CO₂ Payoff of Extensive Green Roofs with Different Vegetation Species

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.

1. Introduction

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. Carbon dioxide (CO₂) is known to be a primary anthropogenic greenhouse gas. In urban areas, according to the Intergovernmental Panel on Climate Change [1], climate change is projected to increase environmental risks for people, assets, economies and ecosystems, including risks from heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scarcity, sea level rise and storm surges. Increasing urban greenspaces is one proposed method of mitigating these problems through urban planning [2,3,4,5,6].

Greenspace in urban areas reduces atmospheric CO₂ through sequestration, shading and evapotranspiration. Shading and evapotranspiration reduce atmospheric CO₂ indirectly by reducing the necessity for air conditioning [7,8,9,10,11], which decreases CO₂ emissions from electric power generation. CO₂ sequestration is the direct removal of CO₂ from the atmosphere through photosynthesis and the fixation of carbon in plant litter and root exudates. The capacity of urban greenspaces (urban forests, parks, trees, etc.) to sequester CO₂ in plants and soils has already been quantified [12,13,14,15,16], demonstrating that an ecosystem can serve as a carbon sink over a sufficient time period.

However, urban areas are covered mainly by impervious surfaces (e.g. streets, parking lots and buildings), which makes it difficult to plant trees and increase urban greenspace. Accordingly, a green roof, which replaces an impervious surface with greenspace, is a key solution to this problem. The total area of green roofs in Japan actually increased about 29-fold between 2000 (135,222 m²) and 2013 (3,875,716 m²). Green roofs also contribute to atmospheric CO₂ reduction through their reduction energy consumption of buildings and sequestration of carbon in plants and substrates. The energy saving potential of green roofs has been widely investigated [17,18,19,20,21,22,23] and Sailor and Bass [24] developed a web tool—the Green Roof Energy Calculator—to readily estimate the annual energy savings for a building with a green roof. By contrast, although Getter et al. [25] and Whittinghill et al. [26] measured the capacity of green roofs to sequester carbon in plants and substrates, efforts to quantify carbon sequestration in green roofs have been limited.

A green roof can be classified as extensive or intensive. The extensive type is characterized by a shallow substrate (<20 cm deep) and requires little maintenance. In contrast, the substrate depth of an intensive green roofs is greater than 20 cm and can support the growth of woody plants. However, an intensive green roof requires careful maintenance and is costly. Because of these reasons, the extensive green roof is currently more common. In particular, 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.

These green roof system components have potential environmental impacts throughout their life cycles (raw material extraction, manufacture, distribution, use, repair and maintenance and disposal or recycling). Several studies have used the life cycle assessment (LCA) methodology to determine the environmental impacts of green roofs [27,28,29]. These studies compared the environmental load and benefits of green roofs and assessed their overall environmental impacts. In particular, a study by Bianchini and Hewage [30] indicated that the annual air pollution (NO₂, SO₂ and O₃) reduction from a green roof will offset the emissions associated with its production after 13 to 32 years. The study calculated the amount of air pollution created by the production of the polymers of a typical green roof system and compared their results with its pollution removal capacity [31]. However, there have been few studies on the CO₂ payoff of modular green roofs and the carbon balance of a modular green roof system—that is, whether it acts as a sink or a source—is therefore open to debate.

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.

In this study, therefore, 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). In addition, we estimated the amount of energy saved of buildings with green roofs using the Green Roof Energy Calculator. We used these parameters to assess the CO₂-payoff for modular green roofs by calculating their CO₂ payback time. We defined the CO₂ payback time of a green roof system as the time it takes total CO₂ reduction by the system to offset the CO₂ emitted during its production and maintenance.

2. Materials and Methods

In order to estimate CO₂ emissions and energy savings from a modular green roof, we set a hypothetical average for green roofs in Japan to a greening area of 200 m² and a substrate depth of 5 cm. We conducted partial LCA for the hypothetical green roofs. The functional unit studied was 1 m² of the modular green roof with a service lifetime of 45 years on a flat concrete roof. We therefore converted our results for CO₂ emissions and reductions into values per m².

2.1. CO₂ Emission from a Modular Green Roof

2.1.1. Definition of Goal and Scope

In this section, we calculated the CO₂ emitted during the production and maintenance of a typical modular green roof.

