Some of those gases in the chimney system such as chlorofluorocarbons found in refrigerants and aerosols and bromine compounds found in products such as fire extinguishers are man-made and can become trapped in the stratosphere, lingering there for years. Those rays can contribute to the destruction of crops as well as human skin cancer.
Other compounds, especially the more reactive bromine compounds, can be made naturally, however. Over the last decade scientists have more closely looked at the amount of bromine in the stratosphere and realized it must have sources beyond the long-lived, man-made compounds. The consensus was that natural bromine compounds were being produced by marine organisms and released into the atmosphere.
These compounds are relatively short-lived, however, so the scientists suggest that they react in the tropical atmosphere to form inorganic bromine containing compounds, such as bromine monoxide, which can eventually lead to ozone depletion. Phytoplankton and other plants in the surface ocean can emit gases containing bromine and also chlorine and iodine into the water, which then escape into the atmosphere. Although the distribution of these emissions is still uncertain, measurements have indicated that the tropical oceans could be major sources, lofting them into the atmosphere where they can ultimately contribute to reactions that control tropospheric and stratospheric ozone.
So, the scientists are actively trying to understand how ocean biology might respond to changes in water circulation, nutrient supply, temperature and other factors, all of which could influence the reactive gas emissions and, in turn, feed into the chemical cycles that further influence climate through changes in greenhouse gases. Besides increasing or decreasing the levels of ozone in the upper atmosphere, some of the chemicals also contribute directly to the greenhouse effect.
For example, added water vapor pumped into the upper atmosphere from the chimney increases the amount of energy trapped there, in turn heating the planet further. Defining those linkages, however, is an obviously complex task, according to Ross Salawitch, a University of Maryland, College Park, professor of atmospheric and oceanic sciences and another principal investigator.
Efforts are underway to conduct similar aircraft observations in the region of the Asian monsoon. The researchers have also identified smaller circulations that could significantly affect atmospheric chemistry such as the North American monsoon and convections over Africa. Different models show similar global Hg deposition patterns, especially near source regions, and can reproduce major features of observed wet deposition Travnikov et al.
However, in some regions, there are substantial differences in simulated attributions of Hg deposition to local versus international sources.
In fact, many background soils also show periods of net Hg 0 deposition Gustin et al. On the other hand, Agnan et al. When considering vegetation uptake of atmospheric Hg, most terrestrial ecosystems turn into net atmospheric Hg 0 sinks see below. Recent global model simulations now reflect reduced soil Hg 0 evasion Amos et al. Over land, Hg II deposition is minor compared to Hg 0 deposition, and the dominant pathway of atmospheric deposition to terrestrial ecosystems is through litterfall.
In addition, further plant depositions occur via woody tissues, including by tree blowdown Mitchell et al. Wright et al. Unfortunately, there is a lack of plant and associated Hg litterfall and throughfall deposition data outside of forest ecosystems, such that estimates for global grasslands, savannas, and shrublands remain highly uncertain. This is likely also true for other areas of the world, for example, the tropics, where some of the highest litterfall Hg deposition is observed Wang et al.
Recent field studies also show evidence for an active Hg 0 sink in soils Sigler and Lee ; Moore and Castro ; Obrist et al. Finally, whole-ecosystem flux measurements using micrometeorological techniques can further confirm the dominant Hg 0 deposition source to terrestrial ecosystems. Previous studies over grasslands also reported a net annual dry Hg 0 deposition [ Other studies, however, have reported inconsistent results, with net terrestrial Hg 0 emissions even in the presence of vegetation Lindberg et al.
These important global hotspot regions for Hg II deposition need to be confirmed by direct deposition measurements. Air—sea exchange of Hg 0 is critical to extending the lifetime of anthropogenic Hg in the atmosphere, terrestrial ecosystems, and the ocean Strode et al. There is, however, substantial uncertainty in the magnitude of air—sea exchange from different ocean regions. Simultaneous measurements of dissolved Hg 0 in seawater and atmospheric concentrations are used to assess this source, with recent latitudinal gradients in both the Atlantic and Pacific Oceans showing distinct spikes in concentrations around the Intertropical Convergence Zone ITCZ.
