Why is the Earth getting darker and what does this have to do with climate change? | Environment and climate

In a new study using satellite data collected since the beginning of the century, researchers discovered that Earth’s reflection of sunlight is gradually decreasing, and that an imbalance arises between north and south, with the former appearing darker and hotter than the latter.

And she points the study Recently published in the journal Proceedings of the National Academy of Sciences, the cycle of winds and ocean currents is closely linked to the distribution of solar energy on the planet.

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According to the team of researchers led by Norman Loeb of NASA’s Langley Research Center in Virginia, the Southern and Northern Hemispheres on average reflect the same amount of solar radiation.

“It’s more of a theory or an open question,” says Norman Loeb. “We don’t know if this is a fundamental feature of our planet or just a coincidence.” The study’s authors confirm that this symmetry has been an interesting topic since it was first observed by satellite in the late 1950s.

Loeb points out that the amount of energy emitted by the sun is not evenly distributed across the globe. At the equator, the sun is more intense, while the energy absorbed is less at the poles. To restore balance, atmospheric circulation (winds, clouds, etc.) and ocean circulation transfer energy from the tropics to the poles.

Previous studies have shown that this rebalancing also occurs from one hemisphere to the other, and that the distribution of clouds or some of the clouds, which are shallower, and reflect the sun’s rays, plays an important role. Without them, the southern hemisphere, which is covered by a large part of the oceans, would be much less bright.

Major imbalance

After analyzing satellite data over a period of 24 years, the researchers discovered that not only are the two hemispheres getting darker, but also that the whiteness of the Northern Hemisphere is decreasing faster than the Southern Hemisphere, causing an asymmetry, that is, an imbalance between these two parts of the Earth.

This was a difference of 0.34 watts per square meter per decade – a small but statistically significant difference – according to the study. Therefore, the north, which absorbs more energy, is warming faster than the south, by about 0.16 degrees Celsius per decade, due to this darkening.

The researchers argue that there are several factors that explain this. Climate change is causing the melting of glaciers and sea ice, which have a high ability to reflect radiation, and this melting is “largest and fastest in the Northern Hemisphere”, which has a greater land cover, as Norman Loeb points out.

Rising temperatures also increase the concentration of water vapor in the atmosphere, which absorbs solar energy instead of reflecting it.

The authors also point to a decline in the Northern Hemisphere of aerosols, fine particles suspended in the air that have been the target of anti-pollution policies in recent years, particularly in China, but also in the United States and Europe.

While these aerosols are indeed pollutants, they also reflect sunlight. Conversely, in the Southern Hemisphere, Australian bushfires and the 2022 eruption of the Honga Tonga volcano in the South Pacific Ocean led to increased particulate matter emissions.

“We have seen decreases in aerosols in the Northern Hemisphere and increases in the Southern Hemisphere, which has exacerbated this disparity,” says Norman Loeb.

Another, more ambiguous explanation lies in the clouds, according to the study, as previous studies indicate that global warming tends to reduce the amount of low-lying clouds, specifically those that reflect solar radiation.

While previous studies and models have shown that north-south imbalances are usually compensated by currents, winds and clouds, the new study suggests that this is no longer true.

“The question of whether the general circulation will adjust to produce a cloud distribution that restores hemispheric albedo uniformity remains open, and has important implications for future climate,” the researchers conclude.

Regarding the possible effects of this discrepancy, researchers are actually observing an increase in precipitation in the northern part of the tropics, specifically in the “tropical convergence zone” located near the equator, which is a very important region for the exchange of energy between the two hemispheres.

Norman Loeb believes that, if this phenomenon is real, it will certainly have repercussions on climate patterns far beyond the tropics.

For her part, Jing Feng, a researcher at Princeton University who specializes in issues of the Earth’s radiative energy balance, confirms that this variation may have major repercussions on climate, such as changes in global precipitation patterns.

Feng believes that the study raises important questions, especially about the ability of climate models to reproduce the observed variation in the hemisphere.

The main shortcoming of the study, according to her, lies in “the relatively short period of observation (24 years), noting that longer observations via satellite are necessary to confirm the continuation of this trend and its impact on the regional and global climate.”