Defining dynamic science-based climate change budgets for countries and absolute sustainable building targets

Given the current climate crisis countries and sectors need to set targets and address reduction potentials for their greenhouse gas (GHG) emissions. The current use of bottom-up benchmarks for the building sector enables relative sustainability comparisons. However


Introduction
In the Paris Agreement under the UNFCCC, 190 countries have expressed the ambition to limit global warming to 1.5 • C or, at most, 2 • C above pre-industrial levels [1].Furthermore, the IPCC report on pathways to achieve the 1.5 • C target shows that rapid decarbonisation is needed, involving all economic sectors and the entire global society [2].In this context, it is necessary to share the effort by defining specific targets at a more granular level than global emissions reductions.Since 2008, the EU has been setting political targets to reduce emissions [3], and most recently, a political agreement has been reached to reduce greenhouse gas (GHG) emissions with 55% relative to 1990 by 2030.The objective for the EU is to reach climate neutrality by 2050 [4].
Buildings are responsible for 37% of global energy-and processrelated GHG emissions, of which 9% originate from the building construction industry [5].Thus, the construction sector plays a pivotal role in achieving global targets.While there has been an intense focus from policymakers to reduce operational energy use in buildings, there has been a lack of requirements towards the embodied impacts in buildings [6].Climate impacts from operational energy in new buildings have been reduced substantially due to the decarbonisation of the energy grid and the construction of highly energy-efficient buildings [7,8].This has led to increased embodied impacts both in absolute and relative terms because of the larger consumption of materials and services required to build energy-efficient buildings [9].An example is the increased use of insulation to meet regulatory requirements on operational energy use.Upfront emissions (related to the production and construction of a building) are the largest contributor to the embodied emissions, accounting for approximately 64% of the life cycle emissions according to a study of 238 Life Cycle Assessments (LCAs) of buildings in literature [7].This burden shift underlines the importance of adopting requirements for whole life assessments of buildings, encompassing both operational and embodied impacts.Widening requirements towards LCAs rather than only operational energy enables that the GHG footprint can become an additional performance criteria for tenders and public procurements.This then brings changes towards increased market demand on environmental product declarations from manufacturers.From a developer perspective, the introduction of limit values enables setting targets that go beyond legal requirements [10].
In Europe, we see a tendency where the countries with the strictest requirements on operational energy are also introducing regulations on embodied impacts in buildings [9].Since 2017 the Netherlands has required new residential and office buildings over 100 m 2 to conduct LCAs and report environmental impacts, which in 2018 led to national environmental limits [11].In the coming years, Denmark, Finland, Sweden, and France will introduce requirements for conducting LCAs which will then be followed by limit values in 2022-2027 [10,[12][13][14].Methodological harmonization has, however, not been accomplished, and while the Swedish method currently only covers upfront emissions (referred to as modules A1-A5 according to EN15978 [15]), the Danish, Finnish and French methods cover both more life cycle stages and building components [10,14].
Today, environmental targets for our buildings are primarily derived from bottom-up benchmarks based on existing building LCAs [16].Multiple examples of bottom-up benchmarks can be found in literature [17][18][19][20].These benchmarks enable relative comparisons but will not ensure that the building design is in line with the Paris Agreement goal of limiting global warming to 1.5 • C [21].Therefore, to tackle global climate challenges, there is a need for evaluating building performance in relation to science-based environmental targets.Top-down benchmarks are typically defined based on political targets such as the Paris Agreement [22].The importance of top-down benchmarks was also underlined at the 71st LCA forum on environmental benchmarks for buildings, where more than 80% of the 33 participants answered that benchmarks for buildings should be developed involving a top-down approach [16].
This study proposes a new procedure for downscaling the global climate budget to a top-down target for a building.The procedure involves identifying the global climate budget, which limits warming to 1.5 • C, defining the climate budget to the construction sector, and further downscaling this to the level of the specific building.Downscaling a global GHG budget to an activity or system involves introducing sharing principles.Sharing principles are based on ethical decisions on how the budget should be split and as different sharing principles represent different theories of justice, it is crucial to communicate the applied allocation principles with transparency [23].The underlying equity principles of the sharing principles applied in this article will be explained along with the introduction of the sharing principles in the method section.Sharing principles are widely used in literature to identify GHG budgets at a national, sector, or company level [24][25][26].
At a building level, there are multiple examples of top-down GHG budgets for single-family buildings, although these examples lack consensus around how the global GHG budget is defined, which sharing principles should be used, as well as scoping in terms of life cycle stages included [27].While most of these examples apply a final per-capita budget, such as 1 tonne CO 2 per capita and then distributed to a share for a building [22,28,29], there are also examples of the global budget being based on the 2030 target or total allowable budget [30,31].However, none of the studies apply a dynamic annual GHG budget accounting for the current emission levels and the possible transition pathways, limiting global warming to 1.5 • C.This study will apply a dynamic annual GHG budget to the Danish building sector to enable building designers, as well as policymakers to set targets in line with the Paris Agreement.By creating dynamic targets, the method this study presents, has the potential of showing the pathway towards climate neutrality in line with the Paris Agreement.The proposed targets seek to provide building designers with targets that can be used for designing buildings that are designed to be climate-wise sustainable and to quantify whether the national legislation in Denmark is ambitious enough.The purpose of this study is to show how top-down targets can be created by exemplifying the procedure in a Danish context.The proposed method could, however, be applied to any country with available data.

