[Update 10/4/21 9:40 am Pacific Time. I received an email from Dr. Steven Koonin, one of the authors. He wanted me to insert this statement-charles]
I am a co-author of this paper, which summarizes twenty years of earthshine observations to measure albedo changes. The paper speaks for itself. If you take the time to read it (it is open access), I think you’ll find it appropriately circumspect about what we measured, with what uncertainties, and the implications of what we found. However, I am in no way responsible for how the paper is cast by the general media, or even the AGU press release. Such is the state of today’s “climate communications”.
There’s a new study published in Geophysical Letters Called: Earth’s Albedo 1998–2017 as Measured From Earthshine
The climate change crisis has cropped up with a vengeance in the last few years, and its ramifications have been unimaginable, extreme and life-changing. Now, a new study claims that climate change has dimmed our planet—literally!
According to the study, the Earth’s brightness has decreased as a result of warming ocean waters. Now you’re probably wondering, “how do you even measure something like that?” But researchers have their ways. They use the Earth’s reflectance or ‘albedo’, which they calculate using something known as the ‘earthshine’, to keep track of, well, the Earth’s shine.
While gazing at the crescent moon immediately after sunset or before sunrise, you may notice that aside from the dazzling crescent, the rest of the moon appears as a dark but faintly glowy disc. The light that bounces off the Earth gives the unlit part of a crescent moon a pale glow, which is referred to as earthshine.
The study indicates that our home planet now reflects almost half a watt less light per square metre than it did 20 years ago, with the majority of the decrease occurring in the last three years of earthshine data. This equates to a 0.5% reduction in the Earth’s reflectance.
Researchers investigated the Earth’s ‘albedo’ by studying earthshine at the Big Bear Solar Observatory in California between 1998 and 2017—that’s over 1,500 nights of data. They could determine how much light is reflected by the planet thanks to this analysis.
The reflectance of the Earth is a fundamental climate parameter that we measured from Big Bear Solar Observatory between 1998 and 2017 by observing the earthshine using modern photometric techniques to precisely determine daily, monthly, seasonal, yearly and decadal changes in terrestrial albedo from earthshine. We find the inter-annual fluctuations in albedo to be global, while the large variations in albedo within individual nights and seasonal wanderings tend to average out over each year. We measure a gradual, but climatologically significant 0.5 decline in the global albedo over the two decades of data. We found no correlation between the changes in the terrestrial albedo and measures of solar activity. The inter-annual pattern of earthshine fluctuations are in good agreement with those measured by CERES (data began in 2001) even though the satellite observations are sensitive to retroflected light while earthshine is sensitive to wide-angle reflectivity. The CERES decline is about twice that of earthshine.
Plain Language Summary
The net sunlight reaching the Earth’s climate system depends on the solar irradiance and the Earth’s reflectance (albedo). We have observed earthshine from Big Bear Solar Observatory to measure the terrestrial albedo. For earthshine we measure the sunlight reflected from Earth to the dark part of the lunar face and back to the nighttime observer, yielding an instantaneous large-scale reflectance of the Earth. In these relative measurements, we also observe the sunlit, bright part of the lunar face.
We report here reflectance data (monthly, seasonal and annual) covering two decades, 1998–2017. The albedo shows a decline corresponding to a net climate forcing of about 0.5 . We find no correlation between measures of solar cycle variations and the albedo variations. The first precise satellite measures of terrestrial albedo came with CERES. CERES global albedo data (2001-) show a decrease in forcing that is about twice that of earthshine measurements. The evolutionary changes in albedo motivate continuing earthshine observations as a complement to absolute satellite measurements, especially since earthshine and CERES measurements are sensitive to distinctly different parts of the angular reflectivity. The recent drop in albedo is attributed to a warming of the eastern pacific, which is measured to reduce low-lying cloud cover and, thereby, the albedo.
H/T Gregory W