Volcanic sulfate deposition to Greenland and Antarctica: A modeling sensitivity study

Reconstructions of the atmospheric sulfate aerosol burdens resulting from past volcanic eruptions are based on ice core-derived estimates of volcanic sulfate deposition and the assumption that the two quantities are directly proportional. We test this assumption within simulations of tropical volcanic stratospheric sulfur injections with the MAECHAM5-HAM aerosol-climate model. An ensemble of 70 simulations is analyzed, with SO2 injections ranging from 8.5 to 700 Tg, with eruptions in January and July. Modeled sulfate deposition flux to Antarctica shows excellent spatial correlation with ice core-derived estimates for Pinatubo and Tambora, although the comparison suggests the modeled flux to the ice sheets is 4–5 times too large. We find that Greenland and Antarctic deposition efficiencies (the ratio of sulfate flux to each ice sheet to the maximum hemispheric stratospheric sulfate aerosol burden) vary as a function of the magnitude and season of stratospheric sulfur injection. Changes in simulated sulfate deposition for large SO2 injections are connected to increases in aerosol particle size, which impact aerosol sedimentation velocity and radiative properties, the latter leading to strong dynamical changes including strengthening of the winter polar vortices, which inhibits the transport of stratospheric aerosols to high latitudes. The resulting relationship between Antarctic and Greenland volcanic sulfate deposition is nonlinear for very large eruptions, with significantly less sulfate deposition to Antarctica than to Greenland. These model results suggest that variability of deposition efficiency may be an important consideration in the interpretation of ice core sulfate signals for eruptions of Tambora-magnitude and larger

Sulfur budget and global climate impact of the AD 1835 eruption of Cosigüina volcano, Nicaragua

Large explosive volcanic eruptions can inject massive amounts of sulfuric gases into the Earth's atmosphere and, in so doing, affect global climate. The January 1835 eruption of Cosigüina volcano, Nicaragua, ranks among the Americas’ largest and most explosive historical eruptions, but whether it had effects on global climate remains ambiguous. New petrologic analyses of the Cosigüina deposits reveal that the eruption released enough sulfur to explain a prominent ca. AD 1835 sulfate anomaly in ice cores from both the Arctic and Antarctic. A compilation of temperature-sensitive tree-ring chronologies indicates appreciable cooling of the Earth's surface in response to the eruption, consistent with instrumental temperature records. We conclude that this eruption represents one of the most important sulfur-producing events of the last few centuries and had a sizable climate impact rivaling that of the 1991 eruption of Mount Pinatubo

Rise in frequency of surface melting at Siple Dome through the Holocene : evidence for increasing marine influence on the climate of West Antarctica

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): D02112, doi:10.1029/2007JD008790.A new melt layer history from Siple Dome, West Antarctica, indicates notable late-Holocene summertime warming. Visual stratigraphic analyses of the 1004-m ice core identified 62 years with melt layers. Melting events began around 11.7 ka, followed by a period of no melting from 8.8–6.6 ka. Melt layer frequency increased from 6.6 ka to the present, with the 1000-year-average melt layer frequency reaching a maximum of 2% at 0.8 ka. We use our millennial-scale archive of melt events as a unique seasonal paleothermometer to elucidate changes in West Antarctic Holocene summer climate. Our calibration suggests the change in melt frequency from 0% to 2% may represent a summer temperature increase of ≥2°C from the middle to late Holocene. This temperature change cannot be explained entirely by local change in ice elevation or summer insolation and is in contrast to East Antarctic climate records, which show peak warmth in the early Holocene followed by stable or decreasing temperature. We interpret the rise in melt frequency as evidence of an increasing marine influence on the Ross Sea sector of West Antarctica. Although the surface elevation of Siple Dome has not changed greatly, the continued lateral retreat of the West Antarctic ice sheet from its Last Glacial Maximum configuration (across the outer continental shelf), and the delayed drawdown in ice thickness from the adjacent coastal Marie Byrd Land region, in conjunction with periods of increased cyclogenesis, perhaps related to variations in ENSO, would allow a moderated maritime climate to more easily reach West Antarctica.This research was supported by NSF grant OPP-9814485 and NASA grant NAG5-7776 to Penn State University and by a NASA Earth System Science Graduate Fellowship and a WHOI Postdoctoral Scholar Fellowship to S. Das. Additional support to R. Alley at PSU is from NSF grants 0440899, 0440447, and 0424589 and the Comer Science and Education Foundation

The Greenland Ice Sheet Project 2 Depth-age Scale: Methods and Results

The Greenland Ice Sheet Project 2 (GISP2) depth-age scale is presented based on a multiparameter continuous count approach, to a depth of 2800 m, using a systematic combination of parameters that have never been used to this extent before. The ice at 2800 m is dated at 110,000 years B.P. with an estimated error ranging from 1 to 10% in the top 2500 m of the core and averaging 20% between 2500 and 2800 m. Parameters used to date the core include visual stratigraphy, oxygen isotopic ratios of the ice, electrical conductivity measurements, laser-light scattering from dust, volcanic signals, and major ion chemistry. GISP2 ages for major climatic events agree with independent ages based on varve chronologies, calibrated radiocarbon dates, and other techniques within the combined uncertainties. Good agreement also is obtained with Greenland Ice Core Project ice core dates and with the SPECMAP marine timescale after correlation through the δ18O of O2. Although the core is deformed below 2800 m and the continuity of the record is unclear, we attempted to date this section of the core on the basis of the laser-light scattering of dust in the ice

Do volcanic eruptions enhance or diminish net primary production? Evidence from tree rings

Low growth rates of atmospheric CO_2 were observed following the 1991 Pinatubo (Luzon) volcanic eruption. One hypothesis for this CO_2 anomaly is that since diffuse light is more efficiently used by forests than direct light, the increase in the diffuse fraction of sunlight due to scattering by volcanic sulfur aerosol in the years following the eruption substantially increased forest net primary production (NPP). However, other observations suggest a decrease in northern forest NPP because of the cooler conditions following the eruption. Here we used a global database of dated tree ring widths (which correlate with forest NPP) to test this hypothesis. Ice core records of sulfur deposition allowed us to identify the timing and magnitude of 23 Pinatubo‐scale eruptions since 1000 CE. We found a significant decrease in ring width for trees in middle to high northern latitudes (north of 45°N) following eruption sulfur peaks. Decreases in tree ring widths were in the range of 2–8% and persisted for ∼8 years following sulfur peaks, with minima at around 4–6 years. Ring width changes at lower latitudes in the Northern Hemisphere (30°N to 45°N) and in the Southern Hemisphere (30°S to 56°S) were not significant. In the tropics (30°N to 30°S) the paucity of tree ring records did not permit the evaluation of NPP changes. Given that elevated aerosol levels and summer cooling last only ∼2–3 years after an eruption, the persistence of declines in northern tree growth for up to 8 years after eruptions implies some additional mechanism that links these shorter‐lived global eruption effects to sustained changes in tree physiology, biogeochemistry, or microclimate. At least for this sample of trees, the beneficial effect of aerosol light scattering appears to be entirely offset by the deleterious effect of eruption‐induced climate change