 1: When the Moon appears as a thin crescent in the twilight skies of Earth it is often possible to see that the rest of the disc is also faintly glowing. This phenomenon is called earthshine. It is due to sunlight reflecting off the Earth and illuminating the lunar surface. After reflection from Earth the colours in the light, shown as a rainbow in this picture, are significantly changed. By observing earthshine astronomers can study the properties of light reflected from Earth as if it were an exoplanet and search for signs of life. The reflected light is also strongly polarised and studying the polarisation as well as the intensity at different colours allows for much more sensitive tests for the presence of life. Image and caption credit: ESO/L. Calçada.

In 2005, a new automated telescope was installed in a small, dedicated dome at the BBSO. The two telescopes, new and old, were then run together from September 2006 through to January 2007, for calibration purposes. Observations made with the more accurate automated telescope were then made through to the end of 2017.

Since the early 2000s, scientists have also been measuring planetary albedo with a series of satellite-based sensors known as Clouds and the Earth’s Radiant Energy System, or CERES. These instruments employ scanning radiometers in order to measure both the shortwave solar energy reflected by the planet - albedo in other words – and the longwave thermal energy emitted by it. The overall aim is to monitor Earth's ongoing energy imbalance caused by our copious greenhouse gas emissions.

The Earthshine project and the CERES satellite-based measurements (2001-present day) both record great variation in albedo. That is as might be expected, because cloudiness is such an important albedo-controlling factor and varies so much. However, a slightly decreasing trend was detected (fig. 1, Goode et al. 2021).

Earthshine annual mean albedo anomalies 1998–2017. 

Figure 2: Earthshine annual mean albedo anomalies 1998–2017 expressed as reflected flux in Wm. The error bars are shown as a shaded grey area and the dashed black line shows a linear fit to the Earthshine annual reflected energy flux anomalies. The CERES annual albedo anomalies 2001–2019, also expressed in Wm, are shown in blue. A linear fit to the CERES data (2001–2019) is shown with a blue dashed line. Average error bars for CERES measurements are of the order of 0.2 Wm/2. From Goode et al. 2021.

The data cover two solar maxima, in 2002 and 2014, plus a solar minimum in 2009. Recorded variations in albedo show no correlation with the 11-year solar cycle, the cosmic ray flux or any other solar activity indices. Therefore, the data do not support any argument for detectable effects of solar activity on the Earth's albedo over the past two decades.

In comparison with the CERES data, both show a downturn in recent years, even though they cover slightly different parts of the Earth (Goode et al. 2021 and references therein). To put some numbers on things, in the earthshine data the albedo has decreased by about 0.5 Wm, while for CERES data, 2001–2017, the decrease is about 1.5 Wm. CERES data shows the sharp downturn to have begun in 2015.

The explanation put forward for the difference in albedo decrease between Earthshine and CERES has been further investigated and calibration-drift, a known issue with satellites, has been discounted. Instead, a recent and appreciable increase in sea surface temperatures off the west coasts of North and South America has been cited. The increase has led to reduced overlying low level cloud-deck cover. That would certainly cause significant albedo-decrease. The sea surface warming is attributed to a flip in the Pacific Decadal Oscillation (PDO), beginning in 2014 and peaking during the 2015–2017 period. It began to decline before the end of the decade.

However, a lot of this is very new, as pointed out by Gavin Schmidt at Realclimate in 2022. The role played by, for example, aerosols is not quantified in any great detail yet. But qualitatively, these developments demonstrate how impacts to the long-wave radiation combined with cloud feedbacks can lead to big shifts in short-wave reflectivity. Needless to say, this complex area is the firm focus of much ongoing investigation and will be for the foreseeable future.

Last updated on 3 March 2024 by John Mason. View Archives

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Comments 1 to 25 out of 133:

