Why is the Earth "shining" less in recent years?


For a few years, the Earth has been “losing its shine”.

That is, our planet is reflecting – or returning – less light from the Sun into space, according to a new study published in the Geophysical Research Letters journal of the American Geophysical Union (AGU) in September.

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

Scientists still hope to pinpoint the causes of the reduction in Earth’s brightness, but they already have some hypotheses.

Here we tell you what they are and what consequences the phenomenon could have on the planet.

What is “albedo”

As is known about light in general, light surfaces reflect it and dark ones absorb it.

The same thing happens with the light of the Sun and the Earth.

Light parts of the Earth, like the poles, reflect light from the Sun. GETTY IMAGES

The dark parts of the planet absorb the light and heat from our star; while the light parts, such as the ice surfaces of the poles and clouds, reflect them and return them towards space.

The amount of light from the Sun that the Earth reflects back into space is known as “albedo“And, on average, it treats about 30% of all sunlight received.

“Changes in ice cover, cloud cover, air pollution or land cover (such as forests or farmland, for example) have subtle effects on global albedo,” explains NASA’s Earth Observatory on its website .

During the last two decades, this reflex or albedo has been reduced.

“The Earth now reflects about half a watt less light per square meter than 20 years ago. That’s the equivalent of a 0.5% decrease in Earth’s reflectance,” says the AGU.

This reduction of the reflex it has been mainly concentrated in the last three years.

“The fall in albedo came as a surprise to us when we looked at the last three years of data after 17 years of near-flat albedo,” said Philip Goode, a researcher at the New Jersey Institute of Technology, USA, and lead author of the study, referring to Earth light data from 1998 to 2017.

But what is the reason for this reduction?

Possible causes

The authors of the study did not detect changes in the brightness of the Sun in the last three years, so the decrease in the reflectance of the Earth is not related to the star, but to causes of the planet.

Scientists detected a reduction in clouds in the eastern Pacific. GETTY IMAGES

The cause that scientists detected here on Earth was a “substantial” variation in the amount of clouds in certain areas of the Pacific Ocean, said Enric Pallé, one of the authors of the study and a researcher at the Instituto de Astrofísica de Canarias (IAC) and from the Department of Astrophysics of the University of La Laguna, Spain, to BBC Mundo.

Now there are fewer clouds – therefore fewer bright white surfaces that reflect light – in the eastern Pacific, off the western coasts of North and South America, according to data from NASA’s Cloud and Earth Radiant Energy System (CERES).

This reduction in clouds is due to an increase in sea temperature, “with probable connections to global climate change,” the AGU said in a statement in September.

But Pallé told BBC Mundo that he does not know “if it is so easy to attribute (the increase in sea temperature) to climate change because the climate system is very complex” and because they have only measured albedo in the last 20 years, while that “natural processes have longer cycles.”

“In other words, I think it is probably due to climate change, but I think it is still premature to assign it. There may be natural cycles of cloudiness that could be changing the albedo,” said Pallé.

“Within the trend of global warming there are episodes of rises and falls (of temperature), so perhaps we are seeing something episodic,” added the researcher.

Climatologist John Nielsen-Gammon, professor in the Department of Atmospheric Sciences at Texas A&M University, told BBC Mundo that “cloud cover is closely linked to temperature and wind patterns in the atmosphere, which are affected by the global warming and natural variability “.

“(But) the 20-year record of Earth’s brightness is not really long enough to separate these two effects,” said Nielsen-Gammon, who was not involved in the study.

Scientists calculated albedo by measuring the light from the Sun that the Earth reflects off the Moon. GETTY IMAGES

To determine exactly what the variation in albedo is due to, “we have to keep measuring how this factor changes in the coming years, measure for a long enough time to see if we can really associate it with climate change or to be sure that this it is not a natural variation, “said Pallé.

Consequences

While investigating the causes of the reduction in Earth’s brightness, scientists do know that the light and heat from the Sun that the Earth stops reflecting to space stays on the planet, in the oceans and in the atmosphere, therefore, it can influence the temperature.

“If the amount of light that the Earth reflects changes over days or decades, it will have an influence on climate change, because it will let in more or less energy from the Sun,” Pallé told BBC Mundo.

“What is clear is that albedo had always been considered in climatic studies as something that was constant, but it is not and we have to continue measuring it because it will greatly affect our ability to predict climate change 20, 30 or 50 years from now, “added the scientist.

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