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2006-2008 Literature Review Archives - Atmospheric Composition

Brook, E. 2008. Windows on the greenhouse. Nature 453 15 May pp 291-292; Luthi, D., M. LeFloch, B. Bereiter et al., 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature 453 15 May pp 379-382; and Loulerge, L., A. Schilt, R. Spahni et al., 2008. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature 453 15 May pp 383-386.
New ice core results show that today's greenhouse gas concentrations are unprecedented in the past 800,000 years.
Scientists involved with the EPICA project in Antarctica have published two articles in the recent edition of the British journal Nature describing the results from the latest ice core study which now extends back 800,000 years. Luthi et al. present the record for carbon dioxide concentrations. The record shows a slightly broader range of pre-industrial concentrations, than the previous 650,000 year record, ranging from 172-300 ppm. The current atmospheric concentration of carbon dioxide is approx. 380ppm. In the companion paper, Loulergue et al. show that methane concentrations varied between approx. 350 and 800 ppb, in comparison to the present level of approx. 1770ppb. Taken together, these results show that current greenhouse gas concentrations have no past analogue in the ice core record, confirming conclusions from earlier studies. In addition, the general long-term behaviour of methane and carbon dioxide is still apparent in the extended record, with atmospheric concentrations rising and falling in tight linkage with temperature.

Canadall, J.G., Le Quere, C., Raupach, M.R. et al., 2007. Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of sinks. PNAS Oct 26 early edition at; Raupach,M.R., Marland,G., Ciais,P. et al., 2007. Global and regional drivers of accelerating CO2 emissions. PNAS 104 (24): 10288-10293.
Recent rapid growth in atmospheric CO2 blamed both on rising emissions and declining role of oceans as sinks.
Between 2000 and 2006, atmospheric CO2 concentrations increased by an average 1.93 parts per million per year, about 30% more rapidly than during the 1990s. Two papers recently published by an international team of scientists in the Proceedings of the National Academy of Science provide some explanations for this acceleration. Authors suggest that the primary reason is the rapid growth in emissions from fossil fuel combustion. All four key socio-economic factors that drive rising CO2 emissions - population growth, per capita economic growth, increase in the energy intensity of national economies and an increase in the carbon content of energy used - are implicated. Although the greatest growth in emissions has occurred in rapidly developing regions of the world (particularly China), no region of the world is currently decreasing the carbon intensity of its energy supply. The net result is a global emission rate that increased from an average 1.3%/year in the 1990s to 3.3%/year since. This places emission rates at the high end of the range of plausible SRES emission projections released by the IPCC in the late 1990s as the basis for policy response discussions. A secondary reason for the rapid rise in CO2 concentrations is a recent decline in the magnitude of land and ocean sinks for carbon. While these sinks removed enough excess CO2 from the atmosphere in the 1990s to annually offset an average 61% of total human emissions, this offset appears to have decreased to about 55% during 2000-2006. If this combination of accelerating human emissions and declining natural sinks continues, the human influences on the climate system are likely to be greater than anticipated.

Deng, F., Chen, J.M., Ishizawa, M. et al., 2007. Global monthly CO2 flux inversion with a focus over North America. Tellus 59B, 179-190.
Canadian and Japanese researchers have developed an improved atmospheric inversion model to assess the regional and seasonal distribution of land and ocean sources and sinks of carbon dioxide. The assessment, undertaken for 2003, shows that Canadian ecosystems were a net sink for about 340 MtC that year, primarily in the forests, grasslands and farmland of south of mid-latitudes.
Atmospheric inversion models can use outputs from an atmospheric circulation model together with data from the global CO2 monitoring station network to estimate regional carbon fluxes between the atmosphere and surfaces. In the past, because of computational challenges, these models have used course spatial resolutions, and have only been able to provide realistic estimates of fluxes at the continental scale. In the new model developed by a University of Toronto-led team of researchers, a high resolution North American land surface model was nested within the global model to allow more detailed analysis of fluxes at the regional level. Results show that, in 2003, the land area of North America collectively removed about 970 million tonnes of C (MtC) from the atmosphere, with about 340 Mt of this carbon being stored in Canadian ecosystems. Strongest Canadian sinks were in Alberta, Ontario and Quebec. Results are broadly similar to those obtained from carbon flux studies using surface flux measurements. On a global scale, the net carbon sink over all land areas for 2003 was estimated to be only 320 MtC, with the strong carbon sink for North America and other mid-latitude regions of the Northern Hemisphere being significantly offset by sources in the tropics and Southern hemisphere land masses.

