Asymmetric warming between day and night impacts all life on Earth

A significant shift in global warming dynamics has been identified, challenging long-held beliefs about the nature of asymmetric warming and temperature changes on Earth.

This discovery, made by researchers at Chalmers University of Technology, Sweden, has profound implications for our understanding of climate change and its impact on all forms of life.

Phenomenon of asymmetric warming

Since the 1950s, scientists believed that global warming did not uniformly affect daytime and nighttime temperatures. Notably, nighttime temperatures were observed to be rising more rapidly.

However, this new study, published in Nature Communications, indicates a dramatic shift in this pattern, with greater daytime warming occurring since the 1990s.

This reversal in temperature trends signifies a growing disparity between day and night temperatures, which could have far-reaching effects on the planet.

Traditionally known as “asymmetric warming,” this phenomenon, characterized by varying warming rates during day and night, has been attributed to both human activities and natural occurrences.

The recent study, however, presents a startling reversal of this trend.

Between 1961 and 2020, researchers found an accelerated warming rate during the day, while nighttime warming rates remained relatively constant.

This shift leads to an expanding temperature difference between day and night.

Asymmetric warming and global brightening

Ziqian Zhong, a post-doctoral researcher at Chalmers, expressed surprise at these findings. The team set out to confirm the dominant trend of greater nighttime warming, but instead found a complete reversal over the past three decades.

“We initially aimed to confirm the previously observed phenomenon of nighttime warming surpassing daytime warming,” says Ziqian Zhong, post-doctoral researcher at Chalmers.

“To our surprise, not only had the asymmetric warming trend ceased, but our analyses, based on state-of-the-art observation-based datasets, indicate a complete reversal of this original warming pattern over the past three decades.”

Zhong points to “global brightening” as a likely cause of this change.

Since the late 1980s, reduced cloud cover has allowed more sunlight to reach the Earth’s surface, resulting in higher daytime temperatures and a widening temperature gap between day and night.

“A likely explanation to this change is a phenomenon called “global brightening”, which has been observed since the late 1980s,” says Ziqian Zhong.

“It is a result of less cloud cover, which causes more sunlight to reach the Earth’s surface, leading to higher daytime temperatures and, as a result, a broader difference between daytime and nighttime temperatures over the recent decades.”

Role of aerosols and environmental factors

The reasons behind changes in cloud cover remain uncertain. The interplay between cloud-free and cloudy atmospheres, along with the impact of aerosols, is complex.

Aerosols, originating from natural sources like sea spray and wildfires or human activities such as fossil fuel burning, significantly influence the environment.

Additionally, the increase in regional droughts and heatwaves points to a weakening of the Earth’s surface cooling effect, typically caused by evaporation, further contributing to the faster rise in daytime temperatures.

The study’s analysis reveals a dramatic shift in temperature patterns across land areas. From 1961 to 1990, the majority of land (81%) experienced greater nighttime warming.

However, from 1991 to 2020, this trend reversed, with 70% of land areas showing more significant daytime warming.

This shift has critical implications for agriculture, public health, and forestry management, as these sectors are deeply affected by temperature variations.

Impacts on health, agriculture, and forestry

The larger temperature difference between day and night could adversely affect crop yields, plant growth, animal well-being, and human health.

An increased temperature gap is known to exacerbate health issues, such as heart rate and blood pressure changes, potentially leading to more severe cardiovascular and respiratory diseases.

“This indicates the need to adjust strategies in different areas affected by temperature variations between day and night, such as agriculture, public health, and forestry management, to address the challenges posed by this climate change”, says Ziqian Zhong.

The impact of this temperature shift varies among tree species in different regions. In humid areas, some tree species might enhance their carbon sequestration capacity due to the increased temperature difference.

Conversely, in dry regions, higher daytime temperatures could lead to soil water deficiency, negatively affecting tree growth.

Zhong highlights the importance of future research to further explore the impacts of this reversed trend in asymmetric warming on tree growth and the carbon cycle.

“In the upcoming research, we will further investigate the impacts of this reversed trend in asymmetric warming on tree growth and carbon cycle”, says Ziqian Zhong.

Implications and future study

In summary, this important research marks a pivotal shift in our understanding of global warming dynamics, revealing a significant reversal in the pattern of daytime and nighttime temperatures.

The team’s discovery, emphasizing the greater increase in daytime warming since the 1990s, underscores the complexity of climate change and its varied impacts on the environment, human health, agriculture, and forestry.

Their findings call for an urgent reassessment of adaptation strategies across different sectors to mitigate the adverse effects of this changing climate.

By highlighting the factors contributing to this shift, such as global brightening and aerosol impacts, the study challenges previous assumptions and paves the way for further research to explore the broader implications of these changes on the planet’s ecosystems and the global carbon cycle.

The full study was published in the journal Nature Communications.

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