Past CO₂ Changes and Tropical Sea Surface Temperatures

The relationship between past CO 2 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 CO 2 levels. Climate models predict that such increases should have been associated with warmer tropical sea surface temperatures (SSTs). However, sparse mid‐Cretaceous δ 18 O‐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 δ 18 O and that interpretation of altered latitudinal δ 18 O 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 δ 18 O records may not always record true SST patterns, as δ 18 O 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 δ 18 O. In order to more rigorously test the paleo‐CO 2 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.