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The albedo effect and global warming

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The skeptic argument...

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)

What the science says...

The long term trend from albedo is that of cooling. In recent years, satellite measurements of albedo show little to no trend.

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.

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Comments

Comments 1 to 3:

moldyfox at 09:08 AM on 21 June, 2010
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...
Forrest Cavalier at 01:28 AM on 16 July, 2010
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 http://www.bbso.njit.edu/Research/EarthShine/ 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
moldyfox at 08:26 AM on 24 July, 2010
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 http://science.larc.nasa.gov/ceres/STM/2005-05/loeb_earthshine.pdf ). 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