Over time, human activities have altered the amount of carbon that flows through and is stored in various reservoirs. The global net flux of carbon to the atmosphere from land use changes may have increased as much as 19 percent since 1850. To stop rising concentrations of CO2 in the atmosphere, countries actively are seeking ways to increase carbon storage capacity on land. Drylands, as an ecosystem with extensive surface area across the globe, have been suggested as a potential candidate for major carbon storage efforts.
Map 31 displays the global variation in the density of carbon storage in live vegetation (both above and below-ground). The carbon values are expressed as a range, in metric tons of carbon per hectare; this map shows storage values at the high end of the range. These estimates of above- and below-ground live vegetation carbon storage are based on those developed at the Oak Ridge National Laboratory (ORNL) and are described as the most commonly used and spatially explicit estimates of biomass carbon densities at a global scale. We applied the ORNL estimates to a land cover database (GLCCD) and the various ecosystems were then matched with the low and high estimates of carbon storage.
In terms of quantity of carbon stored, tropical and boreal forests are visibly outstanding. The values for carbon storage in vegetation in the tropics reach a maximum of 250 metric tons per hectare. Temperate forests and tropical savannas store less than the tropical and boreal forests. Non-woody grasslands and drylands store less than the forested areas, and sparsely vegetated and bare desert areas have the least carbon storage potential.
Map 32 shows the carbon storage in vegetation for drylands only. As described for Map 31, but now much more obvious, drylands generally do not store large amounts of carbon in vegetation. Although there are some exceptions, worldwide drylands store less than 50 metric tons of carbon per hectare.
Map descriptions (Maps 33 and 34)
Map 33 shows potential for global carbon storage in soils. These estimates are based primarily on soil samples taken within 100 centimeters of the soil profile with special reference to the upper 50 centimeters, the depth most directly influenced by interactions with the atmosphere and with land use and environmental change.
To produce this map, over 4,000 soil profiles from the World Inventory of Soil Emission Potentials (WISE) database compiled by the International Soil Reference and Information Centre (ISRIC) were analyzed to determine average soil organic carbon (SOC) at several depths for world soil types defined by the Food and Agriculture Organization of the United Nations (FAO). The SOC content of the soil types found in each 5 x 5 minute grid of the digitized FAO-UNESCO Soil Map of the World were summed and weighted according to the portion of soil type area within each grid cell.
This global carbon storage map for soils, as with the global vegetation carbon map, shows forested areas, especially in northern latitudes with the highest carbon storage potential. Carbon storage values in soils in these boreal regions reach a maximum of 1,250 metric tons of carbon per hectare.
Map 34 shows the carbon storage in soils for drylands only. In this display, extensive dryland areas store up to 300 metric tons of carbon per hectare. This illustrates that dryland soils have larger potential carbon storage than dryland vegetation. And, that unlike tropical forests, where carbon is stored primarily in vegetation, carbon in drylands is stored predominantly in the soil.
Map descriptions (Maps 35 and 36)
Map 35 shows combined estimates for potential carbon storage in vegetation and soils. These estimates range from 1,752 GtC (in the unvegetated regions) to 2,385 GtC (in forested areas). Here again, carbon storage potential appears highest in the tropical and boreal forests (with carbon storage values ranging from 300- 400 metric tons per hectare).
Map 36 shows the distribution and concentration of total carbon stores, in both vegetation and soils, but for drylands only. The carbon storage potential for drylands ranges from greater than 400 to less than 100 metric tons per hectare, although the majority of drylands store less than 225 metric tons per hectare. Thus, while drylands generally store less carbon than forests on a carbon/unit area basis, the total amount of carbon that drylands store is potentially significant because the area of these ecosystems is extensive.
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