Albedo, and why you should care about it

I'm a big fan of albedo. I think this concept is very relevant to the climate system, especially to modern climate change. So this is my attempt to familiarize the reader with this concept and show its practical applicability.

What is albedo? In simple terms, it is the ability of a surface to reflect sunlight. A surface with high albedo reflects more sunlight, a surface with low albedo reflects less sunlight.

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There are more general terms, such as reflectance, which are used in other sciences. For the sake of climate, we are primarily interested in sunlight.

Albedo values lie between 0-1. The overall, average albedo of the Earth is 0.30. This means that 30% of sunlight is reflected back. The rest is absorbed and radiated as heat. Greenhouse gases tend to trap the radiated heat. In the context of a warming planet, it might have been more comfortable if the Earth's albedo was higher. A higher albedo would mean lesser sunlight would be absorbed, lesser heat would be radiated, and hence there would be lesser heat for greenhouse gases to trap.

 Alas, it is not to be so. As the planet warms, the Earth's cryosphere shrinks. "Cryosphere" comes from the Green krios, which means cold. This term refers to frozen water i.e. ice and snow -- and includes continental ice sheets, ice caps, glaciers, permafrost etc. Ice and snow have high albedos, so if the planet is covered with them, the overall albedo is higher. But as global temperatures rise, this ice and snow melt, punching the climate system with a double whammy : it raises sea levels, and it reduces overall albedo.

Many people have trouble relating to the problems of melting of ice sheets. Images of troubled polar bears may not be as evocative to people from tropical regions. But whatever happens at the poles -- i.e. in the Arctic region and the Antarctic region -- affects everyone through this double whammy. Rising sea levels are a problem for every country with a coastline. The reduction of planetary albedo is a problem for every country -- with or without a coastline!

This brings us to the ice-albedo feedback. The term "feedback" is very important to understand climate change. Simply put, this feedback describes the situation that:

(A) rising temperatures lead to melting of cryosphere
(B) melting of cryosphere leads to reduction of planetary albedo
(C) reduction of planetary albedo leads to rising temperatures

There is a term in popular culture to describe this situation: "vicious circle". The ice-albedo feedback is a perfect example. In technical terms, it's called a positive feedback loop. Such loops make climate change non-linear (something that can get out of hand very suddenly).

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This brings us to Polar Amplification. This term essentially describes the situation that as the globe warms, the poles warm faster (more) than the other regions. Populations living in the Arctic Circle are facing climate change at a much stronger rate than the rest of us. As you can imagine, the ice-albedo feedback plays a role in this. Polar amplification has also happened in the paleoclimatic past, when warming was not due to greenhouse gases (especially anthropogenic emissions) but due to insolation changes.

In one of my previous posts, I described how the mid-Cretaceous Period (92-83 million years ago) was so warm, that instead of ice, there were rainforests in Antarctica. Someone remarked, it could have been a better world with more forests. More forests do seem better than the current situation of deforestation, but it would not have been a more comfortable world that way. Albedo values for the cryosphere are around 0.7 - 0.8 (70-80% sunlight is reflected). Albedo values for forests are typically around 0.1 - 0.2. We should very much prefer Antarctica to have ice and reflect more sunlight, than have forests, in the hope that they would absorb some of the anthropogenic carbon dioxide!

Let's talk about the role of albedo in solar geoengineering. The basic idea is to "engineer" the climate and modulate how much sunlight we absorb/reflect, to have more control over climate change. Theoretically, one way to do this is to go for "cloud brightening" -- i.e. making clouds brighter so they can reflect more sunlight. Another way to do this theoretically could be to inject aerosols in the atmosphere so they can reflect sunlight. I emphasize "theoretically" because such attempts could have many unexpected and bad side effects. I do not endorse such initiatives at all, and while a heavy statement like that merits an explanation, it shall have to be kept for another day. (You can read something more here.)

If you noticed the use of the term "aerosol" (fine particles in air), you may be tempted to imagine an overlap between climate change and air pollution. I often find people lumping together all environmental problems into one fuzzy, vague "green" concept in their minds. Such coarse intellectual resolution serves no one. In this particular case, air pollution and climate change seem to be two antagonistic problems. Aerosols cause air pollution, but could theoretically be a climate solution. In fact, when large volcanoes erupt, they release a lot of aerosols which lead to cooling (Mount Pinatubo, in 1991, for example). And air pollution has actually slowed down climate change. Consider the recent reduction in pollutants from shipping. 3 years back, the International Maritime Organization (IMO) decided to regulate air pollution from shipping. These reductions also led to a reduction in the albedo from the aerosols (pollutants) over the oceans. This has contributed to the recent anomalously high warming over oceans.

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Thus, as you can see, air pollution and climate change seem to be almost at loggerheads with each other. As a society, we know that we must lower air pollution, and we must also accept that it will bring greater warming. There are no universal environmental solutions. We can understand the scale of the climate problem in another way -- while the pandemic-related lockdowns led to immediate and visible improvements in air quality, they actually led to a slight warming due to the aerosol reduction. No, the aerosol effect was not balanced by the reductions in carbon emissions! Yes, almost halting our lives for several months did not make even a dent in climate change! (I previously wrote about this here.)

Let's end on a happy note. While albedo has got us all in such a mess, it can also help us. Painting roofs white is a simple, low-level climate solution. Such work is being carried out, for example, by the Mahila Housing Trust in India. This is an encouraging start, and must be expanded as quickly as possible to all of India. Do read more about their work here, and request your local authorities to implement this in your region ASAP. White roofs have higher albedos than the conventional non-white roofs, and can keep a house cooler than it would be otherwise. Taking this a giant step further, researchers are now developing ultra-white paint. What can be whiter than white, you ask? While white paint reflects off visible light as well as some infrared light, the ultra-white paint additionally reflects ultra-violet light (component of sunlight). So far, the product does not seem very expensive and if implemented widely, could even replace air-conditioners. Read more here.

There's a lot more we could discuss about albedo. But hopefully, the reader is now familiar enough with the term to see its relevance to climate change, and the simple yet powerful concept it is.

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