At the planet’s highest northern latitudes, nearly all of the fresh water is frozen. Even the water in the soil is locked away as ice, making it mostly inaccessible to plants. But just a short distance to the south, in the boreal areas of Alaska, Canada, Siberia, and Scandinavia, the landscape comes alive each year after the spring thaw.
The transition is relatively rapid, occurring over just a few weeks, and coincides with increasing sunlight and spring snowmelt. Rapid warming releases liquid water. As liquid water becomes more readily available, plant and animal activity is energized. The land greens up, and animals return to graze.
“I’m always impressed by how rapidly northern landscapes transition from frozen and dormant conditions in the winter to a rapid burst of life and activity in the spring,” said John Kimball, a scientist at the University of Montana.
The transition between frozen and thawed land is something researchers have observed for more than 30 years with satellites. Now, NASA’s Soil Moisture Active Passive (SMAP) satellite is continuing that record.
Data from SMAP’s radar instrument were used to produce this map, which shows the freeze-thaw status of areas north of 45 degrees latitude on March 5, 2015, as spring approached. Frozen land is blue; thawed land is pink. The measurement is possible because frozen water forms crystalline structures that can be detected by satellites.
Kimball and colleagues have mined 30 years of freeze-thaw patterns from the satellite record. In a paper published in 2012, the researchers showed that soils in the Northern Hemisphere thawed for as many as 7.5 days more in 2008 than they did in 1979. The change was primarily driven by an earlier start to the spring thaw and coincided with measureable warming in the region.
“This was a real eye-opener to me,” Kimball said. “We found that the earlier spring-thaw was driving widespread increases in northern growing seasons.” The start and the length of the growing season have implications for how much carbon is exchanged between the land and atmosphere.
Questions still remain. For example: How will larger areas of thaw affect carbon sources and sinks? How stable are areas of permafrost with continued global warming? But scientists are making progress. Freeze-thaw monitoring, according to Kimball, made a major advance thanks to the development of well-calibrated, long-term satellite soil moisture records. As those observations continue, and as they encompass more of the planet, it stands to reason that our understanding of the entire water cycle will improve.
Read more in our feature story: A Little Bit of Water, A Lot of Impact.
NASA Earth Observatory map by Joshua Stevens, using data courtesy of JPL and the SMAP science team. Caption by Kathryn Hansen.