The global patterns of instantaneous CO2 forcing at the
top-of-atmosphere and surface
Abstract
The radiative forcing of carbon dioxide (CO2) at the top-of-atmosphere
(TOA) has a rich spatial structure and has implications for large-scale
climate changes, such as poleward energy transport and tropical
circulation change. Beyond the TOA, additional CO2 increases downwelling
longwave at the surface, and this change in flux is the surface CO2
forcing. Here, we thoroughly evaluate the spatiotemporal variation of
the instantaneous, longwave CO2 radiative forcing at both the TOA and
surface. The instantaneous forcing is calculated with a radiative
transfer model using ERA5 reanalysis fields. Multivariate regression
models show that the broadband forcing at the TOA and surface are
well-predicted by local temperatures, humidity, and cloud radiative
effects. The difference between the TOA and surface forcing, the
atmospheric forcing, can be either positive or negative and is mostly
controlled by the column water vapor, with little explicit dependence on
the surface temperature. The role of local variables on the TOA forcing
is also assessed by partitioning the change in radiative flux to the
component emitted by the surface vs. that emitted by the atmosphere. In
cold, dry regions, the surface and atmospheric contribution partially
cancel out, leading to locally weak or even negative TOA forcing. In
contrast, in the warm, moist regions, the surface and atmospheric
components strengthen each other, resulting in overall larger TOA
forcing. The relative contribution of surface and atmosphere to the TOA
forcing depends on the optical thickness in the current climate, which,
in turn, is controlled by the column water vapor.