Latitudinal temperature gradients and high-latitude temperatures during the latest Cretaceous: Congruence of geologic data and climate models

Abstract

A major challenge in paleoclimatology is disagreement between data and models for periods of warm climate. Data generally indicate equable conditions and reduced latitudinal temperature gradients, while models generally produce colder conditions and steeper latitudinal gradients except when using very high CO 2 . Here we show congruence between temperature indicators and climate model output for the cool greenhouse interval of the latest Cretaceous (Maastrichtian) using a global database of terrestrial and marine indicators and fully coupled simulations with the Community Climate System Model version 3. In these simulations we explore potential roles of greenhouse gases and properties of pre-anthropogenic liquid clouds in creating warm conditions. Our model simulations successfully reproduce warm polar temperatures and the latitudinal temperature gradient without overheating the tropics. Best fits for mean annual temperature are simulations that use 6× preindustrial levels of atmospheric CO 2 , or 2× preindustrial levels of atmospheric CO 2 and liquid cloud properties that may reflect pre-anthropogenic levels of cloud condensation nuclei. The Siberian interior is problematic, but this may relate to reconstructed elevation and the presence of lakes. Data and models together indicate tropical sea-surface temperatures ~5 °C above modern, an equator-to-pole temperature difference of 25-30 °C, and a mid-latitudinal temperature gradient of ~0.4 °C per 1° latitude, similar to the Eocene. Modified liquid cloud properties allow successful simulation of Maastrichtian climate at the relatively low levels of atmospheric CO 2 indicated by proxies and carbon cycle modeling. This supports the suggestion that altered properties of liquid clouds may be an important mechanism of warming during past greenhouse intervals. GSA,

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

“…In consequence, some terrestrial proxy data from Upchurch et al . [] are located over the ocean (and vice versa) on paleogeography of Markwick and Valdes []. In order to provide a consistent model‐data comparison, we move the location of such data points from the ocean (land) to the nearest land (ocean) point.…”

“…The latter includes a selection of initial conditions (e.g., paleogeography), forcing boundary conditions (e.g., CO 2 concentrations in the atmosphere), and model parameterizations (such as cloud parameterization [ Upchurch et al . , ]).…”

The cause of Late Cretaceous cooling: A multimodel-proxy comparison

“…Even though removal of foraminifera leads to warmer tropical SST 137 reconstructions, it does not significantly change the magnitude of cooling from the CEN to the 138 Maa, which is the main focus of this study. We assign an uncertainty of ±2.9°C for planktonic 139 foraminifera based on Holocene core-top data from Crowley and Zachos (Crowley and Zachos, 140 2000) and to be consistent with the work of Upchurch (2015). 141…”

Stomatal Function across Temporal and Spatial Scales: Deep-Time Trends, Land-Atmosphere Coupling and Global Models

“…Results from these studies are revising paleo-CO 2 estimates that were once widely disparate (e.g. 200-2,800 mmol mol 21 for the Paleocene-Eocene thermal maximum; McInerney and Wing, 2011) to well-constrained values that are more compatible with global paleoclimate simulations (Meissner et al, 2014;Upchurch et al, 2015). However, current paleo-CO 2 proxy methods and paleoclimate simulations require refinement and improvement to achieve better agreement across studies and to reduce uncertainty in model outputs.…”

“…This resource has proven to be a valuable key to unlocking the history of vegetationclimate coevolution through the Phanerozoic because it has enabled reconstruction of the evolution of plant gas-exchange capacity and productivity (Beerling and Woodward, 1997;Franks and Beerling, 2009a) as well as global atmospheric CO 2 concentration (McElwain et al, 1999;Beerling and Royer, 2002;Grein et al, 2011;Franks et al, 2014;Montañez et al, 2016), the latter a critical boundary condition for simulating paleoclimates using global climate models (Kiehl and Shields, 2013;Meissner et al, 2014;Upchurch et al, 2015).…”