An additional environmental benefit of a green roof is the ability to sequester carbon. Carbon is the main component of plant matter and is naturally absorbed by plant tissue. The carbon is stored in the plant tissue and the soil substrate through plant litter and root exudates. A study on green roofs in Michigan and Maryland found the above ground biomass and below ground substrate stored on average between 168 g C m and 107 g C m . Variations occurred among the different species of plant used.  Substrate carbon content averaged 913 g C m and after the subtraction of the original carbon content the total sequestration was 378 g C m. The sequestration can be improved by changing plant species, increasing substrate depth, substrate composition, and management practices. In a study done in Michigan above ground sequestration ranged from 64 g C m to 239 g C m for S. acre and S album. Also, by increasing the substrate depth would allow for more area of carbon storage and diversify the types of plants with greater potential of carbon storage. The direct carbon sequestration techniques and methods can be measured and accounted for. Green roofs also indirectly reduce CO given off by power plants through their ability to insulate buildings. Buildings in the US account for 38% of the total carbon dioxide emissions. A model supported by the U.S. Department of Energy found a 2 percent reduction in electricity consumption and 9-11% reduction in natural gas when implementing green roofs.