Differences in polar amplifications between present day and Cretaceous with changes in atmospheric CO2 concentration and orbital parameters

Polar amplification is the phenomenon that external radiative forcing produces a larger change in surface temperature at high latitudes than the global average, which is one of the most robust in climate changes with historical and future increases in atmospheric CO₂. Also, it is known that, polar amplifications occurred during past warm periods due to atmospheric CO₂ concentrations and orbital parameters different from those in the present day. The Cretaceous is known as one of the warmest periods in the Phanerozoic (Foster et al., 2017). The Cretaceous proxy data indicate remarkable temperature amplifications in the high-latitude and polar region (e.g., Jenkyns et al. 2004), resulting in small equator-to-polar temperature difference. Many previous studies using global climate models have invested the relationship between the polar temperatures and the greenhouse gasses, geography, vegetation, and cloud property in order to elucidate the mechanism (e.g., Otto-Bliesner and Upchurch 1997; Upchurch et al., 2015; Niezgodzki et al., 2017). On the other hand, it is not well understood that differences in polar amplifications between present day and Cretaceous with changes in atmospheric CO₂ concentration and orbital parameters. In this study, we systematically investigated the responses of the polar temperatures in the present-day Cretaceous to changes in atmospheric CO₂ concentration and the orbital parameters using an atmospheric-ocean-vegetation fully coupled model MIROC4m-LPJ. Our Cretaceous simulations succeeded in reproducing the polar temperature amplification at that time by considering variations in atmospheric CO₂ concentration and orbital parameters. Furthermore, it was clarified that, due to the differences in geographical conditions between the modern and the Cretaceous, the temperature of the polar regions responded more sensitively to external radiative forcing such as changes in atmospheric CO₂ concentration and orbital parameters in the Cretaceous than in the present-day.