Progress in Projecting Solar Radiation at the Earth's Surface in Climate Models

Abstract:

Projecting changes in solar radiation at the Earth's surface in future
climates is a critical input to forecast surface irradiance for solar
energy. We demonstrate the current state of the art using the
ensemble of opportunity assembled for the Coupled Model
Intercomparison Project (CMIP5) and the Fifth Assessment Report (AR5)
of the Intergovernmental Panel on Climate Change (IPCC).

The reliability of these projections depends upon the accuracy of the
underlying radiation codes, the fidelity of these codes to the
measured optical properties of key radiatively active atmospheric
constituents, and the realism of future projections of these
atmospheric constituents. These constituents include aerosols,
clouds, water vapor, greenhouse gases that absorb near-infrared
sunlight. Since the realism of future projections of anthropogenic
aerosol species is contingent on the underlying scenario, we focus on
the other challenges in forecasting surface irradiance.

Regarding accuracy, we demonstrate that current GCM shortwave
parameterizations often exhibit quite small errors relative to
benchmark radiative transfer codes. In addition, recent work has
bracketed the uncertainties in solar irradiance associated with
complex cloud geometries. There is also an emerging consensus how
cloud radiative effects will evolve in a warmer climate. However,
there is evidence that current GCM codes still exhibit systematic
errors in the near-infrared water vapor bands, particularly for moist
sub-tropical atmospheres. These errors will become more acute as
water vapor feedbacks, combined with global warming, increase the
total precipitable water in the Earth's atmosphere.