Millennial and sub-millennial scale climatic variations recorded in polar ice cores over the last glacial period

Since its discovery in Greenland ice cores, the millennial scale climatic variability of the last glacial period has been increasingly documented at all latitudes with studies focusing mainly on Marine Isotopic Stage 3 (MIS 3; 28–60 thousand of years before present, hereafter ka) and characterized by short Dansgaard-Oeschger (DO) events. Recent and new results obtained on the EPICA and NorthGRIP ice cores now precisely describe the rapid variations of Antarctic and Greenland temperature during MIS 5 (73.5–123 ka), a time period corresponding to relatively high sea level. The results display a succession of abrupt events associated with long Greenland InterStadial phases (GIS) enabling us to highlight a sub-millennial scale climatic variability depicted by (i) short-lived and abrupt warming events preceding some GIS (precursor-type events) and (ii) abrupt warming events at the end of some GIS (rebound-type events). The occurrence of these sub-millennial scale events is suggested to be driven by the insolation at high northern latitudes together with the internal forcing of ice sheets. Thanks to a recent NorthGRIP-EPICA Dronning Maud Land (EDML) common timescale over MIS 5, the bipolar sequence of climatic events can be established at millennial to sub-millennial timescale. This shows that for extraordinary long stadial durations the accompanying Antarctic warming amplitude cannot be described by a simple linear relationship between the two as expected from the bipolar seesaw concept. We also show that when ice sheets are extensive, Antarctica does not necessarily warm during the whole GS as the thermal bipolar seesaw model would predict, questioning the Greenland ice core temperature records as a proxy for AMOC changes throughout the glacial period.

Interhemispheric coupling, the West Antarctic Ice Sheet and warm Antarctic interglacials

Ice core evidence indicates that even though atmospheric CO2 concentrations did not exceed ~300 ppm at any point during the last 800 000 years, East Antarctica was at least ~3–4 °C warmer than preindustrial (CO2~280 ppm) in each of the last four interglacials. During the previous three interglacials, this anomalous warming was short lived (~3000 years) and apparently occurred before the completion of Northern Hemisphere deglaciation. Hereafter, we refer to these periods as "Warmer than Present Transients" (WPTs). We present a series of experiments to investigate the impact of deglacial meltwater on the Atlantic Meridional Overturning Circulation (AMOC) and Antarctic temperature. It is well known that a slowed AMOC would increase southern sea surface temperature (SST) through the bipolar seesaw and observational data suggests that the AMOC remained weak throughout the terminations preceding WPTs, strengthening rapidly at a time which coincides closely with peak Antarctic temperature. We present two 800 kyr transient simulations using the Intermediate Complexity model GENIE-1 which demonstrate that meltwater forcing generates transient southern warming that is consistent with the timing of WPTs, but is not sufficient (in this single parameterisation) to reproduce the magnitude of observed warmth. In order to investigate model and boundary condition uncertainty, we present three ensembles of transient GENIE-1 simulations across Termination II (135 000 to 124 000 BP) and three snapshot HadCM3 simulations at 130 000 BP. Only with consideration of the possible feedback of West Antarctic Ice Sheet (WAIS) retreat does it become possible to simulate the magnitude of observed warming.

Perennial Snow and Ice Variations (2000-2008) in the Arctic Circumpolar Land Area from Satellite Observations

Perennial snow and ice (PSI) extent is an important parameter of mountain environments with regard to its involvement in the hydrological cycle and the surface energy budget. We investigated interannual variations of PSI in nine mountain regions of interest (ROI) between 2000 and 2008. For that purpose, a novel MODIS data set processed at the Canada Centre for Remote Sensing at 250 m spatial resolution was utilized. The extent of PSI exhibited significant interannual variations, with coefficients of variation ranging from 5% to 81% depending on the ROI. A strong negative relationship was found between PSI and positive degree-days (threshold 0°C) during the summer months in most ROIs, with linear correlation coefficients (r) being as low as r = −0.90. In the European Alps and Scandinavia, PSI extent was significantly correlated with annual net glacier mass balances, with r = 0.91 and r = 0.85, respectively, suggesting that MODIS-derived PSI extent may be used as an indicator of net glacier mass balances. Validation of PSI extent in two land surface classifications for the years 2000 and 2005, GLC-2000 and Globcover, revealed significant discrepancies of up to 129% for both classifications. With regard to the importance of such classifications for land surface parameterizations in climate and land surface process models, this is a potential source of error to be investigated in future studies. The results presented here provide an interesting insight into variations of PSI in several ROIs and are instrumental for our understanding of sensitive mountain regions in the context of global climate change assessment.

