1. Introduction
Major greenhouse gas (GHG) reductions are needed right now to reach the globally set target of limiting the global temperature rise to 2.0 °C. This target can only be achieved if two conditions are met. First, before 2050 the overall reduction would need to fall between 50% and 85% relative to the 2000 level. However, the first condition only applies if the second condition is met, namely, that the peaking year of cumulative global emissions must occur between 2010 and 2015. Even a 2.4–2.8 °C rise in the level of the average global temperature would require a reduction of 30%–60% in GHG emissions before 2050, the peaking year being before 2020 [
1]. Even reaching this lower level is a huge challenge for the coming years.
The built environment will be in a central role in the creation of mitigation strategies reaching for these goals. With ever-growing numbers of people residing in cities, and cities already being accountable for the vast majority of global GHG emissions [
2], built environment carbon reduction strategies will predominantly take place through city-level carbon management. Embracing this role, cities worldwide have already set targets for reducing their carbon loads. City-level GHG assessment methods have been extensively studied in recent years and many techniques have been developed [
3,
4,
5,
6,
7]. However, the concept of city carbon management is still very problematic since the city-level assessment methods may be incapable of producing information on phenomena occurring at a community or neighborhood level within cities. Thus, current practices to guide low-carbon development might be incomplete, leading to actions with unknown and potentially even unwanted results.
In this paper we try to create a better understanding of the problems related to low-energy residential construction, typically one of the key strategies of city-level carbon management [
8,
9]. We demonstrate the differences in the assessment results when the perspective is changed from the city level to the new residential development level with a projection of the future use phase emissions. To create a better understanding of the significance of the construction phase emissions in new residential developments, we take the full carbon footprint of the residents into account throughout the analysis.
Currently, while a lot of policy focus has been put on increasing building energy efficiency, the construction phase emissions, including those embedded in the materials and production chains, have been predominantly ignored (e.g., [
10,
11,
12]). This is probably due to traditional LCAs of building GHG emissions showing the use phase to dominate the emissions—with a share of 90 percent [
13,
14]. However, as energy efficiency increases, the construction phase accounts for more and more (up to over 50%) of the overall life cycle GHG emissions [
15], leaving a large share of the building related GHGs outside of the policy focus [
11]. Furthermore, the current city-level assessment models may not even be capable of producing the necessary information for managing the planning process of low-carbon city development due to underestimating the effects of construction on the city-level GHGs [
16,
17], as this study will demonstrate. This is due to one key reason. On the city scale, the GHG emissions related to residential construction seem small compared to the operating emissions of existing buildings since the renewal rate of the building stock is low. However, the situation may be totally different when a single residential development is examined and the temporal allocation of the emissions is taken into account. Then the construction phase emissions seem to dominate for decades and the operation of the building has only secondary importance, even when all the GHG emissions of the residents are taken into account.
It is obvious that all new construction leads to GHG emissions, and thus construction as such can never be about GHG mitigation. However, increasing building energy efficiency is one GHG mitigation strategy [
8,
9], and the construction phase emissions are inevitably an integral part of the buildings’ life cycle emissions. In addition, in Finland and also globally, significant migration from rural areas to cities is taking place, leading to a huge need for new residential construction. Thus, increasing understanding of the impacts of the GHG emissions occurring during major development projects on consumer carbon footprints is very important. For example, considering the above-mentioned time limits of current GHG mitigation targets for reducing GHG emissions, this study demonstrates that regardless of the energy efficiency level of the buildings, new construction cannot be utilized as a means to achieve these targets. Building energy efficiency can be argued to be important, and when new construction takes place the study recommends aiming for high energy efficiency, but it should be understood that, from the climate change perspective, the gains only materialize after decades if the carbon spike from construction remains at the level found in this study.
The paper builds understanding of the role of the construction phase emissions in the residents’ carbon footprints by assessing the life cycle GHG emissions of a new residential development in Espoo, Finland. The assessment is conducted with a LCA framework capable of including all emissions from the construction phase and, later, those from all the consumption of the projected future residents, as well as distinguishing the temporal allocation of the emissions. The functional unit is throughout the paper the inhabitant, i.e., the consumer. With this choice, the results from the different life cycle phases can be compared, which has not been possible with many previous studies. This type of approach also places the emissions from different activities, and taking place at different moments, in their true context and gives perspective on the relative importance of different emission sources, which, the study argues, is an important precondition for effective city carbon management. To demonstrate how city-level assessments may easily fail to produce reliable information to guide low-carbon city development, we present the city-level assessment results and the new residential development assessment results concurrently. While we employ a case study from Finland, the analysis tries to highlight the situation throughout developed countries.
The structure of the remainder of the paper is as follows: in
Section 2 the case is presented, in
Section 3 the assumed future residents are profiled,
Section 4 presents the assessment frameworks and the data utilized,
Section 5 goes through the assessment results,
Section 6 discusses the global representativeness of the case area, and finally
Section 7 discusses the findings and limitations as well as creates a wider context for the study.
7. Discussion
The study was set to analyze the concept of the construction of new low-carbon neighborhoods as an instrument of climate change mitigation in the built environment. A case study of a new residential development in the HMA in Finland was utilized to demonstrate the impacts occurring throughout the developed world. The argument of the study is that the neighborhood level emissions, especially related to new residential developments, are currently incompletely understood due to the prevailing belief that the construction phase emissions would have only minor relevance in the life cycle emissions of residential properties.
A hybrid LCA approach was employed, distinguishing the construction and use phases as well as taking into account the temporal allocation of the emissions over time. This fills the requirements of even a broad understanding of the WRI Scope 3 definition [
28], which is important in measuring the emissions on a neighborhood level since there are few emissions taking place inside the neighborhood. Further, we argue that it is only with the type of approach presented here that the implications of new residential construction can be truly analyzed and effective carbon management in urban areas can emerge, and the results of the study strongly support this argument.
The study found that the construction phase, when allocating the emissions to the future residents, creates a carbon load of over eight times that caused annually by all consumption of the residents—approximately 110 t CO2e compared to 13.8 t. Moreover, compared to the annual energy use related emissions, 2.4 t, the construction phase emissions are in an overwhelmingly dominant position when a single residential development is regarded. The study also highlighted that even with rather energy efficient buildings the emissions of the new area remain as high as those of an average resident in an existing house during the selected 25-year lifespan. This stresses the importance of both including the temporal allocation perspective into the neighborhood level GHG assessments and reducing the construction phase emissions if new construction is to be used for climate change mitigation. Furthermore, if the construction phase emissions remain at the current level, the Finnish average 1% annual renewal rate leads only to an increase in the cumulative emissions up to 2050, the current target year of significant mitigation needs.
By including the full consumption-based carbon footprint of the residents into the assessment, we were able to give context for the housing related emissions and the role of urban development in city-level decision-making. Actually, the traditional geographically restricted production-based approaches may fail to produce accurate information about the emissions even at a national or city level [
43], let alone at a neighborhood level where there is presumably very little or no production. Thus, these type of consumption-based approaches could provide a very valuable complement to the production-based assessments.
When considering the representativeness of the study, there are two perspectives that should be covered. It is likely that a very similar situation with the construction phase carbon spike also exists in developed countries elsewhere, due to construction technologies having only limited variations. The whole context presumably has significant similarities as well, especially in countries with similar energy production GHG intensities. Regarding this perspective, it would seem that the consequences of construction are the most severe in Finland with relatively clean power production (as was shown in
Section 6), but the current development trend of decreasing energy production intensities and increasing consumption volumes [
40] further unifies the areas. In addition, due to the decreasing energy production intensities, the importance of the later use phase emissions decrease even further. In open economies like Finland, where over 50% of the value of consumption goods are imported [
32], the emissions actually accumulate globally, decreasing the impact of local decisions. Ortiz
et al. have also concluded that energy consumption of use phase has been an issue, especially in European countries, indicating that, even without low-energy buildings, the construction phase may account for a much larger share of the total life cycle emissions in countries with a warmer climate than that of Europe [
44].
In Finland, the CHP production method in all major cities gives an additional, interesting perspective on the implications of building energy efficiency. Since heat is also a side product of electricity production in CHP plants, decreasing heat consumption in buildings only affects the overall carbon emissions if electricity generation is reduced as well [
45]. Electricity use, however, is predominantly controlled by the residents and cannot be affected very much by building characteristics in district heated buildings [
46,
47]. The implication is that reducing the energy production GHG intensity and electricity consumption should be the primary targets of the carbon reduction strategies in the built environment instead of energy efficient buildings. However, as new construction will be required to a significant extent in the near future, attention should be paid to the construction materials alongside building energy efficiency. Notwithstanding, the kind of analysis conducted in this study points out that when the life cycle emissions of all consumption are taken into account, significant reductions are not easily attained.
One additional perspective on the true effects of carbon mitigation through energy efficient buildings is the rebound effect. For example, Turner
et al. demonstrate that the emissions may even increase when energy efficiency increases [
48]. This is due to people spending the savings gained by higher energy efficiency. In the initial assessment we didn’t assume the savings to be spent. However, if the annual amount saved on energy consumption would be fully spent, and if we assume the same carbon intensity as that of the overall consumption on average (0.9 kg CO
2e/€), the result is an approximately 0.5 t increase in the annual emissions. Since energy has the highest carbon intensity among all goods, this is not enough to offset the 3.0 t reduction in emissions gained by energy efficient buildings, but still a noteworthy figure.
This type of rebound effect analysis is very uncertain however. There is no way of knowing how the money will be spent, or even what the actual saving truly is. It might well be that the saving on energy is actually invested in the residence and thus already included in the assessment (while still potentially having a rebound effect through increased construction phase emissions, if not invested in low-carbon construction materials, etc.). High investment in the residence may even reduce other forms of consumption. Notwithstanding, the rebound effect further increases the importance of decreasing the carbon intensity of energy production as the main mitigation strategy. With this, the largest single source of carbon emissions can be affected at the city level and no direct rebound effect exists.
There are certain identified limitations and deficiencies in the assessment presented here. One limitation is the scope, including only one case study. This means that the assessment may be sensitive to changes in e.g., construction materials and material replacement rates. Regarding the material choices, the results are not highly sensitive to any single material change, as even the three largest GHG contributors, concrete, bricks and steel, which could to some extent be replaced by wood-based products, together account for less than 25% of the construction phase emissions. This result is supported by a building LCA sensitivity study of Junnila and Horvath who report rather low sensitivity of LCA results to emissions from construction materials [
49]. Concerning the material replacement rates, Junnila and Horvath discuss that LCA’s may underestimate real replacement rates during the building life cycle. In this study the replacement is approximated by the annual average money spent on maintenance, repairs and refurbishments by the projected future resident (under Housing). The amount is based on real monetary consumption data (the input data, see
Section 3 and
Section 4), and should thus not lead to heavy underestimation of the replacement rates.
Second problem relates to the future uncertainties. While the temporal allocation of the emissions is taken into account, no changes in the emissions intensities or the consumption volumes are assumed, except for energy and fuel combustion emissions, which is unrealistic. However, these are also very difficult to predict. The intensities probably decrease over time as new technologies emerge, but the scale of the change is unclear. On the other hand, the consumption volume probably keeps increasing, causing the opposite effect on the carbon emissions that may easily offset the gains from the decreasing GHG intensities. Concerning the scenarios that were brought to the assessment, there are certain important limitations. While it is probable that energy production GHG intensities will fall within the next 25 years, the development is not necessarily similar in certain power plant (nor is the development predicted by the utilized scenario the only prediction available). The scenario utilized in the study presumes linear decrease in the GHG intensity, but in a single power plant the development would likely be in leaps after rather steady periods. It is also possible that the particular power plant utilized in the study will not undergo significant changes during the next 25 years. However, the perspective given by the scenario adds to the generalizability of the results to different urban developments in different locations and moments in time. The fuel combustion scenario may also either under- or overestimate the future change, but suffers less from the above described problem caused by the neighborhood perspective. Furthermore, there is actually a negative discount factor for the near future emissions [
50]. CO
2 persists in the atmosphere for a long time, and the accumulation of the emissions is in fact the key problem instead of the annual magnitudes. Adding this perspective to the analysis should be one important objective of future research.
