Human activities are weakening Earth's air circulation

The summer of 2024 was the hottest on record, an expected outcome given the rising global temperatures. 

In the Northern Hemisphere, hotter and drier summers have led to severe droughts and intense heat waves, particularly in North America and Europe, posing significant threats to society, such as wildfires, crop failures, and serious health risks.

Weakening of Earth’s air circulation 

A contributing factor to these extreme weather events is the weakening of Earth’s air circulation systems, which normally help distribute moisture and heat across the globe. The exact reasons behind this weakening, however, had remained unclear – until now.

In two new studies led by Rei Chemke, an expert in climate dynamics at the Weizmann Institute of Science, researchers have identified human activity as the cause behind the weakening of these critical air circulation systems. 

Vital network of wind patterns 

The studies focused on the wind patterns that form a vital network for transporting wind-borne heat and moisture around the world. Central to this network are storm tracks, which are high- and low-pressure weather systems that move from west to east.

These storm systems play a crucial role in transferring heat, moisture, and air flow momentum within the atmosphere, which in turn shapes the various climate zones on Earth. 

Another key component is the Hadley circulation, a process in which warm air rises near the equator, moves toward the poles, descends in the subtropics, and flows back toward the equator, creating a continuous cycle.

The role of human activities 

While scientists had long known that both storm tracks and the Hadley circulation have been weakening since at least 1980, only the weakening of the Hadley circulation had been clearly linked to human-induced emissions.

The research was conducted in collaboration with Professor Dim Coumou of Amsterdam’s Institute for Environmental Studies 

The first study, published in the journal npj Climate and Atmospheric Science, demonstrated that the weakening of storm tracks is also due to man-made emissions of greenhouse gasses, such as CO₂, and aerosols.

“These emissions warm the air more at the high latitudes than at the low latitudes,” said Chemke. This reduction in the temperature difference between northern and southern latitudes, which drives the storm tracks, has caused the storm tracks to weaken.

Buildup of warm temperatures 

To reach these conclusions, the researchers analyzed vast amounts of climate data from both observations and advanced climate models. 

The experts found that the weakening of storm tracks could only be explained when historical emissions were factored into the climate models. 

Chemke pointed out the significance of these storm systems in cooling landmasses.

“The storms in summertime play an important role in bringing cool air from the ocean to the land. If you reduce the intensity of these storms, you bring in less of this cool air. This leads to a buildup of warm temperatures on the continent, which can lead to increasingly extreme heat events.”

Impacts on air circulation 

Man-made emissions have also been found to impact the Hadley circulation in an unprecedented way. 

While natural factors have historically influenced this circulation, human activity has reversed the direction of these effects – weakening the circulation rather than strengthening it.

In the second study, published in Nature Communications, Chemke and his student Or Hess explored the role of natural forces, such as volcanic eruptions and solar fluxes, in shaping the Hadley circulation over the past millennium. 

Using climate models that encapsulate the physics, biology, and chemistry of the climate system, they simulated how natural factors altered the Hadley circulation in previous centuries.

The findings revealed that natural forces had generally acted to strengthen the Hadley circulation in the past – quite the opposite of what is happening today. Man-made emissions, in contrast, are weakening this vital circulation.

“In the previous millennium, natural factors were dominant, whereas now, man-made emissions play a more dominant role,” Hess said.

“In the past, we had a cooling climate that acted to strengthen the Hadley circulation. Today, we have a warming climate that acts to weaken this circulation.”

Natural changes in Earth’s air circulation 

Chemke and his team emphasize the need for climate models to account more thoroughly for natural factors, which are often overshadowed by the focus on human-induced emissions. 

While human activity undoubtedly plays a larger role in today’s climate changes, natural forces have historically had a significant impact.

Human influence on climate systems

“Our research field mainly deals with man-made emissions and warming climates. Less attention is given to the impact of natural factors such as volcanic eruptions, solar fluctuations, or natural sources of greenhouse gasses,” Chemke explained.

“Yet we found that natural factors can have a major effect on climate, although smaller in magnitude relative to the impact of man-made emissions.” 

“In particular, they seem to affect the Hadley circulation in a way that’s opposite to that of human activity. This was really surprising for us, and it’s something that should be taken into consideration.”

The findings highlight the importance of understanding both natural and human influences on climate systems, especially as the world grapples with increasingly extreme weather events.

By factoring in both, scientists can develop more accurate models and strategies for addressing climate change.

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