Seasonal imprint of Holocene temperature reconstruction on the Tibetan Plateau
Quantitative reconstruction of Holocene paleotemperature is crucial for understanding the evolution of the climate system in an ice-free world and assessing the position of recent global warming within the context of natural climate variability. During the Holocene (between 11.5 and 0 ka BP), the initial compilation of reconstructed global temperature records showed a steady cooling trend, conversely, the simulated Holocene temperature exhibited a general warming trend. The discrepancy between model simulations and proxy reconstructions is colloquially referred to as the “Holocene temperature conundrum”.
The Tibetan Plateau (TP), located in southwest China, is the highest plateau of the Earth and is one of the most sensitive areas to climate changes with twice as much warming as the global average for the past decades. Many efforts have been taken to reconstruct temperature changes using multiple proxies from various types of geological archives on the TP during the Holocene. However, these paleotemperature reconstructions show complex and even contrasting results and the mechanism underlying such contrasting patterns of temperature changes still remains unclear.
A scientific research group, composed of Nanjing Institute of Geography and Limnology, Nanjing University, Institute of Tibetan Plateau Research and other institutions, presents a well-dated, high resolution, quantitative ice-free-season temperature record over the past 19 ka from a small alpine lake on the southeastern TP based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) proxy. The reconstructed temperature displays a long-term ~4°C warming trend during the past 19 ka with a deglacial increase of ~3°C and Holocene increase of ~1°C.
This study reviews 16 published quantitative paleotemperature records since the Last Deglaciation, to better understand the pattern and mechanism of postglacial temperature changes on the TP.
“Results suggest that a general warming pattern without seasonal biases during the Last Deglaciation but divergent trends of seasonal temperatures during the Holocene with a gradual cooling pattern in summer temperature, an overall warming pattern in winter temperature, and a complicated annual temperature,” said ZHANG Can, first author of the study.
The inconsistence among annual temperature records is mainly attributed to the length of ice-free season by comparing the geographic background of proxy-reconstructed data. For the ice-free lakes at low altitude regions, lake water temperature covaries with local air temperature and proxy-reconstructed temperature can really reflect the annul mean temperature changes. In contrast, for the high-altitude lakes with the frozen nature, proxy-reconstructed temperature mainly records air temperature changes during the ice-free season due to the isolation between lake water temperature and atmospheric temperature during the cold season.
According the season differences of proxy-reconstructed temperature, the results reveal an overall cooling pattern for summer temperature, a warming trend for winter temperature, annual temperature, and spring-autumn temperature, and a warming-cooling-warming pattern in warming-season temperature. The reconstructed temperature gradually shifts from the “annual temperature-like” pattern to the “summer temperature-like” pattern with decreasing length of ice-free season.
“In addition, the TraCE-21ka model results show a remarkable similarity with our composite temperature records of proxy reconstructions for all seasons, and further indicate that the Holocene temperature changes are mainly controlled by the coupling of local seasonal insolation and GHGs on the TP,” ZHANG said.
Their findings were published in Earth-Science Reviews.
Paper link: https://www.sciencedirect.com/science/article/pii/S0012825222000113
Location and topographical setting of the Tibetan Plateau. (a) Map showing the location of the Cuoqia Lake (CQ Lake, red circle, No. 1) and other paleotemperature records (No. 2–16) reviewed in this study. Filled circle, triangle, and square denote lake, peatland, and ice core, respectively.
Comparison of the composite and simulated temperature records for different seasons on the TP. (a) summer temperature, (b) winter temperature, (c) annual temperature, (d) ice-free-season (March–October) temperature (TM-O), and (e) warm-season (May–September) temperature (TMJJAS). (f) Global temperature anomaly from proxy reconstruction and model-simulation. (g) Dome C ice-core record of the atmospheric CO2 concentration.
Schematic diagram showing the proxy-reconstructed temperature changes and the driving mechanisms. The length of ice-free season decreases with increasing altitude so that proxy-reconstructed temperature is increasingly skewed toward warm-season. Holocene temperature changes for different seasons are mainly controlled by the coupling of local seasonal insolation and GHGs.TS = summer temperature; TMJJAS = temperature from May to September; TM-O = temperature from March to October; TA = annual temperature; ﹢ = positive correlation; ﹣ = negative correlation.
Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences