Late Pliocene lakes and soils: a global data set for the analysis of climate feedbacks in a warmer world

The global distribution of late Pliocene soils and lakes has been reconstructed using a synthesis of geological data. These reconstructions are then used as boundary conditions for the Hadley Centre General Circulation Model (HadCM3) and the BIOME4 mechanistic vegetation model. By combining our novel soil and lake reconstructions with a fully coupled climate model we are able to explore the feedbacks of soils and lakes on the climate of the late Pliocene. Our experiments reveal regionally confined changes of local climate and vegetation in response to the new boundary conditions. The addition of late Pliocene soils has the largest influence on surface air temperatures, with notable increases in Australia, the southern part of northern Africa and in Asia. The inclusion of late Pliocene lakes increases precipitation in central Africa and at the locations of lakes in the Northern Hemisphere. When combined, the feedbacks on climate from late Pliocene lakes and soils improve the data to model fit in western North America and the southern part of northern Africa

Late Pliocene lakes and soils: a data-model comparison for the analysis of climate feedbacks in a warmer world

Based on a synthesis of geological data we have reconstructed the global distribution of Late Pliocene soils and lakes which are then used as boundary conditions in a series of model experiments using the Hadley Centre General Circulation Model (HadCM3) and the BIOME4 mechanistic vegetation model. By combining our novel soil and lake reconstructions with a fully coupled climate model we are able to explore the feedbacks of soils and lakes on the climate of the Late Pliocene. Our experiments reveal regionally confined changes of local climate and vegetation in response to the new boundary conditions. The addition of Late Pliocene soils has the largest influence on surface air temperatures, with notable increases in Australia, southern North Africa and Asia. The inclusion of Late Pliocene lakes generates a significant increase in precipitation in central Africa, as well as seasonal increases in the Northern Hemisphere. When combined, the feedbacks on climate from Late Pliocene lakes and soils improve the data to model fit in western North America and southern North Africa

Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO

Water isotope-enabled coupled atmosphere/ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ<sup>18</sup>O<sub>coral</sub>) variability arising from Sea Surface Temperature (SST) and the isotopic composition of seawater (δ<sup>18</sup>O<sub>sw</sub>). The unforced behaviour of the isotope-enabled HadCM3 Coupled General Circulation Model affirms that the extent to which inter-annual δ<sup>18</sup>O<sub>sw</sub> variability contributes to that in model δ<sup>18</sup>O<sub>coral</sub> is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ<sup>18</sup>O<sub>coral</sub> variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ<sup>18</sup>O<sub>sw</sub> variability is correlated to that in SST, meaning that local calibrations of the effective local δ<sup>18</sup>O<sub>coral</sub>–SST relationships are likely to be essential. Furthermore, the relationship between δ<sup>18</sup>O<sub>sw</sub> and SST in the central and western equatorial Pacific is non-linear, such that the interpretation of model δ<sup>18</sup>O<sub>coral</sub> in the context of a linear dependence on SST alone may lead to overestimation (by up to 20%) of the SST anomalies associated with large El-Niño events. Intra-model evaluation of a salinity-based pseudo-coral approach shows that such an approach captures the first-order features of the model δ<sup>18</sup>O<sub>sw</sub> behaviour. However, the utility of the pseudo-corals is limited by the extent of spatial variability seen within the modelled slopes of the temporal salinity–δ<sup>18</sup>O<sub>sw</sub> relationship

Sea ice feedbacks influence the isotopic signature of Greenland ice sheet elevation changes: Last interglacial HadCM3 simulations

Changes in the Greenland ice sheet (GIS) affect global sea level. Greenland stable water isotope (δ18O) records from ice cores offer information on past changes in the surface of the GIS. Here, we use the isotope-enabled Hadley Centre Coupled Model version 3 (HadCM3) climate model to simulate a set of last interglacial (LIG) idealised GIS surface elevation change scenarios focusing on GIS ice core sites. We investigate how δ18O depends on the magnitude and sign of GIS elevation change and evaluate how the response is altered by sea ice changes. We find that modifying GIS elevation induces changes in Northern Hemisphere atmospheric circulation, sea ice and precipitation patterns. These climate feedbacks lead to ice-core-averaged isotopic lapse rates of 0.49 ‰ (100 m)−1 for the lowered GIS states and 0.29 ‰ (100 m)−1 for the enlarged GIS states. This is lower than the spatially derived Greenland lapse rates of 0.62–0.72 ‰ (100 m)−1. These results thus suggest non-linearities in the isotope–elevation relationship and have consequences for the interpretation of past elevation and climate changes across Greenland. In particular, our results suggest that winter sea ice changes may significantly influence isotope–elevation gradients: winter sea ice effect can decrease (increase) modelled core-averaged isotopic lapse rate values by about −19 % (and +28 %) for the lowered (enlarged) GIS states, respectively. The largest influence of sea ice on δ18O changes is found in coastal regions like the Camp Century site

