SORCE Contributions to New Understanding of Global Change and Solar Variability

An array of empirical evidence in the space era, and in the past, suggests that climate responds to solar activity. The response mechanisms are thought to be some combination of direct surface heating, indirect processes involving UV radiation and the stratosphere, and modulation of internal climate system oscillations. A quantitative physical description is, as yet, lacking to explain the empirical evidence in terms of the known magnitude of solar radiative output changes and of climate sensitivity to these changes. Reproducing solar-induced decadal climate change requires faster and larger responses than general circulation models allow. Nor is the indirect climatic impact of solar-induced stratospheric change adequately understood, in part because of uncertainties in the vertical coupling of the stratosphere and troposphere. Accounting for solar effects on pre-industrial surface temperatures requires larger irradiance variations than present in the contemporary database, but evidence for significant secular irradiance change is ambiguous. Essential for future progress are reliable, extended observations of the solar radiative output changes that produce climate forcing. Twenty-five years after the beginning of continuous monitoring of the Sun’s total radiative output, the Solar Radiation and Climate Experiment (SORCE) commences a new generation of solar irradiance measurements with much expanded capabilities. Relative to historical solar observations SORCE monitors both total and spectral irradiance with significantly reduced uncertainty and increased repeatability, especially on long time scales. Spectral coverage expands beyond UV wavelengths to encompass the visible and near-IR regions that dominate the Sun’s radiative output. The space-based irradiance record, augmented now with the spectrum of the changes, facilitates improved characterization of magnetic sources of irradiance variability, and the detection of additional mechanisms. This understanding provides a scientific basis for estimating past and future irradiance variations, needed for detecting and predicting climate change.