A comparison of the climates of the Medieval Climate Anomaly, Little Ice Age, and Current Warm Period reconstructed using coral records from the northern South China Sea

Abstract

For the global oceans, the characteristics of high‐resolution climate changes during the last millennium remain uncertain because of the limited availability of proxy data. This study reconstructs climate conditions using annually resolved coral records from the South China Sea (SCS) to provide new insights into climate change over the last millennium. The results indicate that the climate of the Medieval Climate Anomaly (MCA, AD 900–1300) was similar to that of the Current Warm Period (CWP, AD 1850‐present), which contradicts previous studies. The similar warmth levels for the MCA and CWP have also been recorded in the Makassar Strait of Indonesia, which suggests that the MCA was not warmer than the CWP in the western Pacific and that this may not have been a globally uniform change. Hydrological conditions were drier/saltier during the MCA and similar to those of the CWP. The drier/saltier MCA and CWP in the western Pacific may be associated with the reduced precipitation caused by variations in the Pacific Walker Circulation. As for the Little Ice Age (LIA, AD 1550–1850), the results from this study, together with previous data from the Makassar Strait, indicate a cold and wet period compared with the CWP and the MCA in the western Pacific. The cold LIA period agrees with the timing of the Maunder sunspot minimum and is therefore associated with low solar activity. The fresher/wetter LIA in the western Pacific may have been caused by the synchronized retreat of both the East Asian Summer Monsoon and the Australian Monsoon.

Coral Records at the Northern Edge of the Western Pacific Warm Pool Reveal Multiple Drivers of Sea Surface Temperature, Salinity, and Rainfall Variability Since the End of the Little Ice Age

“…Houbihu winter (annual) SST exhibits a range of 0.6 ± 0.2 and 0.45 ± 0.15 °C SST cooling at the end of the LIA (1850 CE, sensu stricto) relative to the twentieth century and the most recent ~30 years, respectively (Table 1). This estimate is within the various LIA cooling estimates from the northern SCS (e.g., 1.1 to 1.5 °C; T. Chen et al, 2018; Deng et al, 2017; H. Yan, Soon, & Wang, 2015); the tropical western Pacific (e.g., 0.5 to 1.5 °C; Newton et al, 2006; Oppo et al, 2009); subtropical western Pacific (e.g., 0.4 °C; W. Wu et al, 2012); and the south Pacific (e.g., 1.4 °C; Corrège et al, 2001) during the peak LIA period (~1700 CE), consistent with those from the tropical and subtropical Atlantic records (i.e., 1 to 2 °C; Goodkin et al, 2008; T. Watanabe et al, 2001). SST cooling during the LIA is currently considered a synchronous global event (e.g., Hendy et al, 2002; Rosenthal et al, 2013) linked to minimum solar irradiance and high volcanic activity (Bradley & Jones, 1993; Crowley, 2000).…”

“…Our wet season rainfall shows a consistent trend where average annual rainfall is lower by ~50 and 60 mm year −1 at the end of the LIA relative to the twentieth century and the most recent ~30 years, respectively (Table 1). The start of our records, 1788, to 1850, is past the peak period of the LIA (i.e., 1700 CE) when most marine‐based records in the northern SCS and the western Pacific exhibit fresher surface waters and/or wetter conditions relative to the late twentieth century (e.g., 1 to 1.5 psu, Deng et al, 2017; Newton et al, 2006; Oppo et al, 2009). Wetter climate during the LIA is attributed to the strengthening of the Walker Circulation (e.g., H. Yan et al, 2011) and weakening of the EASM (e.g., Deng et al, 2017; Zeng et al, 2012) affecting the hydrological climate over the northern SCS.…”

“…The start of our records, 1788, to 1850, is past the peak period of the LIA (i.e., 1700 CE) when most marine‐based records in the northern SCS and the western Pacific exhibit fresher surface waters and/or wetter conditions relative to the late twentieth century (e.g., 1 to 1.5 psu, Deng et al, 2017; Newton et al, 2006; Oppo et al, 2009). Wetter climate during the LIA is attributed to the strengthening of the Walker Circulation (e.g., H. Yan et al, 2011) and weakening of the EASM (e.g., Deng et al, 2017; Zeng et al, 2012) affecting the hydrological climate over the northern SCS. Additionally, the southward migration of the Intertropical Convergence Zone (e.g., Newton et al, 2006; Oppo et al, 2009; Sachs et al, 2009) and/or contraction of the East Asia‐Australia monsoon region due to reduced solar irradiance (e.g., Griffiths et al, 2016; H. Yan, Wei, et al, 2015) resulted in increased precipitation, and hence lower surface salinity in the tropical Western Pacific during the peak LIA period.…”

“…Temperate and high‐latitude paleoclimate records indicate cooling of about 0.5 to 5 °C relative to present (Goodkin et al, 2008; Mann et al, 1999; Overpeck et al, 1997). Extensive evidence also exists for cooling in the tropics (e.g., T. Chen et al, 2018; DeLong et al, 2012, 2013; Deng et al, 2017; Newton et al, 2006; Oppo et al, 2009; Watanabe et al, 2001; Yan, Soon, & Wang, 2015) and high‐latitude Southern Hemisphere (e.g., Bradley & Jones, 1993). Depending on location, proxy reconstructions mostly from corals, bivalves, marine sediments, and foraminifera in this region reveal a period of cooling, between 0.5 and 2 °C lower than present.…”

“…Paleoreconstructions based on networks of various climate archives are increasingly generated in various time spans to investigate large‐scale atmosphere‐ocean interactions related to the WPWP. For example, several coral‐derived climate reconstructions beyond the LIA (e.g., Calvo et al, 2007; D'Arrigo, Wilson, Palmer, et al, 2006; DeLong et al, 2012; Hendy et al, 2002; Lough, 2011) have complemented other marine and terrestrial‐derived reconstructions in the region (e.g., D'Arrigo, Wilson, Palmer, et al, 2006; Deng et al, 2017; Yan, Wei, et al, 2015) to examine the drivers of spatiotemporal changes in hydrological processes in the western Pacific. However, most of these long (i.e., >100 years) coral‐derived records are concentrated in the south equatorial, western, and central tropical Pacific regions (Tierney et al, 2015, and references therein), with sparse coverage around the northern extent of the WPWP region (e.g., T. Chen et al, 2018; Deng et al, 2013; Deng et al, 2017; Ramos et al, 2019), potentially creating a bias in the observed spatiotemporal variability.…”