As we begin the transition into spring, one topic that often comes up is, “When will it really get warm?” Most of us know that if there is snow on the ground, it can be difficult to experience really warm temperatures. This is a result of two different factors: the natural cooling effect of snow cover and the bright snow reflecting sunlight back into space or what is known as albedo.
Years ago, I looked at the topic of albedo and I figured it was a good time to re-examine the topic and update it a little bit. A good portion of the sun’s energy that reaches Earth is simply reflected away, never getting the chance to do any work. On average, over a whole year, the Earth reflects about 31 per cent of the sun’s energy back into space. This reflection is known as earthshine. If we compare it to the moon, which reflects about six to eight per cent of the sun’s energy, and take into account that Earth is four times wider than the moon, you can quickly see and understand Earth would appear very bright from space. In fact, astronauts often report just how startlingly bright Earth appears.
Scientists are also using earthshine to help understand what might be causing long-term temperature changes. The idea is that if earthshine increases, then Earth is reflecting more of the sun’s energy and as a result, the Earth should cool and vice versa; if earthshine decreases, more energy is being absorbed and temperatures should increase. Of interest is that during the 1980s and ’90s earthshine was on a slow decrease, while during the early 2000s earthshine started to increase. Further research into earthshine over the last 17 or so years has shown that there seems to be no discernible trend. On a side note, the 11-year solar cycle of increasing and decreasing energy output from the sun accounts for about a 0.1 per cent change in earthshine.
So, the Earth actually reflects a fair bit of the sun’s energy. As mentioned above, this reflection of the sun’s energy is determined largely by the brightness of Earth’s surface and this is referred to as albedo. Albedo is the percentage of the sun’s insolation that is reflected back into space. An object that absorbs all incoming solar radiation hitting it would have an albedo of zero per cent, while an object that reflects all of the radiation hitting it would have an albedo of 100 per cent.
The colour of an object has the biggest effect on albedo; the darker the object, the lower the albedo. Along with colour, the texture of the surface also affects albedo, with smooth, flat surfaces having a higher albedo. Finally, when it comes to water, the angle of the sun’s rays produces different amounts of albedo. Low sun angles produce more albedo compared to high sun angles; just think of the last time you watched the sun set over a lake.
In the diagram here you can see some average albedo values for different surfaces on the Earth. As you can see, fresh clean snow and ice can have albedo values as high as 90 per cent, whereas older dirty snow will have values more typically in the 40 to 60 per cent range. These variations can greatly impact temperatures in the spring across our region. For example, after a fresh dump of spring snow, we will often see a couple of days of cold temperatures. Part of this is due to cold air moving in behind the storm system that brought the snow, and part is the fact that the bright fresh snow is reflecting back most of the sun’s energy. As the snow melts and darkens, the albedo drops and more energy is absorbed; this leads to more melting and a further darkening of the surface as the snowpack compacts and soil gets exposed. This positive feedback loop helps to explain why, when the conditions are right, the snowpack will often melt or “disappear” in only four to five days.
The one thing not shown in the diagram is the albedo of clouds. The amount of light reflected by clouds is relatively unpredictable and is one of the toughest things to figure in when making climate models. Clouds, as we all know from cloudy days, prevent a fair bit of the sun’s energy from reaching the Earth’s surface by reflecting this energy back into space. This process is known as cloud-albedo forcing. The more clouds covering Earth, the greater Earth’s albedo, thus the cooler Earth will be.
Unfortunately, it is a little more complicated than this, and I have run out of room for this issue. We’ll revisit this topic soon by looking at what happens to all the energy that reaches Earth’s surface and how that helps to drive most of our weather. In the next issue we’ll take a look at how the melt is going and how the spring flood forecast is shaping up.