The Energy Budget

I’ve decided to take this blog more in the direction of communicating science – there are only so many sociological musings to come up with. This is the first of many planned posts explaining basic climate science so people have better context for what they read in the newspaper.

Every post is a work in progress, and will be continuously edited when necessary, so please leave comments with suggestions on how to improve the accuracy or clarity. Enjoy!

What determines the temperature of the Earth?

The temperature in your backyard, the warmth of the equator, the frigid polar regions, the average global temperature for the whole planet…..they might seem like very different things to measure, but they’re all caused by the same process. It all comes back to energy.

This energy comes from the Sun, but it’s not as simple as a single transfer. Remember, at any time of the day or night, the Sun is shining on some part of the Earth. That energy can’t just stay on our planet, otherwise it would keep building up and up and we would fry after a couple of weeks.

Therefore, incoming energy from the Sun has to be balanced by outgoing energy from the Earth for the planet’s temperature to stay relatively constant. So when the Sun’s rays hit the ground, as a mixture of light, infrared, and UV radiation, the Earth absorbs the energy. Then it converts it to all to infrared radiation, which we perceive as heat when it hits us, and releases it upward.

All objects perform this absorption and emission when they are hit with radiation. If they receive enough energy, they can release some of it in the form of light – think of how a stove element glows when it’s turned on. However, the energy hitting the Earth is nowhere near this level, so it all comes out as infrared.

It is this emission of infrared radiation that determines the temperature of the Earth. The second step, not the first, is the important one, the one that we actually feel and experience. So on a hot summer’s day, it isn’t actually energy coming down from the Sun that’s making the air warm. It’s energy coming up from the Earth.

The air doesn’t warm up instantly, either – there’s a bit of a lag. This allows warm air to be transported away from the Equator and towards the poles, in the global circulation system of wind currents. Without this lag time, many regions of our world would have far more extreme temperatures.

Additionally, not all the radiation the Sun sends down gets absorbed by the Earth. Some of it is bounced back by clouds, which is why sunny days tend to be warmer than cloudy days. Some of it reaches the surface of the planet, but is bounced back too, before it’s even absorbed. This reflection of energy is particularly common when the surface is light in colour. That’s why it seems so bright outside after a snowstorm – because the snow is bouncing the energy back up as light, instead of absorbing it and releasing it upward as heat. It also explains why dark concrete, which absorbs almost all the radiation that hits it, is so much warmer than a light-coloured deck.

The amount of energy that the Sun sends down to us is greater than the amount that the surface of the Earth actually absorbs. However, the amount absorbed has to be equal to the amount released, and the amount released is what we witness as the temperature outside.

The Best Analogies Ever

An analogy is a powerful tool in science communication. Here are two of my favourites to do with climate change.

The first is of my own creation (although it isn’t too original) and came about after I had presented to high school students a few times. As anyone taking high school physics learns pretty quickly, when using the formula F=ma, to find the net acceleration (the actual, observable result) you must always use the net force. If three people are pushing a box three different directions, you can’t just take one person into account. You have to look at all of them to see which way the box will move, and how fast.

Similarly, to analyze observed climate change, you can’t just take one forcing into account. You can’t only look at greenhouse gases and expect that they will track perfectly with the global temperature. You have to look at what the sun is doing, what aerosol levels are doing, where the ENSO cycle is. Climate is influenced by a combination of factors, and it will never track perfectly with any one. But if only one is changing significantly, and the others are staying pretty much steady, it’s obvious which way the box is going to move.

I found the second analogy in David Archer’s book Global Warming: Understanding the Forecast. I’m only two pages in and already I found something that I want to share here!

[The energy budget/climate equilibrium] is analogous to a sink with water flowing in from a faucet. The faucet fills the sink at some constant rate, while outflow down the drain depends on the water level in the sink. The sink fills up until water drains out as fast as it comes in.

It is possible to change the average temperature of the Earth by altering the energy flow either coming in or going out. In our sink, one way to raise the water level is to turn up the faucet and wait a few minutes. The water will rise until it finds a new equilibrium water depth. We can also alter the water level by partly constricting the drain. Egg shells and orange peels work well for this purpose. If the drain is partly obstructed, the equilibrium water level will rise.”

Brilliant, no? I think that Greg Craven had a similar analogy (the “bathtub”) in his book.

What are your favourite analogies – of your own creation or that you heard elsewhere? Share them in the comments below.