Two decades of data shows Earth is darkening as the planet warms

The Earth is darkening, researchers have found, and climate change is likely to blame. As the oceans warm, they seem to generate less bright clouds, which means less sunlight is reflected back into space – and it warms the planet even more.

Researchers measured the reflectance or albedo of the Earth by observing the glow of the earth that illuminates the moon. Almost 20 years of data, from 1998 to 2017, were collected to inform the study’s results.

Measurements showed that the Earth was now reflecting about half a watt less light per square meter compared to 1998, the equivalent of a 0.5% decrease in Earth’s reflectance. In total, our planet reflects about 30% of the sunlight that reaches it.

“The decline in albedo was such a surprise to us when we analyzed the last three years of data after 17 years of near-flat albedo,” says theoretical physicist Philip Goode of the New Jersey Institute of Technology.

The brightness of the Earth depends on both the amount of sunlight that reaches it and the reflectivity of the planet. This study showed that the two factors were not in tandem, so something on Earth is causing the darkening, especially in recent years.

Satellite measurements examined by the research team suggest that a reduction in bright, reflective, low clouds over the eastern Pacific Ocean was a major contributor to the reduction in Earth’s luminosity shown in the data.

And all of this is probably linked to climate change. In the same areas where bright clouds are thinning, ocean surface temperatures are rising, possibly due to the reversal of a climatic condition called the Pacific Decadal Oscillation.

“[Earth’s albedo] is an essential determinant of the Earth’s climate, because, in the broadest sense, climate change results from simultaneous changes in solar intensity, the Earth’s albedo and greenhouse insulation ”, write the researchers in their published article.

Earthshine readings were taken from the Big Bear Solar Observatory in California, with approximately 1,500 nights of usable data collected in total. Earthshine has been recorded intermittently for almost 100 years and was first described by Leonardo da Vinci in the 16th century.

Clouds, water, ice, forests, deserts, and all other types of land reflect sunlight differently, which is why researchers need large pools of data to draw conclusions as the Earth turns. Human pollution can also affect readings.

Now, researchers are calling for more comprehensive measures to be taken over the next few years. It had been hoped that a warming planet could create a greater albedo, thereby mitigating some of the warming – but it seems the opposite is happening.

“Strict data quality standards were applied to generate monthly and annual averages,” the researchers conclude. “These vary widely on monthly, annual and decadal scales, the net being a gradual decline over the past two decades, which has accelerated in recent years.”

The research was published in Geophysical research letters.