Of the fve principal global carbon pools, the ocean pool is the largest at 38.4 trillion metric tons (mt) in the surface layer, followed by the fossil fuels (4.13 trillion mt), soils (2.5 trillion mt to a depth of one meter), biotic (620 billion mt), and atmospheric pools (800 billion mt). If the fuxes among terrestrial pools are combined, annual total carbon fows across the pools average around 60 billion mt, with managed ecosystems (croplands, grazing lands, and plantations) accounting for 57 percent of that total. Thus, land managers have custody of more annual carbon fows than any other group.
Carbon concentration in the atmosphere is increasing at the rate of about 4 billion mt (2 parts per million) per year, with transfer primarily from the fossil fuel, biotic, and soil pools. This increase is a double jeopardy. One, the loss of carbon from the terrestrial pools reduces the ecosystem services and goods that these systems provide. In particular, decline in soil quality adversely affects use effciency of inputs, decreases agronomic yields, and exacerbates food insecurity. Two, increase in atmospheric pools accentuates global warming with the attendant impact on pole-ward shifts of ecosystems and the increase in frequency and intensity of extreme events including droughts, melting glaciers and Arctic ice sheet, rising sea level, and loss of biodiversity. One solution to this problem is to transfer atmospheric CO2 into other long-lived pools (such as the soil and biotic pools); this is called carbon sequestration. Increasing carbon pools in the soil beyond a threshold level (about 1.2 percent in the surface layer) is essential to enhancing soil quality, increasing agronomic productivity, and improving quality of natural waters. The strategy of carbon sequestration in soils and biota is cost effective, safe, and has numerous co-benefts over leaving carbon in the atmosphere or sequestering it in geologic and oceanic strata. Biotic, or plant-based, sequestration is based on a natural process whereby CO2 is photosynthesized into organic substances and stored for the long term in plant products and soil organic matter. The natural rate of photosynthesis in the global biosphere is about 120 billion mt of carbon per year. Fossil fuel combustion emits about 8 billion mt of carbon annually, and deforestation and land-use conversion emit another 1.6 billion to 2 billion mt of carbon per year, for a total of 9.6 to 10.8 billion mt of carbon emissions per year. Thus, if roughly 8 percent of the carbon being photosynthesized by the biosphere is retained within the soil and biotic pools, the global carbon budget would be balanced.
