The Earth has been darkened in the last 20 years, why?




Earth is getting dark. Or put another way, the amount of sunlight that our planet reflects back into space has decreased considerably over the last two decades, something that could have serious consequences on the global climate. This is the surprising conclusion of a study just published in
‘Geophysical Research Letters’
by a team of researchers from the Big Bear Solar Observatory, in California, the University of New York, the Instituto de Astrofísica de Canarias and the University of La Laguna. The scientists obtained these data by measuring the faint glow, caused by the brightness of the Earth, that can be observed in the dark areas of the crescent Moon.

«The reflectance of the Earth – write the researchers – is a fundamental climatic parameter that we have measured from the Big Bear Solar Observatory between 1998 and 2017 observing the light of the Earth with modern photometric techniques to accurately determine the daily, monthly, seasonal changes , annual and ten years in the terrestrial albedo. We measured a gradual but climatologically significant decline of 0.5 watts per square meter in global albedo over the two decades of data. We found no correlation between changes in terrestrial albedo and measurements of solar activity. ‘ Researchers, however, still have enough data to determine if the observed darkening is a temporary problem or if, on the contrary, it is a problem. a long-lasting trend capable of seriously affecting the planet’s climate.

On average, the Earth reflects around 30% of the sunlight it receives. But between 1988 and 2017, explains Philiip Goode, the study’s first signatory, that reflectance has decreased by about 0.5%. Most of the decrease, moreover, occurred during the last three years of the period analyzed. According to the study, the drop in reflectance is due to the rise in temperature along the Pacific coasts in both North and South America, which reduced the volume of low-lying clouds and left the ocean exposed, much darker. and less reflective.

Increased warming

Added to this is also the fact that the progressive decline of polar ice Over the past few decades it has caused the Earth to be absorbing additional heat, which in turn has the effect of warming the oceans and melting more ice, a vicious cycle that feeds on itself and leads to increasing warming.

Collectively, Goode and his colleagues estimate that the decline in Earth’s reflectance from 1998 to 2017 means that every square meter of Earth’s surface is absorbing, on average, Additional 0.5 watts of power. For comparison, the researchers note in their study that greenhouse gases and other human activities during the same period increased the energy input to the Earth by about 0.6 watts per square meter. That is, the decrease in the Earth’s reflectance during that same 20-year period almost doubled the rate of warming.

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