Researchers from Chalmers University of Technology in Sweden have made a significant discovery regarding global warming dynamics. It has long been understood that global temperature rises are not consistent throughout the day and night, with greater nighttime warming being observed. However, a recent study has revealed a shift in these dynamics, with greater daytime warming taking place since the 1990s. This change means that the temperature difference between day and night is widening, which could have far-reaching effects on all life on Earth.
The rise in global average surface temperature is a key characteristic of human-induced climate change. However, it is important to note that the temperature increase is not uniform throughout the day and night. In fact, nighttime temperatures have increased at a faster pace than daytime temperatures in the latter half of the twentieth century. This phenomenon, known as “asymmetric warming,” is believed to be caused by a combination of human activities and natural occurrences.
In a new study published in Nature Communications, an international team of researchers revisited the concept of asymmetric warming and made a surprising discovery. Between 1961 and 2020, global daytime warming has accelerated, while the rate of nighttime temperature increase has remained relatively constant. This reversal in the trend of asymmetric warming has led to a widening temperature difference between day and night.
“We initially set out to confirm the previously observed phenomenon of nighttime warming surpassing daytime warming. To our surprise, not only had the asymmetric warming trend ceased, but our analysis, based on state-of-the-art observation-based datasets, indicates a complete reversal of this original warming pattern over the past three decades,” says Ziqian Zhong, a post-doctoral researcher at Chalmers.
The researchers propose that a phenomenon called “global brightening” may explain this change. Global brightening, observed since the late 1980s, is a result of reduced cloud cover, allowing more sunlight to reach the Earth’s surface. This increased sunlight leads to higher daytime temperatures and a greater difference between daytime and nighttime temperatures in recent decades.
The reasons behind the changes in cloud cover are still uncertain. Global brightening may be attributed to a complex interplay between cloud-free and cloudy atmospheres, as well as the presence of small particles in the atmosphere known as aerosols. These aerosols can come from natural processes like sea spray and wildfires, as well as human activities such as fossil fuel burning. They can have a significant impact on various aspects of the environment.
Aside from global brightening, the researchers suggest that another factor contributing to the reversal in asymmetric warming is the increase in regional drought events and heatwaves. These events indicate a potential weakening of the cooling effect caused by evaporation on the Earth’s surface. As a result, daytime temperatures increase at a faster rate.
The study found that from 1961 to 1990, nighttime warming was greater in 81% of the land areas studied. However, in the subsequent period from 1991 to 2020, there was a shift, with 70% of the observed land areas experiencing greater daytime warming instead.
The larger temperature difference between day and night could have significant effects on crop yields, plant growth, animal well-being, and human health. For example, an increased temperature difference is considered an environmental stressor that can elevate heart rate and blood pressure, increasing the risk of 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.
Certain tree species in humid areas may benefit from the increased temperature difference between daytime and nighttime, as it enhances their capacity for carbon sequestration. However, in dry regions, the increased temperature difference may have a negative impact on trees. Higher daytime temperatures can lead to increased evaporation, resulting in a shortage of soil water and unfavorable conditions for tree growth.
“In future research, we will further investigate the impacts of this reversed trend in asymmetric warming on tree growth and the carbon cycle,” says Ziqian Zhong.
