Emulation of long-term changes in global climate: application to the late Pliocene and future

Abstract

Abstract. Multi-millennial transient simulations of climate changes have a range of important applications, such as for investigating key geologic events and transitions for which high-resolution palaeoenvironmental proxy data are available, or for projecting the long-term impacts of future climate evolution on the performance of geological repositories for the disposal of radioactive wastes. However, due to the high computational requirements of current fully coupled general circulation models (GCMs), long-term simulations can generally only be performed with less complex models and/or at lower spatial resolution. In this study, we present novel longterm "continuous" projections of climate evolution based on the output from GCMs, via the use of a statistical emulator. The emulator is calibrated using ensembles of GCM simulations, which have varying orbital configurations and atmospheric CO 2 concentrations and enables a variety of investigations of long-term climate change to be conducted, which would not be possible with other modelling techniques on the same temporal and spatial scales. To illustrate the potential applications, we apply the emulator to the late Pliocene (by modelling surface air temperature -SAT), comparing its results with palaeo-proxy data for a number of global sites, and to the next 200 kyr (thousand years) (by modelling SAT and precipitation). A range of CO 2 scenarios are prescribed for each period. During the late Pliocene, we find that emulated SAT varies on an approximately precessional timescale, with evidence of increased obliquity response at times. A comparison of atmospheric CO 2 concentration for this period, estimated using the proxy sea surface temperature (SST) data from different sites and emulator results, finds that relatively similar CO 2 concentrations are estimated based on sites at lower latitudes, whereas higher-latitude sites show larger discrepancies. In our second illustrative application, spanning the next 200 kyr into the future, we find that SAT oscillations appear to be primarily influenced by obliquity for the first ∼ 120 kyr, whilst eccentricity is relatively low, after which precession plays a more dominant role. Conversely, variations in precipitation over the entire period demonstrate a strong precessional signal. Overall, we find that the emulator provides a useful and powerful tool for rapidly simulating the long-term evolution of climate, both past and future, due to its relatively high spatial resolution and relatively low computational cost. However, there are uncertainties associated with the approach used, including the inability of the emulaPublished by Copernicus Publications on behalf of the European Geosciences Union. 1540 N. S. Lord et al.: Emulation of long-term changes in global climate tor to capture deviations from a quasi-stationary response to the forcing, such as transient adjustments of the deep-ocean temperature and circulation, in addition to its limited range of fixed ice sheet configurations and its requirement for prescribed atmos...

Sensitivity of the Eocene climate to CO<sub>2</sub> and orbital variability

“…PLASIM-GENIE is to our knowledge the most sophisticated climate model that has been applied to an ensemble of Eocene simulations, but we note that increasing computing power is now enabling ensembles of simulations with moderately higher resolution models, such as HadCM3 (3.75 • × 2.5 • ) (e.g. Araya-Melo et al, 2015;Lord et al, 2017), to be run, although with some limitation in the model years in each simulation. It will never be possible to apply state-of-the-art climate models to large ensembles because, given the continual striving for the highest possible resolution, single simulations with such models will always be at the limits of what is practicable with available computing power.…”

“…An absolute astronomical solution has been computed back to 50 Ma (Laskar et al, 2011), and an absolute age of 55.53 ± 0.05 Ma has been proposed for the onset of the PETM at the start of the Eocene epoch by Westerhold et al (2012). Lourens et al (2005) noted the apparent astronomical pacing of global warming events in the late Palaeocene and early Eocene, with correlations to both the long and short periods of eccentricity. Sexton et al (2011) suggested that although the smaller hyperthermal events of the early Eocene were driven by cycles of carbon sequestration and release in the ocean, paced by the eccentricity cycles, the PETM was likely to have been driven by carbon injection from a sedimentary source.…”

“…Climate simulations with high temporal and spatial resolution can be obtained from general circulation models (GCMs), but the requirement of GCMs for powerful computers and long runtimes makes them difficult to deploy for large ensembles of model simulations and restricts their ability to investigate the large uncertainties in forcings and model parameterisations. Such ensembles are more practical with more heavily parameterised and hence more computationally efficient Earth system models of intermediate complexity (EMICs) (Weber, 2010), although we note that Araya-Melo et al (2015) and Lord et al (2017) have deployed the GCM HadCM3 in ensemble-based studies of orbital forcing effects on climates of the Pleistocene and late Pliocene, respectively.…”

PALEO-PGEM v1.0: a statistical emulator of Pliocene–Pleistocene climate

“…Ice-sheet forcing is the sea-level reconstruction of Stap et al (2017). Carbon dioxide forcing after 800 000 BP is ice core data (Luethi et al, 2008), using the Stap et al (2017) reconstruction in the earlier period.…”

“…To generate a paleoclimate time series, we therefore require time series of the boundary condition inputs e, ω, εCO 2 , and S. For the orbital parameter inputs, we applied the 5 × 10 6year calculation of Berger andLoutre (1991, 1999). We used CO 2 from Antarctic ice cores for the last 800 000 years (Luethi et al, 2008). Prior to 800 000 BP, and for the entire sea-level record, we used the CO 2 and sea-level reconstructions of Stap et al (2017).…”