The relationship between the global mean deep-sea and surface
temperature during the Early Eocene
Abstract
Our current understanding of global mean near-surface (land and sea) air
temperature (GMSAT) during the Cenozoic era relies on paleo-proxy
estimates of deep-sea temperature combined with assumed relationships
between global mean deep-sea temperature (GMDST), global mean
sea-surface temperature (GMSST), and GMSAT. The validity of these
assumptions is essential in our understanding of past climate states
such as the Early Eocene Climate Optimum hothouse climate (EECO, 56–48
Ma). The EECO remains relevant today, because EECO-like
CO2 levels are possible in the
22ndcentury under continued high CO2
emissions. We analyze the relationship between the three global
temperature indicators for the EECO using 25 different millennia-long
model simulations with varying CO2 levels from the
Deep-Time Model Intercomparison Project (DeepMIP). The model simulations
show limited spatial variability in deep-sea temperature, indicating
that local temperature estimates can be regarded representative of
GMDST. Linear regression analysis indicates that compared to GMSST, both
GMDST and GMSAT respond more strongly to changes in atmospheric
CO2 by factors of 1.18 and 1.17, respectively.
Consequently, this model-based analysis validates the assumption that
changes in GMDST can be used to estimate changes in GMSAT during the
EECO. Paleo-proxies of GMDST, GMSST, and GMSAT during EECO show the best
fit with model simulations having an atmospheric CO2
level of 1,680 ppm, which matches paleo-proxies of atmospheric
CO2 during EECO. Similar analyses of other past climate
states are needed to examine whether these results are robust throughout
the Cenozoic, providing insight into the long-term future warming under
various shared socioeconomic pathways.