Etiope, G., Lassey, K.R., Klusman, R.W. and Boschi, E. 2008. Reappraisal of the fossil methane budget and related emissions from geologic sources. Geophysical Research Letters 35, L09307, doi:10.1029/2008GL033623, 2008.
New assessments of average annual emissions of methane from natural geological sources, such as slow seepage from the Earth's crust, release from decaying hydrates and geothermal or volcanic vents suggest these may currently represent about 9% of the total emissions from all sources. This is about 3 times that of earlier estimates, and makes this a significant natural source.
Past isotopic studies of atmospheric methane have been used to estimate that about 20% of the average 582 million tones (Mt) of methane entering the atmosphere each year comes from fossil sources (which have no C14). Since 100 Mt of methane is believed to come from anthropogenic sources (through coal mining and the extraction and transportation of oil and natural gas), this suggested an average natural geological fossil source from deep-ground seepage, gas hydrates and geothermal or volcanic vents of about 15 Mt/year. An international team of researchers has now developed a revised assessment of these estimates, both through more accurate analysis of the fraction of C14 in atmospheric methane and improved estimates of ground sources. Their results suggest that total fossil sources of methane may be 50% greater that previously believed, and that the amount coming from natural geological sources may actually be on the order of 53 Mt per year (more than 3 times that of earlier estimates). This would make geological emissions the second largest natural source of atmospheric methane, after wetlands.

(1) Keppler, F., Hamilton, J.T.G., Braß, M. and Röckmann. 2006. Methane emissions from terrestrial plants under aerobic conditions. Nature 439:187-191.
(2) Schiermeier, Q. Methane finding baffles scientists. Nature 439:128.
(3) Lowe, D.C. 2006. A green source of surprise. Nature 439:148-149.
A new ground breaking study into natural sources of methane by a team of European scientists (Keppler et al.,) indicates that the science community's traditional understanding of natural sources of methane emissions from land biomass may have overlooked a significant and unusual source. Past research has suggested that most natural methane sources involve decay of biomass under oxygen depleted conditions like those found in wetlands, rice paddies and digestive systems of ruminant animals. Researchers involved in this study, however, used a large set of laboratory and field experiments to show that living plants (trees and grasses) emit large quantities of methane even when abundant oxygen is present. When scaled to a global perspective, their results suggest that trees and grasslands may emit some 150 million tonnes (± 60%) of methane each year. That is between 10 and 30% of the current estimate of total global emissions. The study also indicates that emissions increase with temperature, and hence tropical forests and grasslands end up contributing about 70% of this. The mechanism for such emissions is not known. Related commentaries in the same issue of Nature note that the study has provided some good news. First, this new source can help explain why satellite data have recently shown high methane concentrations over tropical forest regions. It may also explain why the rate of increase in atmospheric methane concentrations has decreased dramatically in recent years (reduced emissions from this source because of tropical deforestation). It also helps address some of the imbalances still remaining in the current global methane budget. The bad news is that many of the sources of natural methane emissions as currently understood must be wrong. Furthermore, if emissions rise as temperatures rise, increased methane emissions in response to warmer climates may add another positive feedback to human-induced climate change. This paper is certain to elicit considerable follow-up research.

Kurz, W.A., Stinson, G., Rampley, G. et al., 2008. Risk of natural disturbances makes contribution of Canada's forests to the global carbon cycle highly uncertain. PNAS 105:1551-1555.
A new study into the future role of fire and insect disturbances in Canada's boreal forests concludes that it highly likely that these forests will become a significant net source of carbon dioxide during the first Kyoto Protocol reporting period of 2008-2012. This has important implications for forest management programs aimed at reducing net emissions of greenhouse gases into the atmosphere.
In 2007, Canadian Forestry Service scientists published a scientific paper indicating that fire and insect disturbances are likely to cause Canadian forests to become a net source of atmospheric CO2 in the coming decades. The same team of researchers, led by Werner Kurz, have now published a sequel that looks more closely at the related implications for emissions in the first Kyoto Protocol (KP) reporting period of 2008-2012, and beyond to 2022. They use the latest version of the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to simulate a broad range of plausible future scenarios for net sources and sinks of greenhouse gases related to Canadian forests, using a Monte Carlo type approach. Probability distributions of the resulting 5000 different estimates for nation-wide emissions for 2008-2012 were then developed. All of the 5000 estimates project Canadian forests to be a net source in the first reporting period. The magnitude of this source various between 30 and 245 Mt of CO2. At the upper end, this is equal to about 30% of Canada's current anthropogenic emissions of all greenhouse gases. The model simulations indicate that Canada's forests were still a net sink for CO2 as recently as 2001. However, by 2002, the mountain pine beetle infestation in western Canada had changed this into a net source. This infestation, expected to peak in 2009, will have related effects on emissions through decay, wildfire and accelerated harvesting for several decades. Meanwhile another outbreak of spruce budworm infestation is expected soon in eastern Canada. The authors note that current KP guidelines for emission reporting fail to provide incentives for developing good forest management practices that enhance sinks if that country already has significant natural sources of greenhouse gases. Since the latter is the case for Canada, it has elected not to include results of good forest management practices in its KP reporting of emissions.

Peters, W., Jacobsen, A.P., Sweeney, C. et al., 2007. An atmospheric perspective on North American carbon dioxide exchange: Carbon tracker. PNAS 104:18925-18930.
Despite forest fires, Canadian forests appear to have been carbon sinks in recent years.
For most of the past century, Canadian forests are believed to have been accumulating biomass and hence functioning as a large sink for atmospheric carbon dioxide. Canadian Forestry Service models indicate that, with aging forests, increased wild fire and insect damage, this sink has at best become very small over the past few decades, and that Canadian forests may have actually become a net source of atmospheric carbon dioxide. However, a new study, published in a recent issue of the Proceedings of the National Academies of Science, suggests that, at least between 2000 through 2005, Canadian forests may have once again served as a sink. The study uses an atmospheric transport model, called 'Carbon Tracker', together with observations of variations in atmospheric CO2 concentrations across North America to determine which areas have been net biospheric sources of CO2, and which have been sinks. The calculations remove those emissions released through fossil fuel combustion, but include effects of wild fires. Averaged across the five years, the strongest sink region was the re-growing hardwood forests of the eastern USA. However, the second strongest was the North American coniferous forest, dominated by the Canadian boreal forest region. It removed an average 160 million tonnes of carbon from the atmosphere each year. Prairie grass and crop lands also appear to have provided a significant carbon sink. The magnitude of the coniferous forest sink varied considerably from year to year. Droughts during 2002 resulted in virtually no sink in the boreal region that year, in contrast to above average sinks during 2001 and 2004. While using significantly different techniques than those used for data included in a major report on North American carbon fluxes released in late November, results of this study are qualitatively similar.

Piao, S., Ciais, P., Freidlingstein, P. et al., 2008. Net carbon dioxide losses of northern ecosystems in response to global warming. Nature 451:47-53, and Miller, J.B. 2007. Sources, sinks and seasons. Nature 451:26-27.
Northern ecosystems are shown to be a source of CO2 in the fall season. These ecosystems are thought to be a small sink overall at present; however, if future autumn warming occurs more rapidly than spring warming, then this sink status could change.
In November of 2007, a team of international scientists reported that Canadian forests represent one of the global regions that have been a net sink for carbon in recent years (Peters et al., PNAS 104:18925-189302007). Now, a subsequent paper published in Nature by another group of investigators provides a closer look at the seasonal characteristics of net northern ecosystem carbon fluxes. This group used atmospheric CO2 concentration data collected from a network of monitoring stations to show that the annual transition from high winter CO2 concentration levels (when northern ecosystems are dormant) to low summer levels (when ecosystems act as large sinks) has occurred progressively earlier during the spring season. This is consistent with results from numerous other studies. Surprisingly, however, the transition from low summer to high winter CO2 concentration levels has been occurring earlier in the autumn season, despite an extended period of ecosystem removal of CO2 from the atmosphere through photosynthesis. Their analysis suggests this is because the concurrent increase in seasonal respiration has exceeded the photosynthetic sink, thereby causing a net source of atmospheric CO2 in autumn. While not yet enough to fully offset the enhanced spring sink, the authors caution that if future autumn warming occurs more rapidly than in spring, the net annual flux of northern ecosystems could become a source. An accompanying commentary notes that these results are a reminder of the importance of collecting observational data, particularly in poorly observed regions, and using these data to do reality checks on model simulations.

