Earth is getting dimmer, researchers have found, and climate change is likely to blame. As the oceans get hotter, they appear to be generating fewer bright clouds, which means less sunlight is reflected back into space – and that warms up the planet even more.

Researchers measured the reflectance or albedo of Earth by observing the earthshine that illuminates the Moon. Nearly 20 years of data, from 1998 to 2017, was collected to inform the results of the study.

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

"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," says theoretical physicist Philip Goode from the New Jersey Institute of Technology.

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

Satellite measurements looked at by the research team suggest that a reduction in bright, reflective, low-lying clouds over the eastern Pacific Ocean has been a major contributor to the reduction in Earth's brightness shown in the data.

And it's all likely to be connected to climate change. In the same areas where bright clouds are thinning, ocean surface temperatures are rising, possibly caused by the reversal of a climatic condition called the Pacific Decadal Oscillation.

"[Earth's albedo] is an essential determinant of the earth's climate, since, in the broadest sense, changes in climate arise from the simultaneous evolution of the solar intensity, the Earth's albedo, and greenhouse insulation," write the researchers in their published paper.

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

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

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

"Stringent data quality standards were applied to generate monthly and annual means," conclude the researchers. "These vary significantly on monthly, annual, and decadal scales with the net being a gradual decline over the two decades, which accelerated in the most recent years."

The research has been published in Geophysical Research Letters.