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.
References:
An Atlantic article about Carl Sagan and the Pale Blue Dot: http://www.theatlantic.com/technology/archive/2013/11/the-carl-sagan-of-our-time-reprises-the-pale-blue-dot-photo-of-earth/281414/
All about albedo: http://www.climatedata.info/Forcing/Forcing/albedo.html
Cloud Albedo from the Earth Observatory: http://earthobservatory.nasa.gov/Features/Clouds/clouds.php
Volcanoes and climate effects: http://earthobservatory.nasa.gov/Features/Volcano/
