The aim of this article is to demonstrate how nature-based solutions can contribute to climate change mitigation by reducing energy consumption and, in the long term, by building resilience of ecosystems in a planned multi-dimensional way. The focus is on nature-based solutions in the context of climate-resilient buildings and cities. The built environment is currently facing a number of challenges that can be solved by contemporary technology, but this usually results in excessive energy consumption. Alternatively, these issues can be successfully addressed with nature-based solutions. The building’s adaptation to the ambient conditions resembles biological models, in which such factors as body temperature, humidity, gas and fluid exchange, shape and colour modification, allow organisms to adjust to the environment without harmful effects or resource over-consumption. Natural models enable active metabolism, including air- and water-quality improvement, pollutants filtration, energy and waste management, circularity. At the building and city scale this enables carbon sequestration, natural cooling, humidification and air purification. To increase the resilience of humans and ecosystems it is necessary to change the principles of their coexistence with a view to achieving symbiotic homeostasis beyond 2050. Hybrid interaction of biological processes and technology should become a prototype for climate-resilient development.
Introduction
Dynamic development of technology, including all kinds of electronic devices, in addition to positive effects, contributes to a strong dependence on electricity. This refers in particular to the built environment, responsible for around 40% of global GHG emissions and similar levels of energy consumption. In view of the massive demand for energy, humanity under climate stress must take measures to:
- Accelerate the transition from non-renewable to renewable energy sources (RES);
- Increase synergies of RES combined with efficient energy storage methods;
- Reduce dependence on electricity and fossil fuels for heating and cooling purposes through bioclimatic strategies and nature-based solutions;
- Transform building skins from carbon-emitting to carbon-sequestrating bio-based envelopes or living ecosystems;
- Maximise energy efficiency in buildings, neighbourhoods, urban and rural areas by re-greening them and increasing biodiversity through green ventilation networks connected to water bodies;
- Eliminate AC for cooling public recreational spaces in favour of nature-based solutions (natural ventilation, evaporative cooling, shading, greening).
Searching for methods of climate risks mitigation, we turn to nature-based solutions due to their high potential for resilience and efficiency (European Commission 2015).
To increase the resilience of people and ecosystems, it is important to understand that currently their interactions are mostly based on parasitism (a correlation in which one partner benefits at the expense of the health of the other). It is critical to enhance the transformation towards neutralism and commensalism with the minimum requirement of achieving a level of well-balanced competition which can also be described as sustainability. The basic principle of the relationship between people and other species inhabiting the same ecosystem should be to minimise the negative impact of our activity on the environment, taking into account the long-term effects, and in particular the climate change. The optimum scenario beyond 2050 is to achieve homeostasis on the basis of symbiosis (Widera 2018).
In-depth analysis of the natural systems behaviour allows for a knowledge transfer from biology to architecture, with the purpose to implement natural processes in structures designed by human (Gruber et al. 2011, Anthony et al. 2014). This involves several exchange mechanisms between the external and internal environments (including light, energy, gases and liquids) as a part of nature-based solutions applied in the built environment to solve the problems related to climate change (Widera 2016; Naumann et al. 2014).
This refers in particular to excessive energy consumption for heating, cooling and ventilation, but also to the production and transportation of construction materials.
Thermoregulation
One of the most remarkable features of the natural world is the ability to respond adequately to dynamic environmental conditions. Nature-based solutions used in the built environment combine functions allowing for system resilience and carbon storage, and increased user comfort and safety, simultaneously enhancing ecosystem’s health and biodiversity (Fig. 1). An excellent example of a natural system designed to selectively store and distribute energy is the fat concentrated in the camel’s hump, which enables effective cooling through sweat evaporation on the remaining surface of the body. Under conditions of dehydration, the animal’s mass acts as a heat buffer and regulates body temperature. Comparable thermoregulation in buildings can be provided through the thermal massing strategy. Affordable and effective heat storage is possible with earth building techniques such as stabilized earthen blocks, superadobe (earthbags plastered with clay and lime) or hempcrete. Advanced concepts of building envelopes are designed for active adaptation to external conditions through monitored gas and liquid exchange comparable to evapotranspiration. Heat storage and passive night radiant cooling are very efficient in combination with Phase Change Materials (PCM), such as Passive Infrared Night Cooling technology developed by ZAE Bayern and tested in Center for Advanced Research in Building Science and Energy in Würzburg (Lang, Rampp, Ebert 2014).
