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Estimates of the permafrost-climate feedback vary in magnitude and sign, partly because permafrost carbon stability in warmer-than-present conditions is not well constrained. Here we use a Plio-Pleistocene lacustrine reconstruction of mean annual air temperature (MAAT) from the Tibetan Plateau, the largest alpine permafrost region on the Earth, to constrain past and future changes in permafrost carbon storage. Clumped isotope-temperatures (Δ47-T) indicate warmer MAAT (~1.2 °C) prior to 2.7 Ma, and support a permafrost-free environment on the northern Tibetan Plateau in a warmer-than-present climate. Δ47-T indicate ~8.1 °C cooling from 2.7 Ma, coincident with Northern Hemisphere glacial intensification. Combined with climate models and global permafrost distribution, these results indicate, under conditions similar to mid-Pliocene Warm period (3.3–3.0 Ma), ~60% of alpine permafrost containing ~85 petagrams of carbon may be vulnerable to thawing compared to ~20% of circumarctic permafrost. This estimate highlights ~25% of permafrost carbon and the permafrost-climate feedback could originate in alpine areas. The stability of permafrost carbon is poorly understood. Here the authors use Plio-Pleistocene clumped isotope reconstructions from the Tibetan Plateau and climate simulation to determine that ~85 petagrams of alpine carbon is vulnerable to thawing. [ABSTRACT FROM AUTHOR]

The early Eocene (∼56–48 Myr ago) is characterized by high CO2 estimates (1,200–2,500 ppmv) and elevated global temperatures (∼10°C–16°C higher than modern). However, the response of the hydrological cycle during the early Eocene is poorly constrained, especially in regions with sparse data coverage (e.g., Africa). Here, we present a study of African hydroclimate during the early Eocene, as simulated by an ensemble of state‐of‐the‐art climate models in the Deep‐time Model Intercomparison Project (DeepMIP). A comparison between the DeepMIP pre‐industrial simulations and modern observations suggests that model biases are model‐ and geographically dependent, however, these biases are reduced in the model ensemble mean. A comparison between the Eocene simulations and the pre‐industrial suggests that there is no obvious wetting or drying trend as the CO2 increases. The results suggest that changes to the land sea mask (relative to modern) in the models may be responsible for the simulated increases in precipitation to the north of Eocene Africa. There is an increase in precipitation over equatorial and West Africa and associated drying over northern Africa as CO2 rises. There are also important dynamical changes, with evidence that anticyclonic low‐level circulation is replaced by increased south‐westerly flow at high CO2 levels. Lastly, a model‐data comparison using newly compiled quantitative climate estimates from paleobotanical proxy data suggests a marginally better fit with the reconstructions at lower levels of CO2. Plain Language Summary: Approximately 50 Myr ago, a period known as the early Eocene, atmospheric carbon dioxide levels were significantly higher than today, and were more similar to what they could be in the future, if efforts to reduce human greenhouse gas emissions are unsuccessful. However, rainfall changes during this period are less well understood, especially over data‐sparse regions such as Africa. Here, a collection of state‐of‐the‐art climate models are used to study African rainfall during this period, comparing the simulations first to present‐day African rainfall (to validate the models), second to varying levels of atmospheric carbon dioxide, and lastly to newly compiled reconstructions of early Eocene rainfall (from plant fossils). The main findings are that although the models can reproduce present‐day rainfall over Africa, and compare reasonably well with the reconstructions, there is no clear rainfall signal when atmospheric carbon dioxide is increased. Nevertheless, the combination of a different continental configuration, vegetation, topography, and atmospheric carbon dioxide leads to changing rainfall patterns, connected to temperature and low‐level wind changes. Key Points: State‐of‐the‐art climate models are used to study African hydroclimate during the early Eocene (approximately 50 Myr ago)With increasing levels of CO2, there are changes to African precipitation, due to dynamical changes such as low‐level circulationA comparison between the models and newly compiled climate estimates shows a marginally better match at lower levels of CO2 [ABSTRACT FROM AUTHOR]

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 factorisation, and the 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. [ABSTRACT FROM AUTHOR]

Factorisation 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 three properties of uniqueness, symmetry, and completeness. 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-total factorisation. We show that the linear-sum factorisation and the shared-interaction factorisation reduce to be identical. We present the results of the factorisations in the context of studies that used the previously-proposed factorisations. This reveals that only the linear-sum/shared-interaction factorisation possesses a fourth property - boundedness, and as such we recommend the use of this factorisation in applications for which these properties are desirable. [ABSTRACT FROM AUTHOR]

