Abstract
Green roofs are considered effective in the reduction of atmospheric CO2 because of their ability to reduce energy consumption of buildings and sequester carbon in plants and substrates. However, green roof system components (substrate, water proofing membrane, etc.) may cause CO2 emissions during their life cycle. Therefore, to assess the CO2-payoff for extensive green roofs, we calculated CO2 payback time it takes their CO2 sequestration and reduction to offset the CO2 emitted during its production process and maintenance practices. The amount of CO2 emitted during the production of a modular green roof system was found to be 25.2 kg-CO2·m−2. The annual CO2 emission from the maintenance of green roofs was 0.33 kg-CO2·m−2·yr−1. Annual CO2 sequestration by three grass species with irrigation treatment was about 2.5 kg-CO2·m−2·yr−1, which was higher than that of Sedum aizoon. In the hypothetical green roofs, annual CO2 reduction due to saved energy was between 1.703 and 1.889 kg-CO2·m−2·yr−1. From these results, we concluded that the CO2 payback time of the extensive green roofs was between 5.8 and 15.9 years, which indicates that extensive green roofs contribute to CO2 reduction within their lifespan.
Highlights
Global warming caused by the increase of greenhouse gases in the atmosphere has been identified as one of the most important environmental issues currently faced by human civilization
This index is calculated as the ratio of the CO2 emissions from the production of each power plant to the annual CO2 reduction resulting from the generation of electricity from renewables
CO2 payback time = CO2 e-p / (CO2 r-s + CO2 r-e - CO2 e-m) where CO2 e-p is the amount of CO2 emitted during the production of the modular green roof system, CO2 r-s is the annual CO2 reduction owing to CO2 sequestration, CO2 r-e is the annual CO2 reduction owing to energy savings and CO2 e-m is the annual CO2 emission from maintenance of the hypothetical green roof
Summary
Global warming caused by the increase of greenhouse gases in the atmosphere has been identified as one of the most important environmental issues currently faced by human civilization. Modular extensive green roof systems are used exclusively in Japan They are designed for ease of installation and alteration and generally consist of vegetation mats, substrate, substrate containers, water reservoir trays, water proofing membrane, edge dividers and an irrigation system. In studies estimating the emissions reduction potential of power plants utilizing renewable energy, CO2 payoff is often defined as the CO2 payback time [32,33,34] This index is calculated as the ratio of the CO2 emissions from the production of each power plant to the annual CO2 reduction resulting from the generation of electricity from renewables. We used LCA methodology to calculate the CO2 emissions from the production process and maintenance practices of a modular green roof and investigated the annual CO2 sequestration by several green roof plants (Figure 1). MMaatteerriiaallss aanndd MMeetthhooddss hhyyppooIIttnnhheeoottriricddcaaeellrraattvvooeereerasasgttgiiemmeffoaaotrtreeggCCrreOeOeen22nerermomoooiifssfssssiiionionnnJsaJsapaapannanddnteoteonnaeeargrgggryeryeesensanaivnviniginnggagarsserfaefrraooomofmf202aa00m0mmmoo2dd2auunalldanardraggasrruesebeuesnbntrsrratoortaoeotffde,, ewdwpeeetphsstehoetftoa5af c5mc.mW. eWceoncodnudctuecdtepdarptaiarltiLaCl LACfAorftohrethhyephoythpeottihceatligcarel egnrereonofrso.oTfhs.e Tfuhnecftuionncatliounnailt ustnuidt isetdudwieads 1wmas o1fmth2eofmthoedumlaordugrlaerengrreoeonfrwooitfhwaithsearvsiecrevilcifeetliifmeteimofe 4o5f 4y5eayresarosnoan falafltactocnocnrcerteeterorooof.f.WWeetthheerreeffoorree ccoonnvveerrtteedd oouurr rreessuullttss ffoorr CCOO22 eemmiissssiioonnss aannddrreedduuccttiioonnss iinnttoo vvaalluueess ppeerr mm
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