Earth keeps getting warmer because more energy from sunlight stays trapped in the climate system. Scientists refer to this as Earth’s energy imbalance. When incoming energy becomes greater than outgoing energy, heat builds up in oceans, land, and air.
Many past studies linked recent warming to cleaner air and lower pollution levels. New research now shows a different explanation.
Scientists from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science studied long term satellite data and atmospheric records.
Results show that changes in tiny air particles called aerosols do not explain recent global heating trends.
What is Earth’s energy imbalance?
Earth receives energy from the Sun in shortwave radiation, mostly as visible light. Part of this energy reflects back to space from clouds, ice, and bright land surfaces.
The rest warms oceans and land. Earth also releases energy back to space as longwave heat radiation.
Recent measurements show that rising heat comes mainly from increased absorption of sunlight, not from reduced heat loss.
Shortwave energy plays a much stronger role than longwave energy. Clouds and surface reflectivity control most of this shortwave change.
How air particles affect warming
Aerosols are tiny particles floating in air. Sources include factory pollution, vehicle exhaust, wildfires, ocean spray, and volcanoes. Aerosols affect climate in two main ways.
Aerosol radiation interactions occur when particles scatter or absorb sunlight directly. Aerosol cloud interactions occur when particles change cloud droplet size and number.
More particles can create smaller droplets, brighter clouds, and stronger sunlight reflection. Fewer particles can reduce cloud brightness and allow more sunlight to reach Earth’s surface.
Many climate models assumed aerosol reductions caused a large part of recent warming. Observations now show a more balanced picture.
Northern and southern skies act differently
Data from satellites and atmospheric reanalysis show a strong contrast between hemispheres. In northern regions, air quality rules reduced pollution over time.
Lower sulfate pollution led to fewer cloud forming particles. As a result, clouds reflected less sunlight and allowed extra energy to warm land and oceans.
Southern regions followed a different path. Large natural events increased aerosol levels. Major wildfires in Australia during 2019 and 2020 released massive smoke plumes.
A volcanic eruption near Tonga in 2022 injected particles high into the atmosphere. Winds spread these particles across wide ocean areas.
Extra aerosols in southern skies brightened clouds and increased sunlight reflection. Cooling from southern regions balanced warming from northern regions. Global aerosol impact stayed close to zero.
Tracking air pollution and warming
Researchers used two independent methods to track aerosol changes. One method relied on satellite observations that measure how particles affect sunlight passing through air.
Another method used atmospheric reanalysis data to estimate sulfate particle amounts near cloud forming levels.
Both approaches showed similar patterns. Pollution related aerosols declined in northern industrial regions. Natural aerosols increased across southern oceans. Agreement between methods strengthened confidence in results.
Scientists also studied how aerosols affect low level clouds over oceans, where cloud particle interactions matter most. Calculations showed that aerosol cloud effects produced strong regional cooling and warming patterns but canceled out at a global scale.
Clouds now control warming
Cloud behavior explains most of recent energy imbalance growth. Changes in cloud cover and cloud brightness reduced sunlight reflection across large areas.
Surface warming also reduced ice and snow cover, lowering reflectivity and allowing more energy absorption.
Natural climate patterns, such as ocean temperature shifts, influenced cloud formation and location. Shortwave cloud feedback emerged as a major driver of continued heat buildup.
Implications for climate understanding
Chanyoung Park, lead author of the study, is a doctoral student in the Department of Atmospheric Sciences at the Rosenstiel School.
“Understanding this hemispheric ‘balancing act’ helps society focus on the true forces behind global warming – changes in cloud behavior linked to surface warming and natural climate variability – rather than mistakenly attributing recent warming to cleaner air,” said Park.
Climate models often emphasize pollution reductions in northern regions while missing natural aerosol increases elsewhere. Such gaps can lead to overestimated aerosol warming effects.
Observational evidence shows that global energy imbalance growth comes mainly from cloud changes and surface warming.
What comes next
“Earth’s energy imbalance tells us how fast heat is building up in the climate system,” said Brian Soden, co author of the study and Professor in the Department of Atmospheric Sciences at the Rosenstiel School.
“Many earlier studies suggested that cleaner air might explain much of the recent increase, but our results show that aerosol changes largely cancel out between the Northern and Southern Hemispheres.”
“That means we need to look more closely at changes in clouds and natural climate variability to understand why the planet is continuing to gain heat.”
The study is published in the journal Science Advances.
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