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New Methodology for Robust Assessment of the Change in GHGs Induced by Dams

As far as dams are concerned, they are conventionally perceived as emitters of GHGs in big rivers. But a research group from Peking University in China has disrupted this understanding depending on the whole system thinking applied to the Three Gorges Dam (TGD) in China on the Yangtze River.

New Methodology for Robust Assessment of the Change in GHGs Induced by Dams.
Panoramic view of the Three Gorges Dam on the Yangtze River, China. Image Credit: ©Science China Press.

Under the guidance of Professor Jinren Ni, this study was carried out.

We spent over eight years to complete this work.

Jinren Ni, Professor, Key Laboratory for Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University

Thirty water quality indices were utilized by the researchers for around 25 years and quantified greenhouse gases (GHGs) along 4,300 km of the Yangtze River. Huge datasets and data-driven models were fixed with the help of machine learning tools for riverine greenhouse gases.

This highlighted the impact of initial emission of GHGs on pristine river ecosystems and it also offered a trustworthy methodology for strong evaluation of the change in GHGs induced by dams.

Since the operation began in 2003, the TGD has amended carbonate equilibrium in the reservoir area, improved methanogenesis in the upstream, but restrained methanogenesis, and denitrification through changing anoxic habitats via long-distance scouring downstream.

It is essential to include both the upstream and downstream reaches when analyzing the spatiotemporal scope of the so-called “large-dam effects” of the TGD.

A quantitative division was found between new emissions leading to impoundment and operation of the pristine and TGD emissions that naturally took place before construction. This results in a sudden finding that the TGD has made significant reductions in the annual average emissions of CH4, N2O and CO2, in the Yangtze River.

With the help of a whole-system method, the research group determines that the impact of TGD stretches thousands of kilometers downstream along the Yangtze River. This is far beyond the reservoir and the nearby surroundings (with only the latter taken into account in the majority of the earlier studies performed).

Their approach is based on the logic that a full accounting of GHG consequences of the dam should encompass the full sphere of the dam’s influence. This methodology led to the result that aquatic emissions have declined rather than increased since the dam went into operation.

Emily Stanley, Professor, University of Wisconsin-Madison

Being a clean energy source, hydropower has the potential to help decrease reliance on conventional fossil fuel energy (like oil, coal and natural gas) and considerably decrease GHGs emissions. In the past few years, a few scientists have indicated that dams would increase GHGs emissions from rivers.

This problem received renewed attention, thereby stimulating much debate on variations in GHGs emissions before and after the TGD operation. By offering a panoramic view of CH4, CO2 and N2O fluxes along the Yangtze River, the current study helped handle the debate on GHGs that are induced by big dams.

This means that the TGD facilitates the reduction of GHGs emissions, even when leaving aside the huge benefits of GHGs reduction from fossil fuel substitution by hydropower.

Jinren Ni, Professor, Key Laboratory for Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University

The analysis of the entire system offers new insights into GHGs emissions resulting from the operation of huge dams. These are considered essential prerequisites in comprehending their effects on the biogeochemical cycles of huge rivers.

As Professor Stanley noted, “Although TGD is unique in its size, it is unlikely to be unique in the scale and scope of its influence.”

Journal Reference:

Ni, J., et al. (2022) Three Gorges Dam: friend or foe of riverine greenhouse gases?. National Science Review.

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