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2012, Nature
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The sustained rate of increase over the past century in the combined radiative forcing from the three well-mixed greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) is very likely unprecedented in at least the past 16 kyr. Pre-industrial variations of atmospheric greenhouse gas concentrations observed during the last 10 kyr were small compared to industrial era greenhouse gas increases, and were likely mostly due to natural processes. • It is very likely that the current atmospheric concentrations of CO2 (379 ppm) and CH4 (1,774 ppb) exceed by far the natural range of the last 650 kyr. Ice core data indicate that CO2 varied within a range of 180 to 300 ppm and CH4 within 320 to 790 ppb over this period. Over the same period, antarctic temperature and CO2 concentrations covary, indicating a close relationship between climate and the carbon cycle. • It is very likely that glacial-interglacial CO2 variations have strongly amplifi ed climate variations, but it ...
2016
Authors: von der Heydt A. S., Dijkstra H. A., van de Wal R. S. W., Caballero R., Crucifix M., Foster G. L., Huber M., Kohler P., Rohling E., Valdes P. J., Ashwin P., Bathiany S., Berends T., van Bree L. G. J., Ditlevsen P., Ghil M., Haywood A., Katzav J., Lohmann G., Lohmann J., Lucarini V., Marzocchi A., Palike H., Ruvalcaba Baroni I., Simon D., Sluijs A., Stap L. B., Tantet A., Viebahn J., Ziegler M. Over the last decade, our understanding of climate sensitivity has improved considerably. The climate system shows variability on many timescales, is subject to non-stationary forcing and it is most likely out of equilibrium with the changes in the radiative forcing. Slow and fast feedbacks complicate the interpretation of geological records as feedback strengths vary over time. In the geological past, the forcing timescales were different than at present, suggesting that the response may have behaved differently. Do these insights constrain the climate sensitivity relevant for the present day? In this paper, we review the progress made in theoretical understanding of climate sensitivity and on the estimation of climate sensitivity from proxy records. Particular focus lies on the background state dependence of feedback processes and on the impact of tipping points on the climate system. We suggest how to further use palaeo data to advance our understanding of the currently ongoing climate change.
2010, Quaternary Science Reviews
Global mean surface temperatures are rising in response to anthropogenic greenhouse gas emissions. The magnitude of this warming at equilibrium for a given radiative forcing—referred to as specific equilibrium climate sensitivity (S)—is still subject to uncertainties. We estimate global mean temperature variations and S using a 784,000-year-long field reconstruction of sea surface temperatures and a transient paleoclimate model simulation. Our results reveal that S is strongly dependent on the climate background state, with significantly larger values attained during warm phases. Using the Representative Concentration Pathway 8.5 for future greenhouse radiative forcing, we find that the range of paleo-based estimates of Earth's future warming by 2100 CE overlaps with the upper range of climate simulations conducted as part of the Coupled Model Inter-comparison Project Phase 5 (CMIP5). Furthermore, we find that within the 21st century, global mean temperatures will very likely exceed maximum levels reconstructed for the last 784,000 years. On the basis of temperature data from eight glacial cycles, our results provide an independent validation of the magnitude of current CMIP5 warming projections.
2007, Nature
2020, WHAT CONDUCTS EARTH'S TEMPERATURE VARIATIONS?
The temperature on Earth varied largely in the Pleistocene from cold glacials to interglacials of different warmths. To contribute to an understanding of the underlying causes of these changes we compile various environmental records (and model-based interpretations of some of them) in order to calculate the direct effect of various processes on Earth's radiative budget and, thus, on global annual mean surface temperature over the last 800,000 years. The importance of orbital variations, of the greenhouse gases CO 2 , CH 4 and N 2 O, of the albedo of land ice sheets, annual mean snow cover, sea ice area and vegetation, and of the radiative perturbation of mineral dust in the atmosphere are investigated. Altogether we can explain with these processes a global cooling of 3.9 AE 0.8 K in the equilibrium temperature for the Last Glacial Maximum (LGM) directly from the radiative budget using only the Planck feedback that parameterises the direct effect on the radiative balance, but neglecting other feedbacks such as water vapour, cloud cover, and lapse rate. The unaccounted feedbacks and related uncertainties would, if taken at present day feedback strengths, decrease the global temperature at the LGM by À8.0 AE 1.6 K. Increased Antarctic temperatures during the Marine Isotope Stages 5.5, 7.5, 9.3 and 11.3 are in our conceptual approach difficult to explain. If compared with other studies, such as PMIP2, this gives supporting evidence that the feedbacks themselves are not constant, but depend in their strength on the mean climate state. The best estimate and uncertainty for our reconstructed radiative forcing and LGM cooling support a present day equilibrium climate sensitivity (excluding the ice sheet and vegetation components) between 1.4 and 5.2 K, with a most likely value near 2.4 K, somewhat smaller than other methods but consistent with the consensus range of 2-4.5 K derived from other lines of evidence. Climate sensitivities above 6 K are difficult to reconcile with Last Glacial Maximum reconstructions.
