3.4

# The leaky bucket

How are the flow of energy we get from the Sun and the flow of energy the Earth loses to space related to each other? A useful analogy is a leaky bucket.

In this animation you can see a bucket with a hole in it. At the start, the water flows in from the tap at a constant rate. The flow of water in to the bucket is the same as the flow of water out through the hole, and the level of water in the bucket remains the same. What do you think happens if you open the tap some more?

Initially, more water flows in to the bucket than flows out, and the level of water rises. However, remember that the water pressure is related to the depth of the water. As the water level rises, so does the water pressure and this forces more water out through the hole in the bottom. Eventually, the rate of flow out balances the rate of flow in, and the level of water in the bucket, although higher than before, remains the same.

The tap is like the Sun, with the water coming out of it being like the energy entering the top of the atmosphere from the Sun. The bucket is the Earth, with the flow of water out of the hole being equivalent to the energy the Earth loses to space. This is proportional to T4 (where T is the temperature of the Earth), so if the Earth receives more energy from the Sun (the tap is opened more), the Earth heats up (the water level rises) and loses more energy to space (more water is forced through the hole). Eventually, as you can see in the bucket animation, the amount of energy the Earth receives is balanced by the amount it loses, albeit with the Earth at a warmer temperature.

If you add some extra water to the bucket in one go, without changing the rate of flow from the tap, the level will slowly fall back to where it was originally. Similarly, if an event such as a forest fire, emits heat on the Earth’s surface, this heat will slowly be lost to space and the temperature of the Earth gradually falls to where it was originally. This is why it is the greenhouse gases emitted by burning fossil fuels, rather than the heat itself, which causes climate change.

But, it’s not so simple. This is mainly because the Earth has an atmosphere, which also has an impact on the flow of energy to and from our planet.

The temperature of the Earth’s surface, and the atmosphere above it, are governed by the intricate exchange of energy between the different components of the climate system: atmosphere, cryosphere (glaciers, ice sheets or anything else involving frozen water), oceans and land.

If this exchange of energy, called the Earth’s radiation or energy budget, is in balance, the temperature of the Earth’s surface stays the same. However, if anything happens to tip the balance, the temperature will change, in much the same way as the level of water in the bucket changes in response to changes in the flow rate of water in from the tap and out through the hole.

In the next Step, we’ll look in more detail at the Earth’s energy or radiation budget.