Changes in the timing and duration of the near-surface soil freeze/thaw status from 1956 to 2006 across China

The near-surface soil freeze/thaw status is an important indicator of climate change. Using data from 636 meteorological stations across China, we investigated the changes in the first date, the last date, the duration, and the number of days of the near-surface soil freeze over the period 1956-2006. The results reveal that the first date of the near-surface soil freeze was delayed by about 5 days, or at a rate of 0.10 +/- 0.03 day yr(-1), and the last date was advanced by about 7 days, or at a rate of 0.15 +/- 0.02 day yr(-1). The duration of the near-surface soil freeze decreased by about 12 days or at a rate of 0.25 +/- 0.04 day yr(-1), while the actual number of the near-surface soil freeze days decreased by about 10 days or at a rate of 0.20 +/- 0.03 day yr(-1). The rates of changes in the near-surface soil freeze/thaw status increased dramatically from the early 1990s through the end of the study period. Regionally, the changes in western China were greater than those in eastern China. Changes in the near-surface soil freeze/thaw status were primarily controlled by changes in air temperature, but urbanization may also play an important role.