Downscaling the global budget
In the following section, the downscaling procedure, illustrated in Fig. 1, is presented in steps.The procedure involves defining an annual GHG budget consistent with the Paris Agreement target of limiting temperature increases to 1.5 • C above preindustrial levels (step 1).Then the method will introduce sharing principles that can downscale the budget from global to national budgets (step 2) and further down to building materials (step 3).Step 4 involves defining the share for new buildings, as part of the budget for building materials is also used for civil engineering and renovation and maintenance of existing building stock.

Step 1: defining a global GHG budget
A GHG budget defines the maximum amount of anthropogenic cumulative GHG emissions, which can be released into the atmosphere within a given period [2].Limiting global mean temperature increases to 1.5 • C above pre-industrial levels have been translated into global annual GHG budgets and pathways of various confidence levels [32].Consistent with the work of the IPCC Special Report [2], a study was conducted to explore transition pathways that will limit radiative forcing to 1.9 W m − 2 and consequently keep global mean temperature increases to 1.5 • C in 2100 [33].The study applied the narratives of five Shared Socio-economic Pathways (SSPs) and used six different integrated assessment models and a simple climate model to explore under which SSPs, scenarios consistent with the 1.5 • C target could be produced.The study by Rogelj et al. [33] produced 13 successful scenarios, and these will be applied as a first step to identifying GHG pathways for buildings.Fig. 2 illustrates the average, minimum, and maximum of the 1.5 • C consistent pathways (with 66% probability) according to Rogelj et al. [33].
The pathway reductions rely on actions such as decarbonisation of the energy grid by shifting from fossil fuels towards low carbon energy supplies, reduced energy use, and carbon dioxide removals (CDR) [34].Bioenergy with carbon capture and storage (BECCS) contributes to the most significant part of CDR and is the primary reason why the average GHG emissions become negative around 2070.Long-lived GHGs, such as CO 2 , stay in the atmosphere for many years, and according to IPCC Working Group 1, it could take up to 20-80 years before global temperatures stabilise even with significant and sustained emission reductions [35].However, when the excess anthropogenic carbon releases are no longer in the atmosphere, the level of anthropogenic carbon can return to net zero.

