Trend of surface freeze-thaw cycles and vegetation green-up date and their response to climate change on the Qinghai-Tibet Plateau

Abstract

The Qinghai-Tibet Plateau is sensitive to global climate change because of its special geographical environment. Therefore, analyzing the temporal dynamics and spatial patterns of surface freeze-thaw cycles and vegetation green-up date to determine and predict their response to global climate change is important. A dual-index algorithm is adopted to obtain long-term series of surface freeze-thaw states from passive microwave brightness temperatures observed by SMMR, SSMI, and SSMIS during 1982-2013. The vegetation green-up date during 1982-2013 is derived based on the reconstructed NDVI from the GIMMS global vegetation index dataset by the filter algorithm. On the basis of both datasets, the areal extent, timing, seasonal variations, and inter-annual trend of surface soil freeze-thaw cycles and vegetation green-up date are examined across the Qinghai-Tibet plateau via regression and correlation analyses. Most pixels on the images of the Qinghai-Tibet Plateau showed the frozen state from October 30 to April 2 of the subsequent year. The average start date of surface thawing occurred mainly from May 12 to May 27, and the average green-up date occurred from May 19 to May 29. The green-up date occurred 3.94±5.58 days after the start date of surface thawing on the average, and a significant correlation exists between them (R=0.51, P=0.003). The trends for the start date of surface thawing and green-up date underwent three stages, namely, delay, advance, and delay, from 1982 to 2013. The start date of surface thawing and the green-up date were delayed at speeds of 1.93±1.81 and 0.28±1.01 days/year from 1982 to 1987, advanced at speeds of 0.67±0.20 and 0.13±0.16 days/year from 1987 to 2006, and delayed at speeds of 0.97±0.84 and 1.04±0.52 days/year from 2006 to 2013, respectively. However, the meteorological stations operated by the China Meteorological Administration indicated that the 0 cm ground surface temperature presented a continuous increasing trend, which might be caused by the changes in other environmental factors, such as heterogeneous precipitation on the Qinghai-Tibet Plateau. At the same time, the temperature threshold of vegetation green-up date showed a continuous upward trend (R=0.72, P<0.001) as air temperature increased, which was possibly correlated to the capability of vegetation to self-adapt to climate change.