Nonspecific low back pain should not be viewed as a homogenous condition. Outcomes can be improved when subgrouping is used to guide treatment decision-making.

Abstract. The Pliocene Model Intercomparison Project (PlioMIP) is a co-ordinated international climate modelling initiative to study and understand climate and environments of the Late Pliocene, as well as their potential relevance in the context of future climate change. PlioMIP examines the consistency of model predictions in simulating Pliocene climate and their ability to reproduce climate signals preserved by geological climate archives. Here we provide a description of the aim and objectives of the next phase of the model intercomparison project (PlioMIP Phase 2), and we present the experimental design and boundary conditions that will be utilized for climate model experiments in Phase 2. Following on from PlioMIP Phase 1, Phase 2 will continue to be a mechanism for sampling structural uncertainty within climate models. However, Phase 1 demonstrated the requirement to better understand boundary condition uncertainties as well as uncertainty in the methodologies used for data–model comparison. Therefore, our strategy for Phase 2 is to utilize state-of-the-art boundary conditions that have emerged over the last 5 years. These include a new palaeogeographic reconstruction, detailing ocean bathymetry and land–ice surface topography. The ice surface topography is built upon the lessons learned from offline ice sheet modelling studies. Land surface cover has been enhanced by recent additions of Pliocene soils and lakes. Atmospheric reconstructions of palaeo-CO2 are emerging on orbital timescales, and these are also incorporated into PlioMIP Phase 2. New records of surface and sea surface temperature change are being produced that will be more temporally consistent with the boundary conditions and forcings used within models. Finally we have designed a suite of prioritized experiments that tackle issues surrounding the basic understanding of the Pliocene and its relevance in the context of future climate change in a discrete way.

Abstract. The Pliocene epoch has great potential to improve our understanding of the long-term climatic and environmental consequences of an atmospheric CO2 concentration near ∼400 parts per million by volume. Here we present the large-scale features of Pliocene climate as simulated by a new ensemble of climate models of varying complexity and spatial resolution based on new reconstructions of boundary conditions (the Pliocene Model Intercomparison Project Phase 2; PlioMIP2). As a global annual average, modelled surface air temperatures increase by between 1.7 and 5.2 ∘C relative to the pre-industrial era with a multi-model mean value of 3.2 ∘C. Annual mean total precipitation rates increase by 7 % (range: 2 %–13 %). On average, surface air temperature (SAT) increases by 4.3 ∘C over land and 2.8 ∘C over the oceans. There is a clear pattern of polar amplification with warming polewards of 60∘ N and 60∘ S exceeding the global mean warming by a factor of 2.3. In the Atlantic and Pacific oceans, meridional temperature gradients are reduced, while tropical zonal gradients remain largely unchanged. There is a statistically significant relationship between a model's climate response associated with a doubling in CO2 (equilibrium climate sensitivity; ECS) and its simulated Pliocene surface temperature response. The mean ensemble Earth system response to a doubling of CO2 (including ice sheet feedbacks) is 67 % greater than ECS; this is larger than the increase of 47 % obtained from the PlioMIP1 ensemble. Proxy-derived estimates of Pliocene sea surface temperatures are used to assess model estimates of ECS and give an ECS range of 2.6–4.8 ∘C. This result is in general accord with the ECS range presented by previous Intergovernmental Panel on Climate Change (IPCC) Assessment Reports.

A series of transient global warming events 1,2 occurred during the late Palaeocene and early Eocene, about 59 to 50 million years ago. The events, although variable in magnitude, were apparently paced by orbital cycles 2-4 and linked to massive perturbations of the global carbon cycle 5,6. However, a causal link between orbital changes in insolation and the carbon cycle has yet to be established for this time period. Here we present a series of coupled climate model simulations that demonstrate that orbitally induced changes in ocean circulation and intermediate water temperature can trigger the destabilization of methane hydrates. We then use a simple threshold model to show that progressive global warming over millions of years, in combination with the increasing tendency of the ocean to remain in a more stagnant state, can explain the decreasing magnitude and increasing frequency of hyperthermal events throughout the early Eocene. Our work shows that nonlinear interactions between climate and the carbon cycle can modulate the effect of orbital variations, in this case producing transient global warming events with varying timing and magnitude. From the late Palaeocene to the early Eocene (∼59-50 Myr), Earth's surface and oceans went through an interval of progressive warming, culminating in the early Eocene climatic optimum (EECO, ∼51 Myr; ref. 6; Fig. 1). Superimposed on this gradual warming trend are a series of 'hyperthermal' events-geologically abrupt (<10 kyr) warmings of Earth's surface and deep ocean, the most prominent of which being the Palaeocene-Eocene Thermal Maximum (PETM, ∼56 Myr; ref. 6). Two subsequent smaller events, ETM2 (∼54 Myr; ref. 2), and ETM3 (∼53 Myr; ref. 7) seem to share similar characteristics. Associated with the hyperthermals are large negative carbon-isotope excursions of surficial carbon reservoirs and dissolution of deep-sea carbonate (Fig. 1; refs. 8), consistent with massive injections of 13 Cdepleted carbon into the ocean-atmosphere system. Proposed sources for this carbon include methane hydrates 5 , terrestrial peat deposits 9 , and thermogenic methane 10. Understanding the causes of hyperthermals is important, not least because the amount of the carbon release and magnitude of global warming can be used to estimate climate sensitivity. The hyperthermals also reflect possible threshold events ('tipping points'), with slow changes in boundary conditions giving rise to rapid positive feedback and state transitions in the carbon-climate system 11-13. Cyclostratigraphic evidence from complete marine sections suggest that the PETM, ETM2, and ETM3 events all initiated on maxima in the 100 kyr eccentricity cycle 2-4. This has led to the suggestion that orbital pacing controlled the timing of carbon injection 2,4 and hence the occurrence of the hyperthermal events.

The characteristics of the mid-Pliocene warm period (mPWP: 3.264–3.025 Ma BP) have been examined using geological proxies and climate models. While there is agreement between models and data, details of regional climate differ. Uncertainties in prescribed forcings and in proxy data limit the utility of the interval to understand the dynamics of a warmer than present climate or evaluate models. This uncertainty comes, in part, from the reconstruction of a time slab rather than a time slice, where forcings required by climate models can be more adequately constrained. Here, we describe the rationale and approach for identifying a time slice(s) for Pliocene environmental reconstruction. A time slice centred on 3.205 Ma BP (3.204–3.207 Ma BP) has been identified as a priority for investigation. It is a warm interval characterized by a negative benthic oxygen isotope excursion (0.21–0.23‰) centred on marine isotope stage KM5c (KM5.3). It occurred during a period of orbital forcing that was very similar to present day. Climate model simulations indicate that proxy temperature estimates are unlikely to be significantly affected by orbital forcing for at least a precession cycle centred on the time slice, with the North Atlantic potentially being an important exception.