Warming ocean temperatures have reduced the brightness of the Earth, a new study has found.
Researchers used decades of observations of earthshine-the light reflected from Earth that lights the surface of the Moon-along with satellite data to discover that the Earth's reflectance, or albedo, has decreased significantly over the last two decades.
In a new study published in the AGU journal Geophysical Research Letters, the Earth currently reflects around half a watt less light per square meter than it did 20 years ago, with the majority of the reduction coming in the last three years of earthshine data.
This equates to a 0.5 percent reduction in the Earth's reflectance. The Earth reflects roughly 30% of the sunlight that falls on it.
The brightness of the Sun and the reflectivity of the planet have an impact on the net sunlight reaching the Earth. The researchers discovered that fluctuations in Earth's albedo did not correspond to periodic changes in the Sun's brightness, implying that changes in Earth's reflectiveness are produced by something on the planet.
According to satellite observations taken as part of NASA's Clouds and the Earth's Radiant Energy System (CERES) program, there has been a decrease in bright, reflective low-lying clouds over the eastern Pacific Ocean in recent years.
The dimming of the Earth can also be seen in terms of how much more solar energy is being captured by Earth's climate system. Once this significant additional solar energy is in Earth's atmosphere and oceans, it may contribute to global warming, as the extra sunlight is of the same magnitude as the total anthropogenic climate forcing over the last two decades.
"It's actually quite concerning," Edward Schwieterman, a planetary scientist at the University of California at Riverside who was not involved in the new study, said.
Many scientists had expected for a long time that a warmer Earth would result in more clouds and higher albedo, which would help to temper warming and balance the climate system.
"But this shows the opposite is true," Schwieterman added.
The researchers conclude that albedo analysis using earthshine can suffer from a lack of sensitivity due to long-term calibration concerns, and they call for more research into the planet's brightness.
