Climate change is currently the biggest challenge for the global ecosystem and understanding it requires a deep analysis of atmospheric processes. One of the most important indicators of this state is Earth's energy imbalance (EEI), which represents the net difference between the amount of solar energy absorbed by the Earth and the energy radiated back into space. This imbalance includes incoming shortwave (SW) and outgoing longwave (LW) radiation. According to an analysis of observations from the CERES satellite system, between 2003 and 2023, positive EEI trend at 0.51 ± 0.16 W m⁻² per decade, which means that our planet is accumulating more and more heat.

The role of aerosols in global warming

It has long been assumed that the significant increase in absorbed shortwave radiation in the 21st century was mainly due to decrease in anthropogenic aerosols in the Northern Hemisphere (NH). Aerosols are microscopic particles that affect climate in two ways: directly by scattering and absorbing solar radiation (ARI) and indirectly by modifying cloud properties, increasing their reflectivity and lifetime (ACI).

Reductions in sulfate emissions due to stricter air quality regulations in industrial areas of East Asia and North America have led to the atmosphere becoming "more transparent," allowing more sunlight to be absorbed. Previous modeling studies attributed up to half of the positive trend in shortwave radiation to this decline.

Surprising compensation from the Southern Hemisphere

However, the latest analyses based on observations and reanalyses provide a different perspective. They show that although aerosols have decreased in the Northern Hemisphere, The Southern Hemisphere (SH) has seen an unexpected increase in, which has offset this trend globally. This increase is not due to industrial activity, but natural factors, such as extreme forest fires (e.g. in Australia in 2019-2020) and the massive volcanic eruption of Hung Tonga in 2022.

These events introduced huge amounts of particles into the atmosphere, which increased the reflectivity of clouds over the southern oceans and induced radiative cooling. This hemispheric contrast means that the net global impact of aerosols on the EEI trend over the past two decades has actually been negligible.. While aerosols influenced regional variations, their global contribution to the overall increase in imbalance was minimal.

So what drives the energy imbalance?

If aerosols are not the main culprit behind the global trend, scientists' attention is shifting to other factors. According to sources, they changes in cloudiness up to 67 % overall positive trend shortwave radiation. Other important elements are:

• Surface albedo (25 %): A decrease in the reflectivity of the Earth's surface, for example due to the loss of ice and snow.
• Water vapor (7 %): Increased moisture content in the atmosphere trapping heat.
• Cloud feedback: The reduction of low cloud cover in some areas (e.g. due to changes in sea surface temperature) allows for greater absorption of solar energy.

Shortcomings of climate models

Sources suggest that current climate models (such as CMIP6) may overestimate the impact of aerosols. Models often predict much steeper declines in aerosol concentrations than observed reality, and fail to adequately capture the impact of random natural emissions from fires and volcanoes in the Southern Hemisphere. This discrepancy highlights the need to include natural aerosol variability in future climate projections for more accurate predictions.

Understanding the factors contributing to the Earth's growing energy imbalance is essential for formulating effective climate change mitigation strategies. While regional reductions in air pollution contribute to warming, natural processes on the other side of the world temporarily dampen this impact. However, the main driver of the global trend remains processes related to cloud cover and loss of surface reflectivity, confirming the complexity of our climate system.

We can imagine the energy imbalance of the planet as house with a leaky roof on a summer day. Although we installed windows that let in less light (aerosols in SH), we also exposed some of the insulation on the roof (ice loss and cloud changes), causing the interior of the house to constantly overheat no matter what we do with the windows. JRi

The report was published in the journal science.org