  1. The truth is out there Recent peer review of CERES in-flight calibration show that the CERES solar wavelength response drops in RAPs mode due to exposure to atomic oxygen. The data you show above was corrected using the rev 1 corrections described in 2009 G. Matthews, “In-flight Spectral Characterization and Calibration Stability Estimates for the Clouds and the Earth’s Radiant Energy System” Journal of Atmospheric and Oceanic Technology. Vol 26, Issue 9, pp 1685-1716. This also explains how those corrections did not account for the dimming of the on board lamps and hence over-corrected. CERES data properly calibrated would therefore show a slight drop in albedo from 2000 to 2007 as well as an increase in outgoing long wave flux (as Trenberth's climate models would expect). Read the paper and be critical, I could not fault it...
  2. I like the site overall, but please improve this article. The EarthShine researchers seem to be doing an honest job. For example, they compare to CERES and try to explain discrepancies. Your rebuttal seems like cherry picking and advocacy (that you elsewhere correctly pan as interfering with science.) You can do better, and I await your reply. 1. At they describe the use of two observing stations and an intermittent station. They report that their observations correlate well with satellites. 2. It's simple thermodynamics that temperature change is always and only caused by heat exchange. Temperature is an effect, not a cause. Albedo researchers are trying to measure that process on a global average. Temperature measurements are always and only point samples. If one doesn't agree with the other, that is cause for investigation, but you argue for dismissal. What's up with that? 3. What support do you have for your concluding sentence? Your paragraphs above it support a conclusion along the lines of "the temperature changes due to the albedo forcing are not shown by the reported data." But you wrote a conclusion that is a great leap away from that. I expect better at this site. 4. Even if you throw out 2003, do you admit their 2W/m/m variation in albedo forcing over 4 years, or the monthly/yearly variations in the anomoly graphs? This value is significant, compared to the GHG forcing for all emissions over the last century is estimated 2.4W/m/m. But in this article, you write to admit only that albedo is a "potentially powerful" driver of climate. That's skepticism, not science. Are you also skeptical about CO2's potential impact? They are the same order of magnitude, certainly. 5. The EarthShine project may or may not be valuable for estimating long term trends. It's a very short data series, after all. But the short term year to year variations are natural variations, and swamp CO2 radiative forcing. At the very least, this must be estimated and controlled before drawing conclusions from short term temperature data series (30-100 years) to predict long term trends, leaving out the need to remove uncertainty before embarking on global engineering to counteract it. That's separate. Is anyone doing this control? 6. When you write about temperature drop as "no such event occurred" and then dismiss their data aren't you engaging in the "They didn't explain everything, so their work is irrelevant" tactic of political advocacy that your website is trying to counteract? Maybe there is mitigation by some other process or event. It is certainly a reason to investigate their methods and explain correlations or lack with other data. Looking at the BBSO bibliography I think they are doing that themselves in a more scientific way than your straw man attempts to dismiss. Looking forward to your improvements on this one. - Forrest
  3. Dear Forrest, the Science Palle 2004 Earthshine manuscript is a globally discredited paper and technique. NASA have shown that even with an instrument on the Moon, due to its orbit you could not measure global albedo (as correctly stated above, also see ). The only global measurements are those that come from CERES when properly calibrated using peer reviewed techniques that utilize the fixed climate of the Moon as a calibration standard. These show a statistically significant drop in Earth albedo from 2000-2005 and a statistically significant increase in out going thermal radiance (see Matthews 2009). If you wish to discuss global warming, consider that. Absolutely no conclusions about climate change can or should be made based on Earthshine data. The truth is out there and its peer reviewed, hope that helps. Moldyfox
  4. Has it been proven that the equilibrium temperature of a body in a constant EM radiation field can be altered by altering it's reflectivity (short of perfect reflectivity where equilibrium temperature must remain undefined)? Is it not necessary to demonstrate that in order to prove that albedo or aerosol-based reflectance can influence the global mean temperature?
  5. Yes, Rovinpiper, changing the reflectivity of a body changes the number of photons it absorbs, thereby changing the amount of energy it absorbs. All the formulas you see for calculating equilibrium temperature depend on the energy that is absorbed, not the total of that energy plus the energy that was reflected. It will help if you think of the more elemental mechanisms that are involved. A body emits more radiative energy the hotter that body is. The body gets hotter if it absorbs more energy. But radiation reflected off the body does not get absorbed, and therefore does not make the body hotter. So the body does not radiate more energy in response to incoming radiation that it reflected. Reflected radiation might just as well never have existed, in regards to that body's temperature.