We defined the system boundaries as the production processes for the system components (substrate, substrate containers, water reservoir trays, water proofing membrane, edge dividers, irrigation tubes, irrigation pipes and automatic watering device) and the maintenance practices (irrigation and fertilization) of a modular green roof system. The life cycle system included the extraction and refinement of raw materials and the consumption of natural resources. The production process for vegetation mats in a farm and transportation of the components were not taken into account because of a lack of relevant information.

2.1.2. Inventory Analysis

We collected data from companies and experts about the components and maintenance practices of a typical modular green roof system. We calculated the CO₂ emission factors for each component and maintenance practice using the MiLCA LCA software developed by the Japan Environmental Management Association for Industry. We used inter-industrial relation analysis to calculate the CO₂ emission factors of the irrigation tube and automatic watering device. This analytical method is a form of economic analysis based on the interdependencies between economic and environmental sectors. It enables us to estimate the environmental impacts of products throughout their costs. For all of the other components and maintenance practices, we used a bottom-up approach for building the inventory. We calculated the amount of CO₂ emitted by each component or maintenance practice by multiplying its CO₂ emission factor by the quantity of that component used in the hypothetical green roof.

We set the dimensions of a typical green roof at 12.5 m by 16 m (200 m²) and we calculated the use of each green roof system component in this size. A schematic drawing of a typical modular green roof system (with the irrigation pipes and automatic watering device excluded) is shown in Figure 2. The main raw materials used to produce each component and the quantity of each component used in the hypothetical green roof, are shown in

Table 1. The main raw material and the quantity of each component used for the hypothetical green roof in production and maintenance practice.

System Components and Maintenance	Main Raw Material	Required (for 200 m2)
Substrate	Perlite	1572 kg
Substrate container	Polypropylene	800 kg
Water reservoir tray	Polyvinyl chloride	174 kg
Water proofing membrane	Polyvinyl chloride	55 kg
Edge divider	Aluminum	75 kg
Irrigation pipe	Polyvinyl chloride	5 kg
Irrigation tube	Special polyethylene	208 m (142$)
Automatic watering device	-	1 machine (231$)
Irrigation	Water	161.6 t·year−1
Fertilizer	Compound fertilizer	8 Kg·year−1

.

The substrate contains more than 50% perlite, along with compost and zeolite. We calculated the CO₂ emission factors of each substrate component using MiLCA and weighed their individual content ratios (kg·kg⁻¹). We used the results to calculate a CO₂ emission factor for the substrate.

The substrate containers were made of polypropylene, with numerous small holes for drainage. The substrate containers were 50 cm long by 50 cm wide and 6.5 cm deep. Vegetation mats were planted in the containers after filling with substrate to a depth of 5 cm. Each container was connected to adjoining containers to prevent wind uplift.

Drainage from the containers was collected in the water reservoir trays, which were 50 cm long by 50 cm wide by 1.5 cm deep and made of polyvinyl chloride. Each reservoir tray was also connected to adjoining trays.

The irrigation pipes connected the automatic watering device to the irrigation tubes, which were aligned under every second substrate container (13 lines × 16 m). The irrigation pipe needed to be aligned with the edge of the green roof, so we set the length of the irrigation pipe to 15 m. Irrigation pipe consists mainly of polyvinyl chloride.

The water proofing membrane was also made of polyvinyl chloride and was 0.3 mm thick. This membrane was the final layer of the modular green roof system and served as water proofing and a root barrier. It was intended to protect the building from penetration by water and roots.

The edge dividers were made of aluminum and used for sealing the edge of the green roof (57 m). The edge dividers play a crucial role in locking the green roof components in place and preventing wind uplift.

According to the inter-industry relations analysis implemented in MiLCA, the prices of the irrigation tubes and automatic watering device for the hypothetical green roof were $142 and $231, respectively, at an exchange rate of 110 yen to the U.S. dollar.

The use of water and fertilizer during maintenance of the hypothetical green roof are shown in Table 1. According to an interview with a relevant company, a green roof is irrigated 101 times annually, with 8 L·m⁻² of water used for each irrigation. Fertilizer is applied twice annually, with 20 g·m⁻² used each time.

