Understanding and Measuring Earth's Energy Budget: From Fourier, Humboldt, and Tyndall to CERES and Beyond

This historical perspective on the determination of Earth's energy fluxes, beginning with the classical description of climate, outlines the establishment of the basic physics of the Earth climate system in the nineteenth century. After recalling the early twentieth century ground-based attempts to determine the Earth's energy budget, I review the growing contributions of observations from space to quantifying these exchanges. In particular, space observations have shown that variations of solar luminosity have been extremely small (of order 0.1%) over past decades and probably past centuries and that they play practically no role in present-day climate variations or variations that may be expected in coming decades. Overall geographical structure, diurnal and seasonal cycles, and some of the interannual and interdecadal variations of Earth's energy exchanges with the Sun and space are now quite well determined, but much remains to be done regarding, on the one hand, fluxes at the surface and, on the other hand, variations of clouds. Improvements are essential if scientific assessment of anthropogenic climate change risk is to keep up with the changes themselves.