Radiative forcing is the catch-all stat for factors, other than CO2 emissions, which have an effect on the global temperature. In the real world it can be loosely defined as the change in net irradiance at atmospheric boundaries between different layers of the atmosphere, namely the troposphere and the stratosphere.

Radiative forcing is caused by arctic methane releases, clathrate eruptions, and aerosols.

Arctic Methane effects

Temperature Radiative Forcing
1 < x <= 1.5 0.05
1.5 < x <= 2 0.1
2 < x <= 2.5 0.2
2.5 < x <= 3 0.3
3 < x <= 3.5 0.4
3.5 < x <= 4 0.5
4 < x <= 4.5 0.75
4.5 < x <= 5 0.9
5 < x <= 5.5 1.0
5.5 < x <=6 1.1
6 < x <= 7 1.2
7 < x <= 8 1.3
8 < x <= 9 1.4
9 < x <= 10 1.6
x > 10 1.6

Clathrate eruptions cause 0.5 radiative forcing, whilst aerosols give -0.25 each. The game's data files contain events for a space mirror program, but these are not accessible and have no effect.

Radiative forcing is reset to zero every turn, but the forcing from temperature is then re-applied. Clathrates and aerosols (unless re-applied every turn) therefore have only a short term effect on radiative forcing, whilst temperature changes have a permanent effect.

If I use aerosols to lower my temperature, will the radiative forcing effect from methane releases go back down too?

No. Once you've hit a certain temperature, those effects will be applied every turn for the rest of the game. This won't (as far as testing has determined) cause continual temperature rises, but will result in a higher equilibrium temperature.

If I use aerosols to stop my temperature going over x degrees, will it stop the radiative forcing increasing, even if I would have gone over that temperature if I didn't use them?

Yes. If you don't trigger the event, you won't get the effect.

I'm playing aerosols every turn, but they don't seem to do much after the first couple turns. What gives?

The exact mechanics for radiative forcing are unknown, but testing has shown that approximately 40% of the temperature change is based on the radiative forcing from the previous turn, and another ~40% on the 6-8 turns previous to that. In other words, if you were to play 4 aerosols continuously, you would see an early change of 4x-0.25x0.4 = -0.4 degrees on turn 2, and another -0.4 degrees over the next 6-8 turns. The temperature would then leap up again if you stopped using them.