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2010 Literature Review Archives - Atmospheric Composition

Beer, C., M. Reichstein, E. Tomelleri et al. 2010. Terrestrial gross carbon dioxide uptake: global distribution and covariation with climate. Science, Vol 329, pp 834-838, doi:10.1126/science.1184984;  Also, Mahecha, M.D. M. Reichstein, N. Carvalhais et al. 2010. Global convergence in the temperature sensitivity of respiration at ecosystem level. Science, Vol 329, pp 838-840, doi:10.1126/science.1189587.

Two recent studies explore key aspects of the global carbon cycle. Terrestrial gross primary productivity (GPP) is estimated to be 123 ±8 petagrams of carbon per year in recent years, consistent with previous estimates. However, terrestrial respiration in a variety of ecosystems is found to be lower than reported previously and independent of mean annual temperature, suggesting a less-pronounced climate-carbon sensitivity.

Terrestrial gross primary productivity (GPP; CO2 uptake through photosynthesis) and respiration (release of CO2) are the major processes controlling land-atmosphere CO2 exchange.  Understanding these global fluxes is essential for quantifying the climate-carbon cycle feedback.  Two recent studies published in Science use information from the global network of eddy covariance CO2 flux measurements (FLUXNET) (and diagnostic models in Beer et al.) to evaluate global land-atmosphere carbon exchange in recent years.  In the first paper, Beer and colleagues develop an observation-based estimate of GPP.  Spatial patterns of GPP permit quantification of the climate controls of GPP (precipitation, temperature and short-wave radiation are investigated) in different biomes around the globe.  They estimate a global terrestrial GPP of 123 ±8 petagrams of carbon per year during the period 1998-2005 (in line with previous estimates derived using different techniques).  Tropical forests and savannas account for 60% of the uptake over this period.  The results indicate that over about 40% of the vegetated land surface GPP is controlled by water availability (i.e. precipitation). Within savannahs, shrublands, grasslands and agricultural areas this can be up to 70%, indicating that productivity in these ecosystems is very sensitive to changes in precipitation, while productivity in tropical and boreal forests is shown to be more robust to changes in water availability.  In the second paper, Mahecha et al. explore the sensitivity of terrestrial ecosystem respiration to air temperature seeking to resolve the question of whether carbon cycle models should account for globally varying and environmentally controlled terrestrial ecosystem respiration.  At short timescales (i.e., <3 months), they find that, contrary to previous studies, sensitivity at the ecosystem level does not differ across biomes and is independent of mean annual temperature.  Their estimate suggests a less-pronounced climate-carbon cycle sensitivity than assumed by recent models.  At longer-timescales the authors found a more complex spatial pattern of ecosystem level respiration related to carbon pathways through slow pools. 

Bond-Lamberty, B. and A. Thomson. 2010. Temperature-associated increases in the global soil respiration record. Nature 464:579-583.

Global CO 2 emissions through soil respiration have increased by about 2% over the past two decades.  This is consistent with an accelerating global carbon cycle.

While carbon budget modelers have recognized that warming global temperatures will increase the rate of respiration and related CO2 emissions for global soils, the processes involved and the potential magnitude of the response are not well understood.  Researchers have now used a global database of ecosystem carbon flux observations to provide a better estimate of changes in soil respiration over the past two decades.  The results confirm that changes in the global release of carbon dioxide due to soil respiration are significantly and positively correlated with changes in temperature.  Furthermore, between 1989 and 2008, the annual CO2 release from global soils has increased by an average 0.1 GtC/year, reaching about 98 GtC/year by the end of that period – a net increase over the two decades of about 2%.  The authors acknowledge that this does not necessarily imply a positive climate feedback, since other aspects of the global carbon budget, including CO2uptake through photosynthesis, also respond to changing climate.  However, they argue that the data is consistent with an acceleration of the terrestrial carbon cycle in response to climate change.

Levin,I., T. Naegler, R. Heinz, et al. 2010. The global SF6 source inferred from long-term high precision atmospheric measurements and its comparison with emission inventories. Atmos. Chem. Phys. 10:2655-2662.

