Description: Paleo-climate records and geodynamic modelling indicate the existence of complex interactions between glacial sea level changes, volcanic degassing and atmospheric CO2, which may have modulated the climate system's descent into the last ice age. Between ∼85 and 70 kyr ago, during an interval of decreasing axial tilt, the orbital component in global temperature records gradually declined, while atmospheric CO2, instead of continuing its long-term correlation with Antarctic temperature, remained relatively stable. Here, based on novel global geodynamic models and the joint interpretation of paleo-proxy data as well as biogeochemical simulations, we show that a sea level fall in this interval caused enhanced pressure-release melting in the uppermost mantle, which may have induced a surge in magma and CO2 fluxes from mid-ocean ridges and oceanic hotspot volcanoes. Our results reveal a hitherto unrecognized negative feedback between glaciation and atmospheric CO2 predominantly controlled by marine volcanism on multi-millennial timescales of ∼5,000-15,000 years.

Description: Stable water isotope records from Antarctica are key for our understanding of Quaternary climate variations. However, the exact quantitative interpretation of these important climate proxy records in terms of surface temperature, ice sheet height and other climatic changes is still a matter of debate. Here we report results obtained with an atmospheric general circulation model equipped with water isotopes, run at a high-spatial horizontal resolution of one-by-one degree. Comparing different glacial maximum ice sheet reconstructions, a best model data match is achieved for the PMIP3 reconstruction. Reduced West Antarctic elevation changes between 400 and 800 m lead to further improved agreement with ice core data. Our modern and glacial climate simulations support the validity of the isotopic paleothermometer approach based on the use of present-day observations and reveal that a glacial ocean state as displayed in the GLAMAP reconstruction is suitable for capturing the observed glacial isotope changes in Antarctic ice cores.

Description: Coinciding with global warming, Arctic sea ice has rapidly decreased during the last four decades and climate scenarios suggest that sea ice may completely disappear during summer within the next about 50–100 years. Here we produce Arctic sea ice biomarker proxy records for the penultimate glacial (Marine Isotope Stage 6) and the subsequent last interglacial (Marine Isotope Stage 5e). The latter is a time interval when the high latitudes were significantly warmer than today. We document that even under such warmer climate conditions, sea ice existed in the central Arctic Ocean during summer, whereas sea ice was significantly reduced along the Barents Sea continental margin influenced by Atlantic Water inflow. Our proxy reconstruction of the last interglacial sea ice cover is supported by climate simulations, although some proxy data/model inconsistencies still exist. During late Marine Isotope Stage 6, polynya-type conditions occurred off the major ice sheets along the northern Barents and East Siberian continental margins, contradicting a giant Marine Isotope Stage 6 ice shelf that covered the entire Arctic Ocean.

Description: Glacial climate is marked by abrupt, millennial-scale climate changes known as Dansgaard–Oeschger cycles. The most pronounced stadial coolings, Heinrich events, are associated with massive iceberg discharges to the North Atlantic. These events have been linked to variations in the strength of the Atlantic meridional overturning circulation. However, the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear. Here we show that, in a fully coupled atmosphere–ocean model, gradual changes in atmospheric CO2 concentrations can trigger abrupt climate changes, associated with a regime of bi-stability of the Atlantic meridional overturning circulation under intermediate glacial conditions. We find that changes in atmospheric CO2 concentrations alter the transport of atmospheric moisture across Central America, which modulates the freshwater budget of the North Atlantic and hence deep-water formation. In our simulations, a change in atmospheric CO2 levels of about 15 ppmv—comparable to variations during Dansgaard–Oeschger cycles containing Heinrich events—is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode. Because changes in the Atlantic meridional overturning circulation are thought to alter atmospheric CO2 levels, we infer that atmospheric CO2 may serve as a negative feedback to transitions between strong and weak circulation modes.

Description: Proxy reconstructions of tropical Atlantic sea surface temperature (SST) that extend beyond the period of instrumental observations have primarily focused on centennial to millennial variability rather than on seasonal to multidecadal variability. Here we present monthly-resolved records of Sr/Ca (a proxy of SST) from fossil annually-banded Diploria strigosa corals from Bonaire (southern Caribbean Sea). The individual corals provide time-windows of up to 68 years length, and the total number of 295 years of record allows for assessing the natural range of seasonal to multidecadal SST variability in the western tropical Atlantic during snapshots of the mid- to late Holocene. Comparable to modern climate, the coral Sr/Ca records reveal that mid- to late Holocene SST was characterised by clear seasonal cycles, persistent quasi-biennial and prominent interannual as well as inter- to multidecadal-scale variability. However, the magnitude of SST variations on these timescales has varied over the last 6.2 ka. The coral records show increased seasonality during the mid-Holocene consistent with climate model simulations indicating that southern Caribbean SST seasonality is induced by insolation changes on orbital timescales, whereas internal dynamics of the climate system play an important role on shorter timescales. Interannual SST variability is linked to ocean–atmosphere interactions of Atlantic and Pacific origin. Pronounced interannual variability in the western tropical Atlantic is indicated by a 2.35 ka coral, possibly related to a strengthening of the variability of the El Niño/Southern Oscillation throughout the Holocene. Prominent inter- to multidecadal SST variability is evident in the coral records and slightly more pronounced in the mid-Holocene. We finally argue that our coral data provide a target for studying Holocene climate variability on seasonal and interannual to multidecadal timescales, when using further numerical models and high-resolution proxy data.

