According to new work published in the journal Geophysical Research Letters , the warming of the oceans causes fewer bright clouds to reflect sunlight, and as a consequence the Earth’s climate system traps even less energy.
The researchers used data from several decades of measurements of light reflected from the Earth illuminating the surface of the Moon, as well as measurements from satellites, and found that in the last twenty years there has been a significant drop in the albedo or reflectance of our planet. Specifically, the Earth now reflects about half a watt less light per square meter, with most of the fall occurring in the past three years. That’s the equivalent of a 0.5% decrease in reflectance from Earth. The Earth reflects approximately 30% of the sunlight that illuminates it.
“After 17 years of near-flat albedo, we were surprised to see this drop in data from the last 3 years,” explains Philip Goode, a researcher at the New Jersey Institute of Technology and lead author of the new study, referring to data from the Earth’s light from 1998 to 2017 collected by the Big Bear Solar Observatory in Southern California. When the latest data was added to the previous years, the dimming trend became apparent.
Causes of darkening
Both the brightness of the Sun and the reflectivity of the planet itself affect the net sunlight reaching Earth. Since the changes in the Earth’s albedo observed by the researchers did not correlate with periodic changes in the Sun’s brightness, it follows that they are caused by something on our own planet.
Specifically, there has been a reduction in bright, reflective low clouds over the eastern Pacific Ocean in recent years, based on satellite measurements made as part of the Clouds and the Earth’s Radiant Energy System (CERES) project. That is the same area, off the western coasts of North and South America, where increases in sea surface temperatures have been recorded due to the reversal of a climatic condition called the Pacific Decadal Oscillation, with probable connections to global climate change.
On the other hand, the darkening of the Earth can also be analyzed thinking about the amount of energy that is being captured by the climate system. Once this important additional solar energy is found in Earth’s atmosphere and oceans, it can contribute to global warming, as the additional sunlight is of the same magnitude as the total anthropogenic climate forced over the past two decades.
“These findings are quite concerning,” explains Edward Schwieterman, a planetary scientist at the University of California at Riverside who was not involved in the new study. “Many scientists hoped that a warmer Earth could generate more clouds and a higher albedo, which would then help moderate warming and balance the climate system. But in light of this new study the effect would be just the opposite. “