Step 2: scaling from a global to national level
This study proposes two different sharing principles to scale from a global to national level.The first principle applies an egalitarian principle "equal per capita" (EPC).The use of EPC as the initial step is the most frequently used in literature and follows the interpretation of egalitarianism that every individual is assigned an equal share of the global GHG emissions budget [36].The allocated budget for a country using EPC can be calculated through the following formula: Where NB EPC,t is the allocated a national budget in the year t, GB t is the global budget in the year t, POP nation,t is the national population in year t and POP world,t is the global population in year t.
The country shares are based on a projected median population according to the UN [37].By using these projections, the future growing populations are accounted for.The population growth rates of European countries are projected to be lower than the growth rate in African and Asian countries.Thus, the population-based share assigned to European countries will become smaller over time.The annual GHG budgets applied are presented in Table 1 as an annual GHG budget per capita.
As a second sharing principle, this study will apply Ability to Pay (AP), where the current baseline emissions, as well as the country's capability to reduce, determine the national budget.To determine the capability to reduce, GDP is applied as GDP is often considered an indicator of a country's standard of living.The formula is presented in Refs.[24,26] and calculates a national reduction rather than a share of the global budget: where NR t is the national reduction in the year t, GDP nation and GDP world are the national and global GDP and E is the world baseline emissions.
A correction factor is then calculated to ensure the sum of allocated reductions matches the global budget: Where N is the total number of countries and e is national baseline emissions.
Finally, the national budget (NB AP.t ) can be calculated by subtracting the corrected reduction from the national baseline emissions.

NB AP.t = e − NR t • e corr
Countries with higher GDPs than the global average are considered capable of reducing more and are therefore assigned larger relative reductions compared to low GDP countries.
For calculations, country characteristic data for the year 2018 has been used.Data for baseline emissions, population, and GDP was extracted from worldMRIO [38].Population estimates originate from UN medium projections [37], GDP from the World Bank and baseline emissions are based on consumption-based accounting (CBA) from the Eora multiregional input-output (MRIO) model [39,40].A few countries did not contain POP, GDP, and/or CBA and were therefore disregarded.The emission gap between total current emissions according to Ref. [34] and the sum of all the country's emissions baselines was accounted for by splitting the emissions between the countries according to the current contribution share.

Step 3: downscaling from country to building materials
In Step 3, this study explores two sharing principles representing a utilitarian distributive justice theory and acquired rights.Acquired rights (AR) is also sometimes referred to as the grandfathering principle, as the shares are based on the historical distribution of how large a share the system/product has previously acquired.This study applied the environmentally extended MRIO model Exiobase 3.8 [41] to estimate the GHG emissions related to construction (including direct and indirect emissions) relative to total GHG emissions from Denmark in 2015, which resulted in 11.8% of Danish GHG emissions being attributed to construction.The emissions cover raw material extraction, transport to plant, manufacturing of building materials, transport to building site and installation.
For the utilitarian (U) principle, it was assumed that the money spent by consumers on specific products and services also reflects the things that provide us with the largest utility or well-being.This assumption is Fig. 1.Downscaling from a global GHG budget to building level.

Fig. 2.
Annual GHG emissions for 1.5 consistent pathways, with a 66% probability of no overshoot according to Ref. [33].The arithmetic average is shown with the solid line and the shaded area indicates the range from minimum to maximum emissions.

Table 1
Global GHG budget according to the average pathway consistent with a 1.5 target.only partially justified as our current consumption can be considered unsustainable [23].Indeed, it is likely that our spending habits would substantially change if we were to spend within a confined budget in line with, e.g. the 1.5 • C target.The MRIO model Exiobase [41] was used to estimate the overall direct and indirect spending on different industries, such as construction.This allowed for estimating the share of direct spending on the Danish construction sector, as well as indirect spending through direct spending on other sectors.For instance, the indirect spending on the construction of buildings such as production facilities for food production could be estimated [42].The overall direct and indirect spending on the Danish construction sector relative to other industry sectors was used to determine the share of the Danish GHG budget that should be assigned to the Danish construction sector.