Description: Understanding how the Antarctic ice sheet will respond to global warming relies on knowledge of how it has behaved in the past. The use of numerical models, the only means to quantitatively predict the future, is hindered by limitations to topographic data both now and in the past, and in knowledge of how subsurface oceanic, glaciological and hydrological processes interact. Incorporating the variety and interplay of such processes, operating at multiple spatio-temporal scales, is critical to modeling the Antarctic’s system evolution and requires direct observations in challenging locations. As these processes do not observe disciplinary boundaries neither should our future research.

Description: Highlights: • Temperature anomalies for the Mid-Holocene compared to preindustrial are significantly different in the low- and high-resolution versions of the atmospheric model ECHAM5 • For summer, shortwave cloud radiative forcing emerges as an important factor. • For boreal winter, differences are mainly related to circulation changes. • Anomaly differences are regionally as large as the mid-Holocene minus preindustrial temperature signals. Abstract: This study evaluates the dependence of simulated surface air temperatures on model resolution and orography for the mid-Holocene. Sensitivity experiments with the atmospheric general circulation model ECHAM5 are performed with low (∼3.75°, 19 vertical levels) and high (∼1.1°, 31 vertical levels) resolution. Results are compared to the respective preindustrial runs. It is found that the large-scale temperature anomalies for the mid-Holocene (compared to preindustrial) are significantly different in the low- and high-resolution versions. For boreal winter, differences are mainly related to circulation changes caused by the response to thermal forcing in conjunction with orographic resolution. For summer, shortwave cloud radiative forcing emerges as an important factor. The anomaly differences (low minus high resolution version) in the Northern Hemisphere are regionally as large as the anomalous mid-Holocene temperature signals. Furthermore, they depend on the applied surface boundary conditions. We conclude that the resolution matters for the Northern Hemisphere response in mid-Holocene simulations, which should be taken into account in model-model and data-model comparisons.

Description: Abundant hydroclimatic evidence from western Amazonia and the adjacent Andes documents wet conditions during Heinrich Stadial 1 (HS1, 18–15 ka), a cold period in the high latitudes of the North Atlantic. This precipitation anomaly was attributed to a strengthening of the South American summer monsoon due to a change in the Atlantic interhemispheric sea surface temperature (SST) gradient. However, the physical viability of this mechanism has never been rigorously tested. We address this issue by combining a thorough compilation of tropical South American paleorecords and a set of atmosphere model sensitivity experiments. Our results show that the Atlantic SST variations alone, although leading to dry conditions in northern South America and wet conditions in northeastern Brazil, cannot produce increased precipitation over western Amazonia and the adjacent Andes during HS1. Instead, an eastern equatorial Pacific SST increase (i.e., 0.5–1.5 °C), in response to the slowdown of the Atlantic Meridional Overturning Circulation during HS1, is crucial to generate the wet conditions in these regions. The mechanism works via anomalous low sea level pressure over the eastern equatorial Pacific, which promotes a regional easterly low-level wind anomaly and moisture recycling from central Amazonia towards the Andes.

Description: The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia1. The index that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear. A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic, freshened surface waters, affecting the strength of the meridional overturning circulation. Here we present a Holocene rainfall record from northwest Africa based on speleothem δ18O and compare it against a speleothem-based rainfall record from Europe. The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets.

Description: There has been extensive research into the nonlinear responses of the Earth system to astronomical forcing during the last glacial cycle. However, the speed and spatial geometry of ice sheet expansion to its largest extent at the Last Glacial Maximum 21 thousand years ago remains uncertain. Here we use an Earth system model with interactive ice sheets to show that distinct initial North American (Laurentide) ice sheets at 38 thousand years ago converge towards a configuration consistent with the Last Glacial Maximum due to feedbacks between atmospheric circulation and ice sheet geometry. Notably, ice advance speed and spatial pattern in our model are controlled by the amount of summer snowfall, which is dependent on moisture transport pathways from the North Atlantic warm pool linked to ice sheet geometry. The consequence of increased summer snowfall on the surface mass balance of the ice sheet is not only the direct increase in accumulation but the indirect reduction in melt through the snow/ice–albedo feedback. These feedbacks provide an effective mechanism for ice growth for a range of initial ice sheet states and may explain the rapid North American ice volume increase during the last ice age and potentially driving growth during previous glacial periods.