Abstract
Abstract. The early Eocene, from about 56 Ma, with high atmospheric CO2 levels, offers an analogue for the response of the Earth's climate system to anthropogenic fossil fuel burning. In this study, we present an ensemble of 50 Earth system model runs with an early Eocene palaeogeography and variation in the forcing values of atmospheric CO2 and the Earth's orbital parameters. Relationships between simple summary metrics of model outputs and the forcing parameters are identified by linear modelling, providing estimates of the relative magnitudes of the effects of atmospheric CO2 and each of the orbital parameters on important climatic features, including tropical–polar temperature difference, ocean–land temperature contrast, Asian, African and South (S.) American monsoon rains, and climate sensitivity. Our results indicate that although CO2 exerts a dominant control on most of the climatic features examined in this study, the orbital parameters also strongly influence important components of the ocean–atmosphere system in a greenhouse Earth. In our ensemble, atmospheric CO2 spans the range 280–3000 ppm, and this variation accounts for over 90 % of the effects on mean air temperature, southern winter high-latitude ocean–land temperature contrast and northern winter tropical–polar temperature difference. However, the variation of precession accounts for over 80 % of the influence of the forcing parameters on the Asian and African monsoon rainfall, and obliquity variation accounts for over 65 % of the effects on winter ocean–land temperature contrast in high northern latitudes and northern summer tropical–polar temperature difference. Our results indicate a bimodal climate sensitivity, with values of 4.36 and 2.54 ∘C, dependent on low or high states of atmospheric CO2 concentration, respectively, with a threshold at approximately 1000 ppm in this model, and due to a saturated vegetation–albedo feedback. Our method gives a quantitative ranking of the influence of each of the forcing parameters on key climatic model outputs, with additional spatial information from singular value decomposition providing insights into likely physical mechanisms. The results demonstrate the importance of orbital variation as an agent of change in climates of the past, and we demonstrate that emulators derived from our modelling output can be used as rapid and efficient surrogates of the full complexity model to provide estimates of climate conditions from any set of forcing parameters.
Highlights
In the early Eocene, several episodes of global warming coincided with carbon isotope excursions (CIEs), pulses of isotopically light carbon injected into the atmosphere and oceans, and recorded in high-resolution marine and terrestrial sediments (Kennett and Stott, 1991)
We reduce the dimensionality of the model output by computing simple scalar metrics to denote key climatic features of each ensemble member, and we apply singular value decomposition (SVD) to identify the principal components (PCs) of temperature and precipitation fields in the full ensemble, for comparison with the variation in the forcing parameters
Analysis of the model results has focused on variation in surface air temperature and precipitation in both winter and summer in each hemisphere, it should be noted that our experiment has not been designed such that mean values in our ensemble output represent direct estimates of the Eocene climate mean
Summary
In the early Eocene, several episodes of global warming coincided with carbon isotope excursions (CIEs), pulses of isotopically light carbon injected into the atmosphere and oceans, and recorded in high-resolution marine and terrestrial sediments (Kennett and Stott, 1991). The greenhouse gas (GHG) sources and the duration of the onset phase of the PETM are uncertain, the relatively short timescale and global extent of the PETM strongly suggest that a large and sudden increase in GHGs in the atmosphere was the primary climatic forcing factor (Zachos et al, 2007). Since the PETM is the most recent period in Earth’s history for which estimated atmospheric GHG concentrations are similar in magnitude to those of the present day, and expected to arise from fossil fuel burning, the PETM may provide a valuable analogue for anthropogenic climate change The CIEs of the early Eocene show similar regularity in their timing to periodic changes in the Earth’s orbit around. Keery et al.: Sensitivity of the Eocene climate the Sun (Lourens et al, 2005), and the search for causal relationships between orbital cycles and Paleogene climate is an active area of research (e.g. Lauretano et al, 2015; Laurin et al, 2016; Lunt et al, 2011)
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