Dietmar Wagenbach

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.

Observations and modelling of the global distribution and long-term trend of atmospheric 14CO2.

Global high-precision atmospheric Δ14CO2 records covering the last two decades are presented, and evaluated in terms of changing (radio)carbon sources and sinks, using the coarse-grid carbon cycle model GRACE. Dedicated simulations of global trends and interhemispheric differences with respect to atmospheric CO2 as well as δ13CO2 and Δ14CO2, are shown to be in good agreement with the available observations (1940–2008). While until the 1990s the decreasing trend of Δ14CO2 was governed by equilibration of the atmospheric bomb 14C perturbation with the oceans and terrestrial biosphere, the largest perturbation today are emissions of 14C-free fossil fuel CO2. This source presently depletes global atmospheric Δ14CO2 by 12–14‰ yr−1, which is partially compensated by 14CO2 release from the biosphere, industrial 14C emissions and natural 14C production. Fossil fuel emissions also drive the changing north–south gradient, showing lower Δ14C in the northern hemisphere only since 2002. The fossil fuel-induced north–south (and also troposphere–stratosphere) Δ14CO2 gradient today also drives the tropospheric Δ14CO2 seasonality through variations of air mass exchange between these atmospheric compartments. Neither the observed temporal trend nor the Δ14CO2 north–south gradient may constrain global fossil fuel CO2 emissions to better than 25%, due to large uncertainties in other components of the (radio)carbon cycle.

Sulfate and nitrate firn concentrations on the Greenland ice sheet: 2. Temporal anthropogenic deposition changes

Intercomparison of three new chemical ice core records from northern Greenland (covering the time span from approximately 1500 A.D. to present) with previously published records for southern and central Greenland reveals a uniform timing of anthropogenic changes in sulfate and nitrate firn concentrations over the entire ice sheet. The anthropogenic sulfate increase started around 1890, was interrupted by a transient decrease in the 1930s, and has resumed a major increase since 1950. Since the late 1970s though, a significant 30% decline in Greenland sulfate firn levels can be documented. The maximum anthropogenic increase in northern Greenland sulfate firn concentrations (up to 200–230 ppb) is 2–3 times larger than in southern and central Greenland. Nitrate records show an essentially steady increase since 1950 and, documented for the first time, a slight reduction during most recent years. Maximum nitrate firn levels of 100–130 ppb exceed the preindustrial background by 100% all over the Greenland ice sheet. Comparison with anthropogenic SO2 and NOx emission records indicates that the major increase in sulfate firn concentrations since 1950 can be attributed to Eurasian sources, while firn levels during the first half of this century appear to be dominated by North American emissions. A stronger North American source contribution is indicated over the entire 20th century in the case of nitrate. Applying a macroscopic deposition model separate time series for wet and dry deposition were derived which revealed a close correspondence of wet deposited sulfate with the timing of U.S. emissions, while the temporal evolution of Eurasian emissions is mainly reflected in the dry sulfate deposition record. During this century wet sulfate deposition increased by a factor of two while the total dry sulfate deposition flux increased by more than 500%. Wet and dry nitrate deposition both increased by 100% during the same period.

Little Ice Age clearly recorded in northern Greenland ice cores

Four ice cores drilled in the little investigated area of northern and northeastern Greenland were evaluated for their isotopic (δ18O) and chemical content. From these rather uniform records a stable isotope temperature time series covering the last 500 years has been deduced, which reveals distinct climate cooling during the 17th and the first half of the 19th century. Timing of the preindustrial temperature deviations agrees well with other northern hemisphere temperature reconstructions, however, their extent (∼1°C) significantly exceeds both continental records as well as previous southern and central Greenland ice core time series. A 20–30% increase in the sea salt aerosol load during these periods supports accompanying circulation changes over the North Atlantic. Comparison with records of potential natural climate driving forces points to an important role of the long-term solar influence but to only episodically relevant cooling during years directly following major volcano eruptions.

Coastal Antarctica: Atmospheric Chemical Composition and Atmospheric Transport

Snow fields of the coastal Antarctic region are situated intermediately between the vast environmental regimes of the high antarctic plateau and that of the South Polar Ocean. Since they are directly exposed to moist maritime air masses, a relatively high snow accumulation rate, associated with the deposition of mainly marine derived aerosol species, is expected here. These characteristics make this region unique to recover high resolution ice core records that reveal the environmental and climatic history of the South Polar Ocean. Indeed, several extensive ice core studies have been or are currently performed in coastal antarctic regions (see Figure 1). To “calibrate”, in particular, their glacio-chemical information in terms of the corresponding atmospheric changes, representative long term records of the coastal antarctic aerosol chemistry (including the relevant precursor gases) are needed.

