Why has the Earth been reflecting less light in recent years?

This means our planet is reflecting, or returning, less sunlight to space, according to a new study published in Geophysical Research Letters, by the United States Geophysical Union, in September.

• Study says climate change is darkening the Earth

The research authors, from the United States and Spain, came to this conclusion after analyzing data on the amount of light that Earth reflects on the Moon, collected over the past 20 years by satellites and the Big Bear Solar Observatory in California.

Scientists still hope to determine the reasons for the decrease in the brightness of the Earth, but they already have some hypotheses.

Find out what it is and what consequences this phenomenon can cause on the planet.

As is known about light in general, it is reflected by light surfaces and absorbed by dark surfaces. The same thing happens with sunlight and the earth.

The dark parts of the planet absorb light and heat from our star, the Sun. Optical parts, such as the ice surfaces at the poles and clouds, reflect the light back into space.

The amount of light from the Sun that reflects off the Earth is known as the albedo, and it represents, on average, about 30% of the total sunlight received.

“Changes in ice cover, cloud cover, land cover (such as forests or farmland), and air pollution have minor effects on global albedo,” NASA’s Earth Observatory, the US space agency, explains on its website.

Over the past two decades, albedo has been decreasing. “The Earth now reflects about half a watt of light per square meter than it did 20 years ago. This equates to a 0.5% reduction in Earth’s reflectance,” the United States Geophysical Union said.

This reduction was mainly concentrated in the past three years. Philip Good, a researcher at the New Jersey Institute of Technology in the USA, and lead author of the study said, referring to data of light reflected off the Earth from 1998 to 2017.

But what is the reason for this reduction?

The study authors did not detect changes in the brightness of the sun over the past three years, so the decrease in reflected light is not related to the sun, but rather to Earth-related factors.

The reason scientists on Earth discovered was the “significant” difference in the amount of clouds in certain regions of the Pacific Ocean, Enrique Ballet, one of the study authors and a researcher at the Canary Institute for Astrophysics and the Department of Astrophysics from the University of Laguna, Spain.

There are fewer clouds, and therefore fewer bright white surfaces that reflect light in the western Pacific, compared to the western shores of North and South America, according to data from NASA’s Cloud and Earth Radiant Energy System.

The United States Geophysical Union said in a statement in September that this decrease in clouds was due to rising sea temperatures “with a possible link to global climate change.”

But Balleh told BBC News Mundo (BBC’s Spanish-language news service) he did not know “whether it is very easy to attribute (rising sea temperature) to climate change, because the climate system is so complex” and because only albedo was measured. In the last twenty years, while “natural processes have longer cycles”.

“In other words, I think it’s probably due to climate change, but I think it’s too early to say at the moment. There may be natural cycles of clouds that can change albedo,” Ballet said.

“Under the global warming trend, there are periods of increases and decreases (in temperature), so we may be seeing something occasional,” the researcher added.

“Cloud cover is closely related to atmospheric temperature and wind patterns, which are influenced by global warming and natural variability,” climate expert John Nielsen-Gammon, professor in the University of Texas’ Department of Atmospheric Sciences, told BBC News Mundo.

“[But]the 20-year record of Earth’s brightness isn’t really long enough to separate these two effects,” said Nielsen-Gammon, who was not involved in the research.

To determine exactly what caused the difference in albedo, “we have to continue to measure how it changes in the coming years, and measure it long enough to see if we can really relate it to climate change and make sure it’s not the difference, it’s natural,” Ballet said.

While investigating the causes of Earth’s dimming, scientists learn that light and heat from the Sun that Earth doesn’t reflect back into space remains on the planet’s oceans and atmosphere. Therefore, it can affect the temperature.

“If the amount of light reflected by the Earth changes over the course of days or decades, there will be an impact on climate change, because it will allow more or less energy from the sun to come in,” Ballet told BBC Mundo.