Observations from the last fifty years have shown that the nights have been warming much faster than the days. Analysis of the causes of this more rapid warming at night shows that this is likely to continue in the coming decades.

A new publication led by Nansen Center scientist Dr. Richard Davy 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. They have analysed the causes of these changes from observations and model reconstructions of the climate in the 20^th century. By using model reconstructions they were able to determine how much of this asymmetrical warming could be explained by different processes. The paper Diurnal asymmetry to the observed global warming is published in International Journal of Climatology.

Past efforts to understand the reason for this enhanced warming have focused on changes to climate processes that may have occurred at this time, such as increases in cloud cover, precipitation or soil moisture content. However, Davy and colleagues have shown that part of this more rapid warming at night is innate to the climate system, because the night-time temperatures are inherently more sensitive to the global warming that is caused by humanity’s emissions of carbon dioxide.

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 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. At night there is a much smaller volume of air that gets warmed: so if the same amount of heat is added at night and during the day, then there will be a greater warming during the night.

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.

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. This daily cycle in temperature directly affects human health since night-time temperature extremes can trigger temperature-related fatalities. But it also indirectly affects human health by controlling the growth rates of vegetation, and so affecting the length and stability of crop-growing seasons.

Reference:
Davy, R., Esau, I., Chernokulsky, A., Outten, S. and Zilitinkevich, S. (2016), Diurnal asymmetry to the observed global warming. Int. J. Climatol.. doi:10.1002/joc.4688