nbs as "actions to protect, sustainably manage and restore natural or modified ecosystems, which address societal challenges effectively and adaptively, while simultaneously providing human well-being and biodiversity benefits". this framing puts the need for well-managed and restored ecosystems at the heart of nbs, with the overarching goal of "supporting the achievement of society's development goals and safeguard human well-being in ways that reflect cultural and societal values and enhance the resilience of ecosystems, their capacity for renewal and the provision of services". in the context of the ongoing political debate on jobs and growth (main drivers of the current eu policy agenda), the european commission underlines that nbs can transform environmental and societal challenges into innovation opportunities, by turning natural capital into a source for green growth and sustainable development. within this viewpoint, nature-based solutions to societal challenges are "solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social and economic benefits and help build resilience. such solutions bring more, and more diverse, nature and natural features and processes into cities, landscapes and seascapes, through locally adapted, resource-efficient and systemic interventions". this is a somewhat broader framing of the concept, placing economy and social assets at the heart of nbs on a par with the importance of sustaining environmental conditions. it shares similarities with the definition proposed by maes and jacobs (2015), describing nbs as "any transition to a use of es with decreased input of non-renewable natural capital and increased investment in renewable natural processes". under this definition, the development and evaluation of nbs spans three basic requirements: decreasing fossil fuel input per produced unit; lowering systemic trade-offs and increasing synergies between es; and increasing labour input and jobs. within this definition, therefore, nature is seen as a tool to inspire more systemic solutions to societal problems. whatever the definition used, the promotion of sustainability and the increased role of natural, self-sustained processes relying on biodiversity are inherent characteristics of nbs. they constitute actions easily demonstrable as positive for a wide range of stakeholders, as they bring about benefits at environmental, economic, and societal levels. as a consequence, the concept of nbs is gaining acceptance outside the conservation community (e.g. urban planning) and is now on its way to be mainstreamed into policies and programmes (climate change policy, law, infrastructure investment, and financing mechanisms), although nbs still face many implementation barriers and challenges. the potential of nbs for transformative change towards sustainability has recently been explored. one study found that nbs can drive profound and substantial changes towards sustainability in local social-ecological systems through a combination of nature's values, knowledge, community engagement, and nature protection and sustainable management. further studies that assess the contributions of nbs towards transformative change at broader scales, for example in relation to planetary boundaries, are needed. examples demonstrating the benefits of nature and healthy ecosystems, as well as showcasing the return on investment they can offer, is necessary in order not only to increase awareness, but also to provide support and guidance on how to implement nbs. a large number of initiatives around the world already highlight the effectiveness of nbs approaches to address a wide range of societal challenges. worldwide the following table shows examples from around the world: implementing institutions societal challenges addressed location ecosystem types(s) description nbs approaches used nbs typology source rwanda natural resources authority, world resources institute and iucn food security, water security, disaster risk rwanda forest forest landscape restoration as a national priority forest landscape restoration, ecological restoration, ecosystem-based mitigation intermediate type 1/ type 2 wetlands international, both ends, wwf and iucn climate change, disaster risk, food security indonesia, sri lanka, india, thailand and malaysia coastal community based coastal habitat restoration ecological restoration (coastal habitat restoration) type 1 iucn and local partners water security, disaster risk guatemala & mexico rural, mountain, freshwater ecosystem, river, watershed implementing transboundary water governance through community ecosystem-based action in the tacana watersheds natural infrastructure, ecological restoration, forest landscape restoration, ecosystem-based disaster risk reduction, ecosystem-based adaptation intermediate type 1/type 2 the european commission, wageningen university, ecorys, ecnc, grontmij, wwf climate change czech republic, hungary, poland, romania, serbia, slovak republic and ukraine mountain carpathian integrated assessment of vulnerability to climate change and ecosystem-based adaptation measures ecosystem-based management type 2 carpivia project the nature conservancy climate change, food security north america rural integrative strategy to reduce the vulnerability of agricultural ecosystems to drought and other extreme precipitation events ecosystem-based management type 2 unep; secretariat of the pacific regional environment programme climate change, disaster risk oceania (fuji) coastal, urban a comparative analysis of ecosystem-based approaches and engineering options to safeguard lami town (fuji) from severe storms ecosystem restoration; and ecological engineering type 1 and type 2 city of barcelona; city of lisbon climate change, human health spain; portugal urban developing the barcelona & lisbon green infrastructure and biodiversity plan to mitigate climate change and improve well-being green infrastructure intermediate type 2/type 3 city of london climate change, human health london urban greening london's roof tops to mitigate climate change and improve human health, while increasing biodiversity green roofs type 3 lancaster environment center flood management cumbria, uk floodplain implementing catchments at different scales on flood basins in conjunction with modelling software that allows observers to calculate the factor by which the floodplain expanded during two storm events. diverting higher floods flows into expandable areas of storage in the landscape. type 2 chinese national government; sponge cities program climate change, disaster risk, food management china urban use of green grey infrastructure used in 30 chinese cities as a way to manage pluvial flooding and climate change risk after rapid urbanization. green grey infrastructure type 3 east kolkata wetlands in 2018, the hindu reported that the east kolkata wetlands, the world's largest organic sewage treatment facility, had been used to organically clean the sewage of kolkata for several decades through the use of algae. this