Polar firn air reveals large-scale impact of anthropogenic mercury emissions during the 1970s.

Mercury (Hg) is an extremely toxic pollutant, and its biogeochemical cycle has been perturbed by anthropogenic emissions during recent centuries. In the atmosphere, gaseous elemental mercury (GEM; Hg degrees ) is the predominant form of mercury (up to 95%). Here we report the evolution of atmospheric levels of GEM in mid- to high-northern latitudes inferred from the interstitial air of firn (perennial snowpack) at Summit, Greenland. GEM concentrations increased rapidly after World War II from approximately 1.5 ng m(-3) reaching a maximum of approximately 3 ng m(-3) around 1970 and decreased until stabilizing at approximately 1.7 ng m(-3) around 1995. This reconstruction reproduces real-time measurements available from the Arctic since 1995 and exhibits the same general trend observed in Europe since 1990. Anthropogenic emissions caused a two-fold rise in boreal atmospheric GEM concentrations before the 1970s, which likely contributed to higher deposition of mercury in both industrialized and remotes areas. Once deposited, this toxin becomes available for methylation and, subsequently, the contamination of ecosystems. Implementation of air pollution regulations, however, enabled a large-scale decline in atmospheric mercury levels during the 1980s. The results shown here suggest that potential increases in emissions in the coming decades could have a similar large-scale impact on atmospheric Hg levels.

High-resolution carbon dioxide concentration record 650,000-800,000 years before present.

Changes in past atmospheric carbon dioxide concentrations can be determined by measuring the composition of air trapped in ice cores from Antarctica. So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years. Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present. From previously published data and the present work, we find that atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout eight glacial cycles but with significantly lower concentrations between 650,000 and 750,000 yr before present. Carbon dioxide levels are below 180 parts per million by volume (p.p.m.v.) for a period of 3,000 yr during Marine Isotope Stage 16, possibly reflecting more pronounced oceanic carbon storage. We report the lowest carbon dioxide concentration measured in an ice core, which extends the pre-industrial range of carbon dioxide concentrations during the late Quaternary by about 10 p.p.m.v. to 172-300 p.p.m.v.

Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years.

Atmospheric methane is an important greenhouse gas and a sensitive indicator of climate change and millennial-scale temperature variability. Its concentrations over the past 650,000 years have varied between approximately 350 and approximately 800 parts per 10(9) by volume (p.p.b.v.) during glacial and interglacial periods, respectively. In comparison, present-day methane levels of approximately 1,770 p.p.b.v. have been reported. Insights into the external forcing factors and internal feedbacks controlling atmospheric methane are essential for predicting the methane budget in a warmer world. Here we present a detailed atmospheric methane record from the EPICA Dome C ice core that extends the history of this greenhouse gas to 800,000 yr before present. The average time resolution of the new data is approximately 380 yr and permits the identification of orbital and millennial-scale features. Spectral analyses indicate that the long-term variability in atmospheric methane levels is dominated by approximately 100,000 yr glacial-interglacial cycles up to approximately 400,000 yr ago with an increasing contribution of the precessional component during the four more recent climatic cycles. We suggest that changes in the strength of tropical methane sources and sinks (wetlands, atmospheric oxidation), possibly influenced by changes in monsoon systems and the position of the intertropical convergence zone, controlled the atmospheric methane budget, with an additional source input during major terminations as the retreat of the northern ice sheet allowed higher methane emissions from extending periglacial wetlands. Millennial-scale changes in methane levels identified in our record as being associated with Antarctic isotope maxima events are indicative of ubiquitous millennial-scale temperature variability during the past eight glacial cycles.

Atmospheric trends and radiative forcings of CF4 and C2F6 inferred from firn air.

