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  • 1
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    Online Resource
    Mid-Pliocene El Niño/Southern Oscillation suppressed by Pacific intertropical convergence zone shift
    Springer Science and Business Media LLC ; 2022
    In:  Nature Geoscience Vol. 15, No. 9 ( 2022-09), p. 726-734
    Details
    In: Nature Geoscience, Springer Science and Business Media LLC, Vol. 15, No. 9 ( 2022-09), p. 726-734
    Type of Medium: Online Resource
    ISSN: 1752-0894 , 1752-0908
    URL: Article
    DOI: 10.1038/s41561-022-00999-y
    Language: English
    Publisher: Springer Science and Business Media LLC
    Publication Date: 2022
    detail.hit.zdb_id: 2396648-8
    detail.hit.zdb_id: 2405323-5
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  • 2
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    The Pliocene Model Intercomparison Project Phase 2: large-scale climate features and climate sensitivity
    Copernicus GmbH ; 2020
    In:  Climate of the Past Vol. 16, No. 6 ( 2020-11-04), p. 2095-2123
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 16, No. 6 ( 2020-11-04), p. 2095-2123
    Abstract: 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.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-16-2095-2020
    DOI: 10.5194/cp-16-2095-2020-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2020
    detail.hit.zdb_id: 2217985-9
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  • 3
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    Reduced El Niño variability in the mid-Pliocene according to the PlioMIP2 ensemble
    Copernicus GmbH ; 2021
    In:  Climate of the Past Vol. 17, No. 6 ( 2021-12-01), p. 2427-2450
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 17, No. 6 ( 2021-12-01), p. 2427-2450
    Abstract: Abstract. The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period during which atmospheric CO2 levels were similar to recent historical values (∼400 ppm). Several proxy reconstructions for the mid-Pliocene show highly reduced zonal sea surface temperature (SST) gradients in the tropical Pacific Ocean, indicating an El Niño-like mean state. However, past modelling studies do not show these highly reduced gradients. Efforts to understand mid-Pliocene climate dynamics have led to the Pliocene Model Intercomparison Project (PlioMIP). Results from the first phase (PlioMIP1) showed clear El Niño variability (albeit significantly reduced) and did not show the greatly reduced time-mean zonal SST gradient suggested by some of the proxies. In this work, we study El Niño–Southern Oscillation (ENSO) variability in the PlioMIP2 ensemble, which consists of additional global coupled climate models and updated boundary conditions compared to PlioMIP1. We quantify ENSO amplitude, period, spatial structure and “flavour”, as well as the tropical Pacific annual mean state in mid-Pliocene and pre-industrial simulations. Results show a reduced ENSO amplitude in the model-ensemble mean (−24 %) with respect to the pre-industrial, with 15 out of 17 individual models showing such a reduction. Furthermore, the spectral power of this variability considerably decreases in the 3–4-year band. The spatial structure of the dominant empirical orthogonal function shows no particular change in the patterns of tropical Pacific variability in the model-ensemble mean, compared to the pre-industrial. Although the time-mean zonal SST gradient in the equatorial Pacific decreases for 14 out of 17 models (0.2 ∘C reduction in the ensemble mean), there does not seem to be a correlation with the decrease in ENSO amplitude. The models showing the most “El Niño-like” mean state changes show a similar ENSO amplitude to that in the pre-industrial reference, while models showing more “La Niña-like” mean state changes generally show a large reduction in ENSO variability. The PlioMIP2 results show a reasonable agreement with both time-mean proxies indicating a reduced zonal SST gradient and reconstructions indicating a reduced, or similar, ENSO variability.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-17-2427-2021
    DOI: 10.5194/cp-17-2427-2021-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2021
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  • 4
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    On the climatic influence of CO 2 forcing in the Pliocene
    Copernicus GmbH ; 2023
    In:  Climate of the Past Vol. 19, No. 3 ( 2023-03-31), p. 747-764
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 19, No. 3 ( 2023-03-31), p. 747-764
    Abstract: Abstract. Understanding the dominant climate forcings in the Pliocene is crucial to assessing the usefulness of the Pliocene as an analogue for our warmer future. Here, we implement a novel yet simple linear factorisation method to assess the relative influence of CO2 forcing in seven models of the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) ensemble. Outputs are termed “FCO2” and show the fraction of Pliocene climate change driven by CO2. The accuracy of the FCO2 method is first assessed through comparison to an energy balance analysis previously used to assess drivers of surface air temperature in the PlioMIP1 ensemble. After this assessment, the FCO2 method is applied to achieve an understanding of the drivers of Pliocene sea surface temperature and precipitation for the first time. CO2 is found to be the most important forcing in the ensemble for Pliocene surface air temperature (global mean FCO2=0.56), sea surface temperature (global mean FCO2=0.56), and precipitation (global mean FCO2=0.51). The range between individual models is found to be consistent between these three climate variables, and the models generally show good agreement on the sign of the most important forcing. Our results provide the most spatially complete view of the drivers of Pliocene climate to date and have implications for both data–model comparison and the use of the Pliocene as an analogue for the future. That CO2 is found to be the most important forcing reinforces the Pliocene as a good palaeoclimate analogue, but the significant effect of non-CO2 forcing at a regional scale (e.g. orography and ice sheet forcing at high latitudes) reminds us that it is not perfect, and these additional influencing factors must not be overlooked. This comparison is further complicated when considering the Pliocene as a state in quasi-equilibrium with CO2 forcing compared to the transient warming being experienced at present.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-19-747-2023
