The temperature difference between day and night is widening

The rise in the global average surface temperature is one of the key characteristics of human-induced climate change, with the Copernicus Climate Change Service (C3S) recently confirming that January 2024 was the warmest January on record — extending the run of record-warm months to eight, beginning in June last year. But while night-time temperatures have typically increased at a faster pace than daytime temperatures, an international team of researchers recently reinvestigated the so-called ‘asymmetric warming’ phenomenon and found that the pattern had reversed.

The team’s study, published in the journal Nature Communications, found that 81% of the observed area of land experienced larger night-time warming from 1961 to 1990. However, in the subsequent period from 1991 to 2020, a shift occurred, with 70% of the observed land areas experiencing larger daytime warming instead. This reversed trend in asymmetric warming has led to an increasing temperature difference between day and night.

“We initially aimed to confirm the previously observed phenomenon of night-time warming surpassing daytime warming,” said Ziqian Zhong, a postdoctoral researcher at Chalmers University of Technology. “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.

“A likely explanation to this change is a phenomenon called ‘global brightening’, which has been observed since the late 1980s. 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 night-time temperatures over the recent decades.”

Global brightening may be attributed to a complex interplay between cloud-free and cloudy atmospheres, as well as the effect of aerosols in the atmosphere; these aerosols can be derived from natural processes like sea spray and wildfires, but also from human activities like fossil fuel burning. The researchers have also said the increase in regional drought events and heatwaves suggests a potential weakening of the cooling effect due to evaporation at the Earth’s surface, which would result in a faster increase in daytime temperatures.

The researchers noted that the larger temperature difference between day and night could potentially affect crop yields and plant growth. Certain tree species in humid areas might enhance their carbon sequestration capacity due to the increased temperature difference between daytime and night-time; however, this temperature difference may prove disadvantageous for trees in dry regions, as higher daytime temperatures may increase evaporation, leading to deficiency of soil water and unfavourable conditions for tree growth.

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

Worryingly, an increased temperature difference between daytime and night-time is also recognised as one of the environmental stressors that could lead to elevated heart rate and blood pressure, consequently increasing the mortality and morbidity 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,” Zhong said.

Image credit: Chalmers University of Technology | Christian Löwhagen.