2016, Nature
The Early Eocene Climate Optimum (EECO, which occurred about 51 to 53 million years ago), was the warmest interval of the past 65 million years, with mean annual surface air temperature over ten degrees Celsius warmer than during the pre-industrial period. Subsequent global cooling in the middle and late Eocene epoch, especially at high latitudes, eventually led to continental ice sheet development in Antarctica in the early Oligocene epoch (about 33.6 million years ago). However, existing estimates place atmospheric carbon dioxide (CO2) levels during the Eocene at 500-3,000 parts per million, and in the absence of tighter constraints carbon-climate interactions over this interval remain uncertain. Here we use recent analytical and methodological developments to generate a new high-fidelity record of CO2 concentrations using the boron isotope (δ(11)B) composition of well preserved planktonic foraminifera from the Tanzania Drilling Project, revising previous estimates. Although speci...
2010, Quaternary Science Reviews
2008
Bette Otto-Bliesner and Ricardo Villalba were recently voted on to the PAGES Executive Committee (EXCOM), replacing Pinxian Wang and Rick Battarbee, who rotated off the Scientific Steering Committee (SSC) at the end of 2007. In addition, an organizational change in the SSC has resulted in Heinz Wanner, formerly PAGES Swiss Director, becoming PAGES Co-Chair and now sharing the leadership with Julie Brigham-Grette.
2008, The Open Atmospheric Science Journal
2009
The temperature on Earth varied largely in the Pleistocene from cold glacials to warmer than present interglacials. To contribute to an understanding of the underlying causes of these changes we compile various environmental records (and model-based interpretations of some of them) in order to calculate the direct effect of various processes on Earth's radiative budget and, thus, on global annual
2009
This White Paper represents an outcome of" The Leverhulme Climate Symposium 2008-Earth's Climate: Past, Present and Future", convened by H. Elderfield, M. Bickle, G. Allen and E. Shuckburgh, and held at the University of Cambridge and the Royal Society, London, ...
2007, Progress in Physical Geography
2020, Earth and Planetary Science Letters
Future greenhouse warming projections conducted with coupled climate models still exhibit a substantial spread in response to a given anthropogenic greenhouse gas concentration scenario. In order to constrain this spread and to provide robust warming projections, our understanding of Earth's global-mean surface temperature response to radiative forcing (referred to as climate sensitivity) needs to be further refined. Here we estimate an averaged glacial/interglacial climate sensitivity using 25 transient Earth system model simulations of the Last Glacial Cycle and a global-mean sea surface temperature (SST) reconstruction derived from 64 globally-distributed paleo-proxies of SST. Our results document that Earth's averaged Late Pleistocene equilibrium climate sensitivity is in the order of ∼4.2 K per CO 2 doubling. Using the Representative Concentration Pathway 8.5 for future greenhouse radiative forcing, this value translates into a global-mean surface warming of ∼5.0 K by the year 2100 relative to pre-industrial levels. This estimate is in excellent agreement with the ensemble-mean projection of climate simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our uncertainty analysis reveals further that the lack of robust reconstructions of glacial aerosol forcing is a key contributor to the overall uncertainty of paleo-based estimates of climate sensitivity.
