Sensitivity of coastal southern African climate to changes in coastline position and associated land extent over the last glacial

Paleoclimatic changes in South Africa, especially around the southern Cape region, are of intense interdisciplinary interest; as this is an important area in the context of human evolution, hosting several prominent archaeological sites such as Blombos Cave (located near today's shoreline). In this study, we investigate the sensitivity of climate of coastal southern Africa to changes in glacial variations in sea level and associated land extent, by performing a model sensitivity experiment. Employing a high-resolution (12 km grid spacing) regional climate model (WRF), output from a global model simulation at 70 thousand years ago (ka) is downscaled twice: First using the present-day coastline, then with an estimated coastline position at 70 ka; keeping all other forcing factors identical. Thus we focus on the response of the local climate to an expanded land surface area equivalent to a glacial sea level low stand scenario. Our results reveal that the climate of previously coastal localities shows strong continental characteristics when sea level drops, as the coastline moves away, and a coastal plain is exposed. This is most evident from the year-round warmer days and cooler nights, with up to 6 °C increases (decreases) in daily maximum (minimum) temperatures. This result also extends to extremes. For instance, at the archaeological site of Blombos Cave, temperature extremes (1st and 99th percentiles) of the modelled marine climate become 25- to 75-fold more probable as the coastline shifts. We also find year-round reductions (5–40%) in the amount of precipitation within the region, owing to local modifications in near-surface atmospheric circulation caused by the exposed land. The reductions in precipitation are accompanied by a significant drop in the annual number of rainy days (31 days locally at Blombos Cave). Simulated changes also vary seasonally, with more pronounced and widespread changes in temperature and precipitation occurring during summer. Through isolating and quantifying the effects of land extent variation, our approach demonstrates, for the first time, the role of coastline position in shaping local climate around near-coastal environs. Our results have significant implications for future studies exploring the influence of local coastline changes on early human settlement and mobility patterns.