We agree with Muller (2019) that there are important reasons for building dams, particularly in flood-prone and poor countries. However, we contest that among these is that dam reservoirs may emit less greenhouse gas emissions (GHG) than pre-flood landscapes. Recent work shows that increased fluxes of methane following dam construction can be fully attributed to the reservoir’s presence due to gas bubbling, outgassing at turbines, and fluxes from downstream and drawdown areas (Nakayama and Pelletier 2018).
Science to support Muller’s assertion that “a more nuanced approach to their [wetland] management …” could lead to a reduction in emissions is lacking. Natural wetlands outpace biogeochemical functioning of those created and restored, including carbon sequestration rates. Altering wetlands generally leads to large increases in their radiative forcing (Petrescu et al. 2015), and wetland conservation remains foremost for retaining the wetland carbon sink and other ecosystem services. We agree advances in freshwater sciences need integrating into IPCC guidelines, but assert they must account for new methane emissions facilitated by reservoirs, and recognize our limited ability to replicate the role of natural wetlands in the global carbon cycle.
Muller’s social arguments in support of dams—provisioning for and protecting people—are valuable but must be considered in light of the wide uncertainty associated with socio-environmental futures. A dam presents enormous up-front costs and is an estimated 50-year economic commitment, yet in that timeframe population needs can shift, and environmental change can nullify the dam’s initial design. It is imperative to weigh the current benefits of dam building against the reality that dams potentially “lock-in” future populations to certain economic and political paths when they will likely benefit the most from alterative adaptive paths (Haasnoot et al. 2013).
Kelly Hondula, Graham Stewart, and Margaret Palmer
April 2, 2019