Even though I've been teaching about climate change for 15 years, there's still a lot of the science I don't fully understand, and I'm also always looking for ideas on how best to communicate complicated ideas about the environment to my undergraduate students, and how to engage them more fully in learning. So I happened across this link to Future Learn an on-line learning consortium of British universities, and in particular to an undergraduate oriented course on climate change. It's eight weeks long, free, and so far seems quite engaging. This is not my first on-line learning experience (I also teach mostly on-line classes) but it is my first experience with a MOOC, and with a course that has such high production values.
One of the things that we are encouraged to do in this course is to use a blog to engage in "reflective learning." At first I thought about creating a whole new blog, but that seemed redundant given that Blue Island Almanack was just sitting here unused for the past four years. So here I am!
Week 1 covered a lot of basic material. So I was surprised to find that there were a number of things that were new to me, or that I understood better by the end of the week than previously because of the skill with which they had been explained.
The first little surprise was the explanation for why "greenhouse" is not the best analogy for how our atmosphere holds heat. The glass of a literal greenhouse does not prevent long wave (heat) radiation from leaving the greenhouse. This was new to me, simplistic explanations given to me years ago said that the glass prevented the heat from escaping, turns out that is not quite correct. Long wave (heat) radiation does escape through greenhouse single pane glass. However, the glass does provide a physical barrier to wind that would remove heat by convection. This makes so much sense to me - I spent two summers of my life (1970 and 1971) working in greenhouses planting and taking cuttings from chrysanthemum plants, and know what the heat of a greenhouse is like.
A better analogy for how the earth's atmosphere retains heat is NASA's temperature regulating blankets . This high tech blanket is embedded "millions of invisible microcapsules that absorb excess heat when you are hot and release the stored heat when you are cold, ensuring a comfortable temperature and humidity." In the earth's atmosphere the "microcapsules" that absorb and release heat are molecules of the various greenhouse gases: water vapor, carbon dioxide, methane, ozone, CFC's and nitrous oxide. Which brings up another new factor I encountered this week: I'd never heard water vapor called a "greenhouse" gas previously. It is different from the other greenhouse gases listed, because it changes in concentration as temperature changes. Water vapor does absorb and release heat, but water vapor increases when temperature increases and decreases when temperature decreases, so it is an important feedback greenhouse gas, but not a "forcing" gas that changes concentration due to non-climatic events.
The most interesting thing I got from this week was this diagram that helped me understand several important aspects of our atmosphere and how it promotes life and affects climate.
FAQ 1.1, Figure 1. Estimate of the Earth’s annual and global mean energy balance. Over the long term, the amount of incoming solar radiation absorbed by the Earth and atmosphere is balanced by the Earth and atmosphere releasing the same amount of outgoing longwave radiation. About half of the incoming solar radiation is absorbed by the Earth’s surface. This energy is transferred to the atmosphere by warming the air in contact with the surface (thermals), by evapotranspiration and by longwave radiation that is absorbed by clouds and greenhouse gases. The atmosphere in turn radiates longwave energy back to Earth as well as out to space. Source: Kiehl and Trenberth (1997). URL: http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1-1.html
The nature of energy exchange is that for every watt of energy that comes in from the sun (342 Watts per square meter) an equal number of Watts energy (325 + 107 = 342 Watts per square meter) must be radiated back into space. If all that energy came in and went out directly the surface of the earth would average a temperature of -19 degrees Celsius, which is obviously too cold for human (or most other type of) life. What happens is that greenhouse gasses (listed above but especially water vapor and secondarily carbon dioxide) absorb the heat for a while and bounce it back into the atmosphere. This bounced back radiation called logically "Back Radiation" is crucial for making life livable. The heat gets to bounce around for a while longer, raising the temperature of the atmosphere near the surface to an average of 14 or 15 degrees Celsius - a much more hospitable climate. Ultimate all that heat energy does leave and the total amount emitted does equal the amount that comes in from the sun, but there is this time delay, allows the lower atmosphere to reach a warmer temperature. The upper atmosphere where the final heat exchange does occur is -19 degrees Celsius.
Suddenly it all makes sense!
The other thing that really helped me this week was some nice organizing lists that helped me order some information that I already had. The Radiation Balance of the Earth is the equation that looks at all the factors found in that diagram above - how much energy comes in, how much is reflected, how much is absorbed, how much crucial back radiation there is and of course how much is ultimately radiated back to space. While it is always true that the ultimate amount radiated back to space must equal the incoming solar amount, the proportions that are reflected, absorbed by the surface and Back radiation can vary.
There are three fundamental ways to alter the complex equation that is the radiation balance of the earth. First factor, the amount of radiation in coming from the sun can change. This is due to two things: a) changes in the sun itself that affect the sun's energy output and b) changes in the earth's axis tilt and orbit around the sun which affect the time and angle at which sunlight strikes the earth. Second factor is changes in albedo or reflectiveness of the earth - how much of the sun's short wave radiation (light, ultraviolet, etc.) is reflected back before it can warm the earth's surface. Things that change albedo are: the amount of surface covered by ice and snow (highly reflective), the amount of vegetation on the surface (a forest reflects less than a desert), the amount of cloud cover (tops of clouds reflect light back), and the amount of particulates and aerosols in the air - the more aerosols the greater the reflectivity (particulates and aerosols can be naturally occurring from volcanoes, or man-made from smokestacks and car exhausts). The third factor concerns the altering of long wave (heat) radiation patterns, changes that affect the amount of heat immediately radiated into space versus the amount of Back Radiation there is - the amount of heat held and returned to the atmosphere for a while before it is ultimately dissipated into space. The third factor is affected by the chemical composition of the atmosphere, such as changes in water vapor, carbon dioxide, methane, ozone, CFC's and nitrous oxide.
I knew all those things, but that's a nice organizing schemata!
