Modelling Maastrichtian climate: investigating the role of geography, atmospheric CO 2 and vegetation

Abstract

In this paper we describe the results from an ensemble of palaeoclimate simulations of the Maastrichtian using the fully-coupled dynamic ocean-atmosphere General Cir- culation Model, HadCM3L. Using appropriate Maastrichtian boundary conditions, we 5 investigate the sensitivity of the predicted palaeoclimate to changing atmospheric CO2 levels and modelled vegetation treatment. In addition, we explore the climatic response to the changed geography using a comparison with a pre-industrial experiment. We de- scribe our results alongside the findings of previous modelling studies in particular with consideration to concepts of climate equability. Our findings demonstrate increased 10 global temperatures compared with the pre-industrial experiment, with a 5.9◦C increase in temperatures associated with the change to 1×CO2 Maastrichtian conditions and a further 3.9◦C warming associated with a quadrupling of atmospheric CO2 levels. Com- pared to the pre-industrial we find a latitudinal temperature profile that is reduced in gra- dient and shifted to higher temperatures. Our control 4×CO2 Maastrichtian experiment 15 exceeds the pre-industrial by 6.5–8.6◦C, 7.4–11.2◦C, and 10.1–32.4◦C in the equato- rial, mid and high latitudes respectively. We also find a general pattern of increased thermal seasonality in the high latitudes. In terms of global mean annual tempera- tures we find a range of 18.1–23.6◦C for our 1–6×atmospheric CO2 envelope. Other than in the northern high latitudes we find satisfactory levels of agreement between 20 the ensemble temperature envelope and estimates from palaeotemperature proxies. The inclusion of a dynamic vegetation model (TRIFFID) leads to a further increase in the thermal seasonality at high latitudes, warming in the mid to high latitudes and increased precipitation in the low and mid latitudes.