The morning the Zieglergasse re-opens to traffic, the asphalt is still missing in three places. Where there was a parking lane in 2019, there is now a small cluster of Stadtbäume, planted, irrigated, and stubbornly alive after a summer that crossed thirty-six degrees seventeen times. The pavement around them is a porous mineral mix that absorbs rain like blotting paper. The temperature, on the city sensor mounted to the streetlamp opposite, reads two and a half degrees lower than the same hour, the same week, on the still-asphalted Mariahilfer Straße three blocks away.
This is what the City of Vienna calls the Schwammstadt-Prinzip. The official translation is “sponge city”. The colloquial translation, in the Magistratsabteilungen that have been quietly building it since 2019, is closer to: stop fighting the rain, stop sealing the ground, and stop pretending the climate that built this city is the climate it has now.
The first hot July that nobody forgot
Vienna’s climate-adaptation politics began the way most European climate-adaptation politics begin, which is to say: too late, and on a Wednesday. In late June 2019, a heat dome settled over the Wienerwald and refused to move for nine days. The Hohe Warte weather station logged a 38.8°C peak; the inner districts, where the buildings are tighter and the trees fewer, ran hotter still. Hospitals reported a measurable spike in cardiac admissions. The MA22, Vienna’s environmental department, had been quietly modelling exactly this for half a decade and watching nobody act on it.
By August, the same department had a pilot scheme on the street. The first Coole Straße, on the Ergonomiestraße in the 7th district, was a paint job and a few sun-sails, light-touch, plausibly cheap, designed to test whether residents would accept what was effectively a parking removal. They did. The 2020 rollout added eighteen more streets. The 2021 rollout added another twenty-three. By the end of 2022, the city had committed, in writing, to its Strategie für klimaresiliente Stadtentwicklung: a multi-decade reorganisation of how Vienna’s street space, building stock and green network would be retuned for a climate the city had not been designed for.
What gets unsealed, what gets planted
The Schwammstadt approach has three operational pieces and they are unromantic. The first is unsealing, pulling up asphalt on what used to be parking lanes, side streets and parts of squares, and replacing it with permeable mineral mixes that allow rainwater to percolate. The second is planting, new street trees, but planted in oversized substrate trenches that connect underneath the pavement, so that root systems share moisture instead of competing for it. The third is collecting, redirecting roof and pavement runoff into those substrate trenches instead of into the storm drains, so that the trees drink the rain that the city used to flush away.
Each of these is technically modest. The MA28, which administers Vienna’s street infrastructure, was not exactly thrilled to be told its road-building manuals needed rewriting. But the substrate-trench technique, originally developed in Stockholm and refined in Hamburg’s HafenCity, is now standard on every new Viennese street project of significant scale. The 2023 reconstruction of the Praterstraße, one of the inner city’s wider boulevards, integrated all three components on a single 1.6 km stretch.
Surface temperature on a hot July afternoon
MA22 Wien Heat-Vulnerability index, sealed asphalt vs Coole Strasse
Source: Stadt Wien MA22 Klimakarten 2023, surface-temperature data on Praterstrasse
Stop fighting the rain. Stop sealing the ground. Stop pretending the climate that built this city is the climate it has now.
The Schwammstadt working brief, MA22 Vienna
The political trick
The most-asked question among visiting city-halls is no longer how the engineering works. It is how the politics survived. Vienna is not a uniformly progressive city. The current ruling coalition is SPÖ-NEOS; the FPÖ is the largest opposition party and has spent the past three years arguing, loudly, that every removed parking space is a vote against the working-class outer districts. The Ringstraße debates are still functionally about the 1860s. Adding a sponge-park where there used to be ten parking bays should, on the standard model, be politically lethal.
It hasn’t been. Three things explain that. The first is the Klimastadtrat, Jürgen Czernohorszky, who took the brief in 2020 and made an early decision not to lead with morality. Every Coole Straße announcement since has been framed in the language of summer comfort, child safety and emergency-services access, the kind of vocabulary that survives a chat in a Beisl. The second is incrementalism. No district was forced into the rollout; districts that wanted in could request the next pilot. By the time the FPÖ tried to mount a coordinated objection, four-fifths of the inner districts had a pilot in their territory and the BezirksvorsteherInnen had become quietly invested in keeping them.
