Global Inland Water Greenhouse Gas Emissions: Patterns, Trends, and Anthropogenic Drivers

Inland waters (streams, rivers, lakes, and reservoirs) are important sources of greenhouse gases (GHGs), including carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), to the atmosphere. Their importance has been acknowledged in the IPCC assessment reports and in the regional and global greenhouse budgets coordinated by the Global Carbon Project (GCP). In the framework of the 2^nd phase of the REgional Carbon Cycle Assessment and Processes (RECCAP-2) initiative of the GCP, a comprehensive synthesis of existing estimates of regional to global inland water GHG emissions was conducted (Lauerwald et al., 2023a, 2023b) to support the inclusion of these emissions in (sub-)continental GHG budgets. Although that synthesis was published only two years ago, a number of new global estimates have been published since. Here, we present an updated synthesis of recent, global inland water GHG emissions estimates. Moreover, we go beyond the scope of the RECCAP2 synthesis by analyzing regional patterns in more detail, and summarizing the state of knowledge about long-term trends of inland water GHG emissions in response to changes in climate, land use, wastewater management and river damming. Based on that, we discuss how contemporary inland water GHG emissions are impacted by anthropogenic activities and how they may evolve over the 21^st century.

We estimate that global inland water GHG emissions have a combined warming potential of 8 (5–13) Pg CO₂-eq. yr⁻¹ for a 100-year time horizon (GWP100). CO₂ emissions, primarily from tropical river systems, contribute approximately three-quarters of this total, while CH₄, largely from lakes and reservoirs, accounts for most of the remainder. Notably, boreal and Arctic lakes are important emitters due to their large total area, while nutrient-rich lakes and reservoirs with warmer temperatures in the mid to low latitudes exhibit the highest per-area CH₄ emission rates. Contributions from N₂O emissions are relatively minor.

About one third of CH₄ emissions and about three quarters of N₂O emissions from global inland waters can be attributed to anthropogenic perturbations, primarily through eutrophication. For inland water CO₂ emissions, quantification of the anthropogenic component is more complex. Empirical and modelling studies suggest that global greening also increases terrestrial carbon deliveries to inland waters, and through that, emissions of CO₂ from inland waters. Moreover, changes in streamflow are an uncertain, but very important driver. Most dramatic increases are expected for inland water CH₄ and N₂O emissions, which are projected to strongly increase in response to global warming, while changes in nutrient loads from agricultural runoff may offset or enhance that trend.

References

Lauerwald et al. 2023a, GBC, https://doi.org/10.1029/2022GB007657

Lauerwald et al. 2023b, GBC, https://doi.org/10.1029/2022GB007658