Father Theo goes to climate school 3.
The study of energy flows, which is what climate science is all about, requires a specific vocabulary which I’ll discuss here in part 3 of this basic overview of the science. Here I deal with measures of temperature, energy and Earth’s energy input.
There are three basic scales used for describing temperature, Celsius (oC), Fahrenheit (oF) and Kelvin (oK). Science mostly uses Celsius and Kelvin, but because North America has had a hard time adjusting to that, the use of the traditional Fahrenheit scale is widespread, meaning that translations are in order.
Celsius is based around the freezing and boiling points of water at sea level. Oo C is the freezing point of water; 100o C is the boiling point. The comparable points on the Fahrenheit scale are 32o F and 212o F. Oo Kelvin equals -273o C and denotes absolute zero, or the total absence of energy.
Converting Temperature Scales
To convert C to F, use the following formula:
C = (F – 32) / 1.8
Thus 10o F = -22/1.8 = -12.2o C.
To convert F to C, use this formula:
F = (C x 1.8) + 32
Thus 10o F = 18 + 32 = 50o C
To convert C to K, add 273.
To convert K to C, subtract 273.
To convert K to F, etc., convert to C and use the formulas above.
The basic energy units are joules or calories. A calorie is 4.18 joules. A calorie is the amount of energy it requires to raise the temperature of 1 gram of water 1 degree K. A food calorie is different from an actual calorie, and actually refers to a kilocalorie, a 1000 calories. A food calorie, expressed in joules, equals 4.18 times 1000 or 4180 joules or 4.18 kilojoules (kJ) of energy.
4.18 kJ = 1 kC (food calorie) (C in this case standing for calorie.)
There are 800 “food” calories (800 kC) in a 2 litre bottle of Coca Cola, or 800,000 actual calories. This adds up to 3.344 megajoules, enough energy to bring eight litres of water from freezing point to boiling.
Watts measure energy over time. A watt equals one joule per second. A 60 watt light bulb uses 60 joules of energy per second. The bulb would have to burn for approximately 116 minutes to raise the temperature of a litre of water from freezing to boiling.
Measuring Earth’s energy input.
The amount of energy reaching Earth over a given time over a given area is measured in watts per metre squared (W/m2).
At the top of the atmosphere, where it shines directly down (in the tropics, that is), the Sun transfers energy at the rate of 1360 W/m2. That’s 1360 joules of energy a second or 4.9 megajoules an hour for every square metre. Think of a burning 1360 watt bulb arranged at every square metre at the top of the atmosphere. That’s enough energy per square metre to bring a litre of water from freezing to boiling in just 5 minutes.
The 1360 W/m2 figure is correct only over the tropics, however. Averaged over the whole top of the atmosphere, the figure is closer to 340 W/m2. That’s a bright but much more modest 340 watt bulb continuously burning for every square metre.
If you add what Earth receives from the Sun with the energy derived from greenhouse gases, the average energy input at the Earth’s surface is about 500 W/m2. That’s a 500 watt bulb for every square metre, enough energy per square metre to heat a litre of water from freezing to boiling in slightly under 14 minutes.
Planetwide, that would brew a lot of tea.
Acknowledgements to Coursera and the University of British Columbia and the course Climate Literacy: Navigating Climate Change Conversations, taught by Sarah Burch and Sara Harris, with the caveat that if I botched any of this, it’s my fault, not theirs.