2012, Proceedings of the National Academy of Sciences
2005, Nature
2008, Quaternary Science Reviews
2014, Nature
2008, Nature Geoscience
2010, Nature Geoscience
Glaciation during the late Palaeozoic era (340–250 Myr ago) is thought to have been episodic, with multiple, often regional, ice-age intervals, each lasting less than 10 million years. Sedimentary deposits from these ice-age intervals exhibit cyclical depositional patterns, which have been attributed to orbitally driven glacial–interglacial cycles and resultant fluctuations in global sea level. Here we use a coupled general-circulation/biome/ice-sheet model to assess the conditions necessary for glacial–interglacial fluctuations. In our simulations, ice sheets appear at atmospheric pCO2 concentrations between 420 and 840 ppmv. However, we are able to simulate ice-sheet fluctuations consistent with eustasy estimates and the distribution of glacial deposits only when we include vegetation feedbacks from high-latitude ecosystem changes. We find that ice-sheet advances follow the expansion of high-latitude tundra during insolation minima, whereas ice retreat is associated with the expansion of barren land close to the edge of the ice sheets during periods of high insolation. We are unable to simulate glacial–interglacial cycles in the absence of a dynamic vegetation component. We therefore suggest that vegetation feedbacks driven by orbital insolation variations are a crucial element of glacial–interglacial cyclicity.
One hundred thousand years of ice sheet buildup came to a rapid end ∼ 25–10 thousand years before present (ka BP), when ice sheets receded quickly and multi-proxy reconstructed global mean surface temperatures rose by ∼ 3– 5 • C. It still remains unresolved whether insolation changes due to variations of earth's tilt and orbit were sufficient to terminate glacial conditions. Using a coupled three-dimensional climate–ice sheet model, we simulate the climate and Northern Hemisphere ice sheet evolution from 78 ka BP to 0 ka BP in good agreement with sea level and ice topography reconstructions. Based on this simulation and a series of deglacial sensitivity experiments with individually varying orbital parameters and prescribed CO 2 , we find that enhanced calving led to a slowdown of ice sheet growth as early as ∼ 8 ka prior to the Last Glacial Maximum (LGM). The glacial termination was then initiated by enhanced ablation due to increasing obliquity and precession, in agreement with the Milankovitch theory. However, our results also support the notion that the ∼ 100 ppmv rise of atmospheric CO 2 after ∼ 18 ka BP was a key contributor to the deglaciation. Without it, the present-day ice volume would be comparable to that of the LGM and global mean temperatures would be about 3 • C lower than today. We further demonstrate that neither orbital forcing nor rising CO 2 concentrations alone were sufficient to complete the deglaciation.
Nature
The Earth's climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 degrees C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation approximately 2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that...
2015, Geoscientific Model Development Discussions
The last deglaciation, which marked the transition between the last glacial and present interglacial periods, was punctuated by a series of rapid (centennial and decadal) climate changes. Numerical climate models are useful for investigating mechanisms that underpin the 5 events, especially now that some of the complex models can be run for multiple millennia. We have set up a Paleoclimate Modelling Intercomparison Project (PMIP) working group to coordinate efforts to run transient simulations of the last deglaciation, and to facilitate the dissemination of expertise between modellers and those engaged with reconstructing the climate of the last 21 thousand years. Here, 10 we present the design of a coordinated Core simulation over the period 21–9 thousand years before present (ka) with time varying orbital forcing, greenhouse gases, ice sheets, and other geographical changes. A choice of two ice sheet reconstructions is given, but no ice sheet or iceberg meltwater should be prescribed in the Core simulation. Additional focussed simulations will also be coordinated on an ad-hoc basis by the 15 working group, for example to investigate the effect of ice sheet and iceberg meltwater, and the uncertainty in other forcings. Some of these focussed simulations will focus on shorter durations around specific events to allow the more computationally expensive models to take part.
2010, Journal of Climate
2011, Tellus A
2010, Quaternary Science Reviews
English The cause of the fast rise in atmospheric CO2 concentration seen in ice core data during the last glacial termination is not fully understood. Most studies indicate the deep Southern ocean as a possible source of this carbon released to the atmosphere. In this study, the role of high carbon soils associated with permafrost is considered. We found that changes in summer insolation in high northern latitudes resulted in a permafrost thaw, that then released carbon to the atmosphere, causing the atmospheric CO2 to rise. This mechanism would have been active during all insolation cycles. The extent of ice sheets on the land was found to be a strong controller for the permafrost-carbon response. The deep Southern ocean still had an important role to play for atmospheric CO2 concentrations. The permafrost-carbon mechanism significantly changed the carbon cycle response to climatic changes. It also improved the match between model output and data. Français La cause de l'augmentation rapide de CO2 atmosphérique observée sur les données des carottes de glace au cours de la dernière terminaison glaciaire n'est pas entièrement comprise. La plupart des études indiquent l'océan australe profond comme une source de carbone dans l'atmosphère. Dans cette étude, le rôle des sols riches en carbone associés au pergélisol est étudier. Nous avons trouvé que les changements de l'insolation d'été dans les hautes latitudes ont abouti à un dégel du pergélisol, que puis relâché carbone dans l'atmosphère, provoquant le CO2 atmosphérique à la hausse. Ce mécanisme aurait été actif durant tous les cycles d'insolation. L'étendue des calottes glaciaires sur la terre a été trouvé comme contrôleur important pour la réponse pergélisol-carbone. L'océan australe profond avait encore un rôle important à jouer. Le mécanisme pergélisol carbone a modifié fortement la réponse du cycle du carbone. Il a également amélioré le correspondance entre la modèle et les données.
