The Tibetan Plateau is the highest plateau on Earth, lying some 5 km above sea level and surrounded by some of Asia’s largest mountain ranges, including the Himalayas to the South. This large elevated area plays an important role in establishing the local and global climate, both in the past as well as today.
The uplift of Tibet and the Himalayas came about because of the continental collision between the northward moving Indian plate and the stationary Eurasian plate, occurring around 50 Ma. While it is reasonable to assume therefore, that the uplift of the region has taken place during the past 50 Ma, establishing the actual rate of uplift is more problematical. Techniques to try and establish the rate of uplift are based on changes in surface temperature caused by the changing altitude – as altitude increases, temperature decreases. Two such techniques involve looking at fossil fauna, based on differences in plant morphology between hot and cold-climate species, and leaf-margin analysis of fossil leafs, based on differences between leaf-margins between hot and cold climate species. The first technique is somewhat flawed, mainly due to the fact that extinct species of fauna are based on the nearest living relative. This means no account is taken of evolutionary adaptations to changing climate conditions, which may have been made by fauna. The approach of leaf-margin analysis has shown better, although limited results, due to the difficulty of finding a combination of datable strata and well-preserved fossil leafs. Results from the only known location, in southern Tibet, indicate that by 15 Ma, southern Tibet was at its present day altitude.
The present day climate of the southern Asia region is dominated by the summer monsoon, the strength of which is determined by the pressure gradient. This was increased by the uplift of the Himalayas and Tibetan plateau. Latent heat, released from the condensation of moisture, helps to heat the air passing the leeward northern slopes of the Himalayas onto the Tibetan plateau, so that the air is both dry and warm. Studies involving the use of fossil remains of certain foraminiferan, variations in carbon isotopes from organic debris in sediments and the type of sediment originating from the Himalayas all indicate that strengthening of the summer monsoon occurred between 6-9 Ma. Over the past 50 Ma, global temperatures have markedly decreased due to decreasing levels of atmospheric CO2 concentration. To see what influence the uplift of Tibet has had on global climate, two models have been applied: the GEOCARB model and the mountain-forcing model.
The GEOCARB model assumes that global temperatures are determined by atmospheric CO2 concentrations, which are themselves determined by the volume of CO2 released from volcanic activity. In this model, mountains provide a negative feedback loop to stabilise temperature fluctuations. As CO2 released by volcanic activity causes global warming, chemical weathering rates (at mountains) increase due to the higher temperatures. The increased weathering removes CO2 from the atmosphere causing global cooling and so decreases rates of chemical weathering and helps to stabilise temperatures. The GEOCARB model predicts global cooling over the last 110 Ma due to a decrease in the amount of sea floor production supplying CO2 to the atmosphere.
The mountain-forcing model assumes that the primary control of atmospheric CO2 concentrations is chemical weathering rates. This model predicts that as the uplift of the Himalayas and Tibetan plateau led to increased rates of weathering, global cooling was forced by the long-term removal of CO2 from the atmosphere.
To see which model best describes the changes in global climate over the last 50 Ma, scientists have looked at the isotopic strontium ratios in seawater. The 87Sr/86Sr ratios of seawater have increased over the past 50 Ma. This is taken to indicate an increase in weathering rates because continental rocks have high 87Sr/86Sr ratios. This is supported by the high 87Sr/86Sr ratios of rivers that are currently eroding the Himalayas.
The increased rates of weathering together with global cooling over the past 50 Ma appear to support the mountain-forcing model. It seems clear that the uplift of Tibet has had a significant influence on global climate over the past 50 Ma and that continues to this day.