Step 4: defining a share for new buildings
As embodied impacts originate not only from new build projects, it has been necessary to quantify the proportion of different activities contributing to material consumption, i.e. infrastructure, renovation and maintenance of existing building stock, and construction of new buildings.This has been carried out by analysing the production value of construction activities.The production value is an economic key figure which quantifies the economic activity without deducting the consumption of materials and services in production [43].It is acknowledged that using an economic distribution as a proxy for the distribution of material consumption (and in most cases GHG emissions) can be problematic, as it can be argued that maintenance and renovation costs and new build costs are not necessarily proportional to the use of material.Nevertheless, in the absence of data on the distribution of material consumption, the economic key figure was considered the best available option.
Where S EA is the share for new buildings based on economic activity and EA NB and EA CON is the economic activity for new buildings and the overall economic activity for construction.

Sharing principles
Steps 1 to 4 described in the above sections can be put into the following equations calculating the budget for new buildings in Denmark for the year t.

Estimating a budget per m 2
As this study seeks to find a target value for all new construction and to enable comparability with national legislation, the target will be estimated per square metre.Square metre has been chosen, as it is a common denominator for all buildings and commonly used for legislative purposes e.g.energy performance calculations.Thus, the final step of downscaling the global budget requires estimating how many square metres will be built in the future.This study bases the number of square metres, which will be built, on past construction trends based on construction activity in 2018-2020 [44].The status quo assumption serves the purpose of showing how ambitious target values should be, if the level of construction activity is continued in the future.This is relevant for building owners and designers, as their influence is limited to projects under their control.For policymakers, this is relevant, as it can indicate how ambitious target values should be given the current construction activity.

Climate targets for buildings in the Danish national strategy
In 2021 a national strategy for new buildings in Denmark was proposed.The strategy includes limit values suggested for legislation and a "Voluntary CO 2 Class".The limits cover whole-life GHGs encompassing both operational and embodied GHG emissions and will be introduced in 2023 with a progressive tightening.In Table 2, proposed Danish legislation is presented [13].To only consider upfront embodied GHG emissions, an average contribution from upfront emissions out of whole life emissions has been applied based on a Danish study on 60 building LCAs [17].The contribution analysis excluded buildings with wooden structures as these buildings typically have negative upfront emission impacts, which are then counterbalanced by similar size emissions during End of Life (EoL).It is, however, recognised that the distribution could be different, and that this simplification is just to enable comparison between the national strategy and the calculated values of this study.

Results
Section 3 contains results at different levels during the downscaling procedure presented in the method section.The method enables calculating national dynamic budgets for all countries.Section 3.1 presents GHG budgets at country level for selected countries, and results for all countries can be found in Supplementary Material.To exemplify how the method can be further used to scale to building level, Section 3.2 presents results at building level, however, limited to Denmark.

National dynamic budgets
In this section, results at country level are presented using both the egalitarian principle Equal per Capita (EPC) and using the sharing principle Ability to Pay (AP).Results are calculated with equations presented in Section 2.1.2.To enable examination of how baseline emissions and GDP affect the results, Fig. 3 shows both the applied data (3a and 3b) and the calculated results at country level (3c).The Figure shows selected countries, however, results for all countries can be found in Appendix A. Supplementary Material.Both baseline emissions and GDP (presented in 3a and 3b) represent the year 2018 as this was the most recent available data.Fig. 3c shows the calculated emission budgets per capita both using EPC and AP.For the baseline emissions, consumption-based accounting (CBA) was chosen.Thus, the annual GHG budgets should also be read as consumption-based budgets.To enable comparison across countries, results are presented per capita.
When using the sharing principle AP, countries with higher GDP than the global average are considered capable of reducing more and are therefore assigned larger relative reductions compared to countries with a lower GDP than the global average.However, as the point of departure depends on the baseline emissions, countries with high initial emissions also receive high initial budgets.Moreover, countries with low baseline emissions are kept low, as the formula calculates reductions and thus not allowing any countries to increase emissions.Fig. 3 illustrates the effects of GDP and baseline emissions on the countries' emission budgets per capita.
Fig. 3 also shows the equal per capita (EPC) budget, revealing that most of the countries illustrated in the figure have higher initial budgets when applying AP than EPC because of high baseline emissions for these countries.This emphasises the inequality in how emissions are distributed today.However, countries with high GDPs are required to reduce much faster when applying AP than the EPC distribution.

