Top-of-Atmosphere Radiation over the Last Millennium Reconstructed from Proxies

Abstract Earth’s energy imbalance at the top of the atmosphere is a key climate system metric, but its natural variability is poorly constrained by the short observational record and large uncertainty in coupled climate models. While existing ocean heat content reconstructions offer a longer perspective, they cannot separate the contributions of shortwave and longwave radiation, obscuring the underlying processes. We extend the energy-budget record into the preindustrial period by reconstructing the top-of-atmosphere radiation and related surface variables over the last millennium (850–2000 CE) using data assimilation, combining proxy data and dynamics from a coupled climate emulator. Validation reveals skill in the reconstructed radiation fields, especially in the global mean and the tropics. We find that the well-documented last-millennium cooling trend coincides with persistent energy loss, largest early in the millennium, and a reduction in upper-ocean heat content. The cooling trend differs by season and latitude, and is associated with anomalies in outgoing longwave radiation suggestive of an eastward shift in Indo-Pacific convection. Following large volcanic eruptions, ocean heat content anomalies persist for 10–20 years on average, supporting previous evidence that multidecadal cooling is forced by decadally paced eruptions. The reconstruction also reveals that the current rate of energy gain is unprecedented relative to the period before 1850. Significance Statement We use tree rings, corals, and ice cores to create the first reconstruction of energy entering and leaving the Earth system over the last 1000 years. Such a reconstruction of natural variability in Earth’s energy budget provides context for recent climate change. We find that the rate of energy gain associated with current global warming is unprecedented in the last millennium. Further, we find persistent heat loss associated with a global preindustrial cooling trend, and evidence that multidecadal cooling was forced by clusters of volcanic eruptions.