Saturday, May 17, 2014

Climate Change Part 5: The Parts of the system 2

Note: This program first aired on April 5, 2014.

We talked last week about one of the ways that the amount of energy affecting the Earth’s energy balance can change, that being if the amount of sunlight reaching the Earth changes. Changes over time in solar output, as well as minute changes in the distance of the Earth from the sun can influence the energy balance, but only on very long time scales, or to very small degrees, or both.

Another way that Earth’s energy balance can be disrupted is through a change in albedo. Albedo is the reflectivity of a surface, the higher the albedo, the more light is reflected. Reflection is an important concept to understand when thinking about Earth’s energy balance. The sun’s energy comes to Earth as light, but is quickly transformed when that light is absorbed by the oceans, land and atmosphere and is reradiated as heat. Albedo refers to the portion of the sun’s light that is NOT absorbed, but instead simply reflected back into space as light. It hits us as light, and bounces back to the universe as light. Its what makes us visible as the “Pale Blue Dot” that Carl Sagan and the Voyager space craft made famous. Because the light isn’t absorbed and transformed, it does not play any role in Earth’s energy budget. Currently Earth’s albedo is around 0.3, meaning 30% of the light that hits Earth is immediately reflected back to space.

How does that number change? The easiest way is to change the amount of snow and ice on the surface of the Earth, as snow and ice are white, and thus have the highest albedo of all Earth surfaces. Desert sand and grasslands can also have relatively high albedo, but nothing approaching the bright white of freshly fallen snow. When less surface is covered with highly reflective material, less sunlight is reflected, which means that more sunlight gets absorbed by the Earth’s climate system.

Here’s the thing with the albedo of snow, though, and it allows us to introduce another important concept in climate change, the concept of feed back loops, and it goes something like this: the more snow you have, the more albedo you have, the more light you reflect, the less energy stays in the climate system, the cooler it gets, the more snow you get. Likewise, the less snow you have, the less albedo you have, the less light you reflect, the more heat stays in the climate system, the warmer it gets, the less snow you have. These are feedback loops, in which the results of an interaction then influence subsequent interactions. When the interactions result in ever increasing values (and in this case the value we are looking at is temperature), for example the less snow we have, the less we reflect and the warmer it gets as a result, further influencing the amount of snow and thus reflectivity that can remain,  we call that a positive feedback. When the interactions result in decreasing values, the more snow, the more reflection, the less energy in the system, the cooler it gets, that’s a negative feedback. It adds a layer of complexity to understanding the climate system. When the solar output varies, climate can change, but when Earth’s climate changes, it doesn’t influence solar output. Solar output is a truly independent variable. Albedo varies, but often in response to a change in climate. Albedo can change climate, but can also be changed by climate. Complexities like this are why predicting climate change is so incredibly difficult and requires the world’s most powerful super computers to accurately model.

There are a couple other albedo issues to look at. Snow and ice are white and reflective, but what about clouds? They’re white. And its true, clouds can have high albedo, and result in a negative forcing or net cooling influence on climate. But…but, clouds are also very good at trapping heat, infrared radiation, which results in a positive forcing or net warming influence. The albedo of clouds depends strongly on how thick they are and how high they are, and the formation of clouds depends on how much water vapor is in the atmosphere and how much the atmosphere is cooled (water vapor condenses out of air and forms clouds when an air mass reaches its dew point). The effect of clouds is highly variable, and thought, at this point to have a small negative, or cooling impact on the climate system.

The effect of volcanic aerosols is the other place we talk about reflectivity relative to climate. When a volcano erupts, especially if it is a big eruption and it erupts straight up, it ejects lots of material up into the stratosphere. Some of this material is sulfuric acid, which then forms particles called aerosols. Because these aerosols  have been injected way up into the stratosphere, they can disperse around the planet in a matter of weeks, and stay there for a year or more.  They are light colored and hence increase the albedo very high in the atmosphere, which decreases the amount of light energy that reaches the lower atmosphere and surface. Net global cooling is often observed in the year or two after a major volcanic eruption, due in part to this temporary increase in upper atmosphere albedo.

We’ve looked at solar output and now albedo, next time we will look at another big driver of climate change, the so called green house gasses in the atmosphere.


Cloud Albedo from the Earth Observatory: