3.1. Carbon produced during the construction of the green roof
According to the United States Environmental Protection Agency (US-EPA), the carbon footprint of construction activities will increase by adding any new materials in the construction phase of the buildings (EPA, 2018). Consequently, the implementation of green roofs as new components in the building and due to the manufacturing, transportation, and installation of various equipment increases the amount of CO2 released. Table 2 shows the amount of CO2 to be released during the construction of the considered building with either the green roof or a traditional roof, in both direct and equivalent formats.
Table 2
Carbon emissions due to the construction of the considered building with the green roof and with the conventional roof
Types of roof
|
Embodied carbon
(kg CO2/m2)
|
Equivalent CO2
(kg CO2/m2)
|
Traditional roof
|
19.61
|
20.72
|
Green roof
|
21.80
|
23.13
|
Difference
|
2.19
|
2.41
|
The results show that, because of the use of extra materials and equipment in the green roof (drainage system, soil layer, etc.), building this roof will increase the carbon footprint of the construction. Similar to the method used by previous studies, e.g. Nadoushani and Akbarnezhad (2015) and Mirzababaie and Karrabi (2019), in the present study, the embodied carbon term is related to the direct emission of CO2 itself, but the other greenhouse gas emissions such as the methane were reported by their CO2 equivalent amount. The difference between building the green roof and building a traditional roof in terms of direct CO2 emission and equivalent CO2 emission was calculated as respectively 2.19 kg/m2 and 2.41 kg/m2, which, if multiplied by the net area of the roof (143 m2), gives a total difference of 313.17 kg and 344.63 kg, respectively. The sum of direct CO2 emission and equivalent CO2 emission (from other greenhouse gases) was calculated as 657.8 kg or 4.6 kg/m2.
3.2. Carbon sequestration measurement by IRGA
In essence, green roofs have a positive effect on the carbon footprint by absorbing air carbon. CO2 sequestration by the green roof gradually offset the extra carbon emission of the construction process and the positive effect continues throughout the life of the building. Plants grown on green roofs continuously absorb CO2 for photosynthesis, during which CO2 is consumed to produce glucose (six molecules of CO2 are needed to produce one molecule of glucose). The CO2 that plants release during the night should also be considered in carbon footprint calculations. In general, plants return 50% of the absorbed CO2 to the atmosphere during respiration. They also transfer 90% of the remaining amount (i.e. 45% of the total carbon absorbed) to soil microbes, which eventually returns to the atmosphere upon their death. This means that only 5% of CO2 initially absorbed by the plant is actually consumed (Guo and Lee 2006). Figure 2 shows the CO2 absorption (photosynthesis rate) measured by the infrared gas analyzer for the eight considered plant species at the light intensities of 1000, 1500, and 2000 µmol/m2.s.
Figure 2. CO2 uptake (photosynthesis rate) of the eight considered plant species at the light intensities of 1000, 1500, and 2000 µmol/m2.s
The results show that plant species Sedum acre L, Sedum spectabile boreau, Frankenia laevis, and Vinca major have higher CO2 uptake rates than the others and therefore a higher potential to offset the extra carbon produced during the construction of the green roof. The CO2 uptake of plants increased with light intensity, but this increase was not linear. For example, for Sedum acre L and Vinca major, a change in light intensity from 1500 to 2000 µmol/m2.s led to a greater increase in CO2 uptake than the change from 1000 to 1500 µmol/m2.s. This suggests that these plants perform better during sunny hours (around noon) and in warmer seasons (in summer), when the light intensity is higher. In contrast, species Carpobrotus edulis, Sedum spectabile boreau, Aptenia cordifolia, and Phyla nodiflora showed a roughly proportional increase in CO2 uptake as the light intensity increased from 1000 to 2000 µmol/m2.s. Species Frankenia laevis was found to be particularly responsive to light intensity, which was reflected in a dramatic increase in CO2 uptake. The CO2 uptake of this plant at a light intensity of 1000 µmol/m2.s is similar to that of Vinca major and much lower than that of Sedum spectabile boreau, but at light intensities of 1500 and 2000 µmol/m2.s it outperforms Sedum spectabile boreau and closes the gap with Sedum acre L. The performance of different Sedum species has been researched in several works, including Getter et al. (2009), Collazo-Ortega et al. (2017), and Mirzababaie and Karrabi (2019), which have shown its suitability for use in green roofs from the perspective of cold and water stress resistance. The results of this study indicate that these plant species also have a very good CO2 uptake performance, which makes them an even better option for use in green roofs. As shown in Fig. 2, although Potentilla reptans and Phyla nodiflora have an excellent cold and drought resistance, they lack CO2 uptake performance, which makes them ill-suited for the cases where the primary purpose of the green roof is to reduce CO2 emission, like high-traffic urban areas and industrial zones with high potential air pollution. Among the eight examined plants, Sedum acre L, Sedum spectabile boreau, Frankenia laevis, and Vinca major, which had a high CO2 uptake as well as the necessary level of drought and cold resistance for survival in the study area, were chosen for use in the remainder of the work.
3.3. Compensation of extra carbon emission
The results obtained from IRGA was used to compute the annual CO2 uptake of the roof with Sedum acre L, Sedum spectabile boreau, Frankenia laevis, and Vinca major and also the time it takes for the roof to offset the extra carbon produced in the construction process in each case. The results of these calculations are presented in Table 3.
Table 3
Annual CO2 uptake of the roof with chosen plant species and the time required to compensate for the extra carbon footprint
Plant species
|
CO2 uptake
(kg/year.m2)
|
The time needed to compensate for the extra carbon emission (day)
|
Light intensity (µmol/m2.s)
|
Light intensity (µmol/m2.s)
|
1000
|
1500
|
2000
|
1000
|
1500
|
2000
|
Sedum acre L
|
3.85
|
4.31
|
6.29
|
431
|
384
|
264
|
Sedum spectabile boreau
|
2.39
|
3.04
|
3.79
|
690
|
545
|
438
|
Frankenia laevis
|
0.90
|
3.17
|
4.47
|
1830
|
522
|
369
|
Vinka major
|
0.90
|
1.23
|
1.91
|
1830
|
1350
|
870
|
The highest CO2 uptake was observed for Sedum acre L under a light intensity of 2000 µmol/m2.s. With this plant and under this light intensity, it will take the green roof just 264 days to offset the extra carbon produced during construction and start to yield a net positive effect on carbon release in the environment. The condition under which it would take longer (1830 days) to offset the extra carbon is the use of Vinca major under a light intensity of 1000 µmol/m2.s. As shown in Table 3, the CO2 uptake rate of the selected plant species at different light intensities ranges from 0.9 to 6.3 kgCO2/m2.year. In the study carried out by Whittinghill et al. (2014) on the carbon uptake of various types of Sedum when used as green roof vegetation, the uptake rate over a 12-month period was 3.9 kgCO2/m2. Collazo-Ortega et al. (2017) reported a carbon uptake of 1.8 kgCO2/m2.year for green roofs with Sedum dendroideum and Sedum rubrotinctum. The variations in reported CO2 uptake values may result from difference between the studies in terms of light intensity, plant species, and climate.
3.4. Short-term carbon footprint
In a study by Silva et al. (2015), they stated that maintaining a green roof, including its vegetation, soil layer, drainage system, irrigation system, and thermal insulation, normally requires no activity with significant carbon production in the first 5 years. Following the approach taken by Silva et al., the present study assumed that there would be no extra carbon production due to the maintenance of the green roof during the first five years. Figure 3 shows the CO2 uptake of the green roof with each of the four selected plant species in the years following construction.
Figure 3. CO2 uptake of the green roof with a) Sedum acre L, b) Sedum spectabile boreau, c) Frankenia laevis, and d) Vinca major
The amount of CO2 uptake by selected plants is measured based on their photosynthetic rate under three light intensities. The CO2 absorption process for each plant includes two time periods during the short-term operating phase of the building: (i) the time required to compensate for the extra CO2 emission due to the implementation of the green roof and (ii) the remaining time period when the positive effects on carbon footprint occur. As shown in Fig. 3, under higher light intensities and due to the better CO2 sequestration, it takes less time for the green roof vegetation to compensate the extra CO2 emission. The results show that for lower light intensity ranges (1000–1500 µmol/m2.s), Sedum acre L and Sedum spectabile boreau have the best CO2 uptake performance. For higher light intensities (1500–2000 µmol/m2.s), the best CO2 uptake performance belongs to Sedum acre L and Frankenia laevis. The best improvement in CO2 uptake as light intensity increased from 1000 to 2000 µmol/m2.s was observed in Frankenia laevis, which showed increased CO2 absorption 5 times. The next best results in this respect were seen in the plant species Vinca major with 2.1 times increase, Sedum acre L with 1.6 times increase, and Sedum spectabile boreau with 1.5 times increase (Fig. 4). This result demonstrates the relatively stable CO2 uptake behaviour of Sedum acre L and Sedum spectabile boreau under different light intensity conditions.
Figure 4. Variations of CO2 uptake in different light intensities for the selected plants
According to Fig. 4, the CO2 uptake of the roof with Sedum acre L increases more sharply as the light intensity increases, as it shows greater increase in the second 500-unit increase of light intensity (from 1500 to 2000 µmol/m2.s) than in the first one (from 1000 to 1500 µmol/m2.s). The green roof with Vinca major shows the same trend, i.e. it has a better CO2 uptake performance under sunny conditions. In contrast, for the roof with Sedum spectabile boreau, there is no significant change in the trend of CO2 uptake as the light intensity increases from 1000 to 1500 µmol/m2.s and from 1500 to 2000 µmol/m2.s. For the roof with Frankenia laevis, the increase in CO2 uptake in the first light intensity interval (1000–1500 µmol/m2.s) is greater than in the second one (1500–2000 µmol/m2.s). The poor CO2 uptake performance of Frankenia laevis in low light intensities make this plant ill-suited for areas with few sunny hours and sunny days and for use in green roofs whose main purpose is CO2 absorption. The four examined plants can be divided in two groups based on the climatic conditions of the area where the roof is to be built: the first group comprises the plants that are best to be used in areas where there are more cloudy days than sunny days, and the other group is the plants that exhibit better performance in areas with higher light intensity.
3.5. Long-term carbon footprint in Iran
In recent years, many developing countries, including Iran, have shown increasing interest in the promotion of environment-friendly building technologies i.e. green roofs, as a step toward sustainable development in the building sector. According to the Paris Agreement (UNFCCC), between 2021 and 2030, Iran is obligated to reduce its greenhouse gas emissions by either 4% or 12% depending on whether international sanctions are lifted from this country (Umemiya et al. 2020). Therefore, in the next step, the potential impact of green roofing on Iran’s total CO2 emission was evaluated in three hypothetical scenarios where 25, 50 and 75% of the urban roofed space in the country has green roofing. According to the statistics published by the Ministry of Roads and Urban Development of Iran, by 2030, the total area of roofed buildings in the urban areas of Iran will reach 4.9×107 m2. Assuming that 15% of this total area is access space (La Roche and Berardi 2014; Statistical Center of Iran 2018), green roofs can be built on 85% of this area, i.e. 4.1×107 m2. Based on this assumption, the impact of the three mentioned green roofing scenarios on CO2 emission was estimated to determine how much this technology can contribute to Iran meeting its obligations under the Paris Agreement by 2030. The results are presented in Fig. 5.
Figure 5. Annual CO2 reduction due to the extent of green roofing a) Sedum acre L, b) Sedum spectabile boreau, c) Frankenia laevis, and d) Vinca major
As shown in Fig. 5, the greatest reduction in CO2 emission (1.9×105 tons/year) will take place if 75% of the roofed surfaces in urban areas is covered with green roofs with Sedum acre L and receives the maximum light intensity (2000 µmol/m2.s). On the contrary, the smallest reduction in CO2 emission will occur if 25% of these roofed surfaces is covered with green roofs with Frankenia laevis and Vinca major and receives light with an intensity of 1000 µmol/m2.s.
Considering that the building sector is one of the major producers of greenhouse gas emission in Iran (accounting for 30% of total greenhouse gas production (Jeong et al. 2012; Statistical Center of Iran 2018), this sector is expected to have an at least 30% share in the total greenhouse gas emission reduction of the country. Since the signing of the Paris Agreement (2012), Iran has largely remained under international sanctions, and according to many experts, can be expected to remain sanctioned for the foreseeable future, possibly well after 2030. Thus, according to this agreement, by 2030, Iran should reduce its greenhouse gas emission by 4%. Based on the average per capita CO2 production in Iran (8.3 tons), this 4% amounts to 27.1 million tons of CO2, of which 8.12 million tons should be in the building sector. The results of this study show that in the absolute best-case scenario, nation-wide use of green roofs will allow Iran to meet 2.4% of its international obligations in the building sector. However, it must be noted that any decrease in CO2 emission is favorable from an environmental point of view and contributes to the long-term reduction of the country’s carbon footprint.
To benefit from the advantages of green roofs for a long time, they should be designed and constructed meticulously and maintained with great care. This operation should be able to mitigate the risks that threaten the integrity of the roof, including the decay of components and loss of vegetation, so as to prolong the lifetime of this environment (Peck and Kuhn 2003). Some of the green roof maintenance activities and operations have a noticeable carbon emission, which inevitably leaves a negative impact on the overall carbon footprint of the roof. Therefore, taking into account the amount of carbon that will be produced due to maintenance activities during the life of a green roof can improve the accuracy of the estimation in regards to the actual carbon footprint of the roof. So far, only a few studies have been conducted on the carbon footprint of long-term green roof maintenance operations. Therefore, more extensive and comprehensive studies are still needed to determine the amount of carbon released in each operation and the consequent effect on the overall carbon footprint of the structure. It is noteworthy that, in this study, the process of carbon dioxide absorption is considered a linear function with respect to time. However, determining the linear or nonlinear function of CO2 uptake by plants used on green roofs could provide a better perspective on carbon footprint, particularly over a long period of time.