Analysis of observational data indicates that countries are significantly underreporting their emissions of SF6

Sulphur hexaflouride (SF6) is a very potent greenhouse gas, with an atmospheric lifetime of some 3200 years.  Therefore, it is included as one of the key greenhouse gases within the UNFCCC.  Furthermore, since there are no significant natural sources of SF6, changes in atmospheric concentrations of this gas over time provide a good proxy record of global emissions. New analyses of records of SF6 atmospheric concentrations show that global emissions decreased briefly between 1995 and 1998, but increased again during the past decade.  More importantly, the observations indicate that the collective emissions as reported by Annex I countries are only about 20-30% of that actually released on a global scale.  Authors of the study argue that there is clearly a need for a much better independent auditing of emissions reported by Annex I countries under the UNFCCC.

Mühle, J., A.L. Ganesan, B.R. Miller et al. 2010. Perflourocarbons in the global atmosphere: tetrafluoromethane, hexafluoroethane, and octafluoropropane. Atmos. Chem. Phys., 10:5145-5164.

New atmospheric measurements of three perfluorocarbons (PFCs) provide input to global top-down emission estimates which reveal significant discrepancies with previous bottom-up emission estimates. 

The PFCs tetrafluoromethane (CF4, PFC-14), hexafluroethane (C2F6, PFC-116) and octofluoropropane (C3F8, PFC-218) are among the longest-lived and most potent greenhouse gases regulated under the Kyoto Protocol. In this paper, Mühle et al. present atmospheric measurements of these three PFCs with significantly improved accuracy and precision. The new observations are used along with archived air samples to determine growth rates in atmospheric levels of the three PFCs over three decades.  The authors also present revised estimates of pre-industrial background concentrations based on air extracted from Greenland and Antarctic ice cores.. An inversion model (AGAGE 2-D 12-box model) is used to estimate source strengths for CF4, C2F6 and C3F8 from the atmospheric measurements and these are compared to previous emission estimates obtained from bottom-up studies. They find that CF4 abundances are 6–10% lower than previously reported, with concentrations in the southern hemisphere (SH) increasing from ~49.9 ppt in 1978 to ~76.9 ppt at the end of 2008 and from ~46.3 ppt in 1973 to ~78.0 ppt at the end of 2008 in the northern hemisphere (NH). C2F6 increased from ~0.96 ppt in 1978 to ~3.93 ppt at the end of 2008 in the SH and from ~0.75 ppt in 1973 to ~4.06 ppt at the end of 2008 in the NH. Tropospheric mixing ratios for C3F8 remain very low (<0.1 ppt) but have increased by an order of magnitude over the past three decades. UNFCCC CF4 and C2F6 emission estimates are only 30-70% of the global top-down estimates obtained in this study, and it is noted that data from a number of significant emitting countries (non-Annex 1 countries) are not included in the UNFCCC data. Significant discrepancies with bottom-up emissions estimates from other sources were also reported. 

Rice, A.L., C.L., Butenhoff, M.J. Shearer, et al. 2010. Emissions of anaerobically produced methane by trees. GRL 237,L03807, doi:10.1029/2009GL041565.

Recent research provides an alternative explanation for the finding of high methane emissions from tropical forests. A surprising earlier suggestion had been that methane may be produced aerobically within trees. This work suggests instead that more traditional anaerobic processes may be the cause.
Several years ago, a team of European scientists (Keppler et al, 2006) noted that observed methane emissions from tropical forests were much larger than expected, and suggested that trees in that region may produce methane through aerobic processes. Since past research has consistently suggested that terrestrial emissions of methane from land surfaces arise primarily from anaerobic processes (not involving oxygen) in water saturated soils or wetlands, this proposal met with considerable skepticism from biologists. Now a new study published in Geophysical Research Letters by a group of American researchers confirmed that broad leafed trees in three different flooded tropical forests research plots did indeed release significant amounts of methane from their leaves. The methane thus emitted, scaled up to the global area for flooded forests, could be as much as 10% of all global methane emissions. However, analyses of carbon isotopes in the emitted methane indicate that it is produced in the flooded soils under anaerobic conditions and somehow transported through the plant tissue and leaves into the atmosphere. While the authors note that the transport mechanisms within the trees are as yet not understood, and that the results, by themselves, do not rule out the possibility of biological production of methane through aerobic processes, such processes are not needed to explain the high tropical methane sources reported by Keppler and colleagues.

Shakhova,N., I. Semiletov, A. Salyuk,. et al. 2010.Extensive methane venting to the atmosphere from sediments of the East Siberian Arctic Shelf. Science 327:1246-1250: Heimann,M. 2010. How stable is the methane cycle? Science 327:1211-1212.

