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2009 Literature Review Archives - Atmospheric Composition

Davidson, E.A. The contribution of manure and fertilizer nitrogen to atmospheric nitrous oxide since 1860. Nature Geoscience, Vol 2, doi:10.10.1038/NGEO608.

Animal manure and synthetic fertilizers have been key drivers of increasing atmospheric nitrous oxide concentrations since 1860.
Nitrous oxide (N2O) is the fourth most important anthropogenic greenhouse gas and estimates indicate that it accounts for 6% of total anthropogenic radiative forcing. Atmospheric concentrations of N2O have been increasing since the industrial revolution from 270 parts per billion (ppb) in 1860 to 319 ppb in 2005. The primary source of N2O is microbial production in soils which has increased with the use of synthetic nitrogen (N) fertilizers. However, the fraction of fertilizer-N that is emitted as N2O remains uncertain. Davidson employs a combination of two previously utilised approaches (top down and bottom up) to improve quantitative estimates of the amount of N2O coming from the various anthropogenic sources. Such information is required to prioritize mitigation strategies. Davidson estimates that 2% of manure nitrogen and 2.5% of fertilizer nitrogen were converted to N2O between 1860 and 2005. Fertilizer-N became an important source after 1960 whereas manure-N showed a more steady increase over the period. Contributions from these two dominant sources of concentrated N explain the observed pattern of increasing atmospheric N2O concentrations over this period. Davidson (2009) notes that fertilizer use on feed crops for animals raised for human consumption will likely produce double the N2O that would be produced if fertilizer were used to generate crops for direct human consumption or biofuels. This doubling occurs because N2O is released twice: once when the fertilizer is applied to the animal feed and again when the manure-N is recycled onto the soil. The author concludes that as animal protein consumption increases globally, effective management of manure will be key to reducing anthropogenic N2O emissions.

Khatiwala, S, F. Primeau and T. Hall, 2009, Reconstruction of the history of anthropogenic CO2 concentrations in the ocean, Nature, vol. 462, doi:10.1038/nature08526.
Knorr, W., 2009, Is the airborne fraction of anthropogenic CO2 emissions increasing?, GRL, vol. 36, doi:10.1029/2009GL040613.
Le Quéré, C. et al., 2009, Trends in the sources and sinks of carbon dioxide, ngeo, doi:101038/ngeo689.

Recent analyses suggest that current uptake of CO2 by ocean and terrestrial sinks is not keeping up with the increase in CO2 emissions. However, these results should be taken with caution, as large uncertainties are associated with the calculations.
Improved understanding and estimation of the fate of emitted anthropogenic CO2, in particular its partitioning between different natural sinks, is essential to understanding changes in atmospheric CO2 concentration. However, considerable uncertainties remain as to how these sinks are changing and more importantly, how they might change in the future. Three recent papers have re-examined data over different time periods and tried to quantify changes in the airborne fraction of emitted CO2 and in uptake by terrestrial and ocean CO2 sinks. Le Quéré et al. constructed a global carbon budget for each year of the period 1959-2008 and estimated the evolution of terrestrial and ocean CO2 sinks over this time period. The paper by Knorr focused on estimating trends in the airborne fraction of anthropogenic CO2 (that is, the amount of emitted CO2 that remains in the atmosphere) for the period 1850-2008. Khatiwala et al. examined the history of anthropogenic carbon in the ocean for the period 1765-2008. The three studies provide slightly different pictures of the fate of anthropogenic CO2. While Le Quéré et al report an increasing airborne fraction of 0.3±0.2% per year since 1959, Knorr finds no significant trend over the same period, nor over the longer period since 1850. However, in both studies, authors note that the trend is highly sensitive to estimates of land-use change emissions, which have large uncertainties. A number of studies in recent years have highlighted that both land and ocean sinks seem to be losing part of their ability to sequester anthropogenic CO2 emissions. However, Le Quéré and colleagues found that, while the magnitude of the global land sink increased between 1959 and 2008, the fraction of total anthropogenic CO2 emissions that was absorbed by the land shows no significant trend. This implies that growth in the rate of uptake of atmospheric CO2 is not keeping up with the rate of increase in CO2 emissions. A similar conclusion can be drawn from the results for ocean sinks. Both the studies by Le Quéré and Khatiwala et al. show an increasing global ocean sink since the end of the 1950s, but a weakening of the uptake since at least the 1980s. This indicates that, similar to the land sink, the rate of uptake of atmospheric CO2 by the ocean in recent decades increased at a lower rate than the growth in anthropogenic CO2 emissions. All three groups of authors emphasize the importance of the uncertainties in their calculations and results, and note that further analysis and reduction of the uncertainties are thus needed.

Tarnocai, C., J. G. Canadell, E. A. G. Schuur, P. Kuhry, G. Mazhitova, and S. Zimov (2009), Soil organic carbon pools in the northern circumpolar permafrost region, Global Biogeochem. Cycles, 23, GB2023, doi:10.1029/2008GB003327.

According to a new study, the amount of carbon in the northern circumpolar permafrost is more than double that previously estimated. Thus, the potential for carbon-climate feedbacks from this region, where the greatest global warming is predicted to occur, could have been substantially underestimated.
A recent study gives new estimates for the organic carbon pools in the soils of the northern circumpolar permafrost region. The estimates were calculated using the Northern Circumpolar Soil Carbon Database (NCSCD) developed by Agriculture and Agri-food Canada along with other databases. The estimates take into consideration all types of permafrost-affected soils and also non-permafrost soils in the region, down to a depth of 300 cm. In addition, two deep carbon pools (below 300 cm depth) not accounted for in previous analyses were considered: yedoma and deltaic deposits. The results indicate that the northern circumpolar permafrost region contains 1024 Pg of soil organic carbon in the 0-300 cm depth, including 496 Pg in the 0-100 cm depth, An additional 648 Pg of carbon is locked in deep layers of yedoma (407 Pg) and deltaic (241 Pg) deposits, for a total of 1672 Pg. This total value is significantly larger than previous estimates and indicates that soils in the northern circumpolar permafrost region contain about 50% of the reported global belowground organic carbon pool. In the discussion, the authors note the limitations and uncertainties of the databases used and caution about making direct comparisons with previous studies. However, their estimate for the first 100 cm alone (for which they have a higher level of confidence) is approximately double that reported in previous studies. In addition, their estimate for the total regional carbon pool is almost an order of magnitude higher than that used in many other studies. Therefore, the potential for carbon-climate feedbacks from this region, where the greatest global warming is predicted to occur, may have been substantially underestimated.

Wang, K., Dickinson, R.E. and Liang, S. 2009. Clear sky visibility has decreased over land globally from 1973 to 2007. Science 323:1468-1470.

New satellite data indicate that average aerosol cooling influences through solar dimming (which partially masks global warming effects of rising greenhouse gas concentrations) appear to have continued to increase over global land areas in recent years. This contradicts previous studies that suggest these masking effects are becoming weaker.
Past studies have suggested that changing aerosol content in the atmosphere led to solar dimming during the1960s to 1980s, but that clearer skies have resulted in a solar brightening in the past decade or so. Now new and more detailed data from satellites imply that the reported brightening has been limited to some regions. These data, presented in a study published in the journal Science by a team of American researchers, indicate that visibility has indeed improved over Europe between 1973 and 2007, but that it has steadily decreased over south and east Asia, South America, Australia and Africa. There was little change over North America. The net effect for all global land areas is a dimming over time, which contradicts recent IPCC findings. The study does not address trends over oceans.

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