Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

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Climate Change 2007: The Physical Science Basis.

Collectively, reservoirs created by dams are thought to be an important source of greenhouse gases (GHGs) to the atmosphere. So far, efforts to quantify, model, and manage these emissions have been limited by data availability and inconsistencies in methodological approach. Here, we synthesize reservoir CH4, CO2, and N2O emission data with three main objectives: (1) to generate a global estimate of GHG emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates. We estimate that GHG emissions from reservoir water surfaces account for 0.8 (0.5-1.2) Pg CO2 equivalents per year, with the majority of this forcing due to CH4. We then discuss the potential for several alternative pathways such as dam degassing and downstream emissions to contribute significantly to overall emissions. Although prior studies have linked reservoir GHG emissions to reservoir age and latitude, we find that factors related to reservoir productivity are better predictors of emission.

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Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis

The increased use of hydropower is currently driving the greatest surge in global dam construction since the mid-20th century, meaning that most major rivers on Earth are now dammed. Dams impede the flow of essential nutrients, including carbon, phosphorus, nitrogen and silicon, along river networks, leading to enhanced nutrient transformation and elimination. Increased nutrient retention via sedimentation or gaseous elimination in dammed reservoirs influences downstream terrestrial and coastal environments. Reservoirs can also become hotspots for greenhouse gas emission, potentially impacting how ‘green’ hydropower is compared with fossil-fuel burning. In this Review, we discuss how damming changes nutrient biogeochemistry along river networks, as well as its broader environmental consequences. The influences of construction and management practices on nutrient elimination, the emission of greenhouse gases and potential remobilization of legacy nutrients are also examined. We further consider how regulating hydraulic residence time and environmental flows (or e-flows) can be used in planning and operation from dam conception to deconstruction. River damming can harness hydropower, control flooding and store water, but can also alter biogeochemistry in reservoirs and downstream environments. In this Review, the impacts of dams on nutrient cycling and greenhouse production are discussed, emphasizing the need to consider biogeochemical cycling at all stages of dam lifespan.

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River dam impacts on biogeochemical cycling

Soil water content.

Methane production, oxidation and mitigation: A mechanistic understanding and comprehensive evaluation of influencing factors.

Progress in the studies on the greenhouse gas emissions from reservoirs

Methane (CH4) emissions from the drawdown area of the Three Gorges Reservoir (TGR) have not been thoroughly investigated even though the drawdown area encompasses one third of the reservoir surface. In this study, CH4 emissions from different land uses were measured in the TGR drawdown area. The average diffusive CH4 emissions were 2.61, 0.19, 0.18, and 0.12 mg CH4 m(-2) h(-1) in rice paddies, fallow lands, deforested lands, and croplands, respectively, and were positively related to the duration of the inundated season among the latter three land uses. On average the drawdown areas studied here (except rice paddies) were sources in the inundated season (0.22 +/- 0.26 mg CH4 m(-2) h(-1)) and a sink in the drained season (-0.008 +/- 0.035 mg CH4 m(-2) h(-1)). The water level was the dominant factor that controlled whether the drawdown area was either inundated or drained, which in turn determined whether the drawdown area was a source or sink of CH4 emissions. The average diffusive CH4 emissions from the fallow lands, croplands, and deforested lands increased as the distance from the dam increased from Zigui (0.10 +/- 0.15 mg CH4 m(-2) h(-1)) to Wushan (0.15 +/- 0.29 mg CH4 m(-2) h(-1)) to Yunyang (0.24 +/- 0.27 mg CH4 m(-2) h(-1)), which could reflect different sediment characteristics and water velocities. The total CH4 emission from the drawdown area was estimated to range from 1033.5 to 1333.9 Mg CH4 yr(-1), which would account for 42-54% of the total CH4 emissions from the water surface of TGR.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JD017362

Surface methane emissions from different land use types during various water levels in three major drawdown areas of the Three Gorges Reservoir

