The influence of proxy selection on global annual mean temperature reconstructions during the Common Era

The reconstruction of global annual mean temperatures made by the PAGES 2k Consortium in 2019 represents one of the most influential sequences of global climate variability over the Common Era.However,it is still not clear how the reconstruction can be influenced by the selection of reconstruction methods and the selection of proxy records with different temporal resolutions over different regions.We adopt a widely used Composite-Plus-Scale method to elucidate the effects of the selection of the proxy records on temperature reconstruction.To ensure the uniformity of data,different types of proxy records spanning the past~2000 years from the PAGES 2k proxy network were used to investigate the potential effects of proxy selection in hemispheric and global temperature reconstructions during the past two millennia.The long-term trends,spectral characteristics,and volcanic responses of the annual temperatures were studied based on the reconstructions.Our results reveal a significant cooling trend in the global annual mean temperature using both tree-ring and non-tree-ring records during the 1–1850CE period,and show that the cooling exhibits a stronger trend in the Southern Hemisphere(SH)than that in the Northern Hemisphere(NH).Yet,the long-term trends vary according to different combinations of proxy records.Different reconstructions based on different types of proxies also exhibit different features in terms of volcanic responses and spectral properties.Tree-ring-based temperature reconstructions show stronger cooling responses to tropical volcanic eruptions,while non-tree-ring-based reconstructions suggest less robust volcanic responses,which may be related to dating uncertainties and low temporal resolution of the proxies.Tree-ring width records tend to preserve a substantial proportion of high-frequency(<200 years)variability,whereas non-tree-ring proxies tend to capture a larger fraction of low-frequency(>200 years)variations.Efforts are needed to reduce uncertainties of the temperature reconstruction over the Common Era associated with the insufficient spatiotemporal coverage of the current proxy network,especially for the first millennium and for the SH and tropics,also to develop statistical methods and to improve the signal strength and constrain uncertainties in existing proxy records.

Tree-Ring Latewood Width Based July–August SPEI Reconstruction in South China since 1888 and Its Possible Connection with ENSO

Our understanding of the long-term hydroclimate variations in South China is prohibited by the shortness of me- teorological records. Paleoclimatic proxies, such as tree-rings, can be pursued to extend the meteorological records back for centuries to help us better understand hydroclimatic conditions. In this study, we reconstructed the July--Au- gust standardized precipitafio^evapotranspiration index （SPEIjut Aug） based on a newly developed 127-yr adjusted latewood width chronology from Tsuga longibracteata, South China. The chronology explained 40% of the actual SPEIjul Aug variance in the period 1953-2014. The reconstructed SPEIjuI Aug can represent large-scale July-August SPEI variations over South China, including northern Guangxi, Hunan, and Guizhou provinces. From the perspec- tive of the past 127 years, the extreme summer drought in 2013 was not unusual because more extreme drought events occurred in the first half of the 20th century. A significant 2.0-3.6-yr hydroclimatic cycle existed in the recon- struction, which indicated that the SPEIjulug might be driven by El Nifio-Southern Oscillation （ENSO）. We further checked the time-dependency of the relationship between SPEIJuI_Aug and ENSO and found that it was unstable. Their relationship was weak before the 1950s, became significant from the 1950s to early 1990s, and then dropped to be weak again and even out of phase since the early 1990s, which may be attributable to the significant westward exten- sion of the western Pacific subtropical high.

Two centuries of April-July temperature change in southeastern China and its influence on grain productivity

China is a traditional agriculture based country and one main region for crop production is southeastern China where temperature is a dominant climate variable affecting agriculture. Temperature and social disturbances both influence crop production, yet distinguishing their relative impacts is difficult due to a lack of reliable, high-resolution historical climatic records before the very recent period. Here we present the first tree-ring based warm-season temperature reconstruction for southeastern China, a core region of the East Asian monsoon, for the past 227 years. The reconstruction target was April-July mean temperature, and our model explained 60.6% of the observed temperature variance during 1953–2012.Spatial correlation analysis showed that the reconstruction is representative of April-July temperature change over most of eastern China. The reconstructed temperature series agrees well with China-scale(heavily weighted in eastern China) agricultural production index values quite well at decadal timescales.The impacts of social upheavals on food production, such as those in the period 1920–1949, were confirmed after climatic influences were excluded. Our study should help distinguish the influence of social disturbance and warm-season temperature on grain productivity in the core agricultural region of China during the past two centuries.