Understanding what drove Northern Hemisphere ice sheet melt during the last deglaciation (21-7 ka) can help constrain how sensitive contemporary ice sheets are to greenhouse gas (GHGs) changes. The roles of orbital forcing and GHGs in the deglaciation have previously been modelled, but not yet quantified. Here, for the first time we calculate the relative effect of these forcings on the North American deglaciation by driving a dynamical ice sheet model (GLIMMER-CISM) with a set of un-accelerated transient deglacial simulations with a full primitive equation based ocean atmosphere general circulation model (FAMOUS). We find that by 9 ka, orbital forcing has caused 50% of the deglaciation, GHG 30% and the interaction between the two 20%. Orbital forcing starts affecting the ice volume at 19 ka, 2,000 years before CO2 starts increasing in our experiments, a delay which partly controls their relative effect.
2012, Climate of the Past Discussions
2013, Climate of the Past
2008, Climate of the Past
2014, Climate of the Past Discussions
2013
Abstract Past atmospheric CO2 concentrations reconstructed from polar ice cores combined with its∆ 14 C signature as conserved in tree-rings provide important information both on the cycling of carbon as well as the production of radiocarbon (Q) in the atmosphere. The latter is modulated by changes in the strength of the magnetic field enclosed in the 5 solar wind and is a proxy for past changes in solar activity.
2014, Climate of the Past
2006, Global Biogeochemical Cycles
We present a selection of methodologies for using the palaeo-climate model component of the Coupled Model Intercomparison Project (Phase 5) (CMIP5) to attempt to constrain future climate projections using the same models. The constraints arise from measures of skill in hindcasting palaeo-climate changes from the present over three periods: the Last Glacial Maximum (LGM) (21 000 yr before present, ka), the mid-Holocene (MH) (6 ka) and the Last Millennium (LM) (850–1850 CE). The skill measures may be used to validate robust patterns of climate change across scenarios or to distinguish between models that have differing outcomes in future scenarios. We find that the multi-model ensemble of palaeo-simulations is adequate for addressing at least some of these issues. For example, selected benchmarks for the LGM and MH are correlated to the rank of future projections of precipitation/temperature or sea ice extent to indicate that models that produce the best agreement with palaeo-climate information give demonstrably different future results than the rest of the models. We also explore cases where comparisons are strongly dependent on uncertain forcing time series or show important non-stationarity, making direct inferences for the future problematic. Overall, we demonstrate that there is a strong potential for the palaeo-climate simulations to help inform the future projections and urge all the modelling groups to complete this subset of the CMIP5 runs. Published by Copernicus Publications on behalf of the European Geosciences Union. 222 G. A. Schmidt et al.: Using palaeo-climate comparisons to constrain future projections in CMIP5
2007, Climate of The Past
2015, Atmospheric Chemistry and Physics Discussions
There is evidence of ice melt, sea level rise to +5–9 m, and extreme storms in the prior interglacial period that was less than 1 °C warmer than today. Human-made climate forcing is stronger and more rapid than paleo forcings, but much can be learned by combining insights from paleoclimate, climate modeling, and on-going observations. We argue that ice sheets in contact with the ocean are vulnerable to non-linear disintegration in response to ocean warming, and we posit that ice sheet mass loss can be approximated by a doubling time up to sea level rise of at least several meters. Doubling times of 10, 20 or 40 years yield sea level rise of several meters in 50, 100 or 200 years. Paleoclimate data reveal that subsurface ocean warming causes ice shelf melt and ice sheet discharge. Our climate model exposes amplifying feedbacks in the Southern Ocean that slow Antarctic bottom water formation and increase ocean temperature near ice shelf grounding lines, while cooling the surface ocean...