An illusion of equability: Paleoclimate proxies, environmental variability, and temperature seasonality in greenhouse worlds

The Equable Climate Problem refers to long-standing disagreements between lithologic proxy-based reconstructions and climate model simulations of greenhouse climate systems. One key greenhouse climate parameter upon which proxies and models fail to agree is mean annual range in temperature (MART), which is an important component of terrestrial climate and a key control on biogeography and diversity. Proxy-based MART estimates during greenhouse periods vary significantly depending on proxy type, which has inhibited attempts to resolve the discrepancies in proxy/model MART estimates. Paleobotany-based proxies (e.g., the Climate Leaf Analysis Multivariate Program, known as CLAMP) suggest reduced MARTs relative to the modern, while geochemistry-based proxies (e.g., pedogenic carbonate clumped isotopes) suggest similar to modern MARTs. One potential explanation for these diverging proxy-based MART reconstructions is that they reflect true MART variability due to the unique environmental conditions associated with the different proxies (e.g., differences in relative humidity, soil moisture, etc.). To test this hypothesis, we employ a conditional autoregressive statistical model and global long-term land cover, topographic, and climate data to quantify the influence of different proxy formation environments on MART. We show that land cover differences, which reflect local differences in environmental conditions, have a statistically significant effect on MART magnitude. Thus, after considering location, topography, and precipitation, proxy MART differences are largely explained by differences in land cover. Cretaceous and Eocene MART results from two multi-proxy case studies are consistent with this statistical analysis. These findings suggest that the variable proxy-based MART reconstructions for past greenhouse periods reflect real variations in MART. Perhaps most importantly, this analysis suggests that at any given latitude, greenhouse period MARTs were similar in magnitude and variability to modern MARTs.