Lessons from a high CO2 world: an ocean view from ~3 million years ago

Abstract

<p>A range of future climate scenarios are projected for high atmospheric CO<sub>2</sub> concentrations, given uncertainties over future human actions as well as potential environmental and climatic feedbacks. The geological record offers an opportunity to understand climate system response to a range of forcings and feedbacks which operate over multiple temporal and spatial scales. Here, we examine a single interglacial during the late Pliocene (KM5c, ca. 3.205 +/- 0.01 Ma) when atmospheric CO<sub>2</sub> concentrations were higher than pre-industrial, but similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were almost identical to today, we are able to focus on equilibrium climate system response to modern and near-future CO<sub>2</sub>. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial, by ~2.3 ºC for the combined proxy data (foraminifera Mg/Ca and alkenones), or by ~3.2ºC (alkenones only). Compared to the pre-industrial, reduced meridional gradients and enhanced warming in the North Atlantic are consistently reconstructed. There is broad agreement between data and models at the global scale, with regional differences reflecting ocean circulation and/or proxy signals. An uneven distribution of proxy data in time and space does, however, add uncertainty to our anomaly calculations. The reconstructed global mean sea-surface temperature anomaly for KM5c is warmer than all but three of the PlioMIP2 model outputs, and the reconstructed North Atlantic data tend to align with the warmest KM5c model values.  Our results demonstrate that even under low CO<sub>2</sub> emission scenarios, surface ocean warming may be expected to exceed model projections, and will be accentuated in the higher latitudes.</p>

Atmospheric CO2 during the Mid-Piacenzian Warm Period and the M2 glaciation

“…To address this weakness, PlioMIP2 (phase 2) 5 part of the model evaluations 8 feeding into the 6 th Assessment Report (AR) for the Intergovernmental Panel on Climate Change IPCC AR6, will focus on the KM5c interglacial interval at ~3.205 Ma which has a close-to-modern orbital configuration 4 . Other data compilation efforts with future data-model comparisons in mind, have targeted the interval from 3.3 Ma to 3.205 Ma 1,9 because this also includes marine isotope stage M2, a prominent anomalously cold marine isotope stage (MIS) that provides a cold-to-warm transition within the overall warm background climate state of the late Pliocene (Fig. 1).…”

“…1). A major limitation of these latest efforts, however, is the lack of data on atmospheric CO 2 during these rather narrow time intervals [9][10][11] . For instance, although ~40 δ 11 B-based determinations of CO 2 are available for the 3 to 3.3 Ma window [12][13][14] , the M2 to KM5 transition remains poorly sampled and disagreement between datasets from different analytical techniques persist ( Fig.…”

Plio‐Pleistocene Continental Hydroclimate and Indian Ocean Sea Surface Temperatures at the Southeast African Margin

“…As the high CO 2 concentrations and globally warmer conditions of the mPWP are likely analogous to future climate, there have been extensive coordinated efforts to integrate proxy data and numerical modeling to improve and assess the predictive power of climate models (Haywood et al., 2016). Southeast Africa is a notable geographic gap for both marine (McClymont et al., 2020) and terrestrial (Zhao et al., 2020) paleoenvironmental data in these proxy‐model comparisons for the mPWP. The U1478 record illustrates initially cooler conditions during the mPWP (Figure 2), which may be due to changes in the Indonesian Throughflow.…”

Increased Climate Response and Earth System Sensitivity From CCSM4 to CESM2 in Mid‐Pliocene Simulations

“…A key spatial signature of the MP SST warming is the northern high latitude amplification. SST changes are muted in the tropics and are relatively small in the southern high latitudes (Dowsett et al, 2012; Haywood, Hill, et al, 2013; McClymont et al, 2020). This meridional structure is known to be challenging for CCSM4 to capture (Feng et al, 2017; Rosenbloom et al, 2013) (Figure 12).…”