Lakes and ponds

Lakes and ponds are naturally formed permanent water bodies dispersed on the land. This source category includes natural freshwater lakes but excludes impoundments and reservoirs (water bodies formed by dams), as greenhouse gas emissions from impoundments, reservoirs, and other engineering works are considered to be anthropogenic (paragraph 4.7).

CH4 production rates depend on temperature, organic matter availability (food for the bacteria), and isolation from oxygen; these factors are influenced by climate, lake size and depth, and productivity of microscopic and macroscopic plants and animals, which create organic matter for CH4 production when they die and sink to the bottom. There are four pathways for CH4 emissions from lakes: bubbling, diffusion, plant-mediated transport, and seasonal overturning. Bubbling has been determined to be the dominant pathway for CH4 flux, accounting for more than 90 percent of CH4 emissions from lakes.

Based on recent estimates, lakes emit approximately 30 Tg CH4/yr to the atmosphere. The number and total area of large lakes is well known, but some uncertainty involves the total surface area of small lakes and ponds. Lakes smaller than

1 km2 constitute about 40 percent of the total global lake surface area. Because small lakes and ponds generally emit more CH4 per unit area than large lakes, uncertainties about total surface area are a major factor in the overall uncertainty of the estimate (U. S. EPA, 2010a). Climate warming impacts on permafrost and the development of thermokarst lakes could substantially affect future CH4 emissions from lakes. It is estimated that emissions from lakes north of 45°N will eventually decrease, due to lake area loss and permafrost thaw. Before this long-term decline, though, a period of increased CH4 emissions, associated with thermokarst lake development in the zone of continuous permafrost, would come. CH4 emission rates from northern lakes could rise as high as 50 to 100 Tg CH4/yr during this transitional period, which would last hundreds of years.

Updated: September 24, 2015 — 7:09 am