GEWEX Radiative Flux Assessment (RFA) Volume 1: Assessment A Project of the World Climate Research Programme Global Energy and Water Cycle Experiment (GEWEX) Radiation Panel

Chapter 2 provides an overview of solar irradiances, the dominant energy input driving Earth’s climate. The uninterrupted 33-year long total solar irradiance (TSI) record from space-borne instruments requires sufficient stability for climate-quality studies, helping determine the global and regional sensitivities of Earth’s climate to solar variability. New instrument calibration capabilities have helped explain the offsets between on-orbit TSI measurements and are improving instrument absolute accuracies, favoring a TSI value of 1360.8 W m at solar minimum. Space-borne spectral solar irradiances currently demonstrate stability only over shorter (solar rotation to year-long) time scales, providing insights into the spectral response of the Earth’s atmosphere to solar fluctuations. Empirical proxy models help extend the solar irradiance measurement records back in time for historical climate comparisons, while physicsbased solar models help explain the causes of irradiance fluctuations. Discrepancies between solar insolation computations, which are based on TSI measurements and provide direct inputs for Earth climate studies, are large for different regions of the Earth and are often comparable to the solar variability itself, indicating computational consistency improvements are critically needed. 2.1: Solar Irradiance Measurements The main driver of Earth’s climate, providing 2500 times the amount of energy of all other input sources combined (update of table in Sellers, 1965), is the Sun. Fortunately the Sun’s energy input to the Earth is fairly stable, but even small fluctuations in this energy can affect global and regional climate (Lean and Rind 2008, Gray et al. 2010). Solar variability and other natural influences have historically been the Earth’s primary climate drivers. Even today, when these natural influences are overshadowed by human-caused climate forcings, in order to understand climate and set appropriate regulatory policies, determining the contributions to climate change attributable to solar influences remains a key international priority and requires accurate records of solar radiation incident on the Earth (IPCC 2007). 2.1.1: Total Solar Irradiances Both the total solar irradiance (TSI) and its spectral distribution are measured from space, as small solar fluctuations cannot be accurately measured through the Earth’s highly variable atmosphere over climate-relevant time scales. The TSI is the spatiallyand spectrally-integrated radiant solar energy at 1 Astronomical Unit (1-AU) from the Sun, providing a measure of the incident energy at the top of the Earth’s atmosphere (TOA, usually defined as a spherical shell around the Earth about 50 km above the surface) and thus of the dominant driver of the Earth’s climate system. The TSI determines the incoming portion of the Earth’s radiative energy balance (described by Trenberth and Fasullo, 2010). The 34-year space-borne record of TSI measurements is shown in Error! Reference source not found. and covers nearly three solar cycles. Overlap between successive TSI instruments enables corrections for measurement offsets, which are due to instrument calibration