If earth were nothing but a big ball of rock, then computing the effect of changes in energy balance on our planet’s temperature would be easy. That’s because the only relevant factors would be the shortwave (SW) solar energy coming in, and the longwave (LW) radiation escaping to space, and we’ve got a good handle on that. The Stefan-Boltzmann radiation law enables us to compute that for near earth conditions, an additional 1 watt per square meter (W/m^2) of energy coming in to the planet would increase temperature by about 0.3 deg.C. This is the climate sensitivity to radiative forcing, which is the temperature change due to an additional 1 W/m^2 of climate forcing. It’s not the same as the climate sensitivity to doubling CO2; doing that would increase radiative forcing by around 4 W/m^2, so climate sensitivity to doubling CO2 would be around 4 times greater, about 1.2 deg.C. (Note: this is based on a global average temperature of about 14 deg.C, which is a real-world value, but if earth had no atmosphere it would be considerably cooler and climate sensitivity would be a bit less.)
But: earth has an atmosphere, and oceans, and ice caps, and glaciers, and plants and animals and people, oh my! As a result, the response of the climate system to additional climate forcing is quite a bit more complicated. Temperature change can alter other factors which themselves affect climate forcing, which in turn affects temperature, in a feedback loop. Because of these feedbacks, it becomes difficult to estimate with precision exactly how climate responds to climate forcing because it can be difficult to estimate the feedback factors with precision. Let’s consider some of the known factors, and estimates of their impact on climate sensitivity.
