A new study shows that warming ocean waters have been responsible for a drop in the brightness of the Earth.
Earthshine annual mean albedo 1998–2017 expressed as watts per square meter (W/m2). The CERES annual albedo 2001–2019 is shown in blue. A best-fit line to the CERES data (2001–2019) is shown with a blue dashed line. Average error bars for CERES measurements are of the order of 0.2 W/m2. Image Credit: Goode et al. (2021), Geophysical Research Letters.
Researchers have utilized several years of measurements of earthshine — the light reflected from Earth that lights up the Moon’s surface — as well as satellite measurements to determine that there has been a major drop in Earth’s reflectance, or albedo, over the last 20 years.
According to the study, the Earth is currently reflecting around half a watt less light per square meter compared to what it was two decades ago, with a majority of the drop occurring in the last three years of earthshine data. That is equal to a 0.5% decrease in the Earth’s reflectance. Earth reflects nearly 30% of the sunlight that shines on it.
The study has been reported in the AGU journal Geophysical Research Letters, which publishes high-impact, short-format reports with instant implications covering all Earth and space sciences.
The albedo drop was such a surprise to us when we analyzed the last three years of data after 17 years of nearly flat albedo.
Philip Goode, Study Lead Author and Researcher, New Jersey Institute of Technology
The study refers to the earthshine data collected by the Big Bear Solar Observatory in Southern California from 1998 to 2017. On adding the latest data to the previous years, the darkening trend became apparent.
Two things are responsible for impacting the net sunlight reaching the Earth: One is the brightness of the Sun and the other one is the reflectivity of the planet. The variations in Earth’s albedo noted by the scientists did not connect with periodic changes in the brightness of the Sun. This implies that changes in reflectiveness of the Earth are caused by something on the Earth.
As per the satellite measurements made as part of NASA’s Clouds and the Earth’s Radiant Energy System (CERES) project, there has been a decrease of bright and reflective low-lying clouds over the eastern Pacific Ocean in recent years.
That is the same region, which is off the west coasts of North and South America, where increases in sea surface temperatures have been recorded due to the reversal of a climatic condition known as the Pacific Decadal Oscillation, with liable connections to universal climate change.
Furthermore, the Earth’s dimming can be seen relating to how much more solar energy is being captured by the climate system of the Earth. As soon as this significant additional solar energy is in Earth’s oceans and atmosphere, it may add up to global warming. This happens because the extra sunlight is of the same magnitude as the complete anthropogenic climate forcing over the past 20 years.
It’s actually quite concerning. But this shows the opposite is true.
Edward Schwieterman, Planetary Scientist, University of California at Riverside
Schwieterman was not a part of the new study. For some time, many researchers thought that a warmer Earth may result in more clouds and higher albedo, which would further help to moderate warming and balance the climate system.
Journal Reference:
Goode, P. R., et al. (2021) Earth’s Albedo 1998–2017 as Measured From Earthshine. Geophysical Research Letters. doi.org/10.1029/2021GL094888.