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A reduction in dust over North Africa during the mid-Holocene “Green Sahara” period could have amplified precipitation, helping reconcile climate model simulations with paleo-precipitation reconstructions. Here we test this using general circulation model simulations including interactive dust. We calculate a dust-precipitation amplification factor using three different dust configurations to evaluate the sensitivity to dust optical properties and particle size range. The resultant amplification ranges from −20% to 50%. With more absorbing dust properties, there is a large negative net radiative effect and hence a larger impact on the hydrological cycle. With the inclusion of particles greater than 1 μm in radius, the precipitation amplification is reversed. Based on the simulations which best match observed Saharan dust properties, we conclude that there was a limited enhancement of precipitation due to reduced dust during the mid-Holocene, meaning other aspects of the climate system should be the focus of future research.

During the Quaternary, the Sahara desert was periodically colonized by vegetation, likely because of orbitally induced rainfall increases. However, the estimated hydrological change is not reproduced in climate model simulations, undermining confidence in projections of future rainfall. We evaluated the relationship between the qualitative information on past vegetation coverage and climate for the mid-Holocene using three different dynamic vegetation models. Compared with two available vegetation reconstructions, the models require 500–800 mm of rainfall over 20°–25°N, which is significantly larger than inferred from pollen but largely in agreement with more recent leaf wax biomarker reconstructions. The magnitude of the response also suggests that required rainfall regime of the early to middle Holocene is far from being correctly represented in general circulation models. However, intermodel differences related to moisture stress parameterizations, biases in simulated present-day vegetation, and uncertainties about paleosoil distributions introduce uncertainties, and these are also relevant to Earth system model simulations of African humid periods.

Elucidating the source(s) of Meltwater Pulse 1a, the largest rapid sea level rise caused by ice melt (14-18 m in less than 340 years, 14,600 years ago), is important for understanding mechanisms of rapid ice melt and the links with abrupt climate change. Here, we quantify how much and by what mechanisms the North American ice sheet could have contributed to Meltwater Pulse 1a, by driving an ice sheet model with two transient climate simulations of the last 21,000 years. Ice sheet perturbed physics ensembles were run to account for model uncertainties, constraining ice extent and volume with reconstructions of 21,000 years ago to present. We determine that the North American ice sheet produced 3-4 m global mean sea level rise in 340 years due to the abrupt Bolling warming, but this response is amplified to 5-6 m when it triggers the ice sheet saddle collapse.

The observed rise in atmospheric methane (CH4) from 375 ppbv during the Last Glacial Maximum (LGM: 21,000 years ago) to 680 ppbv during the late preindustrial era is not well understood. Atmospheric chemistry considerations implicate an increase in CH4 sources, but process‐based estimates fail to reproduce the required amplitude. CH4 stable isotopes provide complementary information that can help constrain the underlying causes of the increase. We combine Earth System model simulations of the late preindustrial and LGM CH4 cycles, including process‐based estimates of the isotopic discrimination of vegetation, in a box model of atmospheric CH4 and its isotopes. Using a Bayesian approach, we show how model‐based constraints and ice core observations may be combined in a consistent probabilistic framework. The resultant posterior distributions point to a strong reduction in wetland and other biogenic CH4 emissions during the LGM, with a modest increase in the geological source, or potentially natural or anthropogenic fires, accounting for the observed enrichment of δ13CH4.

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.

We compare multi-ice core data with δ18O model output for the early last interglacial Antarctic sea ice minimum. The spatial pattern of δ18O across Antarctica is sensitive to the spatial pattern of sea ice retreat. Local sea ice retreat increases the proportion of winter precipitation, depleting δ18O at ice core sites. However, retreat also enriches δ18O because of the reduced source-to-site distance for atmospheric vapor. The joint overall effect is for δ18O to increase as sea ice is reduced. Our data-model comparison indicates a winter sea ice retreat of 67, 59, and 43% relative to preindustrial in the Atlantic, Indian, and Pacific sectors of the Southern Ocean. A compilation of Southern Ocean sea ice proxy data provides weak support for this reconstruction. However, most published marine core sites are located too far north of the 128,000 years B.P. sea ice edge, preventing independent corroboration for this sea ice reconstruction. Plain Language Summary The Antarctic isotope and temperature maximum, which occurred approximately 128,000 years before present (B.P.) during the warmer than present last interglacial period, is associated with a major retreat of Antarctic sea ice. Understanding the details of this major sea ice retreat is crucial in order to understand the sensitivity of the Southern Hemisphere sea ice system and to evaluate the performance of climate model simulations in response to future warming. This work uses a multi-ice and ocean core data-model evaluation to assess the magnitude and spatial pattern of this sea ice retreat. Our results suggest that sea ice retreat was greatest in the Atlantic and Indian sectors of the Southern Ocean and less in the Pacific sector. These results may have had serious implications for the stability of marine terminating glaciers around the Antarctic Ice Sheet and their contribution to the last interglacial sea level rise. These results also support a hypothesized slowdown in northward ocean heat transport during the early last interglacial.

