Nights warming faster than days, study

Global warming is affecting daytime and night-time temperatures differently.

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Global warming impacts on the biological world vary across latitudes, habitats, and spatial scales. On the other hand, the time of day at which these changes are happening has received moderately little consideration.

Scientists from the University of Exeter determined warming from 1983 to 2017. They found a significant difference in the mean annual temperature of more than 0.25°C between daytime and nighttime warming in over half of the global land surface.

Days warmed all the more rapidly in certain areas, and nights did in others – however, the complete area of excessively greater nighttime warming was more than twice as large.

Their study indicates that nighttime warming is more common than more significant daytime warming worldwide.

This “warming asymmetry” has been driven primarily by changing levels of cloud cover. Increased cloud cover cools the surface during the day and retains the warmth during the night, leading to more significant nighttime warming. Whereas decreasing cloud cover allows more warmth to reach the surface during the day, but that warmth is lost at night.

Lead author Dr. Daniel Cox, of the Environment and Sustainability Institute on Exeter’s Penryn Campus in Cornwall, said“Warming asymmetry has potentially significant implications for the natural world.”

“We demonstrate that greater nighttime warming is associated with the climate becoming wetter, and this has been shown to have important consequences for plant growth and how species, such as insects and mammals, interact.”

“Conversely, we also show that greater daytime warming is associated with drier conditions, combined with greater levels of overall warming, which increases species vulnerability to heat stress and dehydration. Species that are only active at night or during the day will be particularly affected.”

For the study, scientists modeled different rates of change of daytime maximum and nighttime minimum temperatures. They also considered mean daytime and mean nighttime cloud cover, specific humidity, and precipitation.

They observed changes in vegetation growth and precipitation over the same period and noticed that these differences occur due to rainfall.

Increased nighttime warming led to less vegetation growth where it rained more, likely due to increased cloud cover blocking the sun. Whereas, vegetation growth was limited by water availability due to less rainfall where the days warmed more.

Journal Reference:
  1. Daniel T. C. Cox et al. Global variation in diurnal asymmetry in temperature, cloud cover, specific humidity and precipitation, and its association with leaf area index. DOI: 10.1111/gcb.15336