Few investigations have been made on the impact of elevated ozone (O3) concentration on methane (CH4) emission from rice paddies. Using open-top chambers in situ with different O3 treatments, CH4 emissions were measured in a rice paddy in Yangtze River Delta, China in 2007 and 2008. There were four treatments applied: charcoal-filtered air (CF), nonfiltered air (NF), and charcoal-filtered air with different O3 additions (O3-1 and O3-2). The mean O3 concentrations during the O3 fumigation were 19.7, 22.6, 69.6 and 118.6ppb in 2007 and 7.0, 17.4, 82.2 and 138.3ppb in 2008 for treatments CF, NF, O3-1 and O3-2, respectively. The rice yields, as compared with CF, were reduced by 32.8% and 37.1%, 58.3% and 52.1% in treatments O3-1 and O3-2 in 2007 and 2008, respectively. The diurnal patterns of CH4 emission varied temporally with treatments and there was inconsistence in diurnal variations in CH4 emissions from the paddy field. The daily mean CH4 emissions were significantly lower in treatments O3-1 and O3-2 than those in treatments CF and NF. Compared with CF treatment, CH4 emissions from the paddy field were decreased to 46.5% and 38.3%, 50.6% and 46.8% under treatments O3-1 and O3-2 in the whole growing seasons of 2007 and 2008, respectively. The seasonal mean CH4 emissions were negatively related with AOT40 (accumulative O3 concentration above 40ppb; Po0.01 in both years), but positively related to the relative rice yield (reference to CF; Po0.01 in 2007 and Po0.001 in 2008), aboveground biomass (Po0.01 in both years) and underground biomass (Po0.01 in 2007 and Po0.05 in 2008). The decreased CH4 emission from the rice paddy due to an increased O3 exposure might partially mitigate the global warming potential induced by soil carbon loss under elevated O3 concentrations.

Effects of elevated ozone concentration on methane emission from a rice paddy in Yangtze River Delta, China

The natural rainwater and stormwater runoff samples from three underlying surfaces (rooftop, campus road and ring road) were sampled and analyzed from July to October, 2010 in Beijing. Eight rainfall events were collected totally and thirteen water quality parameters were measured in each event. Grey relationship analysis and principal component analysis were applied to assess composite water quality and identify the main pollution sources of stormwater runoff. The results show that the composite water quality of ring road runoff is mostly polluted, and then is rooftop runoff, campus road runoff and rainwater, respectively. The composite water quality of ring road runoff is inferior to V class of surface water, while rooftop runoff, campus road runoff and rainwater are in II class of surface water. The mean concentration of TN and NH4(+)-N in rainwater and runoff is 5.49-11.75 mg x L(-1) and 2.90-5.67 mg x L(-1), respectively, indicating that rainwater and runoff are polluted by nitrogen (N). Two potential pollution sources are identified in ring road runoff: (1) P, SS and organic pollutant are possibly related to debris which is from vehicle tyre and material of ring road; (2) N and dissolved metal have relations with automobile exhaust emissions and bulk deposition.

Research on evaluation of water quality of Beijing urban stormwater runoff

An automated chamber system was employed to measure the soil CO2 efflux (SCE) in situ for 2 years in a conventional wheat field of the Loess Plateau, China under semi-arid conditions. The annual mean SCE values were 2.44 ± 2.52 μmol m−2 s−1 in 2006 and 2.37 ± 2.33 μmol m−2 s−1 in 2007. Distinct seasonality in the SCE was observed, with significant differences occurring among four periods divided by harvesting, tillage and sowing. In the period from tillage to sowing, the mean SCE values were 2.82 and 2.69 times the annual mean values in 2006 and 2007, respectively, and SCE accounted for 39% and 48% of the annual total within 14% and 18% of the days of the years. Although there were significant exponential correlations between the SCE and soil temperature, and significant linear correlations between the SCE and soil moisture for measurements conducted in the periods before tillage and after sowing each year, the SCE from tillage to sowing was significantly beyond the correlation curves. These findings indicated that seasonal variation in the SCE in a conventional field was controlled not only by soil temperature and moisture, but also by tillage practice. The confounding effects of climate and practice on SCE should be considered when developing ways to mitigate soil carbon loss in conventional cropland in semi-arid regions.

Xiaoping Zhou

Papers

Progress in the studies on the greenhouse gas emissions from reservoirs

Surface methane emissions from different land use types during various water levels in three major drawdown areas of the Three Gorges Reservoir

Effects of elevated ozone concentration on methane emission from a rice paddy in Yangtze River Delta, China

Research on evaluation of water quality of Beijing urban stormwater runoff

Seasonal dynamics of soil CO2 efflux in a conventional tilled wheat field of the Loess Plateau, China