Recent climatic changes and wetland expansion turned Tibet into a net CH4 source

Abstract

Methane (CH4) is the second largest contributor to the greenhouse effect. However, it remains unclear to what extent the CH4 cycle acts as a feedback to climate changes, due to insufficient observational constraints and poor knowledge of wetland extent dynamics. The Tibetan Plateau (TP), which has an average elevation of 4000+ m above sea level, contains one-third of China’s natural wetlands. Rapid climate warming (i.e., ~ 0.5 °C per decade since the 1960s) and increasing precipitation in the region have caused wetlands to dry up and then expand, especially since the 2000s. In this study, we assessed the uncertainty and temporal variation of the CH4 budget during 1979–2012 using a biogeochemical model, in situ measurements and dynamic wetland maps. The results showed that the drying up of wetlands from the 1980s to 1990s completely counteracted the rising CH4 emission rates (0.75 ± 0.18 and 0.77 ± 0.19 Tg CH4 year−1 in the 1980s and 1990s, respectively). However, recent precipitation-induced wetland expansion enhanced emissions to 0.96 ± 0.21 Tg CH4 year−1 in the 2000s, which exceeded the rate of CH4 uptake (0.74 ± 0.06 Tg CH4 year−1 in the 2000s). A nonlinear role played by wetland extent in the CH4 budget was revealed, suggesting that there is a need to incorporate wetland extent dynamics over a longer period into model simulations to understand the variation in wetland CH4 release during past decades. Furthermore, the results also indicate that more hydrological components, e.g., wetland shrinkage and expansion under increasing precipitation and glacial melt, should be taken into consideration when projecting wetland CH4 release on the TP.