Raupach, M.R., G. Marland, P. Ciais, et al., 2007. Global and regional drivers of accelerating CO2 emissions. PNAS published online May 22, 2007; doi: 10 1073/pnas.0700609104.
A new study shows that CO2 emissions have risen since the year 2000 at a greater rate than in the 1990s. Observed global CO2 emissions are at the upper edge of the envelope of IPCC emissions scenarios. The increased growth rate is attributed to a cessation or reversal of earlier declining trends in global carbon intensity (emissions/energy) and global energy intensity (energy/GDP).
The growth rate of global carbon dioxide (CO2) emissions from fossil-fuel burning and industrial processes increased from 1.1%/year in 1990-1999 to >3% in 2000-2005. The actual global CO2 emissions trajectory is close to the highest-emissions scenario in the IPCC SRES scenarios envelope - A1FI. The recent growth rate in emissions was strongest in rapidly developing economies, particularly China, because of very strong economic growth coupled with an earlier decline in energy intensity of GDP. The developing and least-developed economies (80% of world population) accounted for 73% of global emissions growth in 2004 but only 41% of global emissions and only 23% of global cumulative emissions since the mid-18th century. The results of this analysis show the critical importance of global co-operation in reducing emissions of GHGs.

Read, K., A. Mahajan, L. Carpenter, M. Evans, B. Faria, D. Heard, J. Hopkins, J. Lee, S. Mollar, A. Lewis, L. Mendes, J. McQuaid, H. Oetjen, A. Saiz-lopez, M. Pilling and J. Plane. 2008. Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean. Nature, Vol 453, 26 June 2008, doi:10.1038/nature 07035.
Natural sources of bromine and iodine may result in tropospheric ozone destruction over the open tropical Atlantic Ocean. Further study will be needed to understand what impact this new process will have on the global ozone budget.
Tropospheric ozone plays many roles in the atmosphere. It is an important greenhouse gas and it is the main precursor of the hydroxyl radical that cleanses the air of pollutants. Prediction of future trends in tropospheric ozone requires an understanding of both precursor emissions and of ozone destruction processes. It was known that the tropical marine boundary layer is the most important region, globally, for loss of ozone, and also that bromine and iodine play a role in ozone loss in the marine boundary layer. However, previously these halogens were believed to be of importance only in coastal areas where biological sources had been identified (e.g. coastal kelp beds). This new paper has documented that bromine and iodine mediated ozone loss is also occurring in previously unsuspected regions of the ocean. The authors reported on measurements from Cape Verde, a volcanic island void of seaweed beds or other local sources. They found a daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. They also showed that the mean daily observed ozone loss is 50 percent greater than that simulated by a global chemistry model that excludes halogen chemistry. The authors performed a box model calculation that indicated that the observed halogen concentrations induced the extra ozone loss required for the models to match the observations. The authors conclude that halogen chemistry has a significant and extensive influence on photochemical ozone loss over the tropical Atlantic Ocean and that its omission in chemistry models may lead to significant errors. However, the significance of this finding to the global ozone budget is not yet clear. Tropospheric ozone is very dynamic and therefore these new processes may have only a relatively modest impact on the distribution of ozone, globally. Also, from a climate perspective, ozone is most radiatively active in the upper troposphere where the effects of losses in the marine boundary layer would be correspondingly less important.

