ALBERT

Description: The triple-oxygen isotopic composition (Δ17O = δ17O-0.52 × δ18O) of sulfate and nitrate reflects the relative importance of their different production pathways in the atmosphere. A new record of sulfate and nitrate Δ17O spanning the last 2400 yr from the West Antarctic Ice Sheet Divide ice core project shows significant changes in both sulfate and nitrate Δ17O in the most recent 200 yr, indicating changes in their formation pathways. The sulfate Δ17O record suggests that an additional 12–18% of sulfate formation occurs via aqueous-phase production by O3, relative to that in the gas-phase in the present-day compared to the early 19th century. Nitrate Δ17O indicates a increasing importance of RO2 in NOx-cycling between the mid-19th century and present-day in the mid-to-high latitude Southern Hemisphere. The former has implications for the climate impacts of sulfate aerosol, while the latter has implications for the tropospheric O3 production rate in remote low-NOx environments. Using other ice core observations, we rule out drivers for these changes other than variability in extratropical oxidant (OH, O3, RO2, H2O2, and reactive halogens) concentrations. However, assuming OH, H2O2, and O3 are the main oxidants contributing to sulfate formation, Monte Carlo box model simulations require a large (≥ 210%) increase in the [O3]/[OH] ratio over the Southern Ocean in the early 19th century to match the sulfate Δ17O record. This unlikely scenario points to a deficiency in our understanding of sulfur chemistry and suggests other oxidants may play an important role in sulfate formation in the mid-to-high latitude marine boundary layer. The observed decrease in nitrate Δ17O since ~1860 CE is most likely due to an increased importance of RO2 over O3 in NOx-cycling and can be explained by a 48–84% decrease in the [O3]/[RO2] ratio in the extratropical Southern Hemisphere NOx-source regions.

Description: As part of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), we evaluate the historical black carbon (BC) aerosols simulated by 8 ACCMIP models against observations including 12 ice core records, long-term surface mass concentrations and recent Arctic BC snowpack measurements. We also estimate BC albedo forcing by performing additional simulations using offline models with prescribed meteorology from 1996–2000. We evaluated the vertical profile of BC snow concentrations from these offline simulations using the recent BC snowpack measurements. Despite using the same BC emissions, the global BC burden differs by approximately a factor of 3 among models due to differences in aerosol removal parameterizations and simulated meteorology: 34 Gg to 103 Gg in 1850 and 82 Gg to 315 Gg in 2000. However, the global BC burden from preindustrial to present-day increases by 2.5–3 times with little variation among models, roughly matching the 2.5-fold increase in total BC emissions during the same period. We find a large divergence among models at both Northern Hemisphere (NH) and Southern Hemisphere (SH) high latitude regions for BC burden and at SH high latitude regions for deposition fluxes. The ACCMIP simulations match the observed BC surface mass concentrations well in Europe and North America except at Jungfraujoch and Ispra. However, the models fail to predict the Arctic BC seasonality due to severe underestimations during winter and spring. The simulated vertically resolved BC snow concentrations are, on average, within a factor of 2–3 of the BC snowpack measurements except for Greenland and the Arctic Ocean. For the ice core evaluation, models tend to capture both the observed temporal trends and the magnitudes well at Greenland sites. However, models fail to predict the decreasing trend of BC depositions/ice-core concentrations from the 1950s to the 1970s in most Tibetan Plateau ice cores. The distinct temporal trend at the Tibetan Plateau ice cores indicates a strong influence from Western Europe, but the modeled BC increases in that period are consistent with the emission changes in Eastern Europe, the Middle East, South and East Asia. At the Alps site, the simulated BCsuggests a strong influence from Europe, which agrees with the Alps ice core observations. Models successfully simulate higher BC concentrations observed at Zuoqiupu during the non-monsoon season than monsoon season, but models underpredict BC in both seasons. Despite a large divergence in BC deposition at two Antarctic ice core sites, models are able to capture the relative increase from preindustrial to present-day seen in the ice cores. In 2000 relative to 1850, globally annually averaged BC surface albedo forcing from the offline simulations ranges from 0.014 to 0.019 W m−2 among the ACCMIP models. Comparing offline and online BC albedo forcings computed by some of the same models, we find that the global annual mean can vary by up to a factor of two because of different aerosol models or different BC-snow parameterizations and snow cover. The spatial distributions of the offline BC albedo forcing in 2000 show especially high BC forcing (i.e. over 0.1 W m−2) over Manchuria, Karakoram, and most of the Former USSR. Models predict the highest global annual mean BC forcing in 1980 rather than 2000, mostly driven by the high fossil fuel and biofuel emissions in the Former USSR in 1980.

