Climate: how the Earth is darkening

Climate: how the Earth is darkening
Climate: how the Earth is darkening

The Earth, our “pale blue point” in the universe as astronomer Carl Sagan called it, is less and less pale. A recent study published in the journal “AGU – Geophysical Research Letters” shows that the warming of ocean waters has caused a decrease in the luminosity of our planet.

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

The Earth now reflects about half a watt less light per square meter than it did 20 years ago. That’s the equivalent of a 0.5% decrease in Earth’s reflectance, scientists say. A significant drop, as the Earth reflects around 30% of the light that comes from the Sun.

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

“This drop was a big surprise for us, after 17 years of almost flat albedo,” responded Philip Goode, researcher at the New Jersey Institute of Technology and lead author of the study, referring to these data collected between 1998 and 2017 by the Big Bear Solar Observatory in California.

This graph shows the Earth’s average annual luminosity (in black) from 1998 to 2017. The blue line shows the annual CERES albedo, which is another annual measure of the Earth’s reflectivity, from 2001 to 2019. Dashed lines black and blue are fitted averages showing that Earth’s reflectivity has decreased over the past 20 years.Goode et al. (2021) /Geophysical Research Letters

Why this decrease in brightness?

Two things can explain this decrease in luminosity: a decrease in light from the Sun and or a decrease in reflectivity of the planet. The first hypothesis has been ruled out: the observed changes in the Earth’s albedo are not correlated with periodic changes in the Sun’s luminosity. Which means that the explanation is to be found on Earth. And more precisely in the clouds around the Earth.

Because cloud cover plays an important role in the albedo of the planet: the sunlight bounces there and is sent back into space. Logically, as this cloud cover decreases, more of the sun’s rays enter Earth. Our planet therefore appears less bright.

Scientists noted that the decline in albedo did indeed coincide with a reduction in low, bright clouds over the eastern Pacific Ocean in recent years, according to satellite measurements made as part of the Clouds and the Project. Earth’s Radiant Energy System (CERES).

This is the same area, off the western coasts of North and South America, where sea surface temperatures have increased due to a reversal in climatic conditions known as the “Pacific Decadal Oscillation” (PDO ), probably linked to global climate change.

To sum up, parts of the oceans have warmed up, resulting in fewer clouds over the water, which then allowed more sunlight to pass through on Earth (instead of reflecting it back into space. ). Does this effect itself participate in global warming? Scientists have not ventured 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 kind of still guessing. “

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