Reconstructing Earth Energy Imbalance over the Common Era

Many aspects of climate change are related to the net energy imbalance at top of Earth's atmosphere. This measures the difference between incoming solar radiation and outgoing radiation from emission and reflected solar radiation; the net imbalance must be reflected in changes to global energy storage. While the net imbalance has been measured by satellites in recent decades and constrained by ocean heat-content estimates from Argo floats, long-term changes are poorly constrained, even in the 20th century. Here we test the hypothesis that the planetary energy imbalance can be estimated from observations, and proxies, of surface temperature using data assimilation over the Common Era.

We present and evaluate a method for reconstructing outgoing radiation at the top of the atmosphere from assimilation of surface temperature observations. The method is first tested during the satellite era by assimilating HadCRUT surface temperature observations using prior estimates drawn from historical climate-model simulations outside the validation period. Results show higher skill in the reconstructions compared to AMIP simulations for interannual variability in both the outgoing infrared, and reflected solar radiation fields, when compared to CERES measurements. Over longer periods of time, the effective radiative forcing (ERF) from changing greenhouse gases and aerosols must be considered since ERF is, by definition, the radiative response independent of surface temperature changes. Adding estimates of ERF from climate simulations to the reconstructed radiation fields shows good agreement with AMIP simulations over the historical period. We extend the method to the Common Era using PAGES2K temperature-sensitive proxies and the Last Millennium Reanalysis assimilation framework.