Nature News: The albedo effect affects the warming of Earth’s atmosphere

published November 9, 2020

Lonesome Lake with ice cover. photo by Sue Pike

I was up in the mountains this weekend hiking up to North Kinsman Mountain via Lonesome Lake. It was so warm for November. Even though we had packs full of winter gear, even though we started early, a warm breeze had begun to blow and we were down to T-shirts and shorts within the first mile.

This is a beautiful little lake nestled in among the mountains at around 3,000 feet. When we got there, it was still mostly covered with ice. There was even ice and old snow along the shaded parts of the trail. I was struck by how warm it was in the surrounding woods and rocky outcroppings, yet cold by the water.

I am about to teach a big climate unit in my freshman Integrated Earth Science class.   Earth’s energy budget is a big part of this, so the sun and its interaction with the Earth have been on my mind. Walking through this dramatic change in air temperature made me realize that I was experiencing the albedo effect. 

Albedo is the fraction of incoming solar radiation reflected back (instead of being absorbed) by an object. The term “albedo” comes from the Latin for whiteness. The albedo effect is a measure of how much of the sun’s energy is reflected back into space. This is one of many factors that affect the warming of the atmosphere. Solar radiation passes through our atmosphere, strikes the ground and is either reflected back into space or absorbed and re-radiated as heat. So a surface with high albedo that reflects a lot of the solar radiation warms the atmosphere less (if at all) than a surface with low albedo that absorbs solar radiation and transforms it into heat.

Lonesome lake near Mt Kinsmen in the White Mountains. The ice kept this area cooler-albedo! -Sue Pike

When I realized what I was feeling, I got so excited. It was like I was walking through one of my science class experiments. I really wished my students were there with me so we could experience albedo together. Walking through the forest, or further up the mountain, the incoming solar radiation (short wavelength energy) was being absorbed by the dark green forest and granite and changed into longer wavelength energy (heat).  The atmosphere was heating up and we felt warmer. By the lake, that incoming solar radiation was reflecting off the ice, bouncing off at the same wavelength, there was no conversion to heat and so the immediate area was cooler. I can talk about this phenomenon in class, but imagine how cool it would be (pun intended) to be standing next to a big ice-covered lake mirror and make this connection.

Why care about albedo? It is a huge regulator of Earth’s climate. Understanding albedo helps us understand why we are experiencing accelerated warming of the poles. As temperatures warm and ice melts, the open ocean with its low albedo replaces ice cover that has a high albedo, increasing the amount of solar radiation transformed into heat, increasing the temperature. This is a positive feedback loop that only ends when the ice disappears. We are, in effect, rapidly losing a giant northern mirror that reflects solar radiation back into space before it can be turned to heat.

Examples of climate feedbacks, like albedo, are all around us. I guess this is obvious, but it hasn’t always been so to me. Until I learned what albedo was I wouldn’t have recognized what I was experiencing on that hike. I wouldn’t have gotten so excited. This, for me, is one of my biggest motivators as a teacher and a learner. The more we understand the science that underlies the world around us, the better we can appreciate it, which might just motivate us to protect it.