A vicious circle seems to be born. Because the earth reflects less light, the earth heats up even more.

If you look at Earth from space, our planet gives off ‘light’ to a certain extent. This is because snow and ice, as well as clouds, reflect a lot of sunlight. In a new study, researchers have looked at Earth’s brightness. Because to what extent is ‘our’ brightness also subject to change as a result of the current changing climate?

Earthshine

According to the researchers, it is important to take a closer look at this issue. The three fundamental parameters for climate change are the ‘radiation output’ of the sun, the emission of greenhouse gases and the reflection of the earth. “A quarter of a century ago, we learned that the latter was (and still is) the least well understood,” said Phil Goode in an interview with Scientias.nl. “We know that the moon is a mirror that can be used to measure the reflection of the earth using ‘earthshine’ (the effect of the reflection of sunlight from the earth on the moon, ed.). So we modernized an age-old technique and built a specialized small telescope.”

More about earth shine
Although the word ‘earthly’ may sound a bit tricky, we’ve all seen it in practice. “When you look at the night sky around a New Moon, you see a small, bright, crescent moon,” Goode explains. “The rest of our natural satellite is easy to see with the naked eye because it is dimly lit by earthshine; sunlight reflected from the day side of the earth to the moon and then back to the nighttime earth.”

After studying two decades of Earthshine data (from 1998 to 2017), the researchers make a curious discovery. Because the measurements show that the Earth now reflects about half a watt less light per square meter compared to 1998. “During the first 17 years of our 20-year study, there were small year-over-year changes,” Goode says. But the biggest changes the researchers found in the last three years of the data. “In the last three years, the albedo was smaller in each year than any of the previous 17!” says Goode.

Warmth

This has far-reaching consequences. “There’s a lot more heat entering the climate system,” Goode explains. And while the observed decrease in brightness may seem negligible, according to Goode, that is certainly not the case. “While 0.5 W/m2 equates to ‘only’ a 0.5% drop in Earth’s reflectance of visible light, it’s as much as the unrelated anthropogenic warming of this century,” he says.

Cause

The question is, of course, what causes this decrease in brightness. “This is because more sunlight is reaching the Earth,” Goode says. “And that in turn is directly related to a decrease in cloud cover west of the Americas.” Satellite data has shown that at lower elevations over the eastern Pacific Ocean, off the west coast of America, there were significantly fewer clear clouds. And this is exactly where scientists also recorded strong temperature increases at the ocean surface.

clouds

This probably requires a little more explanation. As stated at the beginning of this article, clouds reflect much of the sunlight. But as oceans get hotter, they seem to generate less bright clouds, meaning less sunlight is also reflected back into space. “Changes in Earth’s reflectance are dominated by cloud changes,” Goode said. “And if the Earth is less cloudy, it also becomes less reflective. And that in turn means that the earthshine is diminishing; from space, the light from the earth is weaker.”

How this will develop in the future remains to be seen. The observed developments are unexpected anyway. “You might think that warming the oceans would actually lead to more clouds and thus a brighter Earth,” Goode says. “But that obviously hasn’t happened. It is difficult to say how the climate system will dissipate the extra energy.” If the light is not reflected to space and is thus trapped in the Earth’s climate system, the change in brightness could have major consequences for the future of the climate. It could mean accelerating the pace of human-induced climate change. But that remains to be seen. “As with many things in climate studies, they are always more complicated than they appear at first glance,” Goode concludes.