2.2. CO₂ Sequestration by Several Green Roof Plants

In order to quantify CO₂ sequestration in green roofs, we investigated the annual CO₂ sequestration by three grass species—Cynodon dactylon Pers., Festuca arundinacea Schreb. and Zoysia matrella L. “Himekourai-shiba”—and a flowering plant, Sedum aizoon L. Grasses and Sedum species are the most common green roof vegetation in Japan.

This experiment was conducted at the Center for Environment, Health and Field Sciences at Chiba University over one year. All plants in this experiment were propagated as cuttings in plug flats (128 cells·tray⁻¹) filled with seedling propagation soil (Metro Mix; Sun Gro Horticulture, Agawam, USA). After approximately one month, the plugs were planted in 0.2 L polyethylene pots (44 cm²) filled to a depth of 5 cm with commercial artificial soil for green roofs (the same as the substrate mentioned above) and grown in a greenhouse for two months. They were then placed on the rooftop and acclimated for three weeks. For a more accurate simulation in this experiment, we should have used the same modular green roof system. However, we used polyethylene pots and irrigated by hand sowing because we had to test a number of experimental plants using a limited roof area.

At the start of the experiment, on October 20, 2014, we sampled all species from a total of 15 pots over a period of about 10 days. Green roofs composed of grasses are generally fitted with irrigation systems to prevent drought stress. In contrast, irrigation systems are less common in Sedum green roofs because Sedum uses the CAM photosynthetic pathway and is thus better adapted to drought conditions. The S. aizoon pots were therefore assigned randomly to irrigation and non-irrigation treatments after the first sampling. Plants in the irrigation treatment group were watered once a week from January to March, once every two days from April to June, every day from July to September and once every two days from October to December. The non-irrigation treatment group was never irrigated. The three grass species received only the irrigation treatment, in keeping with general cultivation practice. All treatments received about 20 g·m⁻² (0.1 g·pot⁻¹) of controlled-release fertilizer (8N-8P-8K) on June 13 and August 13, 2015. As the end of the experiment, on October 20, 2015, we harvested 15 pots, including all species and treatments.

All plants were dried at 70 °C for 72 h and then divided into plant and substrate matter. Plant and substrate carbon concentrations were measured using an organic elemental analyzer (2400 SeriesⅡ CHNS/O System; PerkinElmer, Waltham, MA, USA). Carbon content was quantified by multiplying carbon concentration by the dry weight. Annual CO₂ sequestration was calculated by subtracting the total carbon content in October 2014 from the total carbon content in October 2015.

In order to calculate the leaf area index (LAI) of the four species in summer, 15 pots, including all species and treatments, were sampled on August 20, 2015 and divided into leaves and non-leaf parts. Leaves were scanned (LP-A500; EPSON, Nagano, Japan) and image analysis software was used to measure the leaf area (ImageJ [35]). The LAI was calculated as leaf area per 44 cm² (the area of each polyethylene pot).

Data were analyzed using IBM SPSS Statistics version 22.0 (IBM Japan, Tokyo, Japan). Differences in mean values were assessed with a Student's t-test.

2.3. Estimation of the Energy Savings Amount

We used the Green Roof Energy Calculator to estimate the energy saved by a building covered with the hypothetical green roof (200 m² greening area of rooftop, 5 cm substrate depth). This web tool was developed from the Energyplus-based green roof model [24] and requests the following information: state and city, surface area of the roof, building type (old or new, office or apartment), substrate depth (limited to 5 cm < depth < 30 cm), leaf area index (limited to 0.5 < LAI < 5), irrigation flag (yes or no), percentage of roof covered by the green roof system and roofing type for the non-green roof area [black (albedo: 0.15) or white (albedo: 0.65)]. Because this tool requires LAI values, we ran separate simulations for the C. dactylon, F. arundinacea, Z. matrella and S. aizoon green roofs, as was done in the CO₂ sequestration experiment. The growing media characteristics for all green roof simulations were set as follows: thermal conductivity 0.35 W·mK⁻¹, density 1100 kg·m⁻³, specific heat 1200 J·kgK⁻¹, saturation volumetric moisture 0.3, residual volumetric moisture 0.01, initial volumetric moisture 0.1. See the article for further details of GREC [24].