Pollen analysis and 14C age of moss remains in a permafrost core recovered from the active rock glacier Murtèl-Corvatsch, Swiss Alps : geomorphological and glaciological implications

Within the framework of core-drilling through the permafrost of the active rock glacier Murtel–Corvatsch in the Swiss Alps, subfossil stem remains of seven different bryophyte species were found at a depth of 6 m below surface and about 3 m below the permafrost table in samples from massive ice. The composition of the moss species points to the former growth of the recovered mosses in the nearest surroundings of the drill site. A total of 127 pollen and spores captured by the mosses and representing 23 taxa were determined. The local vegetation during deposition time must be characterized as a moss-rich alpine grassland meadow rich in Cyperaceae, Poaceae, Chenopodiaceae and Asteraceae, comparable to today’s flora present around the study site. For l4 C analysis, accelerator mass spectrometry had to be used due to the small sample mass (about 0.5 mg Carbon content). The mean conventional 14 C age of 2250 ± 100 years (1 σ variability) corresponds to ranges in the calibrated calendar age of 470–170 BC and 800 BC to AD 0 at statistical probabilities of 68% and 95%, respectively. This result is compared with the present-day flow field as determined by high-precision photogrammetry and with information about the thickness, vertical structure and flow of the permafrost from borehole measurements. Total age of the rock glacier as a landform is on the order of 10 4 years; the development of the rock glacier most probably started around the onset of the Holocene, when the area it now occupies became definitely deglaciated. The bulk of the ice/rock mixture within the creeping permafrost must be several thousand years old. Characteristic average values are estimated for (1) surface velocities through time (cm a -1 ), (2) long-term ice and sediment accretion rates (mm a -1 ) on the debris cone from which the rock glacier develops, (3) retreat rates (1–2 mm a -1 ) of the cliff which supplies the debris to the debris cone and rock glacier, and (4) ice content of the creeping ice/rock mixture (50–90% by volume). The pronounced supersaturation of the permafrost explains the steady-state creep mode of the rock glacier.

Sulfate and nitrate firn concentrations on the Greenland ice sheet: 1. Large‐scale geographical deposition changes

Eleven shallow firn cores and three deeper ice cores drilled along a meridional line over the Greenland ice sheet from 71° to 80°N have been specifically evaluated for their nitrate and sulfate concentrations. Recent average annual sulfate concentrations strongly increase from around 100 ppb in central Greenland to 175 ppb in the northeastern part of the ice sheet, while nitrate levels remain essentially constant at around 150 ppb. Recent sulfate firn concentrations exhibit a distinct seasonal cycle with maximum concentrations in spring. The seasonal amplitude is 5 times higher over the northern ice sheet compared to the central Greenland region. The corresponding nitrate cycle is only weakly defined with a tendency toward higher summer concentrations. Application of a macroscopic deposition model, developed for irreversibly deposited sulfate as well as reversibly deposited nitrate, shows that geographical variations in sulfate and nitrate firn concentrations can be explained largely by the spatial change in snow accumulation rate over the entire interior of the Greenland ice sheet both for recent and for preindustrial conditions. The recent wet sulfate deposition flux has increased by a factor of 2 since preindustrial times, while the dry deposition flux has risen by a factor of 5. In the case of nitrate, depositon fluxes have increased by a factor of 2 both for wet and for dry deposition. On the basis of the empirically determined model parameters, up to 50% of the initially deposited nitrate appears to be remobilized from the firn column.

Postdepositional change in snowpack nitrate from observation of year‐round near‐surface snow in coastal Antarctica

