Coral-inferred high seasonal sea surface temperature in the South China Sea at 7.4 thousand years ago

Under the background of global warming, investigating the seasonal characteristics of climate change during the middle Holocene Thermal Maximum holds critical significance for understanding the future trend. Monthly-resolved Porites coral oxygen isotope (δ¹⁸O) records and annual growth rates of three modern samples from 1962‒2006 CE and one fossil specimen dated from 7.4 ka (7486‒7407 yr BP) from the northern South China Sea (SCS) were used to quantitatively reconstruct variabilities of seasonal and annual sea surface temperatures (SSTs) in the middle Holocene. Compared with modern values, results show that the middle Holocene SSTs were 0.83 ± 0.44 (1σ) °C higher. Seasonally, all four seasons exhibited warmer conditions relative to modern SSTs, with maximum warming occurring in summer and autumn, while minimum warming occurred in winter. This finding implies that proxy reconstructions based solely on summer or autumn SST would overestimate the annual average by 10.3% and 33.8%, respectively, whereas using winter SST would underestimate it by 48.5%. Under sustained global warming, the climatic characteristics observed at 7.4 ka may potentially reemerge in future climate regimes.