We are told that global warming is leading to increased evaporation of water, which is one of the causes driving the terrible storms of late – there’s more water in the atmosphere. But now a new paper reveals a sudden, startling corollary.
Contradicting expectations, ocean warming has resulted in earthshine (aka earthlight) dimming over the last 20 years, especially in the last three. The result is even more warming, say Philip Goode of the Big Bear Solar Observatory, New Jersey Institute of Technology, Enric Pallé of the Institute of Astrophysics of the Canary Islands and Steven Koonin of New York University, writing recently in Geophysical Research Letters, which is published by the American Geophysical Union.
Our planet reflects about 30 percent of the sunshine that hits it, as measured by the light from Earth illuminating the surface of the moon, the American Geophysical Union explains. Scientists have been measuring this earthshine hitting the moon for decades.
This reflectance, aka albedo, has decreased by 0.5 percent from 1998 to 2017. That is a drop of half a watt of light per square meter and is a significant decrease, the union explains.
“The albedo drop was such a surprise to us when we analyzed the last three years of data after 17 years of nearly flat albedo,” stated Goode, the lead author of the study.
How is this earthshine measured exactly? The Big Bear Solar Observatory measures the light reflected from Earth to the dark part of the lunar surface and back to the observatory.
So, the Earth is reflecting less sunlight than we are accustomed to, which means it is “keeping” more of this energy.
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Could the drop in albedo be an artifact of periodic solar cycles, changing the amount of sunlight reaching Earth? No such correlation was found, the researchers say: ergo, the cause is right here on Earth.
The problem is a decrease in bright, reflective low-lying clouds over the eastern Pacific Ocean – off the west coasts of the Americas – in the last three years, according to satellite measurements. And why is that? Because the ocean surfaces are hotter, and that is because of the reversal of a climatic condition called the Pacific Decadal Oscillation.
In short, the underlying cause is global climate change, the team concludes, and the result of less light being reflected is more warming. The planet, the geophysicists explain, is capturing a greater proportion of solar energy.
It is also the categorical opposite of the great hope of some scientists that the warming waters and increase in evaporation would create a cloud cover that would reflect more sunlight, i.e., increasing the planetary albedo. That was expected to help mitigate warming. Well, that’s not happening, and albedo is a key influence on the planet’s climate.
“Changes in climate arise from the simultaneous evolution of the solar intensity, the Earth’s albedo and greenhouse insulation,” the team writes – now check all three.
Why is this news? Partly because the Earth’s albedo changes seasonally, can vary a great deal within seasons and is enormously tricky to measure. Also, its reflectance varies widely. But the trajectory is now clear – and it’s accelerating. And, as the scientists explain: the sun’s irradiance is only 0.1 percent greater during activity maximums, in the course of its famed 11-year cycle, while the difference in earthshine is 0.5 percent.
Therefore, “on the face of it, these cyclic changes in irradiance and activity are too small and short-lived to do more than leave a weak signature in the climate,” they write.
If anything, data from the CERES satellite indicates an even stronger decline in Earth’s albedo, the team writes. The paper does not provide forecasts.