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.

The recent increase in the frequency of winter cold extremes has received particular attention in light of the climate's warming. Knowledge about changes in the frequency of winter cold extremes requires long-term climate data over large spatial scale. In this study, a temperature-sensitive tree-ring network consisting of 31 sampling sites collected from seven provinces in subtropical China was used to investigate the characteristics of cold-season temperature extremes during the past two centuries. The results show that the percentage of trees in a year that experienced an abnormal decrease in radial growth relative to the previous year can serve as an indicator of interannual change in January–March temperature in subtropical China. The frequency of extreme interannual decreases in cold-season temperature has increased since the 1930s. The change in cold-season temperature was significantly correlated with the intensity of the Siberian high, yet the correlation was much weaker in the period preceding the 1930s. The findings provide evidence of a frequency change in the occurrence of interannual cold-season temperature extremes in the past two centuries for subtropical China. Particularly, the pattern in the variation of cold-season temperature suggests a change in climate systems around the 1930s.

There is a close relationship between solar activity and the Earth's surface temperature, but this relationship has weakened with recent global warming. To better understand this puzzle, temperature records need to be extended, and the relationship between long-term variation in temperature and solar activity needs to be examined. In this study, we reconstruct April–September temperature variation back to 1563 using tree ring maximum late wood density (MXD) data from Balfour spruce in the southeastern Tibetan Plateau (TP). Spatial correlation analysis indicates that our reconstruction is representative of temperature variability over the large-scale TP. On the 22 year time scale, the reconstructed April–September temperature corresponds generally to solar activity over the past three centuries. Spectral analyses also indicate that the significant periodicities of ~11 years, 54 years, and 204 years observed in the MXD chronology correspond to the Schwabe cycle, the fourth harmonic of the Suess cycle, and the Suess solar cycle, respectively. However, disparities between temperature change and solar activity are identified in two periods, the 1880s–1900s and the 1980s–present. These results suggest that solar forcing is the critical driver for long-term temperature variability in the TP, but other factors may uncouple surface temperature and solar activity in some periods. One possible cause of the weak effect of solar activity on temperature during the 1880s–1900s is internal climate variability, while human-activity-induced greenhouse gas emissions have likely superseded solar forcing as the major driver of the rapid warming observed since the 1980s.

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.