The Earth's climate is a complex system. Luckily, we can still understand a lot about the climate even if we do not have an advanced degree in physics. In this chapter, I introduce the important physics that we need to know to understand the climate; in Chapter 4 I will use this physics to create a simple model of our climate.

Temperature and energy

Before we get into the physics of radiation and energy balance, it is useful to talk about the concept of energy. To a physicist, energy is the capacity to do work – such as lifting a weight, turning a wheel, or compressing a spring. The unit of energy most frequently used in physics is the joule, abbreviated as the letter J. Energy is sometimes expressed in units of calories, or cal; 1 cal = 4.18 J. A food calorie is actually 1,000 cal, also called a kilocalorie or large calorie (1 kcal = 1,000 cal or 4,184 J). If you go to Europe, the nutritional label on food packaging has the energy content of the food marked in joules rather than food calories. Thus, a bag of Cheetos with 300 kcal or food calories would instead be labeled as containing 1.3 MJ or megajoules. I prefer food labeled in food calories because 1.3 megajoules sounds more fattening than 300 calories!