What you will learn in Solar Radiation and Heat Balance

1. 1. What arrives

   The Sun delivers 1361 watts to every square metre held square to its beam, and the Earth averages a quarter of that. Where the quarter comes from is the first thing to work out, and the pole in midsummer is the first surprise.

2. 2. Albedo

   Some of the sunlight never counts: it is reflected straight back to space. How much depends on what it lands on, and the brightest, most changeable reflector on the planet is not at the poles.

3. 3. The calculation that fails

   Balance the sunlight a planet absorbs against what it radiates and you get a temperature. For the Earth that temperature is 33 degrees too cold, and the arithmetic is not where the mistake is.

4. 4. Which molecules can absorb, and why

   Try to build the missing warmth out of nitrogen and oxygen, which are 99% of the air. It cannot be done at any amount. Whether a molecule absorbs infrared depends on how it can vibrate, not on how much of it there is.

5. 5. The whole budget

   Ten flows of energy, each measured separately, have to add up three ways at once. The published table does not quite, and the small surplus left over when it is fixed is what a warming planet means as physics.

6. 6. Seasons, imbalance and feedback

   Turn the orbit into a circle and the seasons stay. Stand the Earth upright and they go. Then the year-round surplus at the equator and deficit at the poles, and the difference between a push and an amplifier.