The atmospheric histories of two potent greenhouse gases, tetrafluoromethane (CF4) and hexafluoroethane (C2F6), have been reconstructed for the 20th century based on firn air measurements from both hemispheres. The reconstructed atmospheric trends show that the mixing ratios of both CF4 and C2F6 have increased during the 20th century by factors of approximately 2 and approximately 10, respectively. Initially, the increasing mixing ratios coincided with the rise in primary aluminum production. However, a slower atmospheric growth rate for CF4 appears to be evident during the 1990s, which supports recent aluminum industry reports of reduced CF4 emissions. This work illustrates the changing relationship between CF4 and C2F6 that is likely to be largely the result of both reduced emissions from the aluminum industry and faster growing emissions of C2F6 from the semiconductor industry. Measurements of C2F6 in the older firn air indicate a natural background mixing ratio of <0.3 parts per trillion (ppt), demonstrating that natural sources of this gas are negligible. However, CF4 was deduced to have a preindustrial mixing ratio of 34 -1 ppt (-50% of contemporary levels). This is in good agreement with the previous work of Harnisch et al. (18) and provides independent confirmation of their results. As a result of the large global warming potentials of CF4 and C2F6, these results have important implications for radiative forcing calculations. The radiative forcings of CF4 and C2F6 are shown to have increased over the past 50 years to values in 2001 of 4.1 x 10(-3) Wm(-2) and 7.5 x 10(-4) Wm(-2), respectively, relative to preindustrial concentrations. These forcings are small compared to present day forcings due to the major greenhouse gases but, if the current trends continue, they will continue to increase since both gases have essentially infinite lifetimes. There is, therefore, a large incentive to reduce perfluorocarbon emissions such that through the implementation of the Kyoto Protocol, the atmospheric growth rates may decline in the future.

Stable carbon cycle-climate relationship during the Late Pleistocene.

A record of atmospheric carbon dioxide (CO2) concentrations measured on the EPICA (European Project for Ice Coring in Antarctica) Dome Concordia ice core extends the Vostok CO2 record back to 650,000 years before the present (yr B.P.). Before 430,000 yr B.P., partial pressure of atmospheric CO2 lies within the range of 260 and 180 parts per million by volume. This range is almost 30% smaller than that of the last four glacial cycles; however, the apparent sensitivity between deuterium and CO2 remains stable throughout the six glacial cycles, suggesting that the relationship between CO2 and Antarctic climate remained rather constant over this interval.

Palaeoclimatology: the record for marine isotopic stage 11.

The marine isotopic stage 11 (MIS 11) is an extraordinarily long interglacial period in the Earth's history that occurred some 400,000 years ago and lasted for about 30,000 years. During this period there were weak, astronomically induced changes in the distribution of solar energy reaching the Earth. The conditions of this orbital climate forcing are similar to those of today's interglacial period, and they rendered the climate susceptible to other forcing--for example, to changes in the level of atmospheric carbon dioxide. Here we use ice-core data from the Antarctic Vostok core to reconstruct a complete atmospheric carbon dioxide record for MIS 11. The record indicates that values for carbon dioxide throughout the interglacial period were close to the Earth's pre-industrial levels and that both solar energy and carbon dioxide may have helped to make MIS 11 exceptionally long. Anomalies in the oceanic carbonate system recorded in marine sediments at the time, for example while coral reefs were forming, apparently left no signature on atmospheric carbon dioxide concentrations.

Eight glacial cycles from an Antarctic ice core.

The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long--28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.

Timing of atmospheric CO2 and Antarctic temperature changes across termination III.

The analysis of air bubbles from ice cores has yielded a precise record of atmospheric greenhouse gas concentrations, but the timing of changes in these gases with respect to temperature is not accurately known because of uncertainty in the gas age-ice age difference. We have measured the isotopic composition of argon in air bubbles in the Vostok core during Termination III (approximately 240,000 years before the present). This record most likely reflects the temperature and accumulation change, although the mechanism remains unclear. The sequence of events during Termination III suggests that the CO2 increase lagged Antarctic deglacial warming by 800 +/- 200 years and preceded the Northern Hemisphere deglaciation.