Dynamic targets for buildings (covering only Denmark)
In this section, the national annual GHG budget for Denmark has been further downscaled to the Danish construction sector and will be presented per new build m 2 .Results are calculated with equations presented in Section 2.1.5.In this study, the term target value is used to present the average GHG budget all buildings should stay below.In practice, this means some buildings could perform better with a lower GHG footprint and some worse, as long as the average contribution from all new buildings stays below the budget.The proposed targets apply to the construction of new buildings, including all upstream activities.Thus, relating the targets to the LCA methodology as defined in EN 15978 [15], the targets cover A1-A5 (production of materials and construction).
Table 3 presents target values for each decade until 2050, given in kgCO 2 eq/m 2 , for the four sharing principles described in Section 2.1.5.Values are calculated for the average pathways, and minimum and maximum values are presented in parentheses.Minimum and maximum values originate from the span in values of the global pathways consistent with the 1.5 • C target [33].The values are calculated assuming that construction activity in Denmark will remain at the same levels in the future as in 2018-2020.
The proposed targets can also be presented as pathways representing the development from 2020 and forward (see Fig. 4).In Fig. 4, the national strategy is also illustrated allowing for comparison with the target values.Under the assumptions presented in this study, the comparison of the calculated values of this study and the estimated values of the national strategy indicates that in 2023 the limit value of the legislation exceeds the GB1 with 140%, GB2 with 217%, GB3 with 29%, and GB4

Table 2
National strategy for Danish buildings [13].a To enable a comparison between the national strategy and the calculated values of this study, an average distribution of upfront emissions relative to total life cycle emissions according to average values has been applied [17].Furthermore, the values from the national strategy are multiplied by 50 as the new proposed standard assumes a 50-year reference study period.70%.The Voluntary CO 2 Class shows more promising commitment, as it complies with the average values for GB3, although it still exceeds the average values for the GHG budget with 112% for GB2, 60% for GB1 and 14% for GB4 in 2023.In 2029 the legislation and Voluntary CO 2 Class exceeds the budget slightly less.However, for GB2, the values still exceed the GHG budget with approximately 170% and 80%, respectively.
The targets are progressively stricter through time, in line with the IPCC mitigation pathways [2].An exceedance in the calculated budget one year will require an even steeper slope moving forward, as the pathway is a result of a distributed yearly budget of the total allowable amount of GHG emissions, which will prevent the planet from temperature increases higher than 1.5 • C. Thus, the calculated pathway should be updated regularly.
Results for each step of the downscaling procedure can be found in Appendix A. Supplementary Material.

