Fundamentals
Radiative forcing
The change in Earth's energy balance caused by a factor such as a greenhouse gas, measured in watts per square metre.
Radiative forcing quantifies how much a perturbation shifts the balance between incoming and outgoing energy. Positive forcing warms; negative forcing cools.
It allows different gases and mechanisms to be compared on one scale, which is why methane and nitrous oxide are expressed as carbon dioxide equivalents over a chosen horizon rather than treated separately.
Frequently asked questions
Why do equivalence factors differ by horizon?
Because gases have different atmospheric lifetimes, so their relative impact changes with the period considered.
Does albedo count?
Yes. Surface reflectivity changes from land use affect forcing and are relevant to some project claims.