In recent decades, nights are becoming warmer than days around the globe and researchers have discovered the reason which is likely to continue.
Observations from the last 50 years have shown that the nights have been warming much faster than the days.
Part of this more rapid warming at night is innate to the climate system, because the night-time temperatures are inherently more sensitive to climate forcing, said the study published in the International Journal of Climatology
‘Sensitivity to the temperature rise is linked to the layer of air just above the ground which is thin at night, capturing the heat released during the day.’
The research led by Richard Davy from Nansen Environmental and Remote Sensing Centre in Norway examined the causes of the more rapid warming of nights from observations and model reconstructions of the climate in the 20th century.
The researchers found that sensitivity to warming is linked to the layer of air just above the ground which is known as the boundary-layer as it is essentially separated from the rest of the atmosphere.
At night this layer is very thin, just a few hundred meters, whereas during the day it grows up to a few kilometres. It is this cycle in the boundary-layer depth which makes the night-time temperatures more sensitive to warming than the day, the researchers explained.
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 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 study said.
The number of extremely cold nights has dropped by half during the last 50 years, in contrast to the extreme-cold days which have decreased by a quarter, the researchers pointed out.
Understanding the different sensitivity of night and day-time temperatures is crucial for our understanding of climate change and it's affect on human health.