Late Cretaceous climate simulations with different CO₂ levels and subarctic gateway configurations: A model‐data comparison

Abstract

We investigate the impact of different CO2 levels and different subarctic gateway configurations on the surface temperatures during the latest Cretaceous using the Earth System Model COSMOS. The simulated temperatures are compared with the surface temperature reconstructions based on a recent compilation of the latest Cretaceous proxies. In our numerical experiments, the CO2 level ranges from 1 to 6 times the preindustrial (PI) CO2 level of 280 ppm. On a global scale, the most reasonable match between modeling and proxy data is obtained for the experiments with 3 to 5 × PI CO2 concentrations. However, the simulated low‐ (high‐) latitude temperatures are too high (low) as compared to the proxy data. The moderate CO2 levels scenarios might be more realistic, if we take into account proxy data and the dead zone effect criterion. Furthermore, we test if the model‐data discrepancies can be caused by too simplistic proxy‐data interpretations. This is distinctly seen at high latitudes, where most proxies are biased toward summer temperatures. Additional sensitivity experiments with different ocean gateway configurations and constant CO2 level indicate only minor surface temperatures changes (<~1°C) on a global scale, with higher values (up to ~8°C) on a regional scale. These findings imply that modeled and reconstructed temperature gradients are to a large degree only qualitatively comparable, providing challenges for the interpretation of proxy data and/or model sensitivity. With respect to the latter, our results suggest that an assessment of greenhouse worlds is best constrained by temperatures in the midlatitudes.

IPSL-CM5A2. An Earth System Model designed for multi-millennial climate simulations

“…21d) This simulation, and by extant the overall response of IPSL-CM5A2 to a Cretaceous paleogeography without ice sheets and elevated CO2 levels, would need to be evaluated in more details against previous work and proxy data. Still, the basic analysis presented here demonstrates that the mean climate simulated by our model shares many similarities with that simulated by other investigations of Cretaceous warmth with coupled models (Niezgodzki et al, 2017;Tabor et al, 2016;Upchurch et al, 2015). It therefore suggests that our goal of adapting the IPSL ESM to deep-time boundary conditions has been reached with success.…”

IPSL-CM5A2 – an Earth system model designed for multi-millennial climate simulations

“…Indeed, the water denser than 27.8 kg m −3 flowing southward reaches 1.5 Sv in IPSL-CM5A2, while it was only 0.2 Sv in IPSL-CM5A. It remains largely underestimated as the observations from Olsen et al (2008) indicate around 6 Sv of overflow in total from observation-based estimates over the last few decades. Besides, the poleward shift of the westerlies in the North Atlantic stimulate a northward shift of the boundary between the Atlantic subtropical and subpolar gyres, as shown by the barotropic streamfunction anomaly (Fig.…”

Stripping back the Modern to reveal Cretaceous climate and temperature gradient underneath

“…The simulated SST latitudinal gradients range from 0.32°C/°latitude to 0.55°C/°latitude (Lunt et al, 2012;Tabor et al, 2016;Zhu et al, 2019;Fig. 11) and the atmospheric latitudinal gradients from 0.33°C/°latitude to 0.78°C/°latitude (Huber and Caballero, 2011;Lunt et al, 2012;Niezgodzki et al, 2017;Upchurch et al, 2015;Zhu et al, 2019; See Fig. 11), with the lowest latitudinal gradients being obtained for the highest pCO2 values.…”

“…The signal includes 9°C due to the fourfold increase of pCO2 or a 4.5°C increase for a doubling of pCO2 (assuming that the response is linear), which agrees with the high end of the investigations mentioned above. Whilst large, latest generation of earth system models also show an increasingly higher climate sensitivity to increased CO2 (Golaz et al, 2019;Hutchinson et al, 2018;Niezgodzki et al, 2017), suggesting that the sensitivity could have been underestimated in earlier studies. For example, the recent study of Zhu (2019), using an up-to-date parametrization of cloud microphysics in the CESM1.2 model, proposes an Eocene Climate Sensitivity of 6.6°C for a doubling of CO2 from 3 to 6 PAL.…”