Warmer than present Antarctic and Southern Ocean temperatures during the last interglacial, approximately 128,000 years ago, have been attributed to changes in north-south ocean heat transport, causing opposing hemispheric temperature anomalies. We investigate the magnitude of Antarctic warming and Antarctic ice core isotopic enrichment in response to Northern Hemisphere meltwater input during the early last interglacial. A 1,600 year HadCM3 simulation driven by 0.25 Sv of meltwater input reproduces 50-60 % of the peak Southern Ocean summer sea surface temperature anomaly, sea ice retreat and ice core isotope enrichment. We also find a robust increase in the proportion of cold season precipitation during the last interglacial, leading to lower isotopic values at the Antarctic ice core sites. These results suggest that a HadCM3 simulation including 0.25 Sv for 3,000-4,000 years would reconcile the last interglacial observations, providing a potential solution for the last interglacial 'missing heat' problem.

Development of the Antarctic Circumpolar Current (ACC) during the Cenozoic is controversial in terms of timing and its role in major climate transitions. Some propose that the development of the ACC was instrumental in the continental scale glaciation of Antarctica and climate cooling at the Eocene/Oligocene boundary. Here we present climate model results that show that a coherent ACC was not possible during the Oligocene due to Australasian paleogeography, despite deep water connections through the Drake Passage and Tasman Gateway and the initiation of Antarctic glaciation. The simulations of ocean currents compare well to paleoenvironmental records relating to the physical oceanography of the Oligocene and provide a framework for understanding apparently contradictory dating of the initiation of the ACC. We conclude that the northward motion of the Australasian land masses and the reconfiguration of the Tasman Seaway and Drake Passage are necessary preconditions for the formation of a strong, coherent ACC.

The Antarctic polar ice-core records show large changes in the concentration of dust over glacial-interglacial timescales. This paper explores how much of this variation is due to changes in the transport of dust from arid regions to the ice cores. Back trajectories, initialised from the site of Dome C ice-core, Antarctica, are calculated using an offline trajectory code forced by modelled winds, output from the UKMO Unified Model running under present day and LGM boundary conditions. As well as comparing the present day and LGM back trajectories, their seasonal and interannual variability is also explored. The results suggest that the Patagonian provenance of the Dome C dust can be understood in terms of the atmospheric transport whereas the total change in dust concentration requires changes to the sources or sinks.

Concern about tropical deforestation has often focused on local climate or ecological change. Less research has been carried out on its possible effects on the global climate, although possible mechanisms for such an occurrence have been proposed. Here we use a general circulation model (GCM) to show that complete Amazonian deforestation can result in changes in the climate far afield from the region of deforestation. In particular, the model predicts statistically significant changes to winter rainfall over the NE Atlantic, extending towards western Europe. These are associated with large-scale circulation changes in middle and high latitudes. Simulation of these circulation changes with a simple model confirms the physical mechanism responsible (planetary wave propagation) implying that the result is robust and relatively independent of the GCM used.

International audience; The present work evaluates the influence of the North American ice-sheet orography on the Last Glacial Maximum (LGM, 21,000 years ago) atmospheric circulation and snowfall in the northern mid-latitudes, focusing on the North Atlantic sector. Three Atmospheric General Circulation Model experiments are analysed: a control and an LGM climate simulation, and an LGM run with "fiat" ice-sheets over northern North America. This ice-sheet orography affects lee-cyclogenesis over North America and forces differences in stationary waves and therefore in the baroclinicity of the mean flow. As a result, the Atlantic storm-track is reinforced in the "fiat ice-sheet" experiment compared to the LGM one. This, in turn, has a profound impact on snowfall over northern Europe, implying a coupling between the two ice-sheets.

The ice core record reveals large variations in the concentration of atmospheric methane, CH4, over the last 800 kyr. Amongst the most striking natural features are the large, rapid rises in CH4, of 100-200 ppbv, on timescales of less than 100 years, at the beginning of Dansgaard-Oeschger (D-O) events during the last glacial period (21-110 kyr before present). Despite the potential insight they could offer into the likelihood of future rapid rises in CH4, the relative roles of changes in methane sources and sinks during D-O events have been little explored. Here, we use a global atmospheric chemistry-transport model to explore-for the first time, in a process-based fashion-controls on the oxidizing capacity during an idealized D-O event that features a characteristically rapid rise in CH4. We find that the two controls previously identified in the literature as having had significant (though opposing) influences on the oxidizing capacity between glacial and interglacial periods-changes in air temperature and emissions of non-methane volatile organic compounds from vegetation-offset one another between idealized Heinrich stadial and Greenland interstadial states. The result is, the net change in oxidizing capacity is very small, implying the rapid rises in CH4 at the beginning of D-O events were almost entirely source-driven. This poses a challenge to earth-system models-to generate a sufficiently large increase in methane emissions in response to a simulated D-O event, via a more realistic freshwater forcing impacting the strength of the Atlantic meridional overturning circulation or, possibly, other climate-change mechanisms. Citation: Levine, J. G., E. W. Wolff, P. O. Hopcroft, and P. J. Valdes (2012), Controls on the tropospheric oxidizing capacity during an idealized Dansgaard-Oeschger event, and their implications for the rapid rises in atmospheric methane during the last glacial period, Geophys. Res. Lett., 39, L12805, doi:10.1029/2012GL051866.

