Simulating Miocene warmth: insights from an opportunistic Multi-Model
ensemble (MioMIP1)
Abstract
The Miocene epoch, spanning 23.03-5.33Ma, was a dynamic climate of
sustained, polar amplified warmth. Miocene atmospheric CO2
concentrations are typically reconstructed between 300-600ppm and were
potentially higher during the Miocene Climatic Optimum (16.75-14.5Ma).
With surface temperature reconstructions pointing to substantial
midlatitude and polar warmth, it is unclear what processes maintained
the much weaker-than-modern equator-to-pole temperature difference. Here
we synthesize several Miocene climate modeling efforts together with
available terrestrial and ocean surface temperature reconstructions. We
evaluate the range of model-data agreement, highlight robust mechanisms
operating across Miocene modelling efforts, and regions where
differences across experiments result in a large spread in warming
responses. Prescribed CO2 is the primary factor controlling global
warming across the ensemble. On average, elements other than CO2, such
as Miocene paleogeography and ice sheets, raise global mean temperature
by ~ 2℃, with the spread in warming under a given CO2
concentration (due to a combination of the spread in imposed boundary
conditions and climate feedback strengths) equivalent to
~1.2 times a CO2 doubling. This study uses an ensemble
of opportunity: models, boundary conditions, and reference datasets
represent the state-of-art for the Miocene, but are inhomogeneous and
not ideal for a formal intermodel comparison effort. Acknowledging this
caveat, this study is nevertheless the first Miocene multi-model,
multi-proxy comparison attempted so far. This study serves to take stock
of the current progress towards simulating Miocene warmth while
isolating remaining challenges that may be well served by community-led
efforts to coordinate modelling and data activities within a common
analysis framework.