Clogging Earth's Heat Drain

A Brief Guide to Human-Sized Numbers

One of the things that makes climate science confusing is the huge numbers thrown around. Numbers like Gigatons, Terawatts, or Exajoules. It’s hard for numbers this large to feel real to us.

One way to tackle this problem is to divide a very large number by another large number, to end up with a human-sized number. This is like how the size of an economy is easier to understand when expressed per capita.

For example, the total energy that the Earth absorbs from the sun each day is an unrelatably massive number — 10 Zettajoules. (A Zettajoule is 10^21 Joules, or a thousand billion billion Joules. By comparison, annual human energy consumption is about half a Zettajoule.)

This is an unwieldy number. However, if we divide this by the surface area of the Earth, and by the number of seconds in a day, we find that on average, every square meter of the Earth receives about 240 Joules of solar energy per second (these numbers are all after taking into account that our planet reflects away 30% of sunlight).

The term ‘heat flux’ (or energy flux) tells you how much energy is received or emitted by one square meter of surface area in one second. The more intense the energy flow, the higher the heat flux.

So Earth’s incoming solar flux, averaged over the entire planet, is roughly 240 Joules / second / square meter, or 240 Watts / square meter (240 W/m², for short).

How does this number compare to everyday things? If I sat on the ground, I’d be warming the patch of Earth underneath me with a heat flux of about 50 W/m². If I placed my laptop on the ground while it was running, it would heat the Earth under it with an intensity of 100 W/m².

So measuring things this way puts climate-sized numbers and human-sized numbers in the same ballpark.

Worth remembering: The Earth receives energy at a rate of ~240 W/m² from the Sun. That’s our incoming solar flux.

(If you’ve taken some physics, you might have heard that the solar constant is ~ 1360 W/m². That’s the maximum solar flux at the equator, facing the sun, without any clouds. To take into account day and night, and the variation across Earth’s curved surface, we divide this number by 4.