Fig. 1. Biodiversity scheme of Breathe Austria Pavilion at Expo Milan (2015) by team.breathe.austria. ©Terrain.
Gas and liquid exchange
Nature-based solutions for indoor comfort are based on a symbiosis with green plants which absorb carbon dioxide and produce oxygen through photosynthesis. In addition to vegetated façades, green zones should be introduced into buildings through internal courtyards that further improve daylight distribution and natural ventilation. Correctly designed buildings can serve as carbon sinks and the amount of CO2 absorbed by green surfaces in the built environment is increased 50-100 times when the natural processes, such as evapotranspiration and photosynthesis, are hybridized with technological solutions (Widera 2018). Abundant green zones were introduced into the breathe.austria Pavilion at Expo Milan (2015) as a testbed of efficiency of natural and technological processes. Fans, sprinklers and vapour diffusers, powered by minimal amounts of photovoltaic electricity (including dye-sensitised solar cells), enhanced natural processes to lower the temperature, raise the humidity and sequester the maximum amount of carbon dioxide (Fig. 2). It has been proven that the hybridisation of nature and technology allows the indoor space to be cooled by 5 to 7°C thus replacing conventional air conditioning. The tests carried out in the breathe.austria pavilion showed that green plants in a 560 m2 surface produce 62.5 kg of oxygen per hour, while sequestering 86.9 CO2. This rate of photosynthesis is the equivalent of a 3-hectare natural forest (team.breathe.austria 2015). The built environment serves as a breathing ‘photosynthesis collector’ that contributes to global oxygen production and carbon dioxide sequestration.
Bio-filtration façade systems with appropriately-chosen green plants improve air quality by absorbing pollution. Nature-based hybrid façades with building-integrated or building-applied modules (organic PV cells or microalgae bio-façades) combine sustainable energy production with thermoregulation, recovering the part of solar energy not converted to electricity (Arup 2014).
Fig. 2. Hybridized natural and technological processes in Breathe Austria Pavilion, Expo Milan (2015) by team.breathe.austria. ©Terrain.
Flows and mobility
Organization patterns observed in nature are based on mobility and flows of air, water, energy and matter. Under natural conditions, animals and plants developed the ability to adapt to winds or sea currents. By drawing conclusions from organism responses to the phenomenon of movement we can design buildings which save energy through reduced resistance, like fish, whose body is shaped to minimise the flow-blocking surface in the aquatic environment and which, when exposed to strong currents, position themselves to face the tide. A similar building setting with appropriately designed ventilation ducts can save up to 30% of energy. The analogous application of knowledge about flows in urban design results in improved comfort due to cooling effect and better air quality. Urban cooling is enhanced when correctly combined with water bodies introducing water particles into ventilation channels and cooling the urban structure through evaporation. Trees and other plants embedded in the urban tissue provide chilling shade, carbon sequestration and increased humidity. Due to the lack of space in city centres, pocket parks, green façades and roofs, as well as planted terraces, are recommended.
Nature-based techniques established the basis for passive downdraught evaporative cooling (PDEC). This method has been successfully applied in several buildings, e.g., Frontier Project (2009, Rancho Cucamonga, CA, USA) by HMC Architects or De Anza College (2019, Cupertino, CA, USA) by Winkleman Designs. In arid climates, structures inspired by termite dens with vertical ventilation channels are advisable for buildings and districts, using a combination of ground heat exchangers, ventilation chimneys and cooling towers. Double-layered ventilated roofs with projecting sections for shading contribute to façade temperature reduction and significant energy savings while preventing heat radiation.
Nature-based solutions for energy flows include daylighting, heat harvesting and storage, and electricity production. A distinct feature of plants is their ability to follow the sun and optimise the amount of energy intake by opening up to receive more light and heat, and closing down to avoid overheating or excessive cooling. A direct application of heliotropism is found in the Heliotrope building (1994, Freiburg im Breisgau) by Rolf Disch. The structure rotates to track the sun, gaining the maximum amount of sunlight and warmth. This is combined with energy generation including a dual-axis solar photovoltaic tracking panel, a geothermal heat exchanger, a CHP (heat and power) unit and solar-thermal balcony railings for water heating.