During the Pliocene Epoch, a stronger-than-present overturning circulation has been invoked to explain the enhanced warming in the Nordic Seas region in comparison to low to mid-latitude regions. While marine records are indicative of changes in the northward heat transport via the North Atlantic Current (NAC) during the Pliocene, the long-term terrestrial climate evolution and its driving mechanisms are poorly understood. We present the first two-million-year-long Pliocene pollen record for the Nordic Seas region from Ocean Drilling Program (ODP) Hole 642B, reflecting vegetation and climate in Arctic Norway, to assess the influence of oceanographic and atmospheric controls on Pliocene climate evolution. The vegetation record reveals a long-term cooling trend in northern Norway, which might be linked to a general decline in atmospheric CO 2 concentrations over the studied interval, and climate oscillations primarily controlled by precession (23 kyr), obliquity (54 kyr) and eccentricity (100 kyr) forcing. In addition, the record identifies four major shifts in Pliocene vegetation and climate mainly controlled by changes in northward heat transport via the NAC. Cool temperate (warmer than present) conditions prevailed between 5.03–4.30 Ma, 3.90–3.47 Ma and 3.29–3.16 Ma and boreal (similar to present) conditions predominated between 4.30–3.90 Ma, 3.47–3.29 and after 3.16 Ma. A distinct decline in sediment and pollen accumulation rates at c. 4.65 Ma is probably linked to changes in ocean currents, marine productivity and atmospheric circulation. Climate model simulations suggest that changes in the strength of the Atlantic Meridional Overturning Circulation during the Early Pliocene could have affected atmospheric circulation in the Nordic Seas region, which would have affected the direction of pollen transport from Scandinavia to ODP Hole 642B. [ABSTRACT FROM AUTHOR]

Comparing simulations of key warm periods in Earth history with contemporaneous geological proxy data is a useful approach for evaluating the ability of climate models to simulate warm, high-CO2 climates that are unprecedented in the more recent past. Here we use a global data set of confidence-assessed, proxy-based temperature estimates and biome reconstructions to assess the ability of eight models to simulate warm terrestrial climates of the Pliocene epoch. The Late Pliocene, 3.6-2.6 million years ago, is an accessible geological interval to understand climate processes of a warmer world. We show that model-predicted surface air temperatures reveal a substantial cold bias in the Northern Hemisphere. Particularly strong data-model mismatches in mean annual temperatures (up to 18 °C) exist in northern Russia. Our model sensitivity tests identify insufficient temporal constraints hampering the accurate configuration of model boundary conditions as an important factor impacting on data-model discrepancies. We conclude that to allow a more robust evaluation of the ability of present climate models to predict warm climates, future Pliocene data-model comparison studies should focus on orbitally defined time slices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: A 617 site palaeobotanical dataset for the Mid to Late Miocene is presented. This dataset is internally consistent and provides a comprehensive overview of vegetational change from 15.97 to 5.33Ma. The palaeobotanical dataset has been translated into the BIOME4 classification scheme to enable direct comparison with climate model outputs. The vegetation change throughout the Langhian, Seravallian, Tortonian and Messinian is discussed. The data shows that the Langhian, which includes the end of the Mid-Miocene Climatic Optimum, represents a world significantly warmer than today. The high northern latitudes were characterised by cool-temperate forests, the mid-latitudes by warm-temperate mixed forests, the tropics by tropical evergreen broadleaf forests and Antarctica by tundra shrub vegetation. Cool-temperate mixed forest existed during the Seravallian in the high northern latitudes, a reduction in warm-temperate mixed forests in the mid latitudes and a loss of tundra on Antarctica was noted. Tortonian vegetation distribution indicates that further cooling had occurred since the Seravallian. The major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm-temperate mixed forests. By the Messinian, this cooling trend had eliminated warm-temperate mixed forests from the western USA and Australia and had formed mid-latitude deserts. Despite the cooling trend throughout the Mid to Late Miocene, the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre-industrial conditions. The latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the Langhian would have been significantly shallower than at present and has gradually, but asymmetrically, become more modern towards the end of the Miocene. First the southern hemisphere distribution of bioclimatic zones became more modern, probably due to the climatic effects of a fully glaciated Antarctica. The northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the Miocene and slowly became more modern by the Messinian. [Copyright &y& Elsevier]