The impacts of Tibetan uplift on palaeoclimate proxies DANIEL J. LUNT 1,2 *, RACHEL FLECKER 1 & PETER D. CLIFT 3 1 BRIDGE, School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK 2 British Antarctic Survey, Geological Sciences Division, High Cross, Madingley Road, Cambridge CB3 0ET, UK 3 School of Geosciences, Meston Building, King’s College, University of Aberdeen, Aberdeen AB24 3UE, UK *Corresponding author (e-mail: d.j.lunt@bristol.ac.uk) Abstract: Several palaeoclimate proxy records have been interpreted as representing the direct effects of Tibetan uplift on climate, and particularly the intensity of the Asian summer monsoon. However, there are other possible causes for the transitions and changes which have been observed, such as varying greenhouse gas concentrations, nodes or extremes in orbital forcing, and changing continental configurations. In this study we model the direct effects of Tibetan uplift on sea surface temperatures (SSTs), vegetation, and river discharge. We investi- gate whether these climatic effects of topographic uplift are likely to be detectable in proxy records, and also whether the proxies could be used to distinguish between different paradigms for the history of plateau uplift. We find that the SSTs in the western Pacific, South China Sea and Indian Ocean are generally insensitive to Tibetan uplift; however, vegetation in the region of the plateau itself, and river discharge from the Yangtze, Pearl, and in particular the Ganges/ Brahmaputra, could provide a good test of our understanding of Tibetan uplift history. A key contributor to the characteristics of the East Asian Monsoon (EAM) is the seasonal temperature range of the Tibetan Plateau (e.g. Kutzbach et al. 1989). As a result, the long-term evolving intensity of the EAM is thought to be intrinsically linked to the nature and timing of the tectonic uplift of Tibet (An et al. 2001; Kitoh 2004). However, the wide range of proxy records used to monitor the EAM (e.g. loess, marine, estuarine and lake sedi- ments, ice cores, speleothems and tree rings) are often contradictory, so that estimates of monsoon timing and intensity (both Quaternary and pre- Quaternary) vary considerably (e.g. Raymo & Ruddiman 1992; An et al. 2001; Thamban et al. 2002; Chen et al. 2003; Clift et al. 2004; Liu et al. 2004). An alternative approach initially devel- oped by Hahn and Manabe (1975) is to use well designed experiments with numerical models to evaluate the direct impact of Tibetan uplift on the Asian Monsoon system (e.g. Prell & Kutzbach 1992). In this paper we document the results of simulating Tibetan uplift, and examine two different uplift histories for Tibet. Specifically, focusing on SST, vegetation, and run-off, we consider whether locations previously used to generate proxy records for evaluating past monsoonal varia- bility are those most sensitive to changes induced by Tibetan uplift. In general, previous modelling studies have used very similar methodology: typically, a series of simulations have been carried out with linearly increasing topographic height, either in the region of the Tibetan Plateau alone (e.g. Liu & Yin 2002) or globally (e.g. Kitoh 2004). This experimental design is useful for understanding the mechanisms by which orography influences monsoon systems, but is at odds with more recent data that support earlier models that the Tibetan Plateau likely uplifted with orogeny occurring first in southern Tibet (England & Searle 1986), and spreading northwards and eastwards with time (Clark et al. 2005; Rowley & Garzione 2007). As such, some of the conclusions that such modelling studies reached, for example, that the onset of the monsoon occurred when the Plateau reached 50% of its current height (Kitoh 2004), are possibly misleading because this particular configuration may never actually have existed. The study of Ramstein et al. (1997) simulated more realistic palaeogeographies, but used prescribed rather than computed SSTs, and, as a result, missed the important amplifica- tion of the tectonic-driven climate response (Kitoh 2004), particularly via western Pacific SST changes. Indeed, most previous studies have been carried out with prescribed SSTs (e.g. An et al. 2001; Liu & Yin 2002). Those which have utilized From:Clift, P. D., Tada, R. & Zheng, H. (eds) Monsoon Evolution and Tectonics –Climate Linkage in Asia. Geological Society, London, Special Publications, 342, 279–291. DOI: 10.1144/SP342.16 0305-8719/10/$15.00 # The Geological Society of London 2010.