How Lakes and Inland Waters Shape Global Greenhouse Gas Emissions

Lakes worldwide emit vast amounts of greenhouse gases – carbon dioxide, methane, and nitrous oxide because they accumulate organic matter. Bacteria and fungi that feed on dead plants and animals at the bottom of the lake emit vast amounts of GHG, ending in the atmosphere.

Ten years ago, the Regional Carbon Cycle Assessment and Processes Initiative (RECCAP-1) measured the CO2 emissions from inland water bodies. In a pair of studies published in AGU Journal on April 2023, scientists have calculated emissions again from inland bodies of water – rivers, streams, lakes, and reservoirs in the 2nd phase of the Regional Carbon Cycle Assessment and Processes (RECCAP-2). This time, including the other two greenhouse gases, methane and nitrous oxide.

The first study estimates the GHG gases from inland water bodies at a global scale. The second study matched those estimates with global inland water surface maps to provide regionalised estimates of greenhouse gas emissions.

“The study authors found that inland waters contribute 5.5 petagrams of CO2 per year, of which one-third emanates from South American rivers. Meanwhile, inland waters emit 82–135 teragrams of CH4 annually, one-third of which comes from North American and Russian lakes. N2O emissions were comparatively small at 248–590 gigagrams N2O per year, and a quarter of N2O emissions stem from North American inland waters” (Sidder, 2023).

The authors found that methane emissions from these inland water could represent 20% of the total global methane emissions. However, their carbon dioxide and nitrous oxide emissions contributions are relatively minor.

Another study published in Nature Geoscience on June 2023 focuses on the nitric oxide released in the Tibetan Plateau. The Tibetan Plateau contains thousands of lakes, a critical water source for over 2 billion people in China, India, Pakistan, Nepal, Bhutan, Tajikistan and Kyrgyzstan.

Using satellite observations, researchers could record total nitric oxide emissions of 1.9 metric tons per hour from 135 lakes larger than 50km.

Nitric oxide emissions of 1.9 metric tons per hour are comparable to those from global cities such as Beijing at 7.8 tons of nitric oxide per hour, New York City at 3 tons/hour, London at 1.7 tons/hour and Paris at 0.3 tons/hour.

Researchers have also observed that summer lake warming increases at 0.40°C per decade. The melting of glaciers and ice is also expanding these lakes, and the lake’s elevation at 4 to 5 km above sea level makes them more prone to severe impacts of glaciers and permafrost degradation. During summer, the peak values of nitrogen dioxide emissions are twice as high as in winter.

Large quantities of nitrous oxide are problematic as it contributes to the depletion of the ozone layer, which creates a feedback mechanism for climate change. As the ozone layer gets damaged, more sunlight can penetrate the Earth’s surface, warming the planet.

The studies show that GHG emissions from inland water bodies are a significant contributor to climate change and should be included in future global warming modelling.