Antarctic Warm Events Research Articles

Antarctic control on tropical Indian Ocean sea surface temperature and hydrography

We reconstructed the surface hydrography of the South Equatorial Current in the western Indian Ocean for the last 65,000 years using a marine sediment core record. Results show that tropical Indian Ocean temperatures resemble temperatures from Antarctic ice cores with warm and cold fluctuations synchronous with the Antarctic Cold Reversal and the Antarctic warm events A1–A4. The most likely thermal link involves Subantarctic Mode Water (SAMW) which forms north of the subpolar frontal zone and spreads northward into the Indian Ocean. This subsurface water mass is the prime suspect because of a stronger temperature response in the thermocline (recorded by the foraminifer N. dutertrei) than in surface water (G. ruber).

One-to-one coupling of glacial climate variability in Greenland and Antarctica

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth's climate dynamics. For the last glacial period, ice core studies have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard-Oeschger events in Greenland through the Atlantic meridional overturning circulation. It has been unclear, however, whether the shorter Dansgaard-Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland, the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard-Oeschger events by the bipolar seesaw6. The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

Aerosol deposited in East Antarctica over the last glacial cycle: Detailed apportionment of continental and sea‐salt contributions

The major ions, sodium (Na+), calcium (Ca2+), and chloride (Cl−), deposited in central Antarctica and preserved in ice cores originate from both marine and continental sources. They provide important proxy records, helping to reconstruct past climatic processes. However, it is difficult to clearly separate the individual contributions from the two sources, particularly the continental one during glacial periods. On the basis of Na+ and Ca2+ records at an unprecedented resolution from the European Project for Ice Coring in Antarctica (EPICA) Dome C ice core back to the penultimate glacial period, mean ion mass ratios were deduced for the continental and the sea‐salt aerosol body over East Antarctica. The sea‐salt ion mass ratios are in the range predicted for both wind‐induced bubble bursting of breaking waves on the open ocean and sea ice brine‐derived aerosols, respectively, thus allowing no clear decision on the contribution of sea ice to the central Antarctic sea‐salt aerosol. The continental ion mass ratios point to a substantial contribution by halide aerosols, which is in agreement with the source properties in southern South America, although these ratios do not rule out the continental shelf exposed during glacial stages as an additional source. While during cold glacial periods continental sources accounted for more than 90% of the total Ca2+ input, this contribution was highly variable during the remaining glacial periods covarying with the Antarctic warm events. During the Holocene it was less than 50%, but it was significantly higher during the last interglacial period. The sea‐salt aerosol contribution to the total Na+ input, which was mostly dominant and higher than 90%, was reduced to only two thirds during the last two glacial maxima and the period around 60 ka. Thus the glacial continental Na+ contribution appears to be more important than previously assumed, implying that Na+ records not corrected for continental Na+ do not represent a pure marine signal at the East Antarctic plateau during glacial times.

Simulated changes in vegetation distribution, land carbon storage, and atmospheric CO2 in response to a collapse of the North Atlantic thermohaline circulation

It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon cycle. The Lund–Potsdam–Jena Dynamic Global Vegetation Model is forced with climate perturbations from glacial freshwater experiments with the ECBILT-CLIO ocean–atmosphere–sea ice model. A reorganisation of the marine carbon cycle is not addressed. Modelled NA THC collapses and recovers after about a millennium in response to prescribed freshwater forcing. The initial cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the Hadley Centre model HadSM3 for different periods over the past 21 kyr. Changes in terrestrial storage vary between −67 and +50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric CO2 are 6 and 13 ppmv for glacial and late Holocene conditions. Simulated changes in δ13C are between 0.15 and 0.25‰. These simulated carbon storage anomalies during a NA THC collapse depend on their magnitude on the CO2 fertilisation feedback mechanism. The CO2 changes simulated for glacial conditions are compatible with available evidence from marine studies and the ice core CO2 record. The latter shows multi-millennial CO2 variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4 in the South and cooling in the North.

Eddies and the Seesaw

CLIMATE SCIENCE A series of warm episodes, each lasting several thousand years, occurred in Antarctica between 90,000 and 30,000 years ago. These events correlated with rapid climate oscillations in the Arctic, with Antarctica warming while the Arctic was cooling or already cold. This bipolar seesaw is thought to have been driven by changes in the strength of the deep overturning circulation in the North Atlantic Ocean, but some have questioned how completely that process can account for the fine details of Antarctic warming events. Keeling and Visbeck offer an explanation that builds upon earlier suggestions that include the effects of shallow-water processes as well as deep ones. They suggest that changes in the surface salinity gradient across the Antarctic Circumpolar Current were caused by the melting of icebergs discharged from the Arctic, which allowed increased heat transport to Antarctica by ocean eddies. This mechanism produces Antarctic warming of the magnitude observed in ice core records. — HJS Quat. Sci. Rev. 10.1016/j/quascirev.2005.04.005 (2005).

Northern ice discharges and Antarctic warming: could ocean eddies provide the link?

A mechanism is advanced for explaining the Antarctic warm events from 90 to 30ka BP which involves meltwater-induced changes in the salinity gradient across the Antarctic Circumpolar Current (ACC) and consequent changes in the poleward heat transport by ocean eddies. Based on simple linear scale analysis, the mechanism is shown to yield warming in the Antarctic interior of roughly the magnitude seen in Antarctic ice-core records (∼2 °C) in response to ice discharges into the North Atlantic. Consistent with observations, the mechanism produces gradual Antarctic warming and cooling, as dictated by the time required for salinity anomalies to build up and dissipate across the ACC. The mechanism also allows the onset of warming or cooling to be tied to changes in Atlantic overturning, which is relevant here, not for influencing ocean heat transport directly, but for influencing the routing of meltwater from the North Atlantic into the Southern Ocean. The ideas presented here highlight the possibility that eddy processes in the ocean may play a first-order role in aspects of climate variability on millennial time scales.

Ice core evidence for the extent of past atmospheric CO2 change due to iron fertilisation

An extended high‐resolution ice core record of dust deposition over the past 60 ka from Dome C, Antarctica, is presented. The data are in conflict with the idea that changes in aeolian iron input into the Southern Ocean were the major cause for the 80 ppm glacial‐interglacial CO2 increase. During the deglaciation, the CO2 increase shows a linear relationship with the fall of the logarithm of the nss‐Ca2+ flux, a proxy for dust deposition. However, the very large variations in the nss‐Ca2+ flux related to the glacial Antarctic warm events A1 to A4 were accompanied by small CO2 variations only. Our data‐based analysis suggests that decreased Southern Ocean dust deposition caused at most a 20 ppm increase in CO2 at the last glacial‐interglacial transition. Rapid decreases in dust deposition to the northern Pacific could have been responsible for a maximum of 8 ppm in addition.