Description: Refractory black carbon aerosols (rBC) from biomass burning and fossil fuel combustion are deposited to the Antarctic ice sheet and preserve a history of emissions and long-range transport from low latitudes. Antarctic ice core rBC records may thus provide information with respect to past combustion aerosol emissions and atmospheric circulation. Here, we present six East Antarctic ice core records of rBC concentrations and fluxes covering the last two centuries with approximately annual resolution (cal. yr. 1800 to 2000). The ice cores were drilled in disparate regions of the high East Antarctic ice sheet, at different elevations and net snow accumulation rates. Annual rBC concentrations were log-normally distributed and geometric means of annual concentrations ranged from 0.10 to 0.18 μg kg−1. Average rBC fluxes were determined over the time periods 1800 to 2000 and 1963 to 2000 and ranged from 3.4 to 15.5 μg kg−1 m−2 a−1 and 3.6 to 21.8 μg kg−1 m−2 a−1 respectively. Geometric-mean concentrations spanning 1800 to 2000 increased linearly with elevation at a rate of 0.025 μg kg−1/500 m. Spectral analysis of the records revealed significant decadal scale variability, which at several sites was comparable to decadal ENSO variability.

Description: Desert dust perturbs climate by interacting with incoming solar and outgoing long wave radiation, thereby changing precipitation and temperature, in addition to modifying ocean and land biogeochemistry. While we know that desert dust is sensitive to perturbations in climate and human land use, previous studies have been unable to determine whether humans were in the net increasing or decreasing desert dust. Here we present observational estimates of desert dust based on paleodata proxies showing a doubling of desert dust during the 20th century over much, but not all the globe. Large uncertainties remain in estimates of desert dust variability over 20th century due to limited data. Using these observational estimates of desert dust change in combination with ocean, atmosphere and land models, we calculate the net radiative effect of these observed changes (top of atmosphere) over the 20th century to be −0.14±0.11 W/m2 (1990–1999 vs. 1905–1914). The estimated radiative change due to aerosols is especially strong between the dusty 1980–1989 and the less dusty 1955–1964 time periods (−0.57±0.46 W/m2), which model simulations suggest may have reduced the rate of temperature increase between these time periods by 0.11 °C. Model simulations also indicate strong regional shifts in precipitation and temperature from the desert dust changes, causing 6 ppm (12 Pg C) reduction in model carbon uptake by the terrestrial biosphere over the 20th century. Desert dust carries iron, an important micronutrient for ocean biogeochemistry that can modulate ocean carbon storage; here we show that dust deposition trends increase ocean productivity by an estimated 6% over the 20th century, drawing down an additional 4 ppm (8 Pg C) of carbon dioxide into the oceans. Thus, perturbations to desert dust over the 20th century inferred from observations are potentially important for climate and biogeochemistry, and our understanding of these changes and their impacts should continue to be refined.

Description: Refractory black carbon aerosols (rBC) emitted by biomass burning (fires) and fossil fuel combustion, affect global climate and atmospheric chemistry. In the Southern Hemisphere (SH), rBC is transported in the atmosphere from low latitudes to Antarctica and deposited to the polar ice sheet preserving a history of emissions and atmospheric transport. Here, we present two high-resolution Antarctic rBC ice core records drilled from the West Antarctic Ice Sheet divide and Law Dome on the periphery of the East Antarctic ice sheet. Separated by ~3500 km, the records span calendar years 1850–2001 and reflect the rBC distribution over the Indian and Pacific ocean sectors of the Southern Ocean. Highly correlated over the past 60 yr, the records show that coherent large-scale changes in SH rBC occurred at decadal to inter-annual time scales, notably in ENSO-like periodicities. Decadal trends in the records are similar to inventories of SH rBC emissions from grass fires and biofuels. The combined records suggest a large-scale reduction in rBC from 1950 to 1990 over the remote Southern Hemisphere.

Description: We present and discuss a new dataset of gridded emissions covering the historical period (1850–2000) in decadal increments at a horizontal resolution of 0.5° in latitude and longitude. The primary purpose of this inventory is to provide consistent gridded emissions of reactive gases and aerosols for use in chemistry model simulations needed by climate models for the Climate Model Intercomparison Program #5 (CMIP5) in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). Our best estimate for the year 2000 inventory represents a combination of existing regional and global inventories to capture the best information available at this point; 40 regions and 12 sectors are used to combine the various sources. The historical reconstruction of each emitted compound, for each region and sector, is then forced to agree with our 2000 estimate, ensuring continuity between past and 2000 emissions. Simulations from two chemistry-climate models is used to test the ability of the emission dataset described here to capture long-term changes in atmospheric ozone, carbon monoxide and aerosol distributions. The simulated long-term change in the Northern mid-latitudes surface and mid-troposphere ozone is not quite as rapid as observed. However, stations outside this latitude band show much better agreement in both present-day and long-term trend. The model simulations indicate that the concentration of carbon monoxide is underestimated at the Mace Head station; however, the long-term trend over the limited observational period seems to be reasonably well captured. The simulated sulfate and black carbon deposition over Greenland is in very good agreement with the ice-core observations spanning the simulation period. Finally, aerosol optical depth and additional aerosol diagnostics are shown to be in good agreement with previously published estimates and observations.