The location was entered as Houston, Texas, because the meteorological conditions in that city resemble those in Tokyo, Japan. The percentage of roof covered by the green roof system was set at 50%, which set the surface area of the roof at 400 m². In Japan, about 90% of green roofs are located on new building, so the building type was set as “new office.” In order to embed the LAI values in summer into this model, the results in August 2015 was added. The irrigation flag was set as “yes” for all simulations, expect for that of the non-irrigated S. aizoon roof. The roofing type for the non-green roof area was set as “black.”

The reduction in electricity and gas consumption was estimated in kWh and converted to CO₂ emissions reduction (kg-CO₂) using the CO₂ emission factor (0.505 kg-CO₂·kWh⁻¹) obtained from the Ministry of the Environment in Japan.

2.4. CO₂ Payback Time of Modular Green Roofs

In non-irrigated green roof systems (the non-irrigation treatment), CO₂ emitted during the production of the irrigation system (irrigation tubes, irrigation pipes and automatic watering device) and associated with the annual water supply was excluded from the calculation of the total CO₂ emissions from the modular green roof. The amount of annual CO₂ sequestration by the green roof plants may decrease with their age because carbon in plants and soils eventually reaches a carbon equilibrium, with sequestration offset by decomposition [36,37]. Therefore, we calculated the CO₂ payback time based on two different scenarios. In Scenario 1, we hypothesized that the CO₂ sequestration by green roof plants occurs only during the first year after construction. CO₂ payback time for this scenario was defined as

CO₂ payback time = (CO_{2 e-p} – CO_{2 r-s}) / (CO_{2 r-e} – CO_{2 e-m})

(1)

In Scenario 2, we hypothesized that the same amount of CO₂ sequestration by the green roof plants occurs every year. CO₂ payback time for this scenario was defined as

CO₂ payback time = CO_{2 e-p} / (CO_{2 r-s} + CO_{2 r-e} – CO_{2 e-m})

(2)

where CO_{2 e-p} is the amount of CO₂ emitted during the production of the modular green roof system, CO_{2 r-s} is the annual CO₂ reduction owing to CO₂ sequestration, CO_{2 r-e} is the annual CO₂ reduction owing to energy savings and CO_{2 e-m} is the annual CO₂ emission from maintenance of the hypothetical green roof.

3. Results and Discussion

3.1. CO₂ Emissions from a Modular Green Roof

The CO₂ emission factors and total CO₂ emissions from each component are shown in

Table 2. CO₂ emission factors and CO₂ emissions for each modular green roof system component in this experiment.

System Components	CO2 Emission Factor	CO2 Emission	CO2 Emission
		(kg-CO2·200 m−2)	(kg-CO2·m−2)
Substrate	1.15 kg-CO2·kg−1	1809	9.04
Substrate container	1.89 kg-CO2·kg−1	1512	7.56
Water reservoir tray	3.70 kg-CO2·kg−1	644	3.22
Water proofing membrane	3.29 kg-CO2·kg−1	182	0.91
Edge divider	10.26 kg-CO2·kg−1	769	3.85
Irrigation pipe	3.56 kg-CO2·kg−1	16	0.08
Irrigation tube	0.55 kg-CO2·$−1	78	0.39
Automatic watering device	0.13 kg-CO2·$−1	30	0.15
Total	－	5040	25.2

. The aluminum edge divider had the highest CO₂ emission factor out of all of the components analyzed using a bottom-up approach. The water reservoir tray, water proofing membrane and irrigation pipe had similar CO₂ emission factors because they are all made from the same raw material (Table 1 and Table 2).

Aluminum is a lightweight construction material and is therefore suitable for use in green roofs, whose weights are subject to architectural constraints. However, Bribián et al. [38] found that primary energy demand and global warming potential are higher for aluminum production than for the production of several other common building products. Using materials other than aluminum for edge dividers may therefore decrease the environmental load, including CO₂ emissions, associated with the production of green roof systems.

The amount of CO₂ emitted during the production of substrate was higher than that from any of the other components (Table 2). This was due mainly to the relatively large quantify of substrate used in green roof systems (Table 1). The automatic watering device exhibited the second lowest CO₂ emissions out of all of the components. However, this result is the amount of CO₂ emitted during the production of one machine and, unlike the results for the other components, is independent of the area of the green roof. It is therefore clear that the environmental load per m² of the automatic watering device increases as the green roof area decreases.

