High CO2 increases the freezing sensitivity of plants: implications for paleoclimate reconstructions

A common method for estimating cold month mean temperature (CMMT) in the geologic past involves applying the climatic tolerances of modern plant species to close analogues in the fossil record. This so-called 'nearest living relative' (NLR) approach has proven valuable as a cross-check on temperature predictions derived from general circulation models. Here we show that the leaves of seven modern species commonly used as NLRs to reconstruct paleo-CMMT freeze at warmer temperatures when grown at high CO₂ (800 vs. 400 ppmv). This CO₂ effect was found across all tested plant groups, including palms, taxodiaceous conifers, and two southern hemisphere species. This pervasive response indicates that many NLR-derived estimates of CMMT for times of high CO₂ should be adjusted by at least +1.5 to +3 ^oC. A revision of CMMT estimates towards warmer values further increases the disparity in continental interiors between CMMT indicators from the fossil record and the colder predictions from general circulation models.