Applicability of the calculated targets
This article provides a method for setting national science-based targets for upfront embodied GHG emissions for new construction.The method relies on the application of sharing principles and MRIO models for tracking economic flows among industry sectors.MRIO allows for estimating the direct and indirect final consumption of the construction sectors in a specific country.The final consumption by consumers can be used to approximate our needs and, thus, express the sectors that we prefer.The study presented a proof of concept for the Danish building sector.However, the method can be applied to any country with available data.
The study proposes dynamic target values for upfront emissions for new buildings for the Danish construction sector.By presenting the targets as a step-by-step pathway towards the final end goal of climate neutrality and even negative emissions, the results have the strength of being achievable goals today, and at the same time ambitious regarding what should be achieved in the future.The targets are given in kgCO 2eq/m 2 and can be applied across building uses and types.Furthermore, using square metre as the unit ensures comparability with current limit values.The calculated values can be used as GHG budgets (targets) for new construction for developers and building designers.For this target group, it is considered relevant to base the targets on past construction activity, as it has been done in this study, given the unaffiliated choices of different stakeholders to build, and that one developer has no influence on another developer's decision to build.
For the purpose of addressing policymakers, the targets can be used to set the level of ambition for the regulation of GHG limit values for new construction.Current national strategies in the Netherlands, Denmark, Finland, Sweden, and France all base their limit values on bottom-up data [10][11][12][13][14].However, this study shows the relevance of also including top-down values to enable the evaluation of the level of ambition.To investigate the level of ambition of the current national targets, the assumption of applying past construction trends is considered relevant, as the national targets are not currently supplemented by a strategy of building less in the future.The case example from Denmark showed that to support the Danish construction sector in staying within the calculated Paris Aligned budget, either impact per m 2 would need to be reduced or construction activity would need to decline.For instance, for the sharing principles applied in this study, given the current national legislation value for 2023, building activity would have to decline by almost 70% for GB2, 55% for GB1, 40% for GB4, and 20% for GB3 to ensure the construction of new buildings stays below the assigned average GHG budgets.In this context, expanding the research by including other scenarios for future construction activity could then facilitate discussions of how different means of mitigation strategies could supplement each other.This is further discussed in Section 4.4.
The proposed targets cover only upfront emissions for a number of reasons, which will be elaborated in Section 4.3.However, to avoid the risk of burden-shifting between life cycle stages [45], this article proposes calculating whole life GHG emissions, and accounting for biomaterial without carbon uptake, when comparing the LCA results with the target.The sequestrated carbon absorbed by the wood and other biobased materials through photosynthesis, commonly referred to as biogenic carbon, is at EoL when decomposed or incinerated released back into the atmosphere.As the targets calculated in this study only cover upfront emissions, this article proposes, in accordance with recommendations in literature [46,47], to exclude biogenic carbon from the assessment to avoid the risk of misleading results.

Reflections on proposed targets
This study provides a line of assumptions and decisions on how the global GHG budget can be downscaled to targets for upfront embodied emissions for all new built square metres in Denmark.Allocating budgets by creating sharing principles to downscale a global budget to a country, service or system is both a matter of deciding what is fair and a practical matter of which data is available [23][24][25].In accordance with related literature [23], this study also found that the choice of sharing principles strongly influence whether a system or an activity can be considered sustainable.In 2020 the resulting budgets vary with 176 kgCO 2 /m 2 applying average pathways and even more if minimum and maximum values are considered.Resulting budgets, therefore, cannot be interpreted as "one objective truth".Rather the article seeks to exemplify the downscaling procedure and show how the subjective decision on which sharing principles are applied will affect the resulting GHG budget.It is, therefore, essential to be transparent in the communication of these choices and ideally seek a harmonization and consensus within the industry on how to apply sharing principles.
This study applies four combinations of the sharing principles equal per capita (EPC), ability to pay (AP), utilitarianism (U), and acquired rights (AR), revealing significant variations in the proposed GHG budgets.The largest differences in resulting budgets occur when the budget is scaled to the national level by either EPC or AP.This is illustrated in Fig. 3c, showing the calculated emission budgets per capita.While EPC distributes the budget according to the countries' relative population share, AP calculates the reduction from a baseline emission based on the countries' current emissions and GDP.Countries with high GDP are assigned larger reductions.However, the initial budget in 2020 reflects the current distribution of emissions and thus "rewards" high emitting countries.While this could be perceived as unfair, it might better reflect a realistic scenario on a national level, as reducing impacts cannot happen overnight but has to be a transformation.Nevertheless, from a building perspective, sharing principles based on EPC, are highly relevant if the building design aims to be ethically sustainable with an offset in the mindset of all individuals being equal.When applying AP, high emitting countries such as USA and Australia have initial high budgets but are required large rapid reductions, while a low emitting country such India are assigned a low initial budget and required nearly no reductions in the future.Literature [48] point towards the lack of consideration of historical accountability when applying EPC.Rich countries have industrialised their communities at the expense of large GHG emissions.The same argument could be made when applying AP, as e.g.India might require larger shares of the total GHG budget to accomplish higher living standards.One argument against the use of historical accountability points towards "latecomers advantage" in the possibility of utilising the knowledge and technological progress such as energy efficiency etc. generated in the past [26].
As this study seeks to quantify targets for the future, there are inherent uncertainties embedded in the assumptions about future development, i.e. population growth, construction trends, etc.The GHG budgets presented in this study build on a status quo assumption for the relative contribution of building materials to a nation's total consumption both in terms of GHG emissions and final consumption expenditure.For future studies, these status quo assumptions could be challenged by predictions of societal tendencies.Furthermore, it is considered relevant to investigate how policies and societal tendencies could affect future construction activity.
The GHG targets suggested in this article reflect the global budget as well as the need to achieve net-zero and even negative emissions at some point, to compensate for the GHG emissions already emitted.However, the exact role of the building sector to achieve these targets is yet to be defined.However, as the building sector is responsible for 38% of total global energy-related CO 2 emissions, the sector plays a pivotal role in reducing current GHG emissions, and all actors across the value chain must take the agenda onboard [49].The global GHG pathways applied in this study [33] are characterised by depending on rapid decarbonisation of the global energy supply, reduced energy use, and CDR such as bioenergy with carbon capture and storage (BECCS).As the impact of many building materials is highly dependent on the energy grid composition at the production facility, decarbonisation of the energy grid will also reduce the impact of the embodied impacts in buildings.One study identified the potential of GHG reductions of future construction materials due to switching to renewable energy sources, increasing the share of recycled feedstock materials, and/or including CCS systems in the material production and found that on average GHG emissions could be reduced by 65%.The study covered 14 different construction materials and the future potentials were analysed for a point in time between 2030 and 2050 [50].