Postdepositional loss of nitrate in near-surface snowfall is well known, with mean levels of nitrate in ice cores of around 20 to 80 ng g−1, while nitrate in surface snow may occasionally reach over 300 ng g−1. This has been explained as reemission of nitrate (as nitric acid) during aging, via processes that are not yet clear. However, clear seasonal cycles remain in nitrate profiles from higher accumulation rate ice cores and, across Antarctica, the mean concentration of nitrate is remarkably similar despite widely varying deposition conditions, marking nitrate out as quite different to other major ice core species. This paper examines the year-round deposition of nitrate at the snow surface at a coastal Antarctic site and discusses the degree and timing of nitrate loss. At Halley Station, Antarctica, the mean concentration of near-surface snow was 96 ng g−1 over a 2.6 year daily sampling period, while the mean concentration in newly accumulated snow was 79 ng g−1. At the end of this period, a shallow core integrating the sampling period had a mean nitrate concentration of 65 ng g−1 taken over 2 full years of accumulation. Nitrate concentrations in the surface layer were in general highest during the summer period reaching 400 ng g−1 with a mean of about 150 ng g−1 in each of December, January, and February, and lowest during the winter with a mean of around 50 ng g−1 in June. Fresh snow data from Neumayer Station shows a similar seasonal signal, with a mean nitrate concentration of 77 ng g−1, while year-round aerosol data shows total nitrate (particulate and gas phase) in the air is at a minimum in April to June and reaches a maximum in late November [Wagenbach et al., this issue], slightly out of phase with snowfall nitrate. The observation of a reduction in high nitrate concentrations in new snowfall over a few days does not appear to be general, and at Halley, there is evidence of both uptake and loss of nitrate in the surface snow layer, possibly indicating an equilibrium with changing air concentrations. However, there is attenuation of the nitrate signal over the longer period, with concentrations in the ice core taken at the end of sampling never reaching values seen in the upper surface layer.

The Mineral Dust Record in a High Altitude Alpine Glacier (Colle Gnifetti, Swiss Alps)

Ice-core and snow-pit samples from a non-temperated glacier in the summit range of Monte Rosa, Swiss Alps (4450 m.a.s.l.) has been analyzed for total mineral dust and the size distribution of insoluble particulate matter in the size range 0.63–20 microns. Based on a 50 years-record Saharan dust accounts for two third of the mean mineral dust flux of 60 μgcm-2yr-1. Both, background and Saharan dust influenced samples show a distinct mode in the volume size distribution of insoluble particles over the optical active size range with a typical volume mean diameter of 2.5 and 4.5 μm, respectively. These two size distribution categories are attributed to the insoluble fraction of the long lived background aerosol and to the relatively short lived aerosol dominated by soil derived dust (i.e. ground-level aerosol in aride areas).

Sulfate trends in a Col du Dôme (French Alps) ice core: A record of anthropogenic sulfate levels in the European midtroposphere over the twentieth century

A high-resolution sulfate record from a Col du Dome (CDD, 4250 m above sea level, French Alps) ice core was used to investigate the impact of growing SO2 emissions on the midtroposphere sulfate levels over Europe since 1925. The large annual snow accumulation rate at the CDD site permits examination of the summer and winter sulfate trends separately. Being close to 80±10 ng g−1 in preindustrial summer ice, sulfate CDD summer levels then increase at a mean rate of 6 ng g−1 per year from 1925 to 1960. From 1960 to 1980 the increase continued at a rate of 24 ng g−1 per year. Concentrations reach a maximum of 860 ng g−1 in 1980 and subsequently decrease to 600 ng g−1 in the 1990s. These summer sulfate changes closely follow the course of growing SO2 emissions from source regions located within 700–1000 km around the Alps (France, Italy, Spain, and to a lesser extent, former West Germany). In winter the CDD sulfate levels are 3 to 8 times lower than in summer because of more limited upward transport of air masses from the boundary layer at that season. Being close to 20 ng g−1 in the preindustrial ice, winter levels were regularly enhanced at a mean annual rate of 1.2 ng g−1 from 1925 to 1980. The weak winter change from the preindustrial era to 1980 (a factor of 4 instead of 10 in summer) reflects a limited contamination of the free troposphere which, in contrast to summer, occurs at a larger scale (total Europe/former USSR). Intimately connected to Europe, these long-term changes in the Alps clearly differ in time and amplitude with the ones revealed by Greenland ice cores which indicate an increase by a factor of 3 between 1880 and 1970 in relation with long-range transport of pollutants from Eurasia as well as from North America. Furthermore, because of a lower natural contribution to the total sulfate level the anthropogenic changes can be more accurately derived in the Alps than in Greenland. Using the observed relationship between present-day concentrations in air and snowpack, the CDD ice core record permits reconstruction of present and past atmospheric sulfate concentrations at 4300 m above sea level over Europe in summer and winter. These data are compared with the sulfate levels simulated by current global sulfur models at 600 hPa for which uncertainties still range within a factor of 2. Together with observations made at lower elevation in the early 1990s the atmospheric levels derived for the CDD site (∼20 and 400 ng m−3 STP in winter and summer, respectively) document the vertical sulfate distribution between the ground and 4300 m elevation over western Europe at that time. In this way, data gained at high-elevation Alpine sites are powerful in evaluating the recent role of sulfate aerosol in forcing the climate over Europe.