Abstract
Abstract. A range of future climate scenarios are projected for high atmospheric CO2 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 CO2 exceeded pre-industrial concentrations, but were 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 CO2. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial values, by ∼2.3 ∘C for the combined proxy data (foraminifera Mg∕Ca and alkenones), or by ∼3.2–3.4 ∘C (alkenones only). Compared to the pre-industrial period, 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-CO2 emission scenarios, surface ocean warming may be expected to exceed model projections and will be accentuated in the higher latitudes.
Highlights
By the end of this century, projected atmospheric CO2 concentrations range from 430 to > 1000 ppmv depending upon future emission scenarios (IPCC, 2014a)
Relative to the pre-industrial period, the combined UK37 and Mg/Ca proxy data, using the original calibrations, indicate a KM5c global mean sea-surface temperature (SST) anomaly of +2.3 ◦C and a meridional SST gradient reduced by 2.6 ◦C (Fig. 3)
If only the UK37 data are used, the global mean SST anomaly from proxies is higher than all but three of the PlioMIP2 models, and the UK37 meridional gradient calculations are smaller than all models (BAYSPLINE) or one model
Summary
By the end of this century, projected atmospheric CO2 concentrations range from 430 to > 1000 ppmv depending upon future emission scenarios (IPCC, 2014a). The geological record affords an opportunity to explore key global and regional climate responses to different atmospheric CO2 concentrations, including those which extend beyond centennial timescales (Fischer et al, 2018). Palaeoclimate models indicate that climates last experienced during the mid-Piacenzian stage of the Pliocene (3.1–3.3 Ma) will be surpassed by 2030 CE under high-emission scenarios By integrating multiple warm peaks within the 3.1–3.3 Ma mid-Piacenzian data synthesis windows (Fig. 1), regional and time-transgressive responses to orbital forcing (Prescott et al, 2014; Fischer et al, 2018; Hoffman et al, 2017; Feng et al, 2017) are potentially recorded in the proxy data, which may not align with the more narrowly defined time interval being modelled (Haywood et al, 2013; Dowsett et al, 2016)
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