Comparing target values to building LCA calculations
The longevity of the building's service life introduces some challenges in foreseeing actual whole life GHG emissions.Current LCA standards fail to consider the dynamic of emissions and when in the life cycle emissions occur [27].As this study suggests yearly emission budgets, only emissions occurring in the actual year should be accounted for.Furthermore, there are some methodological reasons why this study focuses on upfront emission rather than whole life embodied GHGs: 1) Production and construction phases (upfront emissions) are based on actual data on materials and construction principles, while replacements and EoL are based on assumptions in terms of replacement cycles, choice of material, etc. 2) Technological developments are not accounted for when calculating emissions for replacements and EoL.
When conducting an LCA of a building, the production and construction stages (upfront emissions) are based on actual data of today's decisions regarding materials and construction principles.The rest of the embodied impacts from the building's life cycle, i.e. replacements and EoL, are based on assumptions such as an expected building service life (also called reference study period, RSP) and rely on technical service lives for materials.These assumptions are specified and fixed when conducting LCAs for certification schemes and regulations to make the LCAs comparable and the task of conducting the LCA practically possible.However, the fixed assumptions regarding maintenance and RSP are considered inappropriate when relating impacts to the annual GHG budget, as in this case, the impacts should reflect actual impacts.
The environmental data we use for conducting building LCAs (according to EN15804 [51] and EN15978 [15]) is not designed to account for future developments but take a precautionary principle assuming "current practice" -current being the year the environmental data is conducted.Therefore, current practice is applied to replacements in the future as well as demolition and waste handling.The datasets used to represent a production of a replacement in e.g. 30 years reflect today's practice rather than a projection of what will or could happen in the future.The same applies to waste handling processes that might even occur in 50, 80 or 120 years.Therefore, it is considered inconsistent to compare LCA results from replacement and EoL with GHG pathways highly depending on future technologies.
It should, however, be noted that the message of the above is not that we should disregard the use and EoL stages in LCAs of buildings, only that it is inconsistent to do so when comparing the LCA result from the standard we use today, to a yearly global budget highly depending on the timing of the emissions, as well as future technological developments.

