New data on the variability of the Earth’s reflectance – its albedo – over the last 16 years has been published in Geophysical Research Letters. The measurements of this key climate variable, obtained from telescopes on the ground, agree with the measurements made from satellites and show a period of stability.
From the Instituto de Astrofísica de Canarias
The Earth’s albedo, or reflectance, is a fundamental atmospheric parameter having deep implications for temperature and climate change. For that reason, experiments have been performed to monitor it over the past two decades to reveal how it evolves. One of these, in which the Instituto de Astrofísica de Canarias is a participant, has brought up to date the observations made since 2007 and adds to, and gives a new systematic record of the Earth’s albedo covering the period between 1998 and 2014 from California. This study, whose first author is IAC researcher Enric Pallé, has been published in Geophysical Research Letters, and shows that although the albedo fluctuates both monthly and annually, but there is no trend over those 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 the 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 the 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.
BBSO is operated by New Jersey Institute of Technology.
The Earth’s albedo is a fundamental climate parameter for understanding the radiation budget of the atmosphere. It has been traditionally measured not only from space platforms but also from the ground for 16 years from Big Bear Solar Observatory by observing the Moon. The photometric ratio of the dark (earthshine) to the bright (moonshine) sides of the Moon is used to determine nightly anomalies in the terrestrial albedo, with the aim of quantifying sustained monthly, annual, and/or decadal changes. We find two modest decadal scale cycles in the albedo, but with no significant net change over the 16 years of accumulated data. Within the evolution of the two cycles, we find periods of sustained annual increases, followed by comparable sustained decreases in albedo. The evolution of the earthshine albedo is in remarkable agreement with that from the Clouds and the Earth’s Radiant Energy System instruments, although each method measures different slices of the Earth’s Bond albedo.
Palle, E., P. R. Goode, P. Montañés-Rodríguez, A. Shumko, B. Gonzalez-Merino, C. M. Lombilla, F. Jimenez-Ibarra, S. Shumko, E. Sanroma, A. Hulist, et al. (2016); Earth’s albedo variations 1998–2014 as measured from ground-based earthshine observations, Geophysical Research Letters, doi:10.1002/2016GL068025.
Instituto de Astrofísica de Canarias news release.