A second deficiency is that the projected resident’s consumption profile may deviate from the true consumption of the future residents significantly. Different areas and building types attract different kinds of residents. For example, in Espoo the average detached house resident is very affluent and consumes much more than city residents living in apartments [
20]. The income level is almost twice the income of an apartment resident and the consumption volume approximately 25% higher. However, reducing the carbon emissions by attracting less affluent residents would not affect the GHGs from a wider perspective, as the more affluent would just reside somewhere else. Notwithstanding, both Majamaa
et al. [
19] and the area qualities suggest that the profile resident is fairly representative of the future residents.
Finally, the assessments are subject to a number of uncertainties that decrease the accuracy of the results. These uncertainties are discussed in detail in Heinonen and Junnila [
21,
22,
23] and in Heinonen
et al. [
16,
24] with the conclusion that they should not compromise the overall findings as long as the aggregate level of the average residents is maintained in the analyses.
For positioning the study among academic research in the field, not many similar approaches could be identified. However, the different perspectives of the study arise from the earlier contributions of many academics. Firstly, it is very well understood in general that rapid urbanization in developing countries is a huge challenge for global GHG mitigation efforts. This is partly due to the increasing consumption of goods and services, but as Minx
et al. recently showed in a case study in China, increasing GHG emissions there are largely due to intensive construction activity taking place in cities [
51]. An increase in the importance of the construction phase emissions also relates partly to the assessment methods; e.g., Toller
et al. argue for the importance of taking into account the emissions embedded in construction materials in GHG mitigation strategies [
52] that an IO based LCA can capture, but which may be left outside of the boundary of process LCAs.
Concerning the planning perspective, Wallbaum
et al. bring up the incompleteness of the sustainability assessment methods when city planning is concerned [
53]. Further, they stress the importance of incorporating the environmental perspectives into the early phase planning. The different significance of the emissions occurring now and in the near future, compared to those taking place later, has been recently stated by e.g., Levasseur
et al. [
54] and Dutil
et al. [
55]. Schwietzke
et al. analyze corn ethanol production with a temporal model, arguing for a higher priority for reducing the early phase emissions [
50]—a situation very similar to buildings from the GHG emissions perspective.
Moreover, many authors have brought up the utility of the consumption perspective on emissions, especially when city-level assessments are concerned, one of the key arguments being that on a local level the consumption perspective is needed to capture the real GHG impacts of the local society [
3,
6,
43,
56,
57]. This argument seems to be stronger and stronger the smaller the concerned area is. With a different perspective and scope on the assessed emissions, the case area presented in this study could seem to cause very little emissions and be of little importance to city-level GHG mitigation. Additionally, as Peters and Hertwich argue, and as can also be concluded from the results of this study, more sustainable consumption habits need to be promoted in order to achieve significant GHG reductions [
58]. This calls for local level consumption-based assessment techniques. Jones and Kammen recently compared different consumption choices from just this perspective [
59]. According to VandeWeghe and Kennedy, the consumption perspective also enables meaningful conclusions for urban planning [
60]. Larsen and Hertwich have developed a local assessment model for Trondheim, Norway using a similar argument; that a consumption-based model would provide better information for local action [
61]. Other local assessment approaches in recent years include e.g., Erickson
et al., Baynes
et al. and Lenzen
et al. [
43,
62,
63]. Satterthwaite further discusses the significance of the allocation perspective of the emissions on the assessment results, which highlight the problematic situation of current city level GHG assessment methods [
64]. Finally, systems thinking and industrial ecology, albeit being less structured approaches, provide similar ideas in suggesting to adopt a wide WRI scope 3 type of perspective in the assessments [
65].
To conclude, the contribution of the paper is primarily attached to two findings. First and foremost, without a significant reduction in the construction phase emissions in the very near future, new residential construction cannot be utilized as an instrument of climate change mitigation in the built environment. According to the study, new construction seems to have a detrimental impact, even on the 2050 reduction targets. Second, the paper presents a framework to assess, understand, and manage GHG emissions on a neighborhood level. Since several indicators implied the phenomenon to occur around the world, the presented case study may serve as an important basis for further development in the future.