Why nights are getting warmer faster than days

Night-time temperatures are more sensitive to climate change, a new study has found which explains why the nights have been warming much faster than the days over the last 50 years around the globe.
Analysis of the causes of this more rapid warming at night shows this is likely to continue in the coming decades, said researchers.
The study led by Richard Davy from Bjerknes Centre for Climate Research in Norway has examined the causes of the more rapid warming at night compared to the day, which has been seen around the globe in recent decades.
Researchers analysed the causes of these changes from observations and model reconstructions of the climate in the 20th century.

They showed that part of this more rapid warming at night is innate to the climate system, because night-time temperatures are inherently more sensitive to climate forcing.
The layer of air just above the ground is known as the boundary-layer, and it is essentially separated from the rest of the atmosphere.

At night this layer is very thin, just a few hundred metres, whereas during the day it grows up to a few kilometres.
It is this cycle in the boundary-layer depth which makes night-time temperatures more sensitive to warming than the day, researchers said.
The build-up of carbon dioxide in the atmosphere from human emissions reduces the amount of radiation released into space, which increases both the night-time and the day-time temperatures.

However, because at night there is a much smaller volume of air that gets warmed, the extra energy added to the climate system from carbon dioxide leads to a greater warming at night than during the day.
This higher sensitivity of night-time temperatures has also affected the number of cold-extreme nights we have seen in recent years.
The number of extremely cold nights has dropped by half during the last fifty years, in contrast to the extreme-cold days which have decreased by a quarter.
This daily cycle in temperature directly affects human health since night-time temperature extremes can trigger temperature-related fatalities, researchers said.

However, it also indirectly affects human health by controlling the growth rates of vegetation, and so affecting the length and stability of crop-growing seasons.
The findings by Davy and colleagues shows that a correct simulation of the nighttime boundary layer depth is the most important component to getting the right temperature change at night.
"An improvement of the boundary-layer physics in climate models would very likely reduce our uncertainty in projections of temperature change," Davy said.
The study was published in the International Journal of Climatology.