natural system, in use since the 1930s, was discovered by dhrubajyoti ghosh, an ecologist and municipal engineer in the 1970s, while he was working in the region. ghosh worked for decades to protect the wetlands. it had been a practice in kolkata, one of the five largest cities in india, for the municipal authorities to pump sewage into shallow ponds (bheris). under the heat of the tropical sun, algae proliferated in these bheris, converting the sewage into clean water, which in turn was used by villagers to grow paddy and vegetables. this almost 100-year-old system treats 750 million litres of wastewater per day, providing livelihoods for 100,000 people in the vicinity. for his work, ghosh was included in the un global 500 roll of honour in 1990 and received the luc hoffmann award in 2016. practical implementation there is currently no accepted basis by which a government agency, municipality, or private company can systematically assess the efficiency, effectiveness, and sustainability of a particular nature-based solution. however, a number of studies and reports have proposed principles and frameworks to guide effective and appropriate implementation,: 5 in order to upscale nbs in practice in a variety of situations. one primary principle, for example, is that nbs seek to embrace, rather than replace, nature conservation norms. nbs are also determined by site-specific natural and cultural contexts that include traditional, local and scientific knowledge. geographic information systems (gis) can be used as an analysis tool to determine sites that may succeed as nbs. gis can function in such a way that site conditions including slope gradients, water bodies, land use and soils are taken into account in analyzing for suitability. the resulting maps are often used in conjunction with historic flood maps to determine the potential of floodwater storage capacity on specific sites using 3d modeling tools. comparison of suitability maps to digital imagery from google earth is also often practiced as a way of ensuring that suitability maps are representative of actual conditions. nbs can be implemented alone or in an integrated manner along with other solutions to societal challenges (e.g. technological and engineering solutions) and are applied at the landscape scale. implementing nbs requires political, economic, and scientific challenges to be tackled. first and foremost, private sector investment is needed to supplement traditional sources of capital such as public funding or philanthropy. the challenge is therefore to provide a robust evidence base for the contribution of nature to economic growth and jobs, and to demonstrate the economic viability of these solutions – compared to technological ones – on a timescale compatible with that of global change. already, multiple case studies have demonstrated that nbs are more economically viable than traditional technological infrastructures. furthermore, it requires measures like adaptation of economic subsidy schemes, and the creation of opportunities for conservation finance, to name a few. indeed, such measures will be needed to scale up nbs interventions and strengthen their impact in mitigating the world's most pressing challenges. projects supported by the european union since 2016, the eu has supported a multi-stakeholder dialogue platform (thinknature) to promote the co-design, testing, and deployment of improved and innovative nbs in an integrated way. the creation of such science-policy-business-society interfaces could promote market uptake of nbs. the project is part of the eu’s horizon 2020 research and innovation programme, and will run for 3 years. there are a total of 17 international partners involved, including the technical university of crete (project leader), the university of helsinki and biodiversa. in 2017, as part of the presidency of the estonian republic of the council of the european union, a conference called “nature-based solutions: from innovation to common-use” was organized by the ministry of the environment of estonia and the university of tallinn. this conference aimed to strengthen synergies among various recent initiatives and programs related to nbs launched by the european commission and by the eu member states, focusing on policy and governance of nbs, research, and innovation. nature-based solutions in the paris agreement the paris agreement calls on all parties to recognize the role of natural ecosystems in providing services such as that of carbon sinks. article 5.2 encourages parties to adopt conservation and management as a tool for increasing carbon stocks and article 7.1 encourages parties to build the resilience of socioeconomic and ecological systems through economic diversification and sustainable management of natural resources. the agreement refers to nature (ecosystems, natural resources, forests) in 13 distinct places. an in-depth analysis of all nationally determined contributions submitted to unfccc, revealed that around 130 ndcs or 65% of signatories commit to nature-based solutions in their climate pledges, suggesting broad consensus for the role of nature in helping to meet climate change goals. however, high-level commitments rarely translate into robust, measurable actions on-the-ground. nature-based solutions at the un climate action summit in september 2019 in the 2019 un climate action summit, nature-based solutions were one of the main topics covered, and were discussed as an effective method to combat climate change. a "nature-based solution coalition" was created, including dozens of countries, led by china and new zealand. efficacy of nbs a global systemic map of evidence was produced to determine and illustrate the effectiveness of nbs. after sorting through 386 case studies with computer programs, the study found that nbs were just as if not more effective than traditional or alternative flood management strategies. 66% of cases evaluated reported positive ecological outcomes, 24% did not identify a change in ecological conditions and less than 1% reported negative impacts. furthermore, nbs always had better social and climate change mitigation impacts. evidence gathered from other case studies supports these claims in that nbs are effective at achieving their desired goals, however one caveat that appears is that green infrastructure, common in nbs practices, must be used in conjunction with traditional grey infrastructure. using green infrastructure alone or grey infrastructure alone are less effective than when the two are used together. caveats of efficacy while nbs are successful in flood management, a majority of case studies and examples of nbs are from the global north, resulting in a lack of data for many medium- and low-income nations. consequently, many ecosystems and climates are excluded from existing studies as well as cost analyses in these locations. further research needs to be conducted in the global south to determine the efficacy of nbs on climate, social and ecological standards. additionally, it is