May 11, 2012
Since 1978 a succession of active cavity radiometers have been monitoring the Total Solar Irradiance (TSI, formerly “solar constant”) from space-borne platforms. Recently the calibration gap between these instruments has been largely closed due to comparisons made with a cryogenic Transfer Radiometer Facility located at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, built by LASP with support from NASA and NIST. The resulting continuous record of TSI promises to be a milestone in improving our understanding of the Sun’s impact on Earth’s climate. Since launch of the Solar Radiation and Climate Experiment in 2003, a new challenge has been raised by unique new measurements of the Solar Spectral Irradiance (SSI) across visible and near infrared wavelengths that impact the lower troposphere and upper ocean. Climate models are sensitive to such variations in SSI, and not to TSI alone, especially the vertical profile of temperature and ozone responses to solar variations. Initial findings from the SORCE Spectral Irradiance Monitor (SIM) indicate that multiyear changes at visible and near-infrared wavelengths may be out of phase with those of TSI, while near-ultraviolet changes in the UV-A are larger than expected. This result needs further study before it may be considered firmly established. To determine the climate sensitivity to such hypothesized changes, we compare climate responses to two classes of SSI variations, both having the same variations in TSI. We find that out-of-phase forcing leads to much larger temperature variations in upper stratosphere, but smaller variations in troposphere and upper ocean. Such differences underscore the importance of efforts now underway to calibrate the new SIM instrument scheduled to fly on a mission called TSIS in 2015 or 2016, as part of the Joint Polar Satellite System (JPSS) of NOAA and NASA. The TSIS SIM is now undergoing comprehensive end-to-end calibrations in the LASP SSI Radiometry Facility (SRF) utilizing the NIST SIRCUS laser system covering 210 – 2400 nm for SSI. With SORCE follow-on missions such as the Total and Spectral Solar Irradiance Sensor (TSIS), we anticipate narrowing uncertainties in SSI variability that will be important to improving our understanding of the climate responses to solar forcing.
Dr. Robert Cahalan is Chief of NASA-Goddard’s Climate and Radiation Laboratory of 60 world-class climate scientists, who advance the understanding of aerosols, modeling, instrumentation, and solar radiation. He serves as Project Scientist for NASA’s Solar Radiation and Climate Experiment (SORCE) and also the joint NASA-NOAA Total and Spectral Solar Irradiance Sensor (TSIS). He is Adjunct Professor at University of Maryland, and advises graduate students in the USA, Canada, and the Netherlands. Dr. Cahalan is President of the International Radiation Commission, 40 commissioners representing 20 countries. Dr. Cahalan has been recognized with prestigious national and international awards, including Outstanding Leadership and Service of the United States Climate Change Science Program, and the NASA Exceptional Service Medal. He is a Fellow of the American Meteorological Society.