The third is the data. The MA22 publishes annual Klimabilanz reports that read more like a building society’s annual statement than a climate document. The numbers are local, neighbourhood-level, comparable across years. The temperature delta on a depaved street, the runoff volume captured in the substrate trenches, the increase in canopy cover by district, all of it is there, in tables, with footnotes. When a district councillor wants to argue against a sponge-park in their territory, they have to argue against the same data the next-door district is using to ask for one.
The hardest streets are still the hardest streets
None of this means the work is easy. The streets that need depaving most urgently are also the streets that are the hardest to depave. The 10th and 11th districts, in the city’s south-east, have the lowest tree cover and the worst summer heat exposure. They are also the districts with the highest car-ownership rates, the longest commute distances and the smallest fiscal margin for street rebuilds. Vienna’s climate strategy doesn’t pretend otherwise; it allocates a higher share of the climate-resilience budget to those districts, but the speed gap between the inner and outer city is widening, not closing.
The same pattern shows up in another corner of the work. New construction can be designed sponge-compliant from the start. Existing courtyards inside the dense Gründerzeit blocks, the four-storey late-19th-century buildings that make up most of inner Vienna, cannot. Most of those courtyards are sealed concrete with a clothes line. Unsealing them requires a body-corporate vote, an agreement with the local utility about runoff, and, often, a complete redesign of where the bins go. The MA22 has begun pilot grants for courtyard greening, but the uptake is, by the department’s own admission, slow.
Who else is reading the manual
Munich is the city most often in the room when Vienna’s sponge-city team takes a delegation. Stadtwerke München and the Referat für Klima- und Umweltschutz launched their own Schwammstadt München programme in 2024, with the explicit acknowledgement that they were copying the Viennese approach where it had been already tested. Hamburg’s HafenCity, where some of the substrate-trench engineering originated, now imports the political framing back from Vienna along with the technical specs. Lyon, Bordeaux and a small number of Italian cities have also opened scoping conversations.
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What none of those cities have done yet is replicate Vienna’s second move, the codification. The City of Vienna published its Klimaresilienz-Standard for streets in late 2023, a binding internal document that says: from now on, every new street project of more than 80 metres in length must integrate at least the substrate-trench and unsealing components, unless the responsible district can demonstrate why not. It looks like a small administrative document. It is the line that turns a pilot into infrastructure.
What it actually feels like
On the Zieglergasse on a Tuesday afternoon in late August, after the last heatwave of the summer, the street is busy in a way it would not have been five years ago. There is a small playground built into a former parking bay where the children of the neighbourhood are negotiating a complicated game involving water bottles, a chalk circle, and the universal grandmother who has appeared from a fourth-floor window with biscuits. The man who runs the small grocer on the corner says the street is now noticeably his quietest hour-by-hour, but the busiest day-by-day, the foot traffic comes in pulses, not in volume, and stays.
That is the part that doesn’t make it into the Klimabilanz. Vienna’s sponge-city is, on the official metrics, an infrastructure adaptation programme. On the unofficial ones, it is a project about what city centres are for in summer. The data is supposed to win the argument; the playgrounds are what makes the data feel obvious in retrospect.
The next decade of Viennese street design will be done under conditions the people who designed Vienna never anticipated. The temperature-delta numbers are real. The carbon numbers are real. The political memory of the 2019 heat dome, six years on, is still real enough to keep the budget lines unchallenged. Whether the rest of the European urban decade follows depends, the MA22 says with characteristic understatement, on whether other cities can copy the patience as well as the engineering.
Sources: Magistrat der Stadt Wien (MA22 Umweltschutz, MA28 Straßenverwaltung), Strategie für klimaresiliente Stadtentwicklung (2022), Klimabilanz-Berichte 2020-2024, Wiener Stadtwerke, ZAMG temperature records, Referat für Klima- und Umweltschutz Landeshauptstadt München.
