Das „Große rote G“ wird grün
Study on Securing the Microclimatic Services of Green Settlements with Regard to the Challenges of Climate Change

Urban growth and an increasing sealing of city spaces result in land use and target conflicts and, as a consequence, in the loss of green (and blue) infrastructure. Changing temperatures and precipitation due to climate change as well as their direct and indirect impacts on urban spaces are key challenges in spatial development in Vienna.

Securing the micro- and urban climatic services and the biodiversity of greened settle-ments of allotment garden zones (Kleingartengebiete), allotment garden zones for year-round living (Kleingartengebiete für ganzjähriges Wohnen), garden settlement zones (Gartensiedlungsgebiete) as well as residential areas of building class I (Wohngebieten der Bauklasse I) while considering the challenges of climate change is the main objective of this research project "Das ‚Große rote G‘ wird grün". The focus of the research project lies on the non-built-up areas. Possible measures and instruments to control the greening and thus the microclimatic and ecological performance of urban green infrastructure elements will be identified. This research project "Das ‚Große rote G‘ wird grün" is a contribution to the implementation of the (political) goal of adaptation to climate change in the city of Vienna.

The “horticultural design” and “strictly necessary extent”

The term „Gärtnerische Ausgestaltung” [engl. “horticultural design”] (§ 5 para. 4 lit. p BO W) is assigned for reasons of urban design and urban ecology. Essentially, the aim is to pre-serve and create green spaces to improve or maintain microclimatic conditions. Addition-ally, the existing green space character of the built-up areas is to be secured and the sealing of surfaces is to be averted (cf. Kontrollamt der Stadt Wien 2006). What the "hor-ticultural design" specifically includes is not further specified. The Viennese allotment garden law (WKlG 1996) provides for horticultural design as well (§ 16 Abs. 1 WKlG 1996). Accord-ingly, at least two thirds of an allotment garden must be of horticultural nature. However, as reality shows, an increasing sealing can be observed even in the low construction classes (W BK I and GS) as well as in the allotment garden zones (Ekl and Eklw).

The increase in sealing results from the maximized utilization of the permitted building possibilities, but also from the increasing sealing of non-built-up areas or those to be designed horticulturally, e.g. through access roads or retaining walls, but also through structures that do not require a permit, such as pools or carports. According to § 79 para. 6 BO W, paved paths, driveways, retaining walls, stairs, ramps, etc. are permitted within the areas to be horticulturally designed, as long as they are constructed to the " strictly necessary extent". As on construction land, sealing is also permitted on green land and its areas to be horticulturally designed to the "strictly necessary extent" (§ 16 para. 2 WKlG 1996). The lack of a definition of the term "strictly necessary extent" causes a similar problem at the level of execution as that of the "horticultural design".

Climate change adaptation and securing biodiversity through the preservation of green settlements

The increasing sealing modifies the character of settlements and by the decay of urban green infrastructure (UGI) its ecosystem services are also lost. Urban green infrastructure has eco-logical, economic, and social benefits through the provision of ecosystem services, in ad-dition to climatic benefits (Artmann et al. 2019; Sturiale & Scuderi 2019; Yu et al. 2016). The negative effects that result from sealing are numerous e. g.:

• Loss of biological functions and threat to biodiversity.
• Loss of particulate matter retention
• Increased risk of floods
• Amplification of the heat island effect (Umweltbundesamt o.J.).

Climate Change Adaptation and Safeguarding Biodiversity as Central Strategies of the City of Vienna

A reduction of the impacts of climate change through adaptation measures as well as the enhancement of the ecosystem services of green settlements are anchored in numerous concepts at the international and national level as well as in the City of Vienna.

With the "World Climate Treaty" (Paris Agreement) ratified by Austria in 2016, Austria has committed to plan, implement and monitor adaptation measures to mitigate the impacts of climate change (Paris Agreement, Article 7, (9)). The "EU Strategy for Adaptation to Cli-mate Change" (EC 2013) as well as the "Austrian Strategy for Adaptation to Climate Change" (BMNT 2017) also call for the integration of adaptation concerns into urban planning through nature- or ecosystem-based approaches, which are highlighted as both effective and cost-efficient.

A separate field of action in the "Climate Protection Program of the City of Vienna" (Stadt Wien 2009) emphasizes the importance of adaptation measures. Adaptation was also in-cluded in the last amendment of the "Smart City Wien Framework Strategy" (Magistrat der Stadt Wien 2019) as an equally important second pillar of urban climate policy. All strat-egy and development concepts such as the "Urban Development Plan Vienna (STEP 2025)" (MA 18 2014), the "Thematic Concept Green and Open Space" (MA 18 2015) or the "Urban Heat Island Strategy City of Vienna" (MA 22 2015) emphasize the climatic function of green spaces and their contribution to climate change adaptation.
With the amendment of the "Bauordnung für Wien” – the construction regulations for Vienna – in 2020 (LGBl. Nr. 61/2020), climate protection and climate change adaptation were introduced as objectives to be considered when modifying the determinations on zoning and development plans (§ 1 Abs. 2 lit. 4 BO Wien). The provision for and preservation of green and water areas as well as the provision for a climate-friendly rainwater management are explicitly anchored in the objectives (§ 1 para. 2 lit. 6).