The CO₂ emission factors and total CO₂ emissions for each maintenance practice are shown in

Table 3. CO₂ emission factors and annual CO₂ emissions for each maintenance practice in the hypothetical modular green roof.

Maintenances	CO2 Emission Factor	CO2 Emission	CO2 Emission
		(kg-CO2·200 m−2·yr−1)	(kg-CO2·m−2·yr−1)
Water	0.36 kg-CO2·t−1	58.8	0.29
Compound fertilizer	0.90 kg-CO2·kg−1	7.2	0.04
Total	－	66.0	0.33

. The total annual CO₂ emissions from maintenance practices were 0.33 kg-CO₂·m⁻²·yr⁻¹. The CO₂ emissions from the maintenance of a non-irrigated green roof were generated by fertilizer alone, at 0.04 kg-CO₂·m⁻²·yr⁻¹.

Total CO₂ emissions from the production of a modular green roof were 25.2 kg-CO₂·m⁻² (Table 2). This result is similar to the results of previous research quantifying the amount of CO₂ emitted during the manufacturing phase of layered green roofs [27]. For a green roof without an irrigation system (i.e., a non-irrigated green roof), the total CO₂ emissions from production process were 24.6 kg-CO₂·m⁻².

3.2. CO₂ Sequestration by Several Green Roof Plant

The plant and substrate dry weight, carbon concentration and carbon content of the different plant species, as recorded on 20 October 2014 and 20 October 2015, are shown in

Table 4. Mean values (n = 15) for plant and substrate dry weight, carbon concentration, carbon content and total annual carbon sequestration by four green roof plants (C. dactylon, F. arundinacea, Z. matrella and S. aizoon).

Species and Treatments			Dry Weight		Carbon Concentration		Carbon Content		Total AnnualCarbonSequestration
			(g·pot−1)		(%)		(g-C·pot−1)
			Oct-14	Oct-15	Oct-14	Oct-15	Oct-14	Oct-15	(g-C·m−2·yr−1)
C. dactylon	irrigation	plant	0.4 ± 0.0z	7.0 ± 0.4*	39.2 ± 0.3	40.7 ± 0.4	0.1 ± 0.0	2.9 ± 0.2*	690
		substrate	30.7 ± 0.7	32.4 ± 0.7	5.1 ± 0.3	5.9 ± 0.2	1.6 ± 0.1	1.9 ± 0.1*
F. arundinacea	irrigation	plant	0.4 ± 0.0	7.2 ± 0.3*	38.5 ± 0.2	36.3 ± 0.5	0.1 ± 0.0	2.6 ± 0.1*	751
		substrate	30.0 ± 0.3	33.6 ± 0.6*	5.7 ± 0.1	7.6 ± 0.3*	1.7 ± 0.0	2.5 ± 0.1*
Z. matrella	irrigation	plant	0.6 ± 0.0	6.6 ± 0.4*	42.7 ± 0.2	43.2 ± 0.5	0.2 ± 0.0	2.9 ± 0.2*	671
		substrate	30.8 ± 1.3	31.8 ± 2.5	6.2 ± 0.2	7.0 ± 0.2*	1.9 ± 0.1	2.2 ± 0.1*
S. aizoon	irrigation	plant	0.6 ± 0.0	3.9 ± 0.2*	38.9 ± 0.5	41.5 ± 0.3*	0.2 ± 0.0	1.6 ± 0.1*	459
		substrate	30.7 ± 0.4	30.9 ± 0.5	5.5 ± 0.2	7.6 ± 0.2*	1.7 ± 0.0	2.3 ± 0.0*
	non	plant	0.6 ± 0.0	3.1 ± 0.1*	38.9 ± 0.5	38.5 ± 0.2	0.2 ± 0.0	1.2 ± 0.0*	336
		substrate	30.7 ± 0.4	30.5 ± 0.3	5.5 ± 0.2	6.9 ± 0.3*	1.7 ± 0.0	2.1 ± 0.0*

^Z represent means ± SE. * represent significant differences between the results of October 2014 and October 2015 (Student’s t-test, P < 0.05).