    DOI: 10.5194/cp-19-747-2023-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2023
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  • 5
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    The warm winter paradox in the Pliocene northern high latitudes
    Copernicus GmbH ; 2022
    In:  Climate of the Past Vol. 18, No. 6 ( 2022-06-23), p. 1385-1405
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 18, No. 6 ( 2022-06-23), p. 1385-1405
    Abstract: Abstract. 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.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    URL: Article
    DOI: 10.5194/cp-18-1385-2022
    DOI: 10.5194/cp-18-1385-2022-supplement
    DOI: 10.5194/cp-18-1385-2022-corrigendum
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2022
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  • 6
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    Multi-variate factorisation of numerical simulations
    Copernicus GmbH ; 2021
    In:  Geoscientific Model Development Vol. 14, No. 7 ( 2021-07-08), p. 4307-4317
    Details
    In: Geoscientific Model Development, Copernicus GmbH, Vol. 14, No. 7 ( 2021-07-08), p. 4307-4317
    Abstract: Abstract. Factorisation (also known as “factor separation”) is widely used in the analysis of numerical simulations. It allows changes in properties of a system to be attributed to changes in multiple variables associated with that system. There are many possible factorisation methods; here we discuss three previously proposed factorisations that have been applied in the field of climate modelling: the linear factorisation, the Stein and Alpert (1993) factorisation, and the Lunt et al. (2012) factorisation. We show that, when more than two variables are being considered, none of these three methods possess all four properties of “uniqueness”, “symmetry”, “completeness”, and “purity”. Here, we extend each of these factorisations so that they do possess these properties for any number of variables, resulting in three factorisations – the “linear-sum” factorisation, the “shared-interaction” factorisation, and the “scaled-residual” factorisation. We show that the linear-sum factorisation and the shared-interaction factorisation reduce to be identical in the case of four or fewer variables, and we conjecture that this holds for any number of variables. We present the results of the factorisations in the context of three past studies that used the previously proposed factorisations.
    Type of Medium: Online Resource
    ISSN: 1991-9603
    URL: Article
    URL: Article
    DOI: 10.5194/gmd-14-4307-2021
    DOI: 10.5194/gmd-14-4307-2021-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2021
    detail.hit.zdb_id: 2456725-5
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  • 7
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    Evaluation of Arctic warming in mid-Pliocene climate simulations
    Copernicus GmbH ; 2020
    In:  Climate of the Past Vol. 16, No. 6 ( 2020-11-23), p. 2325-2341
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 16, No. 6 ( 2020-11-23), p. 2325-2341
    Abstract: Abstract. Palaeoclimate simulations improve our understanding of the climate, inform us about the performance of climate models in a different climate scenario, and help to identify robust features of the climate system. Here, we analyse Arctic warming in an ensemble of 16 simulations of the mid-Pliocene Warm Period (mPWP), derived from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The PlioMIP2 ensemble simulates Arctic (60–90∘ N) annual mean surface air temperature (SAT) increases of 3.7 to 11.6 ∘C compared to the pre-industrial period, with a multi-model mean (MMM) increase of 7.2 ∘C. The Arctic warming amplification ratio relative to global SAT anomalies in the ensemble ranges from 1.8 to 3.1 (MMM is 2.3). Sea ice extent anomalies range from −3.0 to -10.4×106 km2, with a MMM anomaly of -5.6×106 km2, which constitutes a decrease of 53 % compared to the pre-industrial period. The majority (11 out of 16) of models simulate summer sea-ice-free conditions (≤1×106 km2) in their mPWP simulation. The ensemble tends to underestimate SAT in the Arctic when compared to available reconstructions, although the degree of underestimation varies strongly between the simulations. The simulations with the highest Arctic SAT anomalies tend to match the proxy dataset in its current form better. The ensemble shows some agreement with reconstructions of sea ice, particularly with regard to seasonal sea ice. Large uncertainties limit the confidence that can be placed in the findings and the compatibility of the different proxy datasets. We show that while reducing uncertainties in the reconstructions could decrease the SAT data–model discord substantially, further improvements are likely to be found in enhanced boundary conditions or model physics. Lastly, we compare the Arctic warming in the mPWP to projections of future Arctic warming and find that the PlioMIP2 ensemble simulates greater Arctic amplification than CMIP5 future climate simulations and an increase instead of a decrease in Atlantic Meridional Overturning Circulation (AMOC) strength compared to pre-industrial period. The results highlight the importance of slow feedbacks in equilibrium climate simulations, and that caution must be taken when using simulations of the mPWP as an analogue for future climate change.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-16-2325-2020
