October 29, 2018
Nitrous Oxide Emissions from Inland Waters: Are IPCC Estimates Too High?
New modeling approach suggests lower emissions from rivers, reservoirs, and estuaries.
Researchers developed a new modeling approach to estimate nitrous oxide (N2O) emissions from inland waters. They used two modeling structures or scenarios to develop their estimates. Shown here is the total combined N2O emissions from rivers, reservoirs, and estuaries for major watersheds worldwide in year 2000, for default scenario 2 (DS2). Pie charts show the 10 watersheds with the greatest emissions globally, with chart size representing a watershed’s relative emission flux compared with the other nine watersheds shown. (Total magnitude of flux shown in brackets after watershed name). The pie charts show the proportion of emissions from denitrification or nitrification in reservoirs, estuaries, or rivers.
[Reprinted with permission from Maavara, T., et al. “Nitrous Oxide Emissions from Inland Waters: Are IPCC Estimates Too High?” Global Change Biology 25(2), 473-488 (2018). DOI:10.1111/gcb.14504. © 2024 John Wiley & Sons Ltd.]
The Science
Nitrous oxide (N2O) is a key greenhouse gas, but emissions from inland waterways remain a major source of uncertainty in greenhouse gas budgets. The Intergovernmental Panel on Climate Change (IPCC) has proposed emission factors (EFs) of 0.25% and 0.75%, but studies have suggested that both these values are either too high or too low. A new approach to modeling nitrous production concludes that the IPCC EFs are likely overestimated by up to an order of magnitude.
The Impact
Researchers have developed a new mechanistic modeling approach for estimating (N2O production from denitrification and nitrification in water bodies and introduce water residence time as a critical limitation on biological activity.
Summary
The authors calculate global nitrous oxide (N2O) emissions from rivers, reservoirs, and estuaries within a range of 10.6 to 19.8 Gmol of nitrogen (N) per year (148 to 277 Gg N per year). This estimate is more than half, and up to an order of magnitude, lower than most studies based on IPCC guidelines. Despite the much-reduced N2O flux estimates, the research team found that anthropogenic perturbations to river systems have doubled to quadrupled N2O emissions from inland waters. The researchers suggest that IPCC EFs of 0.25% and 0.75% are too high to be applied across all rivers, estuaries, and reservoirs. Instead, the team estimates the following EF ranges: 0.004% to 0.005% for rivers, 0.17% to 0.44% for reservoirs, and 0.11% to 0.37% for estuaries.
Most N2O emissions in estuaries and reservoirs originate from nitrification, while denitrification tends to dominate emissions in rivers because of the shorter residence times. Researchers therefore expect worldwide N2O emissions from inland waters to rise substantially in the coming decades because of the ongoing global boom in dam construction. This construction will nearly double the number of large hydroelectric dams on Earth, increasing water residence within these water bodies.
Principal Investigator
Nick Bouskill
Lawrence Berkeley National Laboratory
[email protected]
Program Manager
Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]
Funding
This work was supported by the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.
References
Maavara, T., et al. "Nitrous Oxide Emissions from Inland Waters: Are IPCC Estimates Too High?." Global Change Biology 25 (2), 473–488 (2019). https://doi.org/10.1111/gcb.14504.