. For all species and treatments, plant dry weight was significantly higher at the end of the experiment than at the start. There were no significant differences in plant carbon content between October 2014 and October 2015, except for that of S. aizoon with irrigation treatment. F. arundinacea was the only species for which substrate dry weight at the end of the experiment was significantly different from that at the beginning. The substrate carbon concentration of all species and treatments increased during the experimental period and C. dactylon was the only species for which it did not increase significantly. In addition, the plant and substrate carbon contents for all species and treatments increased significantly during the experimental period. These results suggest that some carbon from the plant litter and root exudate was fixed in the substrates.

The total annual carbon sequestration by F. arundinacea was higher than that of any other species and was similar to those of the other grasses (Table 4). S. aizoon exhibited higher total annual carbon sequestration with irrigation treatment than with non-irrigation treatment. The results for S. aizoon were similar to results from previous research investigating Sedum green roofs [25]. With the assumption that carbon sequestration resulted only from CO₂ uptake, total annual carbon sequestration was converted to annual CO₂ sequestration. C. dactylon sequestered 2.530 kg-CO₂·m⁻²·yr⁻¹, F. arundinacea sequestered 2.754 kg-CO₂·m⁻²·yr⁻¹, Z. matrella sequestered 2.459 kg-CO₂·m⁻²·yr⁻¹, S. aizoon with irrigation treatment sequestered 1.684 kg-CO₂·m⁻²·yr⁻¹ and S. aizoon with non-irrigation treatment sequestered 1.232 kg-CO₂·m⁻²·yr⁻¹. This experiment thus supports the CO₂ sequestration capacity of several green roof plants.

Kuronuma and Watanabe [39] suggested that competence for carbon sequestration of Sedum under wet and increased nutrient conditions are equivalent to those of other green roof plants (Zoysia matrella and Ophiopogon japonicus). However, it was disaccorded with the present results. This is probably because the present experiment conditions keeping with general cultivation practice was nutrient-poor for Sedum species. From this reason, it is suggested that carbon sequestration of Sedum aizoon was lower than those of the grasses. For the carbon sequestration of green roofs, therefore, more suitable design and maintenance practice of vegetation should be studied further.

The LAI of each specie and treatment, as recorded in August 2015, is shown in

Table 5. Mean values (n = 15) for leaf area index (LAI), energy saved annually and annual CO₂ reduction due to saved energy of modular green roofs with different species (C. dactylon, F. arundinacea, Z. matrella and S. aizoon) and treatments (irrigation and non-irrigation).

Species	Treatments	LAI	Energy Saved Annually	Annual CO2 Reduction
		(m2·m−2)	(kWh·200 m−2·yr−1)	(kg-CO2·m−2·yr−1)
C. dactylon	irrigation	2.21 ± 0.16	696.4	1.758
F. arundinacea	irrigation	3.68 ± 0.21	747.1	1.886
Z. matrella	irrigation	2.45 ± 0.17	713.5	1.802
S. aizoon	irrigation	3.71 ± 0.15	748.2	1.889
	non	3.07 ± 0.19	674.3	1.703

. S. aizoon with irrigation treatment exhibited the highest LAI out of all of the species and treatments and C. dactylon exhibited the lowest.

3.3. Estimation of the Energy Savings Amount

The annual energy savings from the hypothetical green roofs are shown in Table 5. Of the irrigated green roofs (with irrigation treatment), the annual energy saving from the S. aizoon green roof was the highest because the LAI of S. aizoon was higher than that of the three grass species. In contrast, although S. aizoon with non-irrigation treatment exhibited the median LAI of all species and treatments, its annual energy saving was the lowest because its irrigation flag was set as “no.” However, annual energy savings did not differ much between species and treatments and annual CO₂ reductions due to energy savings were between 1.703 and 1.889 kg-CO₂·m⁻²·yr⁻¹ (Table 5), being similar to those in previous research [22].

3.4. CO₂ Payback Time of Modular Green Roofs

The CO₂ payback time calculated from the Scenario 1, which hypothesized the CO₂ sequestration by the green roof plants occurs only first year after construction, was in the range of 14.0 to 15.9 years (

Table 6. CO₂ payback time of modular green roofs with different species (C. dactylon, F. arundinacea, Z. matrella and S. aizoon) and treatments (irrigation and non-irrigation).