An ice-core based history of Siberian forest fires since AD 1250

Forest fires play a key role in the global carbon cycle and thus, can affect regional and global climate. Although fires in extended areas of Russian boreal forests have a considerable influence on atmospheric greenhouse gas and soot concentrations, estimates of their impact on climate are hampered by a lack of data on the history of forest fires. Especially regions with strong continental climate are of high importance due to an intensified development of wildfires. In this study we reconstruct the fire history of Southern Siberia during the past 750 years using ice-core based nitrate, potassium, and charcoal concentration records from Belukha glacier in the continental Siberian Altai. A period of exceptionally high forest-fire activity was observed between AD 1600 and 1680, following an extremely dry period AD 1540-1600. Ice-core pollen data suggest distinct forest diebacks and the expansion of steppe in response to dry climatic conditions. Coherence with a paleoenvironmental record from the 200 km distant Siberian lake Teletskoye shows that the vegetational shift AD 1540-1680, the increase in fire activity AD 1600-1680, and the subsequent recovery of forests AD 1700 were of regional significance. Dead biomass accumulation in response to drought and high temperatures around AD 1600 probably triggered maximum forest-fire activity AD 1600-1680. The extreme dry period in the 16th century was also observed at other sites in Central Asia and is possibly associated with a persistent positive mode of the Pacific Decadal Oscillation (PDO). No significant increase in biomass burning occurred in the Altai region during the last 300 years, despite strongly increasing temperatures and human activities. Our results imply that precipitation changes controlled fire-regime and vegetation shifts in the Altai region during the past 750 years. We conclude that high sensitivity of ecosystems to occasional decadal-scale drought events may trigger unprecedented environmental reorganizations under global-warming conditions.

Contamination of Alpine snow and ice at Colle Gnifetti, Swiss/Italian Alps, from nuclear weapons tests

Plutonium is present in the environment as a consequence of atmospheric nuclear tests, nuclear weapons production and industrial releases over the past 50 years. To study temporal trends, a high resolution Pu record was obtained by analyzing 52 discrete samples of an alpine firn/ice core from Colle Gnifetti (Monte Rosa, 4450 m a.s.l.), dating from 1945 to 1990. The 239Pu signal was recorded directly, without decontamination or preconcentration steps, using an Inductively Coupled Plasma - Sector Field Mass Spectrometer (ICP-SFMS) equipped with an high efficiency sample introduction system, thus requiring much less sample preparation than previously reported methods. The 239Pu profile reflects the three main periods of atmospheric nuclear weapons testing: the earliest peak lasted from 1954/55 to 1958 and was caused by the first testing period reaching a maximum in 1958. Despite a temporary halt of testing in 1959/60, the Pu concentration decreased only by half with respect to the 1958 peak due to long atmospheric residence times. In 1961/62 Pu concentrations rapidly increased reaching a maximum in 1963, which was about 40% more intense than the 1958 peak. After the signing of the "Limited Test Ban Treaty" between USA and USSR in 1964, Pu deposition decreased very sharply reaching a minimum in 1967. The third period (1967-1975) is characterized by irregular Pu concentrations with smaller peaks (about 20-30% of the 1964 peak) which might be related to the deposition of Saharan dust contaminated by the French nuclear tests of the 1960s. The data presented are in very good agreement with Pu profiles previously obtained from the Col du Dome ice core (by multi-collector ICP-MS) and Belukha ice core (by Accelerator Mass Spectrometry, AMS). Although a semi-quantitative method was employed here, the results are quantitatively comparable to previously published results.