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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 that of cooling. In recent years, 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)

Change in the Earth's albedo is a potentially powerful driver of climate. When the planet's albedo or reflectivity increases, more sunlight is reflected back into space. This has a cooling effect on global temperatures. Conversely, a drop in albedo warms the planet. A change of just 1% to the Earth's albedo has a radiative effect of 3.4 Wm-2, comparable to the forcing from a doubling of carbon dioxide. So how has albedo affected global temperatures in recent decades?

Albedo trends before 2000

There are various factors that affect the Earth's albedo. Snow and ice are highly reflective so when they melt, albedo drops. Forests have a lower albedo than open land so deforestation increases albedo (but for the record, no, chopping down all our forests is not the solution to global warming). Aerosols have a direct and indirect effect on albedo. The direct effect is reflecting sunlight back into space, cooling the Earth. The indirect effect is when aerosol particles act as a cloud condensation nucleus, affecting the formation and lifetime of clouds. Clouds in turn influence global temperatures in various ways. They cool the climate by reflecting incoming sunlight but can also warm the climate by trapping outgoing infrared radiation.

All these factors are considered when adding up the various radiative forcings that drive climate. Changes in land use are calculated from historical reconstructions of cropland and pastureland changes. Combinations of satellite and surface-based observations allow us to determine trends in aerosol levels as well as cloud albedo effect. What we observe is that of the various albedo forcings, cloud albedo is the most dominant effect. The long term trend is that of cooling with a radiative forcing from 1850 to 2000 of -0.7 Wm-2.

Figure 1: Globally and annually averaged radiative forcing (Chapter 2 of the IPCC AR4).

Albedo trends after 2000

One way to measure the Earth's albedo is the use of earthshine. This is sunlight reflected from the Earth, then reflected from the Moon back to the nighttime Earth. Earthshine has been measured at the Big Bear Solar Observatory since November 1998 (with some measurements in 1994 and 1995). Figure 2 shows changes in albedo from reconstructed satellite data (black line) and Earthshine measurements (blue line) (Palle 2004).

Figure 2: Albedo anomalies reconstructed from ISCCP satellite data (black) and Earthshine-observed albedo anomalies (blue). The right hand vertical scale shows negative radiative forcing (eg - cooling) (Palle 2004).

The data in Figure 2 is problematic. The black line, reconstructed from ISCCP satellite data, "is a purely statistical parameter that has little physical meaning as it does not account for the non-linear relations between cloud and surface properties and planetary albedo and does not include aerosol related albedo changes such as associated with Mt. Pinatubo, or human emissions of sulfates for instance" (Real Climate).

Even more problematic is the spike in albedo around 2003, shown by the blue earthshine line. This is in sharp contrast to satellite measurements which showed little to no trend over the same period. To put this in perspective, consider the Pinutabo volcanic eruption in 1991 which spewed aerosols into the atmosphere. These aerosols reflected incoming sunlight, causing a negative radiative forcing of 2.5 Wm-2. This led to a dramatic drop in global temperatures. The earthshine data indicate a radiative forcing of nearly -6 Wm-2 which should cause an even greater drop in global temperatures. No such event occured (Wielicki 2007).

In 2008, the reason for the discrepancy was discovered. The Big Bear Solar Observatory installed a new telescope in 2004 to measure earthshine. With the new and improved data, they recalibrated their old data and updated their earthshine albedo results (Palle 2008). Figure 3 shows the old albedo data (black) and the updated albedo (blue). The anomalous 2003 spike disappears. Nevertheless, a trend of increasing albedo remains from 1999 to 2003.

Figure 3: 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).

How accurate is the earthshine method in determining global albedo? The earthshine method doesn't give a global albedo estimate. It covers about one third of the Earth at each observation occasion and certain areas can never be ‘‘seen’’ from the measurement site. Furthermore the measurements are sparsely sampled in time, and only made in a narrow wavelength band of 0.4 to 0.7 µm (Bender 2006).

In contrast, satellite data such as CERES is a global measure of the Earth’s reflected shortwave radiation, including the effects of all atmospheric and surface properties. It covers a broader spectrum than earthshine (0.3–5.0 µm). An analysis of the CERES data finds no long term trend in albedo from March 2000 to June 2005. A comparison with 3 independent sets of satellite data (MODIS, MISR and SeaWiFS) also finds "remarkable consistency" between the 4 satellite results (Loeb 2007a).

Figure 4: Monthly anomalies in global mean CERES SW TOA flux and MODIS cloud fraction (Loeb 2007b).

Albedo has had an effect on global temperatures - mostly a cooling effect on long term trends. As for recent albedo trends, earthshine data shows increasing albedo from 1999 to 2003 but little to no trend from 2003. Satellites show little to no trend since 2000. The radiative forcing from albedo changes in recent years appears to be minimal.

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 26 October 2016 by John Cook. 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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