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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

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

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Comments 101 to 101 out of 101:

  1. I have no idea why the "hot link" problem occurs. When a simple click didn't get me to the graph, I tried copying the URL. When that worked, I thought I'd let others know.

    The sum of the trends isn't that far off 0:The balance is ET = Pr - Q - dS/dt, or 2.30 = 1.00 + 1.01 + 0.75 ==> 2.30 = 2.76, so only off by 0.46.

    (Pr, Q, and dS/dt are defined in the figure in MAR's comment.)

    I was able to download the Pascolini-Campbell et al paper through work. They do discuss the uncertainty in trends. On p 544, they say:

    "Propagated uncertainty in the trend is greatest for Pr (±0.41 mm yr−1 ), followed by Q (±0.32 mm yr −1), and smallest for dS/dt (±0.05 mm yr −1). This leads to a bounded ET trend estimate of 2.30 ± 0.52 mm yr−1 (determined by summing the square of the error in the trend of each component). The fractional uncertainty from Pr is 61%, 38% from Q and 1% from dS/dt. From this analysis it follows that the ET trend is positive and significant in light of the propagated error, and ranges from 1.78 mm yr−1 to 2.82 mm yr−1."

    This is getting off-topic for albedo, though.

  2. I was reluctant to look into the values of Cloud Radiative Effect by location as up-thread the idea that added cloud & associated albedo came without added warming from water vapour seemed to be too difficult to accept by an insistent commenter and I wasn't sure how supportiive the result would turn out to be.

    However, Calisto et al (2014) does provide in its Fig 7 the positive and negative components of CRE by latitude for both Land & Ocean and they can be easilyare here adapted to show net CRE as in the following graphic (assuming the graphic is visible to others when I link to it). The net CRE by latitude is the gap between the bold red trace & the green/blue trace in the upper panels.

    Calisto et al (014) fig 7 adapted

  3. MAR:

    The graphs are a bit hard to read, but your link to the paper at ResearchGate provides a way to download the entire paper as a PDF, and the graphs (and others) can be easily read there.

    The most interesting aspect of the graphs in that paper (in the context of the horribly over-simplified views of the insistent commenter) is the complexity they show. Both the diagrams such as the ones you point to, where we see zonal averages (function of latitude), and in the maps that they provide (showing two-dimensional variability).

    Of course, several people tried to explain some of that complexity to the insistent commenter, to no avail. The insistent commenter's claim of increases in surface evaporation leading directly to proportional increases in cloud cover and changes in radiative forcing (based on a global diagram), remind me of this well-know cartoon. (Hint: the insistent commenter is the one that is trying to foist the miracle as as if it is a proper explanation.)

    A miracle occurs

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