New data on variability of Earth's reflectance over the last 16 years

The effect of the albedo is important for life on Earth because it affects the energy budget of the planet: it plays a key role in the process by which the planet achieves an equilibrium between the solar radiation which enters the atmosphere and the radiation emitted as heat into space. The albedo is defined as the fraction of sunlight that Earth reflects back directly into space, and it varies according to cloud composition, ice, snow, and the nature of the surface onto which the sun's rays fall. The term comes from the Latin "albus" (white), and a rise or fall in the albedo will have consequences for the planet's global warming or cooling.

To check how the albedo has evolved, since the 1990's it has been measured from space with instruments such as ERBE and CERES, whose estimations are made using absolute photometric measurements, which could be affected by any degradation in the instrument while in space.

However, a complementary way to measure the reflectance, which does not suffer the same calibration errors since it is a relative measurement, is from the ground, using telescopes that observe the so-called earthshine (the light reflected from Earth to the night-time face of the Moon). This method has been used during the period 1998-2014 from the Big Bear Solar Observatory (BBSO) in California, and also, since 2007 from the Earthshine telescope at the Teide Observatory (Tenerife), to measure this climate parameter. These observations were aimed at increasing the temporal coverage of the measurements, and reducing the measurement errors.

The result of this study is that, applying strict quality criteria to the measurements of the earthshine, and after re-calibrating the measurements taken from the CERES instrument in space, the variations found in the value of the albedo not only agree in magnitude but also show identical, small annual variations over the 14 years that the two types of observations overlap. Philip Goode, lead earthshine researcher at BBSO explains that "Although the measurements that we have made of the albedo over the past 16 years show monthly and annual variations, there is virtually no change in the long term. This also coincides with a stabilization in the mean temperature of the planet," he says.

Even so, although a consensus has been reached about the results of the two types of measurements, the origin of the anomalies is not completely clear. "We need to continue the experiments to measure this phenomenon accurately and see where we get to in a few more years. For example, the construction of a global network of robotic telescopes around the world or the launching of a micro-satellite dedicated to the study of earthshine will give us data to improve our knowledge of changes in the albedo, and see how they affect the climate," concludes Pallé.