Earth Is Getting Darker, Literally, and Scientists Are Trying To Find Out Why

It’s not the vibes; Earth is literally getting darker. Scientists have discovered that our planet has been reflecting less light in both hemispheres, with a more pronounced darkening in the Northern hemisphere, according to a study published on Monday in Proceedings of the National Academy of Sciences. 

The new trend upends longstanding symmetry in the surface albedo, or reflectivity, of the Northern and Southern hemispheres.  In other words, clouds circulate in a way that equalizes hemispheric differences, such as the uneven distribution of land, so that the albedos roughly match—though nobody knows why.  

“There are all kinds of things that people have noticed in observations and simulations that tend to suggest that you have this hemispheric symmetry as a kind of fundamental property of the climate system, but nobody’s really come up with a theoretical framework or explanation for it,” said Norman Loeb, a physical scientist at NASA’s Langley Research Center, who led the new study. “It’s always been something that we’ve observed, but we haven’t really explained it fully.”

To study this mystery, Loeb and his colleagues analyzed 24 years of observations captured since 2000 by the Clouds and the Earth’s Radiant Energy System (CERES), a network of instruments placed on several NOAA and NASA satellites. Instead of an explanation for the strange symmetry, the results revealed an emerging asymmetry in hemispheric albedo; though both hemispheres are darkening, the Northern hemisphere shows more pronounced changes which challenges “the hypothesis that hemispheric symmetry in albedo is a fundamental property of Earth,” according to the study.  

Loeb and his colleagues suggest that asymmetry is primarily driven by the effects of climate change, reductions in aerosol pollution, and natural disasters like volcanic eruptions and wildfires. Since snow and ice are highly reflective, the thinking goes, the melting of glaciers and ice sheets due to anthropogenic gas emissions is causing a reduction in albedo, especially in the Northern hemisphere. 

Meanwhile, aerosols—which stimulate the formation of clouds—are causing uneven regional albedo changes. For example, the international effort to remove harmful commercial aerosols from the atmosphere has resulted in a drop in these substances over the Northern hemisphere, and therefore cloud cover, exacerbating the darkening effect. In the Southern hemisphere, aerosol-heavy clouds generated just over the past few years by disasters like the 2019-2020 Australian bushfires and the 2021 to 2022 Hunga Tonga volcanic eruption may have brightened the albedo relative to the Northern hemisphere.

“The amount of aerosols has been increasing in the Southern hemisphere, and they’ve been decreasing in the Northern hemisphere,” explained Loeb. “Since aerosols reflect solar radiation, that would give you this asymmetry where you’re seeing darkening in the Northern hemisphere compared to the southern Hemisphere.”

“All of these pieces added together give you this trend,” he continued. “But what was mysterious to me was that the clouds weren’t compensating. If this hemispheric symmetry is a fundamental property of the system, the clouds should be giving you more reflection in the Northern hemisphere to compensate for the non-cloud properties. And I don’t see that—at least, not yet.”

Loeb’s team was able to spot this trend thanks to the long-term observations collected by CERES, a program that dates back to the late 1990s. The program has monitored the evolution of albedo in high resolution over decades, enabling the scientists to spot the new divergence from the normal symmetry.

“CERES has really opened up a new avenue of research that we couldn’t do before,” Loeb said. “We had some measurements of Earth’s radiation budget, but we struggled to have the same level of quality of the data.”

“Right now it’s wonderful because we have very precise measurements over 25 years from CERES,” he continued. “It’s a unique opportunity for us to study things like this symmetry in a new light.”

To that end, Loeb and his colleagues plan to continue monitoring the asymmetry with CERES and probing its possible causes with more sophisticated climate models. The researchers are watching for signs that the symmetry might reemerge in the future, or if asymmetry is perhaps the new normal. 

The overall darkening of Earth’s albedo is already accelerating the effects of climate change, and an asymmetric hemispheric darkening could produce its own complex impacts, including disruptive shifts in precipitation. 

It’s very difficult to tease out the individual components that merge to create such complicated dynamics (Loeb calls it “unscrambling the egg”). To make matters worse, NASA is facing major cuts from the Trump administration, especially to its Earth observation satellites. CERES is due for one more launch in 2027, but these instruments are getting “long in the tooth,” Loeb said, and another program will eventually have to take up the mantle. Until then, researchers across disciplines will puzzle over why Earth is anomalously darkening, and what it might mean if this asymmetry is here to stay.

“We’ll keep measuring and keep studying it, and I think this study should open the avenue for others to look at it,” Loeb concluded.