We know from the ice record that the concentration of atmospheric methane, [CH4], at the Last Glacial Maximum (LGM) was roughly half that in the pre-industrial era (PI), buthow much of the difference was source-driven, and how much was sink-driven, remains uncertain. Recent developments include: a higher estimate of the LGM-PI change in methane emissions from wetlands―the dominant, natural methane source; and the possible recycling of OH consumed in isoprene oxidation―the principal methane sink. Here, in view of these developments, we use an atmospheric chemistry-transport model to re-examine the main factors affecting OH during this period: changes in air temperature and emissions of non-methane volatile organic compounds from vegetation. We find that their net effect was negligible(with and without an OH recycling mechanism), implyingthe change in [CH4] was almost entirely source driven―a conclusion that, though subject to significant uncertainties,can be reconciled with recent methane source estimates.

[1] Variations in carbon-14 to carbon-12 ratio in the atmosphere (Δ14Catm) provide a powerful diagnostic for elucidating the timing and nature of geophysical and anthropological change. The (Atlantic) marine archive suggests a rapid Δ14Catm increase of 50‰ at the onset of the Younger Dryas (YD) cold reversal (12.9–11.7 kyr BP), which has not yet been satisfactorily explained in terms of magnitude or causal mechanism, as either a change in ocean ventilation or production rate. Using Earth-system model simulations and comparison of marine-based radiocarbon records from different ocean basins, we demonstrate that the YD Δ14Catm increase is smaller than suggested by the marine archive. This is due to changes in reservoir age, predominantly caused by reduced ocean ventilation.

[1] Sunshade geoengineering - the installation of reflective mirrors between the Earth and the Sun to reduce incoming solar radiation, has been proposed as a mitigative measure to counteract anthropogenic global warming. Although the popular conception is that geoengineering can re-establish a ‘natural’ pre-industrial climate, such a scheme would itself inevitably lead to climate change, due to the different temporal and spatial forcing of increased CO2 compared to reduced solar radiation. We investigate the magnitude and nature of this climate change for the first time within a fully coupled General Circulation Model. We find significant cooling of the tropics, warming of high latitudes and related sea ice reduction, a reduction in intensity of the hydrological cycle, reduced ENSO variability, and an increase in Atlantic overturning. However, the changes are small relative to those associated with an unmitigated rise in CO2 emissions. Other problems such as ocean acidification remain unsolved by sunshade geoengineering.

[1] Accounting for the natural 40% reduction in methane during the Last Glacial Maximum (LGM) represents one of the most challenging problems posed by polar ice core studies. Inverse modelling, constrained by observations, suggest it is driven by a decline in wetland sources, but this is not supported by forward modelling. Using a comprehensive Earth system model, we show that closure of the methane budget requires reduced emissions and an amplified sink caused by reduction in emissions of volatile organic compounds (VOCs) from LGM forests. Our study emphasizes a key role for the terrestrial biosphere in the methane budget during the Quaternary.

International audience; The mid‐Holocene (6000 years before present) North Atlantic Oscillation (NAO) from nine models in the Paleoclimate Modeling Intercomparison Project Phase 2 is studied, primarily through principal component analysis of winter time North Atlantic sea level pressure (SLP). Modeled mid‐Holocene NAO and mean SLP show small changes compared to pre‐industrial control runs, with a shift in mean state towards a more positive NAO regime for three of the models. Modeled NAO variability shows little change, with a small increase for some models in the fraction of time spent in the NAO‐negative phase during the mid‐Holocene. Proxy based reconstructions of the NAO indicate a more positive NAO regime compared to present day during the mid‐ Holocene. We hypothesise that there was a small NAO+ like shift in mean state during the mid‐Holocene.

[1] During the last c. 300,000 years mega-lakes with areas an order of magnitude greater than those of today occurred in the monsoon region of northern Australia. The results of an Atmosphere Global Circulation Model (AGCM) sensitivity experiment show that the repeated occurrence of mega-lake phases during the Pleistocene, and the strong monsoon flow that this implies, can be related to Milankovitch insolation forcing. The AGCM experiment is centred on the Southern Hemisphere low-latitude, summer insolation high of 115,000 years ago. The details of the results indicate a strong response of the Australian monsoon to the higher insolation at that time, with a longer monsoon season, increased precipitation and higher resultant runoff than at present.

[1] Ice and ocean core data suggest an increase in atmospheric dust loading at the Last Glacial Maximum, 21,000 years ago, relative to the present day. We present results from a new dust model; in contrast to previous modelling studies, we correctly simulate the observed provenance data in both Antarctica and Greenland, and obtain reasonable agreement with the dust deposition data. The observed increases are attributable to decreases in vegetation, increased surface wind strength, and increased particle lifetime (due to the drier LGM climate). The Patagonian provenance of Antarctic LGM dust is linked to a decrease in soil moisture and an extension in area of the source region, and a high transport efficiency between the two regions. The results show a high degree of interannual variability, which must be taken into account when interpreting previous modelling results.