Earth’s Energy Imbalance Measured From Space

Abstract

The direct measurement of earth’s energy imbalance (EEI) is one of the greatest challenges in climate research. The global mean EEI represents the integrated value of global warming and is tightly linked to changes in hydrological cycle and the habitability of our planet. Current space-born radiometers measure the individual radiative components of the energy balance with unprecedented stability, but with calibration errors too large to determine the absolute magnitude of global mean EEI as the components’ residual. Best estimates of the long-term EEI are currently derived from temporal changes in ocean heat content at ~0.7 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> . To monitor EEI directly from space, we propose an independent approach based on accelerometry that measures nongravitational forces, such as radiation pressure, acting on earth-orbiting spacecrafts. The concept of deriving EEI from radiation pressure has been considered in the past, and we provide analysis that shows today’s capabilities are sufficiently accurate to answer the question: At what rate is our planet warming? To measure global mean EEI to within at least ±0.3 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> requires spacecraft(s) of near-spherical shape and well-characterized surface properties to reduce confounding effects. The proposed concept may provide the basis for a data record of global and zonal mean EEI on annual and potentially monthly timescales. It is not meant to replace the existing concepts designed to measure energy balance components or ocean heat storage, but to complement these by providing an independent estimate of EEI for comparison and to anchor data products and climate models that lack energy balance closure.