Last time, we talked about the energy budget – the process of radiation coming in from the sun, being absorbed by the Earth, and then emitted as infrared radiation, which we perceive as heat when it hits us. Remember that this outgoing emission of energy is what determines the temperature of the Earth.
So how can the temperature of the Earth be changed? Naturally, there is a lot of year-to-year variation. For example, when the oceans absorb radiation from the sun, they don’t always emit it right away. They will store energy for a long time, and sometimes release lots at once, during El Nino. This kind of internal variability makes the average global temperature very zig-zaggy.
We need to revise the question, then. The question is not about the average global surface temperature – it’s about the amount of energy on the planet. That’s generally how the climate is changed, by increasing or decreasing the amount of energy the Earth emits as infrared radiation, and consequently, the temperature.
There are two ways to do this. The simplest method is to change the amount of incoming energy. By increasing or decreasing the amount of solar radiation that hits the Earth – either directly, by changing the sun’s output, or indirectly, by increasing the albedo or reflectivity of the Earth – the amount of infrared radiation emitted by the surface will also increase or decrease, because incoming has to be equal to outgoing. The change in outgoing radiation will often take a bit of time to catch up to the change in incoming radiation. Until the two reach a new equilibrium, the Earth will warm or cool.
Another way to change the Earth’s temperature is by artificially changing the amount of incoming energy. The same amount of solar radiation reaches the Earth, but when it is absorbed and emitted, some of the emitted infrared energy gets bounced back so the Earth has to absorb and emit it again. By processing the same energy multiple times, the temperature is a lot warmer that it would be without any bouncing. We refer to this bouncing as the “greenhouse effect”, even though greenhouses work in a completely different way, and we will be discussing it a lot more later. By increasing or decreasing the greenhouse effect, the temperature of the Earth will change too.
A change in incoming energy is referred to as a radiative forcing, because it “forces” the Earth’s temperature in a certain way, by a certain amount. It is measured in watts per square meter (W/m2), and it doesn’t take very many watts per square meter to make a big difference in the Earth’s temperature. The resulting change in temperature is called a response.
My favourite analogy to explain forcing and response uses one of the most basic physics equations – F=ma. Mass (m) is constant, so force (F) is proportional to acceleration (a). Applying a forcing to the Earth is just like pushing on a box. If the force is big enough to overcome friction, you get an acceleration – a response.
It’s also very important to use net force, not just any force. If there are two people pushing on the box in different directions with different amounts of force, the acceleration you observe will be equal to the result of those forces combined. Similarly, there are often multiple forcings acting on the climate at once. The sun might be getting slightly dimmer, the albedo might be decreasing, the greenhouse effect might be on the rise. The response of the climate will not match up to any one of those, but the sum of them all together.
Here is a video I made last year, in collaboration with Climate Change Connection, about this very analogy:
In future posts, I will be discussing different forcings in more detail. Stay tuned!