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Our Earth is getting darker – it may be the oceans

The Earth, when seen from space, is no longer as bright as it was 20 years ago. On average, sunlight reflects about 0.5 percent less, as indicated by astronomical measurements of Earth’s glow and satellite data.

The researchers believe that the likely cause of this low albedo could be the warming of the oceans and the resulting thinning of the low, bright clouds suspended in the ocean. However, it is unclear whether this is due to natural or human causes.

The Earth throws about 30 percent of incoming short-wave sunlight back into space. This light makes our planet appear a bit blue and white from space, and is even enough to light up the dark side of the new moon a little bit facing us. brightness of this earth glow It depends mainly on the cloud cover of the planet and also on the ice cover and ice. Because there is more land in the northern hemisphere and these factors vary more strongly during the seasons, land albedo is higher in northern winter than in summer.

Earth light and whiteness in the show

But among those dated weather Seasonal fluctuations appear to mask a long-term trend, as measurements by Philip Good of New Jersey Institute of Technology and his team now reveal. For their study, they evaluated data from the Big Bear Solar Observatory in California, which has been operating regularly since 1998 Mouth Before and after the new moon, the Earth’s luminosity scales are reversed. In addition, they used measurement data from the CERES instrument of the Earth observation satellites Aqua- and Terra, which has been regularly measuring Earth’s surface reflection since 2000.

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In order to be able to record the effect of solar radiation, the researchers chose a period of more than 20 years for their evaluation. “During an eleven-year cycle, the Sun is on average 0.1% brighter than minimum at its maximum activity,” Judd and colleagues explain. This also affects the brightness and glare of the Earth and was included in the analyses.

0.5 percent darker in 20 years

The surprising result: the Earth has gotten much darker in the past two decades. Today, an average of 0.5 percent of light is shed into space again than it did 20 years ago. The scientists stated that “according to the Earth’s luminosity data, the albedo of the Earth decreased by 0.5 watts per square meter during this time.” Satellite data indicates a larger decrease of 1.6 watts per square meter.

“This drop in albedo was a surprise to us,” Good says. The researchers explain: “It is remarkable how closely the interannual geothermal anomalies match those of CERES. Both show a decrease, especially in recent years, although they cover slightly different regions of the Earth.”

waning sea clouds

But what caused the darkening of the earth’s glare? It would be reasonable to assume that the solar radiation changed during this time. However, the comparison with solar data did not show any relationship between solar activity and the number of sunspots or irradiation with trends in Earth’s appearance. “Our data finds no evidence that solar activity has had a direct or indirect effect on Earth’s reflection over the past two decades,” Good and colleagues write.

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Instead, researchers suspect terrestrial causes of our planet’s diminishing albedo, including its ice cover, as well as long-term changes in sea temperatures. It is known from previous studies that low, bright ocean clouds over subtropical ocean regions reflect 30 to 60 percent of short-wave solar radiation back into space. Thus, these ocean clouds play an important role in regulating climate and terrestrial albedo.

Natural fluctuations or the impact of climate change?

But as CERES data shows, these clouds in particular have diminished in recent years. Because of rising sea temperatures in the eastern Pacific, cloud cover has decreased significantly, especially off the western coasts of North and South America, and with it the albedo of these areas. “So far, it is unclear whether these changes are due to internal fluctuations in climate or whether they are part of reactions to external influences,” Good and his team say.

In theory, both climate change and natural climate variability such as the Pacific Decadal Oscillation (PDO) could be responsible for warming ocean regions and retreating ocean clouds — or both. 2019 resulted in model simulationThe sharp rise in carbon dioxide values ​​and the associated climate change lead to severe thinning of ocean clouds.

Impact also on climate

If the albedo of the Earth decreases even more, this will in any case have consequences for the Earth’s climate. Because if less sunlight is returned to space, more energy stays in the Earth’s system – and thus heat, too. Therefore, if the albedo actually falls in the long run due to melting of the ice surfaces, less sea clouds and other influences, this could lead to a further intensification of climate heating.

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“This is very concerning,” comments non-teaching geologist Edward Schwitterman of the University of California, Riverside. For a long time it was hoped that a warmer Earth would have brighter clouds and therefore higher albedo – thus some compensation for anthropogenic warming. “But these findings suggest it may be the other way around,” Schwettermann said. (Geophysical Research Letters, 2021; doi: 10.1029/2021GL094888)

Quelle: American Geophysical Union

This article was written by Nadja Podbregar