  6. Hi Tom, Thanks for replying to my question. Do you have a solid source for a proof of that? I just read about Kirchoff's Law and it seems to say that if the Earth becomes more reflective it becomes less emissive by an equal amount and so temperature remains unchanged.
  7. Hi, Rovinpiper. Good questions you're asking. Kirchoff's Law refers to absorptance and emissivity at the same wavelength -- i.e., an object's emissivity at a given wavelength will equal its absorptance at the same wavelength. In the case of a planet (e.g., earth), almost all the radiation it receives from the sun is at short wavelengths (UV, visible, and near-infrared). In contrast, all the radiation it emits is at long wavelengths (> 3 micrometers). 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. When this happens, the planet then warms or cools until the outgoing radiation is once again in balance with the incoming radiation. Hopefully that's clear. It's around midnight here and I'm not really a night person, so my explanations may not be all that coherent......
  8. Rovinpiper (bagpipes?), try playing with this calculator.
  9. And, back to the previous question: "Has it been proven that the equilibrium temperature of a body in a constant EM radiation field can be altered by altering it's reflectivity [...] Is it not necessary to demonstrate that in order to prove that albedo or aerosol-based reflectance can influence the global mean temperature?" There are actually quite a few different ways you can see this operating in the real world. If you live in a place where it snows in the winter, you might notice dirty snow melting faster than clean snow -- because its lower albedo causes it to absorb more sunlight and warm up faster. The same principle is what makes ice ages cold ... as the large continental ice sheets expand, they reflect more sunlight back to space, which makes the local climate cooler, which helps the ice expand further. (When they begin melting, at the end of each glacial episode, the same process happens in reverse -- the loss of ice makes the landscape absorb more sunlight, making it warmer, which melts the ice further....)
  10. Hi Ned, There's something I don't understand in your explanation of Kirchoff's Law. You say that emissivity is equal to absorptance at any given wavelength, yet the Earth absorbs light in visible wavelengths and then emits that energy as infrared, doesn't it. How can the emissivity be equal to absorptance at the visible wavelengths if the energy is getting converted into infrared? Thanks again.
  11. That's a great question, Rovinpiper. Think about an object at normal Earth temperature, and assume it's floating in a vacuum. This object has an absorptance in the visible (a_vis) and an emissivity in the visible (e_vis). It also has an absorptance in the thermal-infrared (a_tir) and an emissivity in the thermal-infrared (e_tir). Now, Kirchoff's Law tells us that [a_vis must equal e_vis], and [a_tir must equal e_tir]. With me so far? OK, now, as long as this object is at normal Earth temperatures, e_vis is basically irrelevant -- because it's too cold to emit anything in the visible. It still has a value for emissivity in the visible spectrum, but it never gets a chance to use that. So, under normal conditions, the object absorbs visible solar radiation (sunlight) according to a_vis. If we assume it's floating in a vacuum, it only loses energy by emitting thermal-infrared, in proportion to e_tir. Consider a substance familiar to most of us: paint. Typically, paint will have an emissivity of around 0.90 to 0.96 in the thermal-infrared, but the range is mostly a function of the type of paint, not its color. Anyway, that painted surface would also have an absorptance of 0.90-0.96 for thermal radiation. But, in the visible spectrum, that painted surface might have an absorptance way below 50% (for white paint) or almost 100% (for black paint). What about its emissivity in the visible spectrum? If you could somehow heat the painted surface up to 6000 K without changing its structure and composition, the black-painted surface would emit much more radiation than the white-painted one, because in the visible spectrum it would have a higher emissivity. So ... to get back to your question from a few days ago -- if the Yellowstone Supervolcano were to erupt tomorrow, and eject gigatons of aerosols into the stratosphere, that would increase the Earth's albedo (reflectance) in the solar spectrum. But it wouldn't make a corresponding reduction in the Earth's thermal-infrared emissivity. With less radiation coming in, and the same amount going out, the climate would not be at equilibrium, and things would start to get cold. The colder planet would then emit less infrared radiation, and the equilibrium would return, with the planet at a lower temperature (until all the aerosols wash out of the stratosphere...) Let's hope that doesn't happen any time soon!
  12. Rovinpiper, Kirchoff's Law refers to a material's capacity to absorb and emit radiation at a specific wavelength, not the actual amount that is absorbed or emitted at that wavelength. The total amount of radiation emitted at a specific wavelength does not need to match the amount of radiation absorbed at that same wavelength. It is no violation of the law to have the majority of radiation absorbed in one wavelength while the majority of radiation emitted is in another. After all, materials don't "remember" how their energy was received.