These spikes are thought to result from deep convection and intense precipitation in these regions that increase atmospheric Hg II deposition Soerensen et al. Using high-frequency measurements of atmospheric and aquatic Hg 0 concentrations, Soerensen et al. This would effectively lower the magnitude of the global evasion flux of Hg 0 from the ocean because scavenging of Hg from surface waters would reduce the pool of Hg II available for reduction and conversion to Hg 0. Louis et al. Recent Hg stable isotope analyses have been able to differentiate among inorganic Hg sources to freshwater. Similarly, analyses of sediment of the Laurentian Great Lakes indicate that atmospheric Hg II deposition is the dominant Hg source to Lakes Huron, Superior, and Michigan, while terrestrial catchment and industrial effluent are the dominant Hg sources to Lakes Erie and Ontario Jackson et al.
Contrasting results have been reported for a boreal forest catchment in Sweden, where Hg stable isotope analyses suggest that Hg in forest runoff originated from the deposition of Hg 0 through foliar uptake, rather than from precipitation Jiskra et al. Analyses of dated lake sediment cores and catchment soils using geochemical tracers can also be used to tease apart changes in atmospheric and terrestrial Hg II inputs over time Fitzgerald et al. Recent analyses of Hg stable isotopes in dated lake sediment cores have also been used to examine temporal changes in Hg sources Sonke et al.
For pelagic ocean regions, the dominant source of Hg is atmospheric deposition Soerensen et al. Rivers comprise only a small fraction of Hg input to most ocean basins because the majority of Hg is in the particle phase and settles in ocean margins. Zhang et al. The large shelf and relatively smaller surface area of the Arctic Ocean make riverine inputs of Hg a more important source.
For example, modeling studies have used atmospheric observations to infer a large missing Hg source from rivers and coastal erosion Fisher et al. As illustrated in Fig. Critical factors affecting future anthropogenic emissions include energy use and Hg emission control strategies [e. One of the largest anthropogenic sources of Hg, ASGM emissions, will critically affect future Hg emissions, but is also associated with the largest uncertainties in future emission estimates.
A study by Giang et al. Declining Hg emissions in China through could, however, be offset by increasing Indian emissions Giang et al. Rafaj et al. Pacyna et al. Lei et al. Although less important, atmospheric processes can also be directly impacted by climate change; for example, the oxidation rate constant of the Hg—Br reaction is sensitive to temperature. Changes in precipitation patterns can change spatial distribution, magnitude, and seasonal variation of Hg II deposition. Megaritis et al.
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Hansen et al. Major impacts on global Hg cycling are expected due to shifts in global biomes, hydrology, fire patterns, water table depth, soil moisture, and redox conditions. In addition, forestry practices and deforestation can affect watershed Hg processes and MeHg exposures and biomagnification, although responses can be variable between different watersheds discussed in detail in Hsu-Kim et al. Based on the importance of vegetation for atmospheric Hg deposition, predicted shifts in biomes will directly impact terrestrial and aquatic Hg distribution and impacts.
Modeling studies have predicted a strong sensitivity of Hg 0 dry deposition to changes in vegetation Krabbenhoft and Sunderland ; Zhang et al. Regionally decreased Hg 0 dry deposition may occur in South Asia and Africa, for example, where projected increases of agricultural land area will lead to losses of leafy areas. Strong shifts in soil Hg accumulation have been predicted as well. Hararuk et al. The combined effects of increased CO 2 , increased temperature, and increased or decreased precipitation will likely lead to pronounced regional differences in soil Hg changes. Anticipated shifts in coniferous versus deciduous forest abundance will also likely impact terrestrial Hg distribution.
Richardson and Friedland suggested that anticipated losses of 2. These anticipated effects of climate change and ecosystem properties on Hg cycling can be seen clearly in archive studies.
