Past CO2 Changes and Tropical Sea Surface Temperatures

Abstract

The relationship between past CO2 variations and tropical climate change on time scales of millions of years is reviewed. Explanations for Cretaceous and early Cenozoic climate change often involve higher atmospheric CO2 levels. Climate models predict that such increases should have been associated with warmer tropical sea surface temperatures (SSTs). However, sparse mid‐Cretaceous δ18O‐based SST estimates from low‐latitude calcareous plankton are ∼ 1.0–1.5‰ heavier than predicted. Initial evaluations indicate that changes in salinity may not alleviate model‐data discrepancies for calcareous δ18O and that interpretation of altered latitudinal δ18O gradients in terms of increased poleward heat transport due to warm, saline bottom water (WSBW) formation would require very large levels of WSBW production (at least 60 Sv). Calcareous δ18O records may not always record true SST patterns, as δ18O analyses from phosphatic groups, which are less sensitive to diagenesis, indicate SSTs ∼5°C higher. This latter conclusion is supported by a number of other fossil groups, which indicate temperatures warmer than those estimated from calcareous δ18O. In order to more rigorously test the paleo‐CO2 hypothesis, and indeed the models used to forecast future climates, it is desirable to intensify investigations of past climate change in low latitudes in order to determine whether these preliminary conclusions stand the test of further scrutiny.