The concept of climate-responsive screens inspired by nature is used in kinetic façade systems such as Al Bahar Towers (2012, Abu Dhabi) by Aedas Architects (Fig. 3). This results in a 50% reduction of solar gain and a significantly reduced demand for air conditioning. An even better performance of a kinetic building inspired by a falcon’s wings being a “symbolic interpretation of the flow of movement” (Stouhi 2021) was achieved in the UAE Pavilion (Expo 2020 Dubai, UAE) by Santiago Calatrava. The roof hybrid system, between a shell and a portal frame, consisted of 28 movable carbon fiber wings which, when open, allowed daylight penetration to fully expose the photovoltaic panels beneath them to solar radiation, and when closed, protected against wind and sandstorms. Cantilevered wings created a pleasant ambience around the pavilion, with water ponds naturally cooling the air and native greenery enhancing biodiversity and reducing reflected heat (Calatrava 2021) (Fig. 4).
Fig. 3. Kinetic façade system in Al Bahar Towers (2012), Abu Dhabi by Aedas Architects. Photo: B. Widera.
Circularity
The solutions encountered in the natural environment are based on circular economy principles. They are characterised by a lack of waste, since the metabolic products of some organisms are part of the food chain for others. In today’s construction sector, a zero-waste approach is extremely important for reducing a negative environmental footprint. The most efficient nature-based concepts use technologies that can be found in nature. They include phytopurification, soaking and aerating applied to the conservation of soil, water, woodlands and wetlands. To save drinking water sources, rainwater should be fully utilised and the use of salt water maximised, especially in conditions where freshwater is limited. Rainwater harvesting and phytopurification were successfully applied in the Children’s Center (2011-2012, Um al Nasser, Gaza Strip) by Arcò and Mario Cucinella Architects. Coastal desert areas can benefit from salt water for plantations using biological desalination methods (e.g., quinoa) and farming such as fish ponds for water purification and fertilization.
Fig. 4. Kinetic building designed for optimal performance and inspired by a falcon. UAE Pavilion at Expo Dubai (2020), UAE, by Santiago Calatrava. Photo: B. Widera.
Efficiency
Natural structural systems (e.g., spider webs) are characterized by high efficiency and performance, being thin, lightweight and fully fit for purpose. A similar approach was used by Gaudí, Fuller and Nervy, and nowadays it is supported by parametric design. Genetic algorithms apply a repeated trial-and-error method based on the evolutionary process experience. Natural-based performance optimisation of structural systems diminishes consumption of raw materials and resources, contributing to a significant reduction in CO2 emissions.
Structures such as beehives or anthills inspire optimal use of materials, space organization and building operation. Animals build their homes from organic materials available in the immediate vicinity and adapt their operation to the shifting external conditions. Climate adaptation and resilience of the built environment can be enhanced by responsive façade systems (kinetic, green or PCM modules) which can be combined with food production adding to the most efficient use of space (Fig. 5).
An observation derived from ecosystem organization is that species share the territory using only as much as they need. Contemporary preference for large open space in buildings may be satisfied through inclusive, shared facilities. A nature-based approach should promote green areas and the extra space can be provided through gardens, green atria, shaded terraces or similar carbon-absorbing zones. Moreover, gardens and green façades can be transformed into semi-public areas cultivated by those who can dedicate their time and resources to benefit from sustainable urban farming.
Fig. 5. Kinetic green façade for sustainable food production. American Food 2.0 at Expo Milan (2015) by James Biber. Photo: B. Widera.
