The urgency of assessing the greenhouse gas budgets of hydroelectric reservoirs in China

Abstract

NATURE CLIMATE CHANGE | VOL 3 | AUGUST 2013 | www.nature.com/natureclimatechange C hina overtook the United States and became the world's largest electricity producer and consumer in 2011. The total electricity generation reached 4,722 TW h in 2011, with thermal power and hydropower contributing to 82.5% and 14.0% of the generated electricity, respectively 1 . Meanwhile, over 90% of the thermal electricity is generated by highly polluting coal-fired generation, which had a high grid emission factor of 804 g CO 2 equivalent (CO 2 e) per kW h in 2010 2 . Coal-fired plants are not only the largest stationary source emitters of greenhouse gases (GHGs) in China, but they also bring serious air pollution problems with releases of SO 2 , NO x and fine particles. Hydropower, with more than 200 GW of total installed capacity, accounts for approximately 6% of the total energy supply and over 80% of all electricity generated from renewable sources in China. Joining the global efforts to combat climate change, China is committed to increase the share of non-fossil fuels in its total energy mix to 15% by 2020. Accordingly, the production capacity of hydropower should increase by, on average, 10 to 15 GW yr −1 to help achieve this goal. Cascades of dams are currently being built, planned or proposed on the major river valleys in southwest China, where over two-thirds of the country's hydroelectric resources are located (Fig. 1). With physical water scarcity, and spatially and temporarily uneven distribution of water resources 3 , reservoir development is also a key component of China's recent heavy investment in water infrastructures to adapt to climate change, droughts and floods, and to improve food security 4 .Although hydropower is widely claimed to be a 'clean' energy source, critics have long warned of the environmental impact of large hydroelectric projects. In particular, research over the past two decades has revealed that the decay of organic matter (OM) in reservoirs could release significant amounts of CO 2 and CH 4 , and the emissions from tropical reservoirs characterized by high levels of OM and shallow depths are much higher compared with those in boreal and temperate regions [5][6][7][8][9][10][11][12][13] . Methane is 25 times more effective at trapping heat in the atmosphere than CO 2 over a 100-year period, and is responsible for over 20% of the change in the radiative forcing of the climate system 14 . It has been estimated that large dams release approximately 104 Tg CH 4 annually, or 20% of the total emissions from natural and anthropogenic sources 15 , although current estimations on GHG emissions from global hydroelectric reservoirs bear large uncertainties 13,16 .Carbon dioxide, methane and nitrous oxide are the major GHGs that can be produced in reservoirs, but nitrous oxide emissions are The urgency of assessing the greenhouse gas budgets of hydroelectric reservoirs in China Yuanan Hu, Hefa Cheng* Already the largest generator of hydroelectricity, China is accelerating dam construction to increase ...

Enhanced greenhouse gas emission from exposed sediments along a hydroelectric reservoir during an extreme drought event

“…Lakes and reservoirs are important sources of C evasion from inland waters to the atmosphere; recent estimates of the global CO 2 evasion from standing water systems range from 0.32 to 0.53 Pg C yr −1 (Tranvik et al 2009, Raymond et al 2013. Hydroelectric reservoirs account for 25% of the global area used for artificial freshwater systems, yet their role as a C source has been underappreciated (Barros et al 2011, Hu andCheng 2013). Large C emissions from flooded soils and vegetation following dam construction can offset the reduction of greenhouse gas (GHG) emission associated with hydroelectricity generation (Giles 2006, Fearnside and Pueyo 2012, de Faria et al 2015.…”

“…Large C emissions from flooded soils and vegetation following dam construction can offset the reduction of greenhouse gas (GHG) emission associated with hydroelectricity generation (Giles 2006, Fearnside and Pueyo 2012, de Faria et al 2015. Although CO 2 and CH 4 emissions from hydroelectric reservoirs have been suggested to comprise a small fraction of the global C evasion from inland waters (Barros et al 2011), uncertainties remain unresolved regarding spatiotemporal variations in the production and consumption of CO 2 and CH 4 across seasons and reservoir components (Fearnside andPueyo 2012, Hu andCheng 2013). While high GHG production in flooded vegetation and soil organic matter usually occurs during the initial flooding phase (Abril et al 2005, Teodoru et al 2010, C evasion rates can also increase drastically in drawdown areas in response to periodic flooding and draining (Chen et al 2009, Hu andCheng 2013).…”

Estimating greenhouse gas emissions from future Amazonian hydroelectric reservoirs

“…Moreover, although the literature about the C balance in reservoirs has advanced considerably in the last decades, predicting the C budget for future reservoirs is still challenging because of the difficulty in representing the spatial and temporal variability of the C fluxes (Galy-Lacaux et al 1999, Roland et al 2010). Given the high number of dams planned in the Amazon region and in other countries like China, it is imperative to develop models to estimate the C balance of large hydropower projects in order to support decisionmaking before the dam construction (Hu and Cheng 2013).…”

“…The GHG flux rates into the atmosphere from a tropical reservoir depend on a complex combination of physicochemical, meteorological, and reservoir features (St. Louis et al 2000, Abril et al 2005, Guérin et al 2006, Kemenes et al 2007, 2011, Barros et al 2011, Demarty and Bastien 2011, Fearnside and Pueyo 2012, Goldenfum 2012. Part of the difficulty of quantifying the C balance spatial and temporal variability of future reservoirs resides in an incomplete understanding of the physical, chemical, and biological processes involved in the production, consumption, and C outgas from reservoirs (Hu and Cheng 2013). For example, GHG production rate and C fate from flooded trunks is still undetermined (Guérin et al 2008).…”

Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems: Concepts, emerging trends, and research challenges

“…Although large pulses of GHGs are released from the flooded vegetation and soil OM during the initial flooding phase (Abril et al, 2005;Chen et al, 2009;Hu and Cheng, 2013;Deshmukh et al, 2016Deshmukh et al, , 2017, sedimentation can accumulate a growing amount of C in reservoir sediments, greatly decreasing the rate of CO2 release from aging reservoirs (Barros et al, 2011). 325…”