RESUMO
The annual cycle of extra-tropical surface air temperature is an important component of the Earth's climate system. Over the past decades, a reduced amplitude of this mode has been observed in some regions. Although attributed to anthropogenic forcing, it remains unclear when dampening of the annual cycle started. Here we use a residual series of tree-ring width and maximum latewood density from the Tibetan Plateau >4,000 m asl to reconstruct changes in temperature seasonality over the past three centuries. The new proxy evidence suggests that the onset of a decrease in summer-to-winter temperature difference over the Tibetan Plateau occurred in the 1870s. Our results imply that the influence of anthropogenic forcing on temperature seasonality might have started in the late nineteenth century, and that future human influence may further contribute to a weakening of the annual temperature cycle, with subsequent effects on ecosystem functioning and productivity.
RESUMO
Based on temperature data from 79 meteorological stations, we estimate the warming rate by season on the Tibetan Plateau (TP) during 1984-2013. The warming rate was spatially heterogeneous across seasons over the past 30 years. The northern TP (NTP) experienced more warming than the southern TP (STP) (divided near 33 °N) in all seasons. The greatest north-south difference in warming was 0.70 ± 0.11 °C for summer (June-August), while the smallest difference was 0.27 ± 0.14 °C for the cold season (November-April). Such seasonal and spatial heterogeneity in the warming rate is consistent with the seasonal precipitation patterns of the NTP and the STP. One possible cause for this phenomenon is that more precipitation occurs in the STP than in the NTP (especially for summer), accompanied by more low cloud cover, which may have slowed the warming rate. Our results imply that dry regions on the TP will possibly experience greater temperature increase than wet regions under future global warming, and this will be more prominent in summer.