Seasonally frozen ground across the former Soviet Republic extended 31.9 cm shallower in winter of 2000 than in the 1930s. That’s according to a comprehensive assessment of soil thermal conditions in the Eurasian high latitudes, which also found that the trend for decreasing freeze depth began in the late 1960s and appeared to stop in the early 1990s.
“We fully expected to find accelerated and amplified changes in these seasonally frozen ground regions, as is the case for virtually all other cryospheric variables in recent decades,” Oliver Frauenfeld of Texas A&M University, US, toldenvironmentalresearchweb. “Observing this slowdown in the response of Northern soils despite continued climate warming is a significant finding in itself.”
According to Frauenfeld, much of the observed warming and cryospheric change from the 1970s to 1990s, including changes in seasonal freeze depths, can be explained by a prolonged and anomalous positive mode of the North Atlantic Oscillation (NAO).
“However, most other cryospheric variables have continued their decline despite the absence of strong NAO forcing from the mid-1990s onward,” he said. “Seasonally frozen ground apparently has not. It is important to note that while seasonal freeze depth has not continued to decrease, it has not returned to its pre-1970 levels.”
Together with Tingjun Zhang of the University of Colorado at Boulder, US, and Lanzhou University, China, Frauenfeld found that the freezing index – a cumulative total of the number of degrees that temperatures fell below freezing each day – was the most significant factor linked to seasonal freeze depth, followed by annual air temperature and snow depth.
“Our approach of analysing the variability of seasonal freeze depth was motivated by the fact that climate change has primarily affected and occurred during the cold season,” said Frauenfeld. “The maximum depth of frost penetration is a unique indicator of the magnitude and length of the cold season, and therefore gauges climate change in high-latitude land areas.”
The pair used 71 years’ worth of data from 387 sites on seasonally frozen ground. “Thanks to US National Science Foundation-funded data rescue efforts with Russian collaborators beginning in the mid-1990s, we were able to assemble this exceptional database of soil temperature observations across the former Soviet Union,” said Frauenfeld. “Some of these observations date back as far as the 1880s and allow us to characterize changes in frozen ground across a region that is traditionally very data sparse.”
Frauenfeld believes that less freeze, and the associated smaller amplitude and duration of the freeze–thaw cycle in winter, could actually reduce damage to utilities such as water and sewer pipes, building foundations, roads, airports and utility poles.
But there may be other effects. “Despite the apparent slowdown of cold-season changes in Eurasian soils in recent years, high-latitude land areas are significantly warmer now than they were 20 to 30 years ago,” he said. “As northern soils remain unfrozen later into the cold season, this seasonal redistribution of energy may result in significant land-atmosphere interactions.”
The team is now quantifying these aspects of changing seasonality and energy redistribution in the Eurasian high latitudes, as well as looking to expand the soil thermal regime database.
“Current station records are only available to us through 2000,” said Frauenfeld. “While we have a sense of what the more recent changes may be from our research, obtaining the actual updated station records is vital.” The pair will also look to expand their database with additional station records from various archives in Russia, so that they can more accurately characterize the spatial variability of soil thermal regime changes across Eurasia.
The scientists reported their work in Environmental Research Letters (ERL).