The Earth’s radiation budget can be analysed both at the top of the atmosphere and at the surface (Figure 2.7).
At the top of the atmosphere, the long-term equilibrium condition is that absorbed solar radiation must balance outgoing radiation to space. However, under present-day conditions, this balance is slightly positive: the Earth system currently absorbs a little more energy than it emits. This small residual imbalance is the signature of ongoing warming.
At the surface, the energy budget is more complex because it includes not only solar radiation, but also downward infrared radiation from the atmosphere, as well as non-radiative heat fluxes such as latent and sensible heat transfer. Nevertheless, the same physical conclusion applies: the current imbalance implies that the climate system is accumulating energy.
Quantitatively, this means that the planet is still warming and is moving toward a new equilibrium state.
Figure 1: Simplified view of the Earth’s radiation balance at the top of the atmosphere and at the surface. Greenhouse gases reduce the efficiency of outgoing infrared radiation, creating a small positive radiative imbalance that leads to energy accumulation in the climate system and, consequently, to global warming.
Conclusion on the role of greenhouse gases
The Sun–Earth system tends toward radiative equilibrium. The Earth continuously dissipates the energy it receives from the Sun by emitting infrared radiation to space. Greenhouse gases modify this process by absorbing and re-emitting part of this outgoing infrared radiation, thereby reducing the efficiency of radiative cooling.
As greenhouse gas concentrations increase, the radiative balance of the Earth system is disturbed, and the climate responds through an increase in surface and tropospheric temperatures. In this sense, global warming can be understood as the adjustment required to restore radiative equilibrium in the presence of enhanced greenhouse gas concentrations.
The attribution of recent global warming to anthropogenic greenhouse gas emissions has been progressively strengthened over successive IPCC assessment reports. Today, there is an overwhelming scientific consensus that the observed warming of the climate system is primarily caused by human activities, especially the increase in greenhouse gas concentrations.