Application of solar max ACRIM data to analysis of solar-driven climatic variability on Earth

Abstract

Climatic change caused by solar variability has been proposed for at least a century, but could not be assessed reliably in the past because the uncertainty in solar irradiance measured from the Earth's surface is too large. Now satellite measurements by such instruments as the Active Cavity Radiometer Irradiance Monitor (ACRIM) permit a preliminary assessment. The satellite data exhibit irradiance variations over a spectrum of shorter timescales, but the first 5-yr overall trend indicates slightly decreasing luminosity. The global temperature response to monthly-mean ACRIM-measured fluctuations from 1980–1984 was computed from the NYU 1D transient climate model - which includes thermal inertia effects of the world oceans - starting from an assumed pre-existing steady state, and the results compared with observations of recent global temperature trends. The modeled surface temperature evolution exhibited a complex history-dependent behavior whose fluctuations were an order of magnitude smaller than observed, primarily owing to oceanic thermal damping. Thus solar variability appears unlikely to have been an important factor in global-scale climate change over this period. The possibility of using the measurements to develop simple correlations for irradiance with longer term solar activity observable from the surface, and therefore to analyze historical effects, was considered, but is not supported by the satellite data. However, we have used a model of solar irradiance variation with time (Schatten, 1988), covering the period 1976–1997 in order to assess our model's response to forcing whose fluctuation timescale is comparable to the thermal relaxation time of the upper ocean. Continuous monitoring of solar flux by space-based instruments over timescales of 20 yr or more, comparable to timescales for thermal relaxation of the oceans, and of the solar cycle itself, is probably needed to resolve issues of long-term solar variation effects on climate.