Energy flows in Earth's climate system
Authors
Stephen E. Schwartz — Brookhaven National Laboratory
Bjorn Stevens — Max Planck Institute for Meteorology
Wei Wu — NOAA National Ocean Service
Category
Radiation
Description
Recent research in observing and modeling energy flows in Earth’s climate system is reviewed with emphasis on Earth’s energy balance and its susceptibility to perturbations, particularly the roles of clouds and aerosols. More accurate measurements of the total solar irradiance and the rate of change of ocean enthalpy help constrain the energy budget at the top of the atmosphere (TOA) to less than 4 W m-2. Earth reflects substantially less solar radiation and emits more terrestrial radiation than was believed even a decade ago. High precision measurements of the energy budget at the TOA provide new opportunities to track Earth’s energy flows on timescales of days to years. The principal limitation in the estimate of secular trends now lies in the natural variability of the Earth system itself. The average planetary energy imbalance, central to interpretation of climate change over the industrial period, is estimated as 0.9 ± 0.3 W m-2 (one-sigma).
Constraining the energy budget at the surface is much more difficult than at the TOA. Although satellite instruments afford the opportunity for reproducible measurements with global coverage and high spatial and temporal resolution, important quantities such as cloud fraction depend strongly on the measurement approach. Collocated measurements of cloud fraction by multiple approaches yield results that differ can differ profoundly at a single time, in monthly averages, and in the seasonal pattern; monthly anomalies in time series of cloudiness also show less sensitivity to measurement technique. Differences among measurement techniques make unambiguous determination of long-term trends difficult and potentially sensitive to observational inhomogeneities.
Rapid responses (adjustments) of elements of the climate system are central to the definition of the forcing that results from a change in atmospheric composition. Uncertainty in this adjustment, in addition to uncertainty in the secular compositional perturbation, limits accurate determination of radiative forcing. Changes in clouds contribute importantly to this adjustment and thus contribute to uncertainty in estimates of both radiative forcing and climate system response. Advances in tracking Earth’s energy flows and compositional changes on daily through decadal timescales are key to advancing model development and evaluation.