Inclusiveness and Symbiosis
Examples of inclusiveness and symbiosis can be found in numerous ecosystems. One of these is the coral reef which, when healthy, represents an exceptionally rich biodiversity. Corals are marine animals that form colonies secreting calcium carbonate skeletons hosting algae living within coral polyp cells. Algae absorb the sunlight and use it in photosynthesis, providing energy for the coral. Symbiotic algae protect the coral from excessive ultraviolet radiation (Baptista, Parker, Conant 2021). The coral reef is home to millions of fish, anemones, crabs and shrimps defending their corals from predators such as starfish. This ecosystem is as complex as the city, but also very sensitive. The negative effects of climate change, which threaten the safety of the planet, are lethal for reefs. The process begins with coral bleaching caused by sea warming. When temperatures rise, symbiotic algae produce oxygen at toxic levels and are expelled or die, revealing white skeletons. Corals can recover if conditions improve and they are repopulated by algae. If conditions do not change, the corals will starve and eventually die, and so will the reef. A temperature rise of +1 to 2°C can stress corals. Currently, water temperature equal or higher than 30°C represents a threat to most coral species (Hughes et al. 2018). Moreover, ocean pollution results with the lowered pH. As oceans absorb a quarter of anthropogenic CO2 and water becomes acidic, the coral structure weakens and the skeletons break. The warning from this observation is that humanity must not allow the temperature to exceed a critical limit. However, the latest observations show that this process is reversible. In 2020 and 2021 the amount of CO2 emitted into the atmosphere by plane and car travels was significantly lower due to the COVID-19 pandemic limitations and lockdowns. Research performed in September 2021 on the coral reefs in the Indian Ocean, on Northern Atoll in Maldives, revealed that the average annual water temperature at a depth range of 1 to 30 meters dropped to 29°C, which was 1°C lower than the temperature measured in the same area for 3 consecutive years. This resulted in the improvement of the ecosystem’s health and the scientists noted that previously bleached corals restarted their growth processes reaching about 5-8 mm of annual increment. Experimental attempts to increase the biodiversity of local ecosystems showed that microalgal symbionts with improved thermal tolerance also increase coral resilience and bleaching tolerance (Buerger et al. 2020). While further research is necessary, it is initially estimated that coral growth in symbiotic relation with heat-evolved algae can be 30-40% faster than under conditions of limited biodiversity. This leads to the conclusion that improving biodiversity has a positive impact on reversing the negative effects of climate change.
The temperature rise beyond 2°C that causes lethal stress to coral reefs is exactly the same as the critical temperature limit calculated for global climate change (IPCC 2022). The author of the paper believes that in nature there are parallels rather than coincidences and, since the sources of negative change in ecosystems are the same – increasing temperatures and acidification of the environment due to excessive carbon dioxide emissions – then the effective countermeasures observed in the ocean environment may also be key to tackling global climate change. The nature-based resilience model assumes that the surface of the coral resembles a building covered with vegetation that provides shelter for other species in the city biotope. This kind of symbiosis needs to be developed in conjunction with a model for carbon dioxide sequestration and air pollutants bio-filtration, simultaneously contributing to the increased energy efficiency of buildings, and sustainable vertical urban farming.
Conclusions
Climate-responsive building adaptation to ambient conditions resembles biological models in which temperature, humidity, gas and fluid exchange, shape and colour modifications allow organisms to naturally adjust to the environment. This enables active metabolism, including the improvement of air and water quality, pollutants filtration, energy and waste management, and circularity. Building functioning on the basis of solar technologies and active metabolism results in CO2 absorption, oxygen production, natural cooling, humidification and air purification. Hybrid interaction of biological processes and technology should become a prototype for climate resilient development. This approach perfectly complements the philosophy of transforming the built environment in the context of the New European Bauhaus: sustainable, beautiful, together.
Interdisciplinary research and knowledge transfer from underwater biology to the built environment and climate science allows us to understand that a limit of 2°C on global temperature increase, which has been identified as critical to the survival of civilization in its current form, is also the limit beyond which coral reef ecosystems in tropical waters will be destroyed. Moreover, the factors disturbing the stability of the system are the same, namely increasing temperatures and acidification of the environment due to excessive carbon dioxide emissions. The author of the paper argues that the effective countermeasures observed in the ocean environment may also be key to tackling global climate change. This leads to the conclusion that symbiotic relations and biodiversity conservation and restoration, combined with effective carbon sequestration, are the most important elements in the process of combating climate change.
The nature-based resilience model for the built environment assumes that living building surfaces provide symbiotic shelter for other species in the city biotope and can be combined with sustainable food production, thus contributing to climate change mitigation.
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