Sir

Stewart Brand's Essay 'Whole Earth comes into focus' (Nature 450, 797; 2007) makes a strong case for continuous satellite observations of the “whole Earth”. A key reason is that existing observations are inadequate to monitor changes in global albedo — the amount of sunlight reflected by Earth and a key determinant of Earth's climate.

A comparison of existing albedo measurement programmes, which are based on polar-orbiting and geostationary satellites, shows large discrepancies in trends taken over several years, as well as poor correlations in monthly anomalies (N. G. Loeb et al. J. Clim. 20, 575–591; 2007). In addition, there is a significant difference between the historical reflected flux data of the Earth Radiation Budget Experiment and the Clouds and the Earth's Radiant Energy System (CERES), and a large imbalance in the amount of incoming and outgoing radiation derived from the CERES measurements (F. A. Bender et al. Tellus 58A, 320; 2006). The causes of these discrepancies are unknown and call for independent high-quality data.

DSCOVR, the radiometric satellite that Brand mentions — which is “mothballed” but ready to launch — would provide the data needed. From its position 1.5 million kilometres away at the Lagrange-1 point, it would orbit the Sun in synchrony with Earth and provide a continuous, well-calibrated proxy for global albedo by observation of the sunlit side of Earth. Understanding this albedo proxy could be helped by simultaneous diagnostic observations from the CALIPSO satellite (which measures the reflected laser light) and from the suite of instruments comprising the A-train satellite constellation, which includes CERES. It is therefore a matter of urgency to launch DSCOVR soon, in order to achieve synergy with existing satellites and to provide a bridging link with future systems.