    DOI: 10.5194/cp-16-2325-2020-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2020
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  • 8
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    Lessons from a high-CO〈sub〉2〈/sub〉 world: an ocean view from  ∼ 3 million years ago
    Copernicus GmbH ; 2020
    In:  Climate of the Past Vol. 16, No. 4 ( 2020-08-27), p. 1599-1615
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 16, No. 4 ( 2020-08-27), p. 1599-1615
    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.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-16-1599-2020
    DOI: 10.5194/cp-16-1599-2020-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2020
    detail.hit.zdb_id: 2217985-9
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  • 9
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    Mid-Pliocene West African Monsoon rainfall as simulated in the PlioMIP2 ensemble
    Copernicus GmbH ; 2021
    In:  Climate of the Past Vol. 17, No. 4 ( 2021-08-27), p. 1777-1794
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 17, No. 4 ( 2021-08-27), p. 1777-1794
    Abstract: Abstract. The mid-Pliocene warm period (mPWP; ∼3.2 million years ago) is seen as the most recent time period characterized by a warm climate state, with similar to modern geography and ∼400 ppmv atmospheric CO2 concentration, and is therefore often considered an interesting analogue for near-future climate projections. Paleoenvironmental reconstructions indicate higher surface temperatures, decreasing tropical deserts, and a more humid climate in West Africa characterized by a strengthened West African Monsoon (WAM). Using model results from the second phase of the Pliocene Modelling Intercomparison Project (PlioMIP2) ensemble, we analyse changes of the WAM rainfall during the mPWP by comparing them with the control simulations for the pre-industrial period. The ensemble shows a robust increase in the summer rainfall over West Africa and the Sahara region, with an average increase of 2.5 mm/d, contrasted by a rainfall decrease over the equatorial Atlantic. An anomalous warming of the Sahara and deepening of the Saharan Heat Low, seen in 〉90 % of the models, leads to a strengthening of the WAM and an increased monsoonal flow into the continent. A similar warming of the Sahara is seen in future projections using both phase 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). Though previous studies of future projections indicate a west–east drying–wetting contrast over the Sahel, PlioMIP2 simulations indicate a uniform rainfall increase in that region in warm climates characterized by increasing greenhouse gas forcing. We note that this effect will further depend on the long-term response of the vegetation to the CO2 forcing.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    DOI: 10.5194/cp-17-1777-2021
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2021
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  • 10
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    Amplified surface warming in the south-west Pacific during the mid-Pliocene (3.3–3.0 Ma) and future implications
    Copernicus GmbH ; 2023
    In:  Climate of the Past Vol. 19, No. 7 ( 2023-07-13), p. 1359-1381
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 19, No. 7 ( 2023-07-13), p. 1359-1381
    Abstract: Abstract. Based on Nationally Determined Contributions concurrent with Shared Socioeconomic Pathways (SSPs) 2-4.5, the IPCC predicts global warming of 2.1–3.5 ∘C (very likely range 10–90th percentile) by 2100 CE. However, global average temperature is a poor indicator of regional warming and global climate models (GCMs) require validation with instrumental or proxy data from geological archives to assess their ability to simulate regional ocean and atmospheric circulation, and thus, to evaluate their performance for regional climate projections. The south-west Pacific is a region that performs poorly when GCMs are evaluated against instrumental observations. The New Zealand Earth System Model (NZESM) was developed from the United Kingdom Earth System Model (UKESM) to better understand south-west Pacific response to global change, by including a nested ocean grid in the south-west Pacific with 80 % greater horizontal resolution than the global-scale host. Here, we reconstruct regional south-west Pacific sea-surface temperatures (SSTs) for the mid-Pliocene warm period (mPWP; 3.3–3.0 Ma), which has been widely considered a past analogue with an equilibrium surface temperature response of +3 ∘C to an atmospheric CO2 concentration of ∼350–400 ppm, in order to assess the warming distribution in the south-west Pacific. This study presents proxy SSTs from seven deep sea sediment cores distributed across the south-west Pacific. Our reconstructed SSTs are derived from molecular biomarkers preserved in the sediment – alkenones (i.e. U37K′ index) and isoprenoid glycerol dialkyl glycerol tetraethers (i.e. TEX86 index) – and are compared with SSTs reconstructed from the Last Interglacial (125 ka), Pliocene Model Intercomparison Project (PlioMIP) outputs and transient climate model projections (NZESM and UKESM) of low- to high-range SSPs for 2090–2099 CE. Mean interglacial equilibrium SSTs during the mPWP for the south-west Pacific sites were on average 4.2 ∘C (1.8–6.1 ∘C likely range) above pre-industrial temperatures and show good agreement with model outputs from NZESM and UKESM under mid-range SSP 2–4.6 conditions. These results highlight that not only is the mPWP an appropriate analogue when considering future temperature change in the centuries to come, but they also demonstrate that the south-west Pacific region will experience warming that exceeds that of the global mean if atmospheric CO2 remains above 350 ppm.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    DOI: 10.5194/cp-19-1359-2023
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2023
    detail.hit.zdb_id: 2217985-9
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