The study of past climates informs us on the causes, amplitude and mechanisms of climate change, which is necessary to our ability to predict future changes, and build the necessary infrastructure to ensure the resilience of our society to these changes. Local climate records contain both large scale and local signals, from both external forcing, such as radiative forcing, and internal climate variability. In order to understand the response of the climate system to global drivers, we must average out the local signals into hemispheric or global variables. This process has been difficult because we lack the spatial coverage in large areas of the Earth, including the oceans, and much of the Southern Hemisphere. The work presented here aims at improving our knowledge of the climate by producing a new temperature time series from the center of West Antarctica for the last 1000 years, a region previously unexplored. This temperature reconstruction is based on a new method, combining borehole temperature measurements with inert gas isotopes from the WAIS-Divide ice cores into a single inverse problem. Borehole temperature measurements constrain the long term changes in the climate, while inert gas isotopes record decadal to centennial scale changes. Together, they produce a temperature estimate that is independent of the traditional water isotope proxy [Delta]¹⁸O of ice, and provide a way to calibrate it. WAIS-Divide experienced a long term cooling trend from 950 to 1850 A.D., which ended abruptly by warming by 2.3°C in 30 years. More recently, WAIS-Divide has been warming by more than 1.5°C since 1957, which refutes the idea that Antarctica would not experience the current warming seen elsewhere. The long term cooling trend was superimposed on centennial scale variations in the climate, including two warming events, notably between 1315 and 1395 A.D., 1596 and 1626 A.D., with a warming rate of 0.24 and 0.32°C/decade. This evidence shows that the current rate of warming at WAIS- Divide, of 0.23°C/decade for the last 50 years is rare but not unprecedented. This record is consistent with the idea that the decrease in solar radiation from 1400 to 1800 A.D. induced widespread cooling in mid and high latitudes of both hemispheres

Polar temperature is often inferred from water isotopes in ice cores. However, non-temperature effects on δ18O are important during the abrupt events of the last glacial period, such as changes in the seasonality of precipitation, the northward movement of the storm track, and the increase in accumulation. These effects complicate the interpretation of δ18O as a temperature proxy.Here, we present an independent surface temperature reconstruction, which allows us to test the relationship between δ18Oice and temperature, during Dansgaard-Oeschger event 8, 38.2 thousand yrs ago using new δ15N and δ40Ar data from the GISP2 ice core in Greenland. This temperature reconstruction relies on a new inversion of inert gas isotope data using generalized least-squares, and includes a robust uncertainty estimation.We find that both temperature and δ18O increased in two steps of 20 and 140 yrs, with an overall amplitude of 11.80 ± 1.8°C between the stadial and interstadial centennial-mean temperature. The coefficient α=dδO18/dT changes with each time-segment, which shows that non-temperature sources of fractionation have a significant contribution to the δ18O signal. When measured on century-averaged values, we find that α=dδO18/dT=0.32±0.06‰/°C, which is similar to the glacial/Holocene value of 0.328‰/°C. © 2014 Elsevier B.V.

Melt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt event.

Understanding the causes of recent climatic trends and variability in the high-latitude Southern Hemisphere is hampered by a short instrumental record. Here, we analyse recent atmosphere, surface ocean and sea-ice observations in this region and assess their trends in the context of palaeoclimate records and climate model simulations. Over the 36-year satellite era, significant linear trends in annual mean sea-ice extent, surface temperature and sea-level pressure are superimposed on large interannual to decadal variability. Most observed trends, however, are not unusual when compared with Antarctic palaeoclimate records of the past two centuries. With the exception of the positive trend in the Southern Annular Mode, climate model simulations that include anthropogenic forcing are not compatible with the observed trends. This suggests that natural variability overwhelms the forced response in the observations, but the models may not fully represent this natural variability or may overestimate the magnitude of the forced response.

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.