Climate change is on the conservation agenda again and so I thought I would share with you some course-notes from the University of Exeter on the principles of it.
You’ve probably heard of the greenhouse effect. It’s fundamental to understanding what keeps our planet warm and why our climate changes over time. The greenhouse, like the rest of the planet, receives incoming radiation from the sun, which passes easily through the glass, just as it does through the atmosphere. Some of that incoming sunlight is absorbed by the surfaces inside the greenhouse. And like all objects, the plants and the soil in the greenhouse re-emit heat radiation at long wavelengths that are invisible to us. The heat radiation coming off objects inside the greenhouse tries to escape to space, and some of that heat is trapped on the way out by the glass, but only a little bit. The problem is that what really warms the greenhouse up is the prevention of air flow, stopping the loss of heat by convection. Whereas what helps keep the Earth warm is the presence of certain gases in the atmosphere that absorb heat radiation and re-emit some of it back down to the surface. These are known as greenhouse gases, and the result is called the greenhouse effect. But really these gases function more like a blanket around the planet.
Visible shortwave radiation comes through the Earth’s atmosphere. Some is reflected by clouds, or is scattered by particles in the atmosphere and a little bit is absorbed by ozone and other gases. But the remainder reaches the Earth’s surface. There it can either be absorbed or reflected back through the atmosphere towards space. The fraction that is reflected depends on the nature of the Earth’s surface and is called the albedo. Some surfaces like ice and snow reflect a lot of radiation. They have a high albedo. Other surfaces like the ocean absorb a lot of radiation. They have a low albedo.
The Earth emits radiation as heat, then some of that heat radiation is at the right wavelength to be absorbed by gases in the atmosphere. These gases in turn give off heat radiation, and some of it comes back to the Earth’s surface. So what are those key gases? Carbon dioxide is what most of us would identify as the most important gas in this blanket. But actually its water vapour that plays the most important role in keeping our planet warm. Other important gases are methane, ozone, and nitrous oxide.