Climate and atmospheric circulation during the Early and Mid-Holocene inferred from lake-carbonate oxygen-isotope records from western Ireland

The Early to Mid-Holocene experienced marked climate change over the northern hemisphere mid-latitudes in response to changing insolation and declining ice volume. Oxygen isotopes from lake sediments provide a valuable climate proxy, encoding information regarding temperature, hydroclimate and moisture source. We present oxygen-isotope records from two lakes in western Ireland that are strongly influenced by the North Atlantic. Excellent replication between the records suggests they reflect regional, not local, influences. Carbonate oxygen-isotope values peaked at the start of the Holocene, between 11.2 and 11.1 cal ka bp, and then decreased markedly until 6 cal ka bp at both sites. Palaeoecological evidence supports only modest change in temperature or hydroclimate during this interval and we therefore explain the decrease primarily by a reduction in the oxygen-isotope composition of precipitation (δ18Oppt). We show a similar decrease in δ18O values in a forward model of carbonate isotopes between 12–11 and 6–5 cal ka bp. However, the inferred reduction in δ18Oppt between the Early and Mid-Holocene in the model is mainly linked to a decrease in the δ18O of the ocean source water from ice sheet melting whereas the lake carbonate isotope records are more consistent with changes in the transport pathway of moisture associated with atmospheric circulation change as the dominant cause

Ocean circulation drifts in multi-millennial climate simulations: the role of salinity corrections and climate feedbacks

Low-resolution, complex general circulation models (GCMs) are valuable tools for studying the Earth system on multi-millennial timescales. However, slowly evolving salinity drifts can cause large shifts in climatic and oceanic regimes over thousands of years. We test two different schemes for neutralising unforced salinity drifts in the FAMOUS GCM: surface flux correction and volumetric flux correction. Although both methods successfully maintain a steady global mean salinity, local drifts and subsequent feedbacks promote cooling (≈ 4 °C over 6000 years) and freshening (≈ 2 psu over 6000 years) in the North Atlantic Ocean, and gradual warming (≈ 0.2 °C per millennium) and salinification (≈ 0.15 psu per millennium) in the North Pacific Ocean. Changes in the surface density in these regions affect the meridional overturning circulation (MOC), such that, after several millennia, the Atlantic MOC (AMOC) is in a collapsed state, and there is a strong, deep Pacific MOC (PMOC). Furthermore, the AMOC exhibits a period of metastability, which is only identifiable with run lengths in excess of 1500 years. We also compare simulations with two different land surface schemes, demonstrating that small biases in the surface climate may cause regional salinity drifts and significant shifts in the MOC (weakening of the AMOC and the initiation then invigoration of PMOC), even when the global hydrological cycle has been forcibly closed. Although there is no specific precursor to the simulated AMOC collapse, the northwest North Pacific and northeast North Atlantic are important areas that should be closely monitored for trends arising from such biases

Modelling the enigmatic Late Pliocene Glacial Event: Marine Isotope Stage M2

The Pliocene Epoch (5.2 to 2.58Ma) has often been targeted to investigate the nature ofwarmclimates. However, climate records for the Pliocene exhibit significant variability and show intervals that apparently experienced a cooler than modern climate. Marine Isotope Stage (MIS) M2 (~3.3 Ma) is a globally recognisable cooling event that disturbs an otherwise relatively (compared to present-day) warm background climate state. It remains unclear whether this event corresponds to significant ice sheet build-up in the Northern and Southern Hemisphere. Estimates of sea level for this interval vary, and range from modern values to estimates of 65 m sea level fall with respect to present day. Here we implement plausibleM2 ice sheet configurations into a coupled atmosphere–ocean climate model to test the hypothesis that larger-than-modern ice sheet configurations may have existed at M2. Climate model results are compared with proxy climate data available for M2 to assess the plausibility of each ice sheet configuration. Whilst the outcomes of our data/model comparisons are not in all cases straight forward to interpret, there is little indication that results from model simulations in which significant ice masses have been prescribed in the Northern Hemisphere are incompatible with proxy data from the North Atlantic, Northeast Arctic Russia, North Africa and the Southern Ocean. Therefore, our model results do not preclude thepossibilityof the existenceof larger icemasses duringM2 in the Northern or SouthernHemisphere. Specifically they are not able to discount the possibility of significant icemasses in the Northern Hemisphere during the M2 event, consistent with a global sea-level fall of between 40 m and 60 m. This study highlights the general need for more focused and coordinated data generation in the future to improve the coverage and consistency in proxy records for M2, which will allow these and future M2 sensitivity tests to be interrogated further

