"Dark Heat" and Earth's Energy Imbalance

To understand why global warming happens, we need to think about Earth’s energy balance.

Here’s a map of the incoming solar energy that Earth receives.

The blue stuff is the energy that reaches the surface, and the bright white spots are either clouds, snow, or ice, which reflect the Sun’s light and shade our planet.

If this was the end of the story, we’d keep getting hotter and hotter, with no limit (although I know it might feel like that these days). Somehow, we also need to get rid of all this energy.

The way we do this is by glowing with invisible light, through what the mathematician Joseph Fourier poetically called ‘dark heat’ (or rather, he called it chaleur obscure, because he was French).

Today, we use the rather less poetic term infrared radiation. Climate scientists sometimes refer to this as our ‘outgoing longwave radiation’. But don’t let the names confuse you. We absorb visible light and we emit dark heat.

If a thing gets hot enough, like lava, or the red-hot embers after a fire, then the heat becomes so intense it’s no longer dark. It leaks into the visible range. But on average, Earth’s relatively moderate temperature is safely inside the range of dark heat. (Interestingly, although this heat is dark to our eyes, it’s visible to heat-sensing snakes.)

Here’s what this dark heat looks like, as seen by satellite. This is Earth’s invisible glow.

Seen from this high view, clouds obscure Earth’s heat and show up as blue or white cold spots. That’s because, in addition to their role in providing shade, clouds can also act like a blanket, trapping our heat and keeping the Earth warm.

Clouds are complicated. When it comes to incoming solar energy, they cool us down with their shade. When it comes to our outgoing heat energy, they warm us up with their blanket-like behavior. (Or as a physicist might say, clouds are white in visible light but act as black bodies in the infrared spectrum).

Which of these wins out — cooling or warming — has to do with the specifics of the type of cloud. So to predict future climate, scientists have to model the nitty-gritty details of how clouds will form in the future. This is one of the big reasons why climate predictions are hard.

But let’s take a step back and talk about temperature.

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