Mitigation strategies
Mitigating impacts from a nation's embodied GHG emissions in buildings can be through lowering the GHG intensity (kgCO 2 -eq/m 2 ) of new buildings, as well as limiting the number of square metres being built.The approach this article has taken, is to show how much we should reduce per m 2 if Denmark keeps building as many square metres was the case in 2018-2020.Several studies have made efforts to show how embodied impacts can be reduced [52,53].
Apart from lowering the GHG intensity, a second strategy is to build fewer but more efficient square metres and to use the existing building stock better.The relevance of the second strategy in a Danish context is supported by the fact that Denmark has the second-highest built-up area per capita in the world after the US, according to OECD statistics [54].In 2014, the built-up area per capita in Denmark was 408 m 2 /capita compared to a global built-up area of 107 m 2 /capita and an average in the EU of 264 m 2 /capita.Studies comparing the environmental impact of case studies of one family house show that an effective way to reduce environmental impacts is to reduce the size of the house, as this reduces both material consumption and operational energy [29,55].Another initiative that could even avoid the need for new buildings, is to use the existing building stock more efficiently.Examples of this could be to transform redundant buildings for other uses, introduce evening classes at educational facilities etc.These measures are out of scope for this study to address.However, one way of addressing the efficiency issue when setting limit values, is to analyse, how a GHG budget could be distributed according to the use of the building, differentiating the budget for different building uses and typologies.Ideally, a share of the global budget would be set, in relation to the utility that it is fulfilling [23].Thus, the function of the building should justify the magnitude of the share that the building can use.Using m 2 as the target's unit, has the advantage of being operable and comparable across building types and uses.However, it does not give an incentive to design less but more efficient square metres.In fact, using intensity based indicators such as per m 2 could incentivise construction of larger buildings as smaller buildings tend to have higher impact per m2.This is a known issue with intensity based indicators and options for complementing with other more relevant or absolute indicators should be investigated.Examples of target units reflecting the use could be emissions per number of residents, number of fulltime employees, number of hospital beds etc.Furthermore, it could be feasible to introduce different targets for different types of use.It could, for instance, be argued that it would be reasonable to have a larger budget for residential buildings, as they fulfil a basic human need [56].Further work should look into how this differentiation could be achieved e.g. by differentiating the functional unit depending on the building use.

Conclusion
In the industry today, targets and limit values for the built environment are set through bottom-up benchmarks created by analysing the performance of existing building LCAs.However, these benchmarks are not related to the global climate targets defined in the Paris Agreement.In this study, a downscaling procedure for setting science-based topdown targets for upfront emissions in new buildings has been presented and tested in a Danish context.To do so, a combination of four sharing principles was combined revealing significant variations in the proposed GHG budgets depending on the chosen sharing principle.At national level, EPC and AP's application revealed very different GHG budget pathways.While EPC reflects the relative population share for a country, AP assigns large initial budgets to high emitting countries.However, it requires very steep reduction pathways for countries with high GDPs.The results indicated that when comparing the resulting GHG budgets in Denmark to the Danish national strategy, legislation (becoming effective in 2023) will exceed the average budget for all four sharing principles with up to 220%.Following the ambition of the Danish building sector wanting to comply with the 1.5 • C target, this would therefore require building activity (measured in new build square metres) in Denmark to decline with 20%-70% depending on the sharing principle applied compared to construction trends in 2018-2020.The proposed downscaling method and targets have the potential of being applied by developers and building designers to set targets for their building designs and for policymakers to set the level of ambition to support a sustainable transformation of the built environment to ultimately achieve global climate targets.

Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Lise Hvid Horup reports financial support was provided by Innovation Fund Denmark.Lise Hvid Horup reports financial support was provided by Realdania.

Fig. 4 .
Fig. 4. Budget for upfront embodied GHG emissions.EPC = equal per capita, AP = ability to pay, AR = acquired rights, EA = economic activity, U = utilitarian.The transparent area illustrates the uncertainty in the calculated budgets due to the range in emissions in the global pathway applied.

Table 3
Budget for upfront embodied GHG emissions per m 2 of new building.Values are calculated for the average pathway and minimum and maximum are presented in parentheses.