24 years ago, the earth was symmetrical. Now the northern hemisphere is “unequivocally” darker than the southern hemisphere

NASA’s last 24 years of satellite data reveal an “unequivocal” trend: the earth has lost its balance, and now the northern hemisphere reflects less light than the southern hemisphere.

How is that? Until a few years ago, our planet maintained an almost perfect symmetry in regard to its albedo: its reflectivity from the perspective of an observer in the earth’s orbit.

Despite its obvious differences (the north dominated by terrestrial masses and the south by oceans), both hemispheres reflected practically the same amount of sunlight to outer space. Now that doesn’t happen anymore. The northern hemisphere is absorbing more solar energy than the southern hemisphere, breaking the balance that had been maintained for a long time.

In figures. The new study, published in Proceedings of the National Academy of Sciencesquantifies this divergence at 0.34 wm – 2 per decade. It is a statistically significant value that points to a deep change in the energy engine of our planet.

And why? The study, led by Norman Loeb, from the NASA Langley Research Center, points to a combination of factors; The first one, somewhat paradoxical. According to Loeb, the main engine of the growing asymmetry is the aerosols, the tiny particles suspended in the atmosphere against which we have been fighting for some time.

Thanks to environmental protection measures, fine particle pollution has decreased significantly in Europe, the United States and China in recent years. Less contamination means a cleaner air and, therefore, less particles that reflect sunlight. The result is that more radiation reaches the surface and is absorbed.

In the southern hemisphere it has happened just the opposite. Mass events, such as Australian forest fires of 2019-2020 or the eruption of the Hunga Tonga volcano in 2022, injected huge amounts of aerosols into the atmosphere, temporarily increasing the reflectivity.

There is something else. To the aerosols we must add the change in the Albedo of the surface itself. The northern hemisphere is losing snow and sea ice at an accelerated pace, in this case because of global warming. If white and bright surfaces (which reflect the light) are replaced by water and dark terrain (that absorb it), the hemisphere is further darkened.

Clouds are missing. What has most bewildered scientists is the role of clouds. For a long time it was theorized that clouds would act as a natural compensatory mechanism for this phenomenon: if a hemisphere darkened by external factors, atmospheric circulation would adjust cloudiness to reflect more light and restore balance.

However, the data shows that this is not what is happening. The study concludes that the contribution of clouds to the difference in reflectivity between hemispheres is surprisingly small. The reason is complex: it seems that the changes in the clouds of the tropics are being canceled with the changes in the highest latitudes, questioning one of the fundamental hypotheses on the self -regulation of the earth’s climate.

A problem. That a hemisphere hotly gets more than the other is not a simple academic curiosity. The Earth’s energy balance is the engine that drives atmospheric and oceanic circulation; that is, our climate and our weather patterns. This imbalance is already having consequences.

The northern hemisphere not only heats up faster than the south, but is also seeing an increase in rainfall in tropical latitudes. If the intertopical convergence zone, the land rain belt, moves north, the consequences will be hard for billions of people.