According to a new paper published in Science, current carbon accounting methods significantly overstate the amount of carbon that can be absorbed by forests, plains, and other terrestrial ecosystems. That is because most current carbon accounting methods do not consider the methane and carbon dioxide released naturally by rivers, streams, and lakes. This new paper suggests that rivers, streams, and lakes emit the equivalent of 2.05 billion metric tons of carbon every year. (By comparison, all the terrestrial ecosystems on the world’s continents are thought to absorb around 2.6 billion metric tons of carbon each year). This is, as the lead author of the paper said, is a “major accounting error”.
Each year, rivers, streams, and lakes release 1.4 billion metric tons of carbon into the atmosphere in the form of carbon dioxide. In addition, as this new paper explains, rivers, streams and lakes also emit 103 million metric tons of methane every year—equivalent to about 650 million metric tons of carbon. (Methane is an extremely potent greenhouse gas; a single ton of methane has much larger impact on the atmosphere than an equivalent ton of carbon dioxide.) It remains difficult to measure methane emissions precisely; methane bubbles may emerge from river and lake sediments suddenly, or at very irregular intervals. Given this uncertainty, these numbers may actually be under-estimates. Even so, the methane emissions calculations alone suggest that we have been overestimating the absorptive ability of the continents by 25 percent.
This paper makes it clear that freshwater ecosystems play a valuable role in the world’s carbon cycles—especially given how little of the earth’s surface they cover. In addition, this paper also highlights just how little we know about natural carbon cycles. Previous papers have suggested that freshwater ecosystems may also be storing large quantities of carbon dioxide—perhaps as much as 600 million metric tons. There is an urgent need for further study, as precise measurements of natural carbon sources and sinks are vital for shaping policies on conservation, deforestation, and other issues.
Citation: Bastviken D, Tranvik LJ, Downing JA, Crill PM, Enrich-Prast A. Freshwater methane emissions offset the continental carbon sink. Science. 2011 Jan 7;331(6013):50.