Eastern Siberian Arctic Shelf waters show evidence of significant escape to the atmosphere of old methane from ocean sediments.  This implies another important positive climate system feedback that is as yet not included in model projections for future climates.

Methane stored within frozen permafrost on land and beneath the ocean floor is one of the wild cards in predicting future climate change since there is a risk that warmer climates could trigger the rapid release of large volumes of such methane.  This would significantly enhance the change in climate currently projected by climate models.  Analysis of observational data from the East Siberian Arctic Shelf region suggests that some release of methane may already be happening.  The analyses of more than 5000 observations at many different locations over the region indicate that 80% of the bottom waters in this part of the Arctic Ocean are supersaturated with methane.  Since average water depth in the region is only about 45 meters, much of the methane makes its way to the surface before being oxidized, resulting in 50% of surface waters also being supersaturated with methane.  The net annual flux of methane from these waters into the atmosphere, both through diffusion from the surface and through bubbles rising directly from the bottom, is estimated to be about 8 Mt of C.  The authors note that this is similar in magnitude to past estimates for total methane emissions from all global ocean surfaces. They also note that although the oceanic methane flux needs revision, their results do not drastically alter the contemporary global methane budget. However, this feedback should be added as an important feedback within earth system models.

Zaehle, S., P. Friedlingstein, and A.D. Friend. 2010. Terrestrial nitrogen cycle feedbacks may accelerate future climate change. GRL 37,L01401, doi:10.1029/2009GL041345. Also, Jain, A., X. Yang, H. Kheshgi, et al. 2009. Nitrogen attenuation of terrestrial carbon cycle response to global environmental factors. Global Biogeochemical Cycles23, GB4028, doi:10.1029/2009GB003519; Also, Wang, Y.-P. and B.Z. Houlton. 2009. Nitrogen constraints on terrestrial carbon uptake: Implications for the global carbon-climate feedback. GRL 36,L24403, doi:10.1029/2009GL041009.

Three independent studies all indicate that the global nitrogen cycle has an important influence on the response of the global carbon cycle to rising atmospheric CO2 concentrations and changing climates. In general, they suggest dynamic inclusion of the nitrogen cycle in model simulations reduces net global terrestrial carbon sinks, which enhances the rate of increase in greenhouse gas radiative forcing relative to simulations that assume unlimited nitrogen availability.

Past research has identified the global climate-carbon cycle feedback as an important aspect of the response of the Earth's climate to rising concentrations of greenhouse gases. In general, studies have determined that rising carbon dioxide concentrations cause a fertilization effect that increases the uptake of carbon in ecosystems, but that rising temperatures eventually increase soil respiration and drier climates can significantly increase carbon loss from ecosystems through wild fire. However, most of these studies have not yet included dynamic interactions between the carbon cycle and the nitrogen cycle. In particular, most assume the available nitrogen supply for ecosystem growth is unlimited. Three new studies, presented by independent research teams, now shed some light on the importance of including dynamic nitrogen responses in coupled climate-carbon cycle simulations. One study, (Jain et al.) compared the response of the global carbon cycle to changes in climate and atmospheric CO2 concentrations during the 20th century in simulations with two versions of the Integrated Science Assessment Model (ISAM) - one that did not include a fully dynamic nitrogen cycle and one that did. They discovered that, while the total global carbon sink during the 20th century was similar, the response of the components of the carbon cycle as well as spatial distribution of net carbon flux differed significantly. Including the N cycle, for example, caused the CO2 fertilization effect during the 1990s to be significantly weaker but also reduced the carbon loss due to rising temperatures and changing precipitation. It also increased the net sink in high latitudes and decreased those in areas such as the southeastern USA. A second study (Wang and Houlton) used a theoretical analysis to conclude that most models that fail to include limitations of nitrogen supply as a constraint significantly overestimate land carbon uptake and hence underestimate the rate of rise in atmospheric CO2 concentrations in response to global emissions from fossil fuel combustion and land use change. Thirdly, Zaehle et al. report that their simulations with a dynamic vegetation model show that incorporating N dynamics reduces terrestrial carbon storage due to CO2 fertilization between 1860 and 2100 by about 50%. This is partially offset by a 16% reduction in soil C loss, primarily at high latitudes. The net effect remains a reduced global carbon sink that, by 2100, enhances the global radiative forcing caused by rising greenhouse concentrations by a modest 0.29 W/m2. All of these studies underscore the importance of including the dynamic nitrogen cycle in future coupled earth system model simulations.

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