Rigby, M. et al., 2008. Renewed growth in atmospheric methane. GRL, Vol 35, L22805, doi:10.1029/2008GL036037, 2008.
Atmospheric methane concentration shows first sign of an increase in a decade.
Methane concentration is largely a function of the balance between methane emissions (from both anthropogenic and natural sources) and methane destruction in the atmosphere by hydroxyl free radicals (OH). While methane concentrations have increased significantly over the past century, during the past decade or so, concentrations have stabilized, for reasons that are not yet well understood. In this paper, data from two global monitoring networks (AGAGE and CSIRO) are presented. These show renewed growth in atmospheric methane from the end of 2006 or beginning of 2007 until the most recent measurements. This change appeared almost simultaneously at all latitudes. The authors suggest two possible reasons for this change: 1) that an increase in emissions is solely responsible, in which case this would have had to occur simultaneously in both hemispheres, or 2) a decrease in the OH sink may have occurred, potentially reducing the required rise in emissions from one or both hemispheres. They used an inverse modeling approach to test these two theories, either maintaining a constant annual-average OH sink or varying the OH sink. In the latter experiments, OH concentrations were derived from measurements of atmospheric methyl chloroform. The authors were unable to draw definitive conclusions about which of these processes is most likely responsible for the recent rise in methane concentration, although there is some evidence of a decreasing OH sink.

Weiss, R.F., J. Muhle, P.K. Salameh, and C.M. Harth. Nitrogen triflouride in the global atmosphere. GRL, Vol 35, L20821, doi: 10.1029/2008GL035913, 3 pp.
The first measurements of the concentration of nitrogen triflouride (NF3) in the atmosphere show that the concentration is higher than previously assumed.
Nitrogen triflouride (NF3) is a very potent greenhouse gas that is increasingly being used in the electronics industry. A paper published earlier this year in the same journal reported that the global warming potential of NF3 over a 100-year time period is about 17,000 times that of CO2. This paper presents the first measurements of the concentration of NF3 in the atmosphere. The authors used stored samples of clean, background air collected from two sites in the Northern Hemisphere (Trinidad Head and La Jolla, California) over the period 1978 to 2008, and one site in the Southern Hemisphere (Cape Grim, Tasmania) in 1995 and again in 2005. The authors support their assumption that these sites can be considered representative of their respective hemispheres by noting the good agreement between these sites and real-time background trace gas measurements from the AGAGE network. The NH concentration data show a quasi-exponential growth curve, and while the data were too sparse in the SH to establish a trend, concentrations lagged those in the NH, consistent with sources being overwhelmingly in the NH. Based on the NH concentration data, the authors used a simple model to determine SH concentrations and the flux of NF3 into the atmosphere over a 30 year period. The mean global tropospheric NF3 concentration at the start of their measurement record in 1978 was 0.02 ppt. The model results yield a July, 2008 concentration of 0.454 ppt. This corresponds to an atmospheric burden of 5,380 metric tons of NF3. These first measured values of NF3 show significantly higher current concentrations than had been predicted based on very low estimates of how much of the NF3 in use is being emitted to the atmosphere. The authors conclude that the rise in NF3 concentration corresponds to about 620 metric tons of emissions per year, or about 16% of current global annual NF3 production. The authors contend that these results strengthen the case for inventorying and regulating NF3 emissions.

Zimov, S.A., S.P. Davydov, G.M. Zimova, A.I. Davydova, E.A.G. Schuur, K. Dutta, and F.S. Chapin III. 2006. Permafrost carbon: stock and decomposability of a globally significant carbon pool. GRL Vol. 33, L20502, doi: 10.1029/2006GL027484.
The authors provide, in this paper, the first quantitative estimate of carbon stocks in Siberian loess permafrost. In so doing, they address a gap in global carbon inventories, which to date have not included soil carbon from such soils, although they occupy extensive areas (1 million km2) of Siberia. These deep deposits accumulated during the Pleistocene when a steppe-tundra ecosystem dominated these lands. Sampled loess permafrost was analyzed for carbon content. They calculate that Siberian loess permafrost contains a large carbon pool of about 450GtC, based on their measured C concentration and bulk density, and on published reports on the areal extent, thickness and ice content of this soil type. Experimental work was also done to investigate decomposition rates for the soil carbon. They show that Siberian permafrost carbon decomposes quickly when thawed, both in in-situ and laboratory incubations. The typical release time for the C in Siberian permafrost would be in the order of a few decades after thaw, if conditions were similar to the experimental incubations, a finding that is of concern given recent and projected warming at high latitudes.

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