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Are Natural Lakes and Man-Made Reservoirs Significant Sinks for Anthropogenic Carbon?
In a review and analysis of the degree to which lakes and reservoirs may serve as the final resting place of the carbon that makes its way into the atmosphere as a consequence of the activities of man, Einsele et al. (2001) come to some eye-opening conclusions.  They determine, for example, that lake basins - which cover only about 0.8% of the surface area covered by the world's seas - bury an amount of carbon that annually "reaches more than one fourth of the annual atmospheric carbon burial in the modern oceans."

How is this amazing feat accomplished?  Mainly, say the authors, "by the rapid accumulation of lacustrine sediments and a very high preservation factor (on average 50 times higher than that in the oceans)."  In addition, smaller lakes exhibit much greater accumulation rates of atmospheric-derived organic carbon than do larger lakes (about eight times greater); and there is an enormous number of small lakes in the world.  Also, man-made reservoirs are more effective yet, exhibiting atmospheric-derived organic carbon burial rates that are more than an order of magnitude (approximately 12.5 times) greater than those of small lakes.  Hence, as the authors note, the planet's man-made reservoirs bury more organic carbon each year than do all natural lake basins combined.

So how do these lake and reservoir sequestration rates compare with anthropogenic emission rates?  According to the authors, present-day carbon emissions due to the burning of fossil fuels amount to about 5.5 x 109 tons C per year, while lakes and reservoirs remove about 0.3 x 109 tons C per year, which is 5.5% of what we put into the atmosphere annually as a consequence of our utilization of coal, gas and oil.  Hence, as the authors note, "the contribution of lakes and artificial reservoirs in counteracting man-made CO2 emissions should not be neglected," or, as might have been done in the past, discounted as effective ameliorative measures.

But what about the future?  How are these numbers likely to change if, as most climate models predict, earth's air temperature rises and its hydrologic cycle intensifies?  The authors findings suggest that lake and reservoir organic carbon burial rates will only grow larger under these conditions; for they say that "carbon burial rates in lakes commonly increase with change to wetter and warmer climate," due to increasing lake size, higher rates of carbonate precipitation, more stratified lake water and, hence, more oxygen-deficient bottom water, which retards decomposition.

What about the increasing number of people on the planet?  How might humanity alter the picture?  For one thing, more people would be expected to construct more reservoirs; and the authors note that this is indeed the case, with the total area of reservoirs throughout the world "steadily increasing."  In addition, as people and lakes inevitably come into closer contact with each other, both the production and preservation of organic carbon in lake basins rise, often by a factor of three to four.  Agriculture and other types of land use, for example, tend to promote soil erosion, which leads to an increase in the mass accumulation rate of lake sediments and a consequent higher organic carbon burial rate and preservation potential.  Simultaneously, say the authors, "enhanced riverine nutrient supply can augment primary organic production in the lakes," as has been observed "in many lake basins close to densely populated and industrialized regions."

Clearly, these several negative feedback phenomena have the potential to increase the carbon sequestering prowess of lakes and reservoirs several-fold as time progresses; and a several-fold increase in their current ability to remove 5.5% of the atmospheric carbon that results from the burning of fossil fuels is truly a force to be reckoned with.  Also not to be forgotten is the fact that this important self-regulating feature of the totality of life processes operating on the planet's surface comes courtesy of the natural progression of what climate models are predicting for the future (a warmer, wetter world), as well as what history suggests will be the likely forward trajectory of human population and technological enterprise, demonstrating once again that the biosphere - even with "dreaded man" in the picture - seems quite capable of maintaining its health and vitality with respect to climatic conditions conducive to the continued flourishing of life.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Einsele, G, Yan, J. and Hinderer, M.  2001.  Atmospheric carbon burial in modern lake basins and its significance for the global carbon budget.  Global and Planetary Change 30: 167-195.