Dams have a net negative impact on climate change, as they release twice as much carbon as they store, researchers have found.
With dams serving as reservoirs for drinking water, agricultural irrigation, or the operation of hydropower plants, dams were believed to act as net carbon stores.
Researchers from the Helmholtz Center for Environmental Research (UFZ), along with Spanish scientists from the Catalan Institute for Water Research (ICRA) in Girona, now think this is not the case.
Streams typically transport large amounts of carbonaceous material such as leaves, branches or algae. As the water is dammed, the material gradually settles and accumulates at the bottom of the water. “Due to lack of oxygen, the decomposition processes there are much slower.
This releases less carbon dioxide. The carbon it contains is stored in the sediment of the dam for a longer period of time, ”says Dr. Matthias Koschorreck, a UFZ biologist. “Dams were thought to store about as much carbon as they are released as greenhouse gases.
” However, not only the zones covered by water play a role in the carbon balance of water bodies, but also the zones that temporarily dry out due to a drop in the water level.
When the carbonaceous material that was previously covered with water comes into contact with oxygen from the air, decomposition processes and thus the formation of carbon dioxide are strongly driven.
“Areas that dry out therefore emit considerably more carbon than areas that are under water,” says Philipp Keller of the UFZ. “If large amounts of water are released at a dam, large areas are suddenly released.
But these areas were not taken into account when calculating the carbon balance. This is the knowledge gap that we close with our work ”.
The researchers used a database based on satellite images for their research. It contains monthly data on the size of the water surfaces of approximately 6,800 dams worldwide between 1985 and 2015.
During these 30 years, scientists were able to determine exactly when, where and for how long the dams were not completely filled and the size of the arid areas. On average, 15 percent of the total reservoir surface was not covered with water.
The scientists used this figure to further calculate the carbon release from these areas. “Our calculations show that CO2 emissions from dams were significantly underestimated. Globally, they emit twice as much carbon as they store, ”said Koschorreck.
“Their image as net carbon stores in the global carbon cycle needs to be reconsidered”. The data also shows that the magnitude of dams’ water level fluctuations depend on both their use and their geographic location.
“Fluctuations were more pronounced in dams used for irrigation than in dams used for hydropower generation,” said Keller. “In places where the annual precipitation pattern is more even – such as at the poles and around the equator – there were less large fluctuations in water levels than in the middle latitudes, where larger parts of the dams were often dry for much longer”.
To better understand the carbon balance of dams, Koschorreck’s research team plans to take a closer look at both carbon dioxide and methane emissions, as well as the role of vegetation on the carbon cycle of areas that have become arid.