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In a remote lake in Patagonia, for example, pre-anthropogenic changes in sediment loads up to a factor of four were found, comparable to recent anthropogenic forcing Hermanns et al. Similarly, Rydberg et al. Other ecosystem disturbances are expected to affect Hg cycling, but in unknown directions. For example, changes in wildfire frequency and abundance will impact atmospheric Hg emissions and watershed fluxes in a nonlinear fashion given the complex relationships between Hg emissions and fire intensities, pre-fire Hg accumulation, and post-fire Hg mobilization.
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Wildfire Hg emissions now likely include substantial emissions of anthropogenic legacy Hg accumulated in litter and soils, and climate and land use change impacts on fire frequency and activity need to be considered as part of human impacts Westerling et al. A recent modeling study Kumar et al.
Corresponding land use changes may amplify or alleviate this effect, e. In addition, land management practices such as burning of agricultural fields, grassland, and brush fires contribute to Hg emissions, although little information is available on their global contribution. Land use changes such as shifts in land management and forestry practices also have the potential to mobilize terrestrial Hg pools, via increased erosion, changes in hydrological pathways, and changes in yields Kocman et al.
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Both oceans and freshwater ecosystems will be affected by changing Hg emissions and climate-induced alterations. Modeling studies show that response times for lakes can range from a few years to many decades Harris et al. Recovery of freshwater ecosystems to decreased atmospheric Hg emissions is expected when reduced atmospheric Hg loadings translates into a substantial decrease in DOC-bound MeHg and inorganic Hg catchment runoff Graham et al.
A recent long-term mass balance study in New York state suggested that Hg inputs to lakes from forested catchments could be very responsive to decreased Hg emissions Gerson and Driscoll A common disturbance of freshwater Hg cycling results from widespread reservoir creation, which generally amplifies MeHg production; reservoir effects are discussed in detail in Eagles-Smith et al.
Most studies of climate change impacts on freshwater Hg cycling have been conducted in sensitive ecosystems experiencing accelerated changes e. Jonsson et al. Peat mesocosm experiments suggest that changes in hydrological regimes and shifts in vascular plant communities may have a significant impact on Hg cycling in peatlands Haynes For example, lower, more variable water tables and the removal of Ericaceae shrubs significantly enhanced inorganic Hg and MeHg mobility in peat pore waters and MeHg export from snowmelt, likely from enhanced peat decomposition and internal regeneration of electron acceptors related to water table changes Haynes et al.
The effect on Hg of large-scale changes in the marine environment expected from climate change was explored by Krabbenhoft and Sunderland Increased seawater temperatures may enhance organic matter remineralization and the propensity for MeHg production in some regions of the ocean. Rapid changes in sea-ice cover and seasonality in the polar oceans are likely to exert a major impact on air—sea exchange of Hg in these regions, and further work may be needed to understand how temperature and sea-ice dynamics alter Hg dynamics in the Arctic Angot et al.
Changes in microbial community structure and ocean productivity will propagate through marine food webs in potentially unexpected ways, altering bioaccumulation. Fisher et al. They concluded that the dominant climate mode of the future in the Arctic may result in a lower reservoir of Hg in the Arctic Ocean because of enhanced air—sea exchange.
This work did not consider the effects of a changing melting terrestrial landscape, which may dramatically increase Hg input to the Arctic from the terrestrial environment. Soerensen et al. Stern et al. Finally, recent modeling suggests that the response times of marine fish tissue burdens to changes in Hg input will depend on the locations of MeHg production Li et al. Benthic sediment will respond much more slowly than the upper Ocean and estuarine surface waters Sunderland et al. Amos et al. We reviewed how our understanding of Hg global cycling has advanced in the last decade, focusing on environmental reservoirs and processes within and between these reservoirs.
With emerging large global datasets, in combination with improved models and analytical techniques, new constraints are possible on the magnitude of reservoirs and fluxes.