Climate: How the Earth is turning black

Posted on October 9, 2021 at 8:29 am

Earth, our “light blue dot in the universe,” as astronomer Carl Sagan called it, is less and less yellow. A recent study published in the journal “AGU – Geophysical Research Letters” suggests that the warming of ocean waters has reduced our planet’s brightness.

The researchers used decades of measurements of Earth’s brightness – calculated based on the brightness reflected from Earth that illuminates the dark side of the Moon – as well as satellite measurements to detect a significant drop in Earth’s reflection. – also called albedo – over the last two decades.

Earth now reflects about half a watt less light per square meter than it did 20 years ago. Scientists say that this is equivalent to a 0.5% reduction in Earth’s reflectivity. A significant drop, as the Earth reflects about 30% of the light coming from the Sun.

The change was sudden, to the extent that it confused the scientific team. During the first 17 years of measurement, the albedo data seemed more or less the same, to the extent that researchers were reluctant to continue with the rest of the study. Then there was a sudden decline during the last three years.

“This decline was a big surprise to us, after nearly 17 years of nearly flat albedo,” said Philip Goode, a New Jersey Institute of Technology researcher and lead author of the study. Referring to Bear Solar Observatory in California.

This graph shows Earth’s average annual brightness (in black) from 1998 to 2017. The blue line shows the annual CERES albedo, another annual measurement of Earth’s reflectivity from 2001 to 2019. The black and blue dashed lines are showing averages. That the Earth’s reflectivity has decreased over the past 20 years.Goode et al. (2021) /Geophysical Research Papers

Why this reduction in brightness?

Two things can explain this decrease in brightness: a decrease in light from the Sun and or a decrease in the planet’s reflectivity. The first hypothesis is rejected: the observed changes in Earth’s albedo are not related to periodic changes in the Sun’s brightness. Which means that the interpretation is to be found on earth. And more precisely in the clouds that surround the Earth.

Because cloud cover plays an important role in the planet’s albedo: sunlight bounces off there and is sent back into space. Logically, as this cloud cover decreases, the Sun’s rays penetrate the Earth more. That’s why our planet appears less bright.

The scientists noted that according to clouds and satellite measurements carried out as part of the project, the decline in albedo in recent years actually coincides with a decrease in low, bright clouds in the eastern Pacific Ocean. Earth’s Radiant Energy System (CERES).

This is the same region, off the western coasts of North and South America, where a reversal in climatic conditions known as the “Pacific Decadal Oscillation” (PDO) has led to an increase in sea surface temperatures, possibly linked to global climate change.

In short, parts of the oceans have warmed, resulting in less cloud cover over the water, allowing more sunlight to pass through to Earth (rather than reflecting it back into space). Does this effect itself participate in global warming? Scientists have not dared to answer this question. Philip Goode, interviewed by CNN, simply said, “Of course the Earth gets an extra half a watt per square meter, but what the Earth does with that energy, we’re still speculating.”

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About the Author: Tad Fisher

Prone to fits of apathy. Music specialist. Extreme food enthusiast. Amateur problem solver.

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