Long-term variability of shortwave absorption under abrupt-4xCO2 climate forcing and its visible and near-IR contributions

While ongoing global warming is largely the result of reduced outgoing longwave radiation (OLR), climate feedbacks associated with changes in atmospheric water vapor and surface albedo are expected to enhance the absorption of shortwave radiation (ASR) and to sustain global warming on centennial time scales beyond the OLR modulations. These feedbacks as well as positive cloud feedbacks reduce the reflected shortwave (SW) flux at the top-of-atmosphere (TOA) and are a result of scattering and absorbing processes that differ by their near-infrared (NIR) and visible (VIS) contributions. Since direct measurements of broadband NIR (~0.7-5 mm) and VIS (~0.2-0.7 mm) radiation flux do not exist, we utilize UKESM1 simulations to study SW, NIR, and VIS climate feedbacks under preindustrial and abrupt-4xCO₂ climate forcing.

Besides its global long-term behavior, the spatial variability and key physical controls of ASR are not well characterized either. A prominent example is the unexplained hemispheric symmetry in planetary albedo that is consistently missed by current global climate models yielding unrealistic precipitation and circulation patterns. Although energetically equivalent, the observed hemispheric albedos differ spectrally, reflecting the uneven distribution of clouds and land masses. We use the same UKESM1 simulations to contrast inter-hemispheric differences in SW, NIR and VIS, and their relation to changes in clouds, the gaseous atmosphere and surface properties to shed light on processes relevant to the present-day symmetry, model biases, and potential future changes.