Abrupt change in tropical Pacific climate mean state during the Little Ice Age

Abstract

The mean state of the tropical Pacific ocean-atmosphere climate, in particular its east-west asymmetry, has profound consequences for regional climates and for the El Niño/Southern Oscillation variability. Here we present a new high-resolution paleohydrological record using the stable-hydrogen-isotopic composition of terrestrial-lipid biomarkers (δDwax) from a 1,400-year-old lake sedimentary sequence from northern Philippines. Results show a dramatic and abrupt increase in δDwax values around 1630 AD with sustained high values until around 1900 AD. We interpret this change as a shift to significantly drier conditions in the western tropical Pacific during the second half of the Little Ice Age as a result of a change in tropical Pacific mean state tied to zonal sea surface temperature (SST) gradients. Our findings highlight the prominent role of abrupt shifts in zonal SST gradients on multidecadal to multicentennial timescales in shaping the tropical Pacific hydrology of the last millennium, and demonstrate that a marked transition in the tropical Pacific mean state can occur within a period of a few decades.

Long‐term ecological responses of a lowland dipterocarp forest to climate changes and nutrient availability

“…The north-eastern Luzon has a dominant rainfall season occurring during autumn and is a member of the global 'Autumn Monsoon precipitation' region (Ramesh et al, 2021). ENSO is the main driver of interannual rainfall variability at Bulusan, resulting in reduced precipitation during El Niño years and increased precipitation during La Niña years (Prohaska et al, 2023). Bulusan Lake is a 0.28 km 2 lake lying at 360 m asl on the foothills of Mt Bulusan, a stratovolcano at the southeast end of the Bicol Volcanic Arc (Delfin et al, 1993;Fig.…”

“…All proxy records in this study come from the two sedimentary sequences and are constrained by a high-resolution age-depth model (Fig. S1; Prohaska et al, 2023) based on AMS 14 C dates from terrestrial plant macrofossils (Table S1; see Methods S3 for details about the age-depth modelling).…”

“…Given the long life span (multicentennial) and generation time (multidecadal) of large rainforest trees, predicting future responses of tropical rainforests, such as lowland dipterocarp forests, to changing conditions is extremely challenging. However, distinct changes in precipitation amount, some of which linked to ENSO dynamics, along with changing fire intensity and nutrient availability, have been recorded over the past millennia (Cobb et al, 2003(Cobb et al, , 2013Conroy et al, 2008;Overpeck & Cole, 2010;Marlon et al, 2013;McLauchlan et al, 2013;Prohaska et al, 2023). Comparing palaeoclimate records with fossil pollen and other fossil proxies that capture plant responses to these changes therefore holds great potential for understanding and predicting the effects of future climate change on lowland dipterocarp forests.…”

“…We examine the response of individual dipterocarp forest plant taxa, including the dominant dipterocarps, and the overall forest vegetation cover, to a range of abiotic factors. These independent variables, representing climatic and nutrient variability in the Bulusan sequence, are reconstructed from three main sources: a recently generated record of hydrogen isotope values of leaf wax alkanes (dD wax , &) as a proxy record of precipitation variability (Prohaska et al, 2023); fossil macrocharcoal influx rates (particles cm À2 yr À1 ) as a proxy record of local fire activity (Clark, 1988;Mooney & Tinner, 2010); and stable nitrogen isotope composition of bulk sediments (sedd 15 N, &) and phosphorus concentration of bulk sediments (sedP conc , XRF counts per second; Croudace et al, 2006) as proxy records of (Prohaska et al, 2023). Data from PhilGIS (2017), map generated in QGIS v.3.10.14 (QGIS Development Team, 2020).…”

“…(c) Map showing Bulusan Lake (in blue) within the larger landscape, with Mt Bulusan volcano to the northwest (OpenStreetMap, 2023). (d) Optical and radiographic profiles of the two overlapping sedimentary sequences, BUL1 and BUL2, obtained from the Bulusan Lake (Prohaska et al, 2023). The radiograph images depict the level of organic content, with higher values shown as lighter bands.…”