The role of atmospheric CO2 in controlling sea surface temperature change during the Pliocene,

We present the role of CO2 forcing in controlling Late Pliocene sea surface temperature (SST) change using six models from Phase 2 of the Pliocene Model Intercomparison Project (PlioMIP2) and palaeoclimate proxy data from the PlioVAR working group. At a global scale, SST change in the Late Pliocene relative to the pre-industrial is predominantly driven by CO2 forcing in the low and mid-latitudes and non-CO2 forcing in the high latitudes. We find that CO2 is the dominant driver of SST change at the vast majority of proxy data sites assessed (17 out of 19), but the relative dominance of this forcing varies between all proxy sites, with CO2 forcing accounting for between 27 % and 82 % of the total change seen. The dearth of proxy data sites in the high latitudes means that only two sites assessed here are predominantly forced by non-CO2 forcing (such as changes to ice sheets and orography), both of which are in the North Atlantic Ocean.We extend the analysis to show the seasonal patterns of SST change and its drivers at a global scale and at a site-specific level for three chosen proxy data sites. We also present a new estimate of Late Pliocene climate sensitivity using site-specific proxy data values. This is the first assessment of site-specific drivers of SST change in the Late Pliocene and highlights the strengths of using palaeoclimate proxy data alongside model outputs to further develop our understanding of the Late Pliocene. We use the best available proxy and model data, but the sample sizes remain limited, and the confidence in our results would be improved with greater data availability

The warm winter paradox in the Pliocene northern high latitudes

Reconciling palaeodata with model simulations of the Pliocene climate is essential for understanding a world with atmospheric CO2 concentration near 400 ppmv (parts per million by volume). Both models and data indicate an amplified warming of the high latitudes during the Pliocene; however, terrestrial data suggest that Pliocene northern high-latitude temperatures were much higher than can be simulated by models. We focus on the mid-Pliocene warm period (mPWP) and show that understanding the northern high-latitude terrestrial temperatures is particularly difficult for the coldest months. Here the temperatures obtained from models and different proxies can vary by more than 20 ∘C. We refer to this mismatch as the “warm winter paradox”. Analysis suggests the warm winter paradox could be due to a number of factors including model structural uncertainty, proxy data not being strongly constrained by winter temperatures, uncertainties in data reconstruction methods, and the fact that the Pliocene northern high-latitude climate does not have a modern analogue. Refinements to model boundary conditions or proxy dating are unlikely to contribute significantly to the resolution of the warm winter paradox. For the Pliocene high-latitude terrestrial summer temperatures, models and different proxies are in good agreement. Those factors which cause uncertainty in winter temperatures are shown to be much less important for the summer. Until some of the uncertainties in winter high-latitude Pliocene temperatures can be reduced, we suggest a data–model comparison should focus on the summer. This is expected to give more meaningful and accurate results than a data–model comparison which focuses on the annual mean

Lake isotope records of the 8200-year cooling event in western Ireland: Comparison with model simulations

The early Holocene cooling, which occurred around 8200 calendar years before present, was a prominent abrupt event around the north Atlantic region. Here, we investigate the timing, duration, magnitude and regional coherence of the event as expressed in carbonate oxygen-isotope records from three lakes on northwest Europe's Atlantic margin in western Ireland, namely Loch Avolla, Loch Gealáin and Lough Corrib. An abrupt negative oxygen-isotope excursion lasted about 200 years. Comparison of records from three sites suggests that the excursion was primarily the result of a reduction of the oxygen-isotope values of precipitation, which was likely caused by lowered air temperatures, possibly coupled with a change in atmospheric circulation. Comparison of records from two of the lakes (Loch Avolla and Loch Gealáin), which have differing bathymetries, further suggests a reduction in evaporative loss of lake water during the cooling episode. Comparison of climate model experiments with lake-sediment isotope data indicates that effective moisture may have increased along this part of the northeast Atlantic seaboard during the 8200-year climatic event, as lower evaporation compensated for reduced precipitation