Soil temperature influences land-atmosphere interaction within the Earth system, which is a primary component of "thermal regime" of the land, or a regular pattern of temperature change within the soil.

Global surface warming has slowed down during the first decade of the 21st century. Whether or not soil temperatures will respond to slower surface warming remains unclear.

"Soil temperatures normally are closely coupled with the upper surface air temperature," said Dr. ZHANG Haoxin from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences, the lead author of a study published in Advances in Atmospheric Sciences. "There are also many other factors including solar radiation, snow cover, soil memory, etc., that may alter soil temperatures."

During the recent global warming hiatus, the strongest warming slowdown throughout China occurred during winter. Observed surface air temperatures and soil temperatures at multiple depths showed consistent cooling trends at many observation stations in China.

However, in northeastern China, surface air temperature seemed less influential on soil temperature. In this region, soil data retrieved from 0 cm to a depth of 80 cm suggested a continuing warming trend despite the surface air warming hiatus. "The enhanced thermal insulation effect of the snow cover reduced the heat loss from the soils," said Dr. ZHANG.

The researchers investigated snow cover along with other direct and indirect soil temperature influences in northeastern China. They found that the increasing snow depth in northeastern China may be the main reason for the continued warming trend in soil temperatures. Dr. ZHANG further stated "This is important for agriculture."

Crocuses waiting to bloom, surrounded by snow. (Image from Pixabay) 

In addition to the thermal insulation effect of snow cover, the ability for soil to record anthropogenic changes and environmental influences, or "soil memory", is also important, especially at greater depths.

"This means the effects of various factors may be 'memorized' in deep soil temperatures, which in turn contribute to the seasonal climate prediction," said Dr. YUAN Naiming, the corresponding author of the study. "The thermal conditions from previous seasons have stronger impacts than those from the surface air temperatures or the snow cover of the considered season at deeper layers, around 160cm or deeper."