  13. I just realized that some people may not be that familiar with the terminology here. There's a very important distinction between * "absorptance" and "absorbed energy" and likewise between * "emissivity" and "emitted energy" "Absorptance" is a unitless fraction (from 0 to 1) that says how efficient something is at absorbing radiation. It's defined as alpha = L_a / L_i where L_a = absorbed energy and L_i = incident energy Note that as L_i fluctuates, (say, as the sun rises and sets), L_a fluctuates too, but alpha stays constant. Similarly, M = e * s * T^4 where M, the total amount of emitted energy, is a function of emissivity (a unitless fraction from 0-1 that says how efficiently something is able to emit, compared to a blackbody) and T is temperature in kelvins. So, the amount of energy that gets absorbed by an object (L_a) is determined by how much energy is incident on it and its innate absorptance (the unitless fraction "alpha"). Likewise, the amount of energy that gets emitted by an object (M) is determined by its temperature and its innate emissivity (the unitless fraction "e"). Okay, here's the reason I just walked through all that verbiage: Kirchoff's law says that an object's emissivity (at a given wavelength) must be equal to its absorptance (at the same wavelength). It does *not* say that the object's emitted energy (at a given wavelength) must be equal to its absorbed energy (at the same wavelength). In my experience, people (i.e., undergrads in the first week of my class) can easily get tripped up by this. Bottom line -- the amount of solar energy the Earth absorbs is determined by its shortwave albedo (alpha) and by total solar irradiance. The amount of energy the Earth emits is determined by its longwave emissivity (e) and its temperature. The two quantities are not necessarily moving in lockstep ... thus, the climate can warm or cool.
  14. Ha. While I was writing that all out, the appropriately-named "e" snuck in and expressed it much more concisely.
  15. Tom, Ned, e, Yeah, Tom. I'm a bagpiper. Thanks for your help. Kirchoff's Law makes sense to me now. You know, in the book "Jurassic Park", the chaos theorist character, Ian Malcolm, asserts that someone wearing black clothing will be just as comfortable as someone wearing a light color because of black body radiation. Now, Crichton's written "State of Fear". I wonder if his misconception of black body radiation is an important factor in his views on global warming.
  16. You know, in the book "Jurassic Park", the chaos theorist character, Ian Malcolm, asserts that someone wearing black clothing will be just as comfortable as someone wearing a light color because of black body radiation. Really? I must have missed that, though it's been a long time since I read those books. Yes, Dr Malcolm is forgetting about the wavelength-dependence of absorptance and emissivity. Kind of surprising, given that people have known for a long time that dark-colored objects will heat up much faster in the sunlight than light-colored objects.
  17. #16: "Dr Malcolm" And who wrote Jurassic Park? Same guy who did this bit of work. At last we see how those deniers work, moving so seamlessly that one cannot tell where their non-fiction ends and their fiction begins.
  18. Hey Ned, What is "s" in your equation for energy emitted? Thanks, David
  19. I am facing that most intractable of global warming deniers, the old physicist. Faced with what we just discussed about Kirchoff's Law he states that we must integrate over the whole spectrum. How do you do that?
  20. Hi, Rovinpiper. Sorry to have missed your first question: What is "s" in your equation for energy emitted? It should be a "sigma" ... it's the Stefan-Bolzmann constant. Since it's constant, the equation tells us that emitted energy at a given wavelength is a function of just the object's temperature and its emissivity (fraction) at that wavelength. [...] he states that we must integrate over the whole spectrum. Must integrate over the whole spectrum to do what? What's he "skeptical" about? The spectral distribution of incoming solar radiation is very different than the spectral distribution of outgoing longwave radiation. The former is almost entirely at short wavelengths (probably > 99% of it is below 3 micrometers) , while the latter is almost entirely long wavelengths (definitely > 99% of it longer than 3 micrometers). The latter is why the Earth doesn't glow in visible light (lava flows and forest fires excepted...). So you don't really need to integrate across the entire spectrum (or integrate anything, really) to answer the questions you were talking about earlier in this thread. Changing the visible-wavelength albedo of an object will change how much it absorbs, without necessarily implying a corresponding change in the efficiency with which it emits longwave radiation. In that case, the object will warm up or cool down until it reaches a new equilibrium. Dunno if this helps at all.
  21. #19, I recommend Climate modeling through radiative-convective models (Ramanathan 1978) equation 16 (absorption and scattering of solar radiation) which integrates over wave number, angle of incidence, etc.
  22. Here's a link to that paper /news.php?n=481&p=2#34079
  23. Rovinpiper not sure I understood your mate's question. If referred to Kirchoff law, it is valid at each wavelength and need not be integrated. Integration, instead, is performed when computing the radiative balance.
  24. Hi Ned, For our purposes he is "skeptical" about the ability of light-reflecting aerosols to lower Global Mean Temperature. He seems to be saying that a change in the reflectance of an object in a constant electromagnetic field will not change its equilibrium temperature. This is because the emissivity of said object will increase. He says that his personal friend Ferenc Miskolczi has a paper positing this which has never been refuted. I have a link to Miskolczi's work. Unfortunately, the material is too complicated for me to read. It might as well be written in context free grammar as far as I'm concerned.
  25. Re: Rovinpiper (24) Barton Paul Levenson has addressed some of Ferenc Miskolczi's misconceptions here. The Yooper

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