Global variation in diurnal asymmetry in temperature, cloud cover, specific humidity and precipitation and its association with leaf area index

• Published in: Global change biology 2020-12, Vol.26 (12), p.7099-7111
• Main Authors: Cox, Daniel T. C., Maclean, Ilya M. D., Gardner, Alexandra S., Gaston, Kevin J.
• Format: Article
• Language: English
• Subjects: activity patterns; Asymmetry; Climate; Climate Change; climate change asymmetry; Climatic data; Cloud cover; Clouds; Darkness; Daytime; daytime warming; Diurnal; Divergence; Ecosystem; Heat stress; Heat tolerance; Hot Temperature; Humidity; Leaf area; Leaf area index; Leaves; Night; night‐time warming; Plant Leaves; Precipitation; Spatial variations; Specific humidity; Surface temperature; Temperature; Time of use; vegetation cover; Vulnerability; warming asymmetry; water cycle; Water resources
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.15336

The impacts of the changing climate on the biological world vary across latitudes, habitats and spatial scales. By contrast, the time of day at which these changes are occurring has received relatively little attention. As biologically significant organismal activities often occur at particular times of day, any asymmetry in the rate of change between the daytime and night‐time will skew the climatic pressures placed on them, and this could have profound impacts on the natural world. Here we determine global spatial variation in the difference in the mean annual rate at which near‐surface daytime maximum and night‐time minimum temperatures and mean daytime and mean night‐time cloud cover, specific humidity and precipitation have changed over land. For the years 1983–2017, we derived hourly climate data and assigned each hour as occurring during daylight or darkness. In regions that showed warming asymmetry of >0.5°C (equivalent to mean surface temperature warming during the 20th century) we investigated corresponding changes in cloud cover, specific humidity and precipitation. We then examined the proportional change in leaf area index (LAI) as one potential biological response to diel warming asymmetry. We demonstrate that where night‐time temperatures increased by >0.5°C more than daytime temperatures, cloud cover, specific humidity and precipitation increased. Conversely, where daytime temperatures increased by >0.5°C more than night‐time temperatures, cloud cover, specific humidity and precipitation decreased. Driven primarily by increased cloud cover resulting in a dampening of daytime temperatures, over twice the area of land has experienced night‐time warming by >0.25°C more than daytime warming, and has become wetter, with important consequences for plant phenology and species interactions. Conversely, greater daytime relative to night‐time warming is associated with hotter, drier conditions, increasing species vulnerability to heat stress and water budgets. This was demonstrated by a divergent response of LAI to warming asymmetry. Spatial variation in warming asymmetry across the diel cycle between 1983 and 2017. In total c. 54% of the land surface has experienced warming asymmetry of >0.25°C, with more than twice the area of land warming more during the night‐time than the daytime. Driven primarily by changing levels of cloud cover this is associated with a wetting (increased night‐time warming) and drying (increased daytime warming) of the clima