The Earth is getting darker. This is not good news.

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Here’s what you’ll learn from reading this story:

• The radiation balance between the northern hemisphere and the southern hemisphere continues to expand.

• A new study suggests that this discrepancy is due to radiation interaction, decreasing albedo, and changes in water vapor, all of which create fewer clouds that can reflect solar radiation from the Sun.

• Even though the difference between these budgets only increases by about 0.34 watts per square meter per decade, it is statistically large enough to be considered in future climate models.

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Thanks to Earth’s slow rotation at a speed of 1,040 miles per hour, if two points are equidistant from our equator, they should be heated relatively equally by solar radiation. But of course, the reality is not that simple.

A new NASA-backed study, drawing on data from the Clouds and the Earth’s Radiant Energy System (CERES) satellite, suggests that the Northern Hemisphere (NH) is actually getting darker than the Southern Hemisphere (SH), and that could be a big problem as the world races to break the climate change-induced fever.

A team of scientists, led by Norman Loeb of NASA’s Langley Research Center in Virginia, has discovered that the Northern Hemisphere experiences a net loss of radiative energy compared to the Southern Hemisphere. This means that the “top” half of the planet is darker in the sense that it absorbs more light and therefore reflects less light. (If you looked at it from space, the northern hemisphere would appear darker due to this lack of reflection). The results of the study were published in the journal PNAS.

All of this affects Earth’s “radiation budget,” or the amount of light the planet absorbs and re-emits into space as outgoing longwave radiation (OLR). Typically, this imbalance is offset by ocean currents that carry energy from the Southern Hemisphere to the Northern Hemisphere, but surface changes have tipped the balance so much that ocean currents have not made up the difference over the past two decades.

“The emerging darkening of the NH relative to the SH is associated with changes in hemispheric differences in aerosol-radiation interactions, surface albedo, and changes in water vapor,” the authors wrote. “How clouds respond to this hemispheric imbalance has important implications for future climate. »

One of the main factors in this change is the albedo, or reflectivity of the Earth’s surface. The most striking component is Arctic sea ice, whose rapid decline is causing reflective surfaces (snow and ice) to be rapidly replaced by more light-absorbing surfaces (land and seawater). This increases solar absorption, thereby increasing surface temperatures, and is the main reason why Earth’s polar regions heat up four times faster than other parts of the world.

Clouds are another major source of reflectivity. Recent studies have shown that clouds, especially low-lying ones, have decreased in recent decades. To add insult to injury, part of this could be because the northern hemisphere pollutes less than before. Aerosols (the tiny particles that produce various forms of air pollution) can act as seeds in cloud formation, and fewer seeds means fewer clouds.

In 2024, the National Oceanic and Atmospheric Administration (NOAA) detailed how reducing emissions from transoceanic shipping could actually accelerate climate change due to a large reduction in reflective clouds known as “ship tracks.” Of course, more pollution is not the solution, as these gases still trap heat on the planet, but this discovery has led some scientists to consider experimenting with geoengineering techniques such as injecting stratospheric aerosols or brightening marine clouds.

In the case of divergent radiation budgets between the two hemispheres, while environmental laws in the Northern Hemisphere have led to some progress in reducing pollution, the Southern Hemisphere has experienced extreme episodes of aerosol injection from many sources, from the Australian bushfires to the eruptions of the Hunga Tonga volcano in 2021 and 2022, according to Phys.org.

As the distance between these radiation budgets slowly increases (by about 0.34 watts per square meter per decade), the differences will be enough to significantly impact climate models and distort our view of the planet’s future. Hopefully, with time and lots of innovation, we’ll find a way to get these outreach budgets back on track.

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