Greenhouse warming of Earth’s surface and lower atmosphere is driven by radiative forcing,
F , the difference between the flux of thermal radiant energy from a black surface through a
hypothetical, transparent atmosphere, and the flux through an atmosphere with greenhouse
gases, particulates and clouds, but with the same surface temperature[1]. Radiative forcing is
often specified in units of watts per square meter (W m−2). Forcing depends on the altitude,
z, and on how the temperature and greenhouse-gas concentrations vary with altitude.
This paper has been written for readers with a strong background in quantitative sciences,
who know little about radiation transfer in Earth’s atmosphere. So we include material that
is common knowledge to a small number of experts, but little known to the larger scientific
community.
The two goals of this review were: (1) to rigorously review the basic physics of thermal
radiation transfer in the cloud-free atmosphere of the Earth; and (2) to present quantitative
information about the relative forcing powers of the the naturally-occurring, greenhouse-gas
molecules, H2O, CO2, O3, N2O and CH4.
The most striking fact about radiation transfer in Earth’s atmosphere is summarized by
Figs. 10–12. Large relative changes of the concentrations of greenhouse gases from current
values cause relatively small changes in forcings. Doubling the current concentrations of
the greenhouse gases CO2, N2O and CH4 increases the forcings by only a few percent for
cloud-free parts of the atmosphere.