Species	Treatments	CO2 Payback Time (years)
		Scenario 1	Scenario 2
C. dactylon	irrigation	15.9	6.4
F. arundinacea	irrigation	14.4	5.8
Z. matrella	irrigation	15.5	6.4
S. aizoon	irrigation	15.1	7.8
	non	14.0	8.5
Means ± SE		11 ± 4.3
Standard uncertainty		1.4

Note: Scenario 1 was hypothesized that the CO₂ sequestration by the green roof plants occurs only in the first year following construction. Scenario 2 was hypothesized that the same amount of annual of CO₂ sequestration by the green roof plants occurs every year.

). On the other hand, Scenario 2 was hypothesized that the same amount of annual CO₂ sequestration by the green roof plants occur every year. Thus, the CO₂ payback time calculated from the Scenario 2 was in the rage of 5.8 to 8.5 years and shorter than that calculated from the Scenario 1. For the S. aizoon green roofs, the amount of CO₂ emitted during the production of a non-irrigated green roof was lower than that from an irrigated green roof. Therefore, the CO₂ payback time calculated from Scenario 1 was longer for the irrigated green roof than for the non-irrigated green roof. However, the CO₂ payback time calculated from Scenario 2 was longer for a non-irrigated green roof than for an irrigated one because the increase through in CO₂ reduction from irrigation is greater than the amount of CO₂ emitted by the addition of an irrigation system and annual water supply (Table 4 and Table 6). However, these results indicate that there were not significant differences among the CO₂ payback time of modular green roofs with different species and treatments in this experiment. Our results for the hypothetical green roofs indicate that CO₂ reduction from the combination of CO₂ sequestration and energy savings by a green roof can offset the CO₂ emitted during its production and maintenance after 5.8 to 15.9 years (Table 6). The lifespans of green roof components are generally thought to be between 40 and 50 years [28,29,30,40], so it is clear that the lifetime CO₂ reduction of modular green roofs offsets the CO₂ emitted during their production and maintenance. Accordingly, our results suggest that modular green roofs are an effective way to reduce atmospheric CO₂ and mitigate global warming. In addition, Tripanagnostopoulos et al. [34] clarified CO₂ payback time of solar photovoltaic systems which is between 1.3 and 4.1 years. Thus, CO₂ payback time will be one of the index which could compare the carbon balance of green roofs with that of rooftop solar photovoltaic systems [41].

In this study, we focused on the production processes and maintenance practices for green roofs. However, CO₂ sequestration in plants and substrates seems to be greatly influenced by their disposal phase. In addition, the period for which green roofs serve as carbon sinks may be shorter than those of other urban greenspaces (e.g., urban forests, parks and trees) because of the more frequent repair and demolition of buildings. Therefore, the disposal phase also needs to be discussed to ensure the sustainable design of green roofs although the CO₂ payback time calculated from energy savings alone was shorter than the system component’s lifespan (17.6 years for C. dactylon, 16.2 years for F. arundinacea, 17.1 years for Z. matrella, 16.2 years for S. aizoon with irrigation treatment and 14.7 years for S. aizoon with non-irrigation treatment).

This study did not cover all green roof types (e.g., different green roof systems, system components, vegetation plants, substrate depth and locations) that could influence the CO₂ payback time results. Thus, the CO₂ emissions from a green roof and CO₂ reductions from secondly environmental benefits should be studied further. Our results will serve as a baseline for future research on the CO₂ payoff of green roofs.

4. Conclusions

This study quantified the CO₂ emitted during the production and maintenance of a hypothetical modular green roof and estimated the CO₂ reduction from energy savings and CO₂ sequestration. The results of the study show that CO₂ emissions are offset through CO₂ sequestration and energy savings after 5.8 to 15.9 years, which indicates that extensive modular green roofs contribute to atmospheric CO₂ reduction and global warming mitigation within their lifespan. In addition, this study provided a method to assess the CO₂ payoff of green roofs. It is hoped that the findings presented in this paper will contribute to the development and design of green roofs more suitable for CO₂ reduction and global warming mitigation.