Today, more than 50% of the global population lives in cities. Cities are attractive with job opportunities and cultural experiences, but increasing urbanization also put pressure on status and lead to many urban sustainability challenges. For example, the old water and sewer systems were built during a time when the population was much smaller and cities cover less area. Many cities, therefore, lack capacity to handle me to cloudburst which may result in floods. Another example is the high number of commuters in an out of cities, which causes congestion and increasing air pollution. We also have an increasing competition for land that puts a high pressure on urban green space, which affects both human well being and urban biodiversity. To reduce the pressure on the city, we can work with gray solutions like putting new sewers and waste water pipes in the ground, but we can also work with nature-based solutions to solve some of these challenges. Example of urban nature-based solutions include parks, street trees, ponds, green roofs, urban and domestic gardens. To support decision-making in relation to the implementation of nature-based solutions, we need to assess what kind of social, environmental and economic benefits they can deliver. If we want to use nature based solutions for stormwater management, for example, one option is to build a pond that can hold water to prevent floods and this is an example of an environmental benefit. And when we prevent floodings, we prevent damages to surrounding properties which can be an example of an economic benefit. But the pond may not only contribute to stormwater management, it can also provide social benefits by being an opportunity space for recreation, social activities and sports. Ponds are for example, use for ice skating in cities where it gets cold enough during winter time, but we also need to consider the nature-based solution may create negative impacts. A pond can be considered dangerous to children or if it's not properly managed, it can become contaminated. In decision-making, it's important to consider both the positive and the negative impacts that can be created by nature based solutions. There are several different ways to assess how nature-based solutions can contribute to different urban sustainability challenges. Depending on what and how you assess, you will get qualitative, qualitative or monetary values. We can take street trees as an example. If we're interested in assessing how the trees contribute to local climate regulation, it's possible to quantify temperature differences between services with and without tree shadow. But if we're interested in how trees contribute to human well-being instead, we may ask citizens about the perceived benefits of the trees like providing shade or being aesthetically pleasing. Here we could get both quantitative and qualitative values, depending on how the question is formulated. Another way to assess the value of trees could be to estimate your economic value. For example, the cost to put trees in cities or how much people would be willing to pay for trees outside their apartment or maybe energy savings due to increased temperature in urban areas. To do assessments, we need indicators. If we take local climate regulation, for example, indicators might be changes in temperature or perceived firmer comfort experienced by people. It's important to select indicators are both scientific and relevant. Depending on what we assess, we need different types of data. For example, if you want to study water retention of ponds, we could use precipitation data. But it could also be that we need to measure to the retention capacity of an area. In contrast, if we want to study local climate impacts of urban vegetation, we could use city wide geographical data about green space cover or maybe measure the temperature reduction bus to increase in a field study. It takes time and money to do assessments. Therefore, it's ideal if we can generalize the result from one place to another. For some nature-based solutions and benefits, this is possible. We can assume that urban street trees will reduce air temperature in a similar way given the same climatic conditions. But for many benefits, it's not possible to generalize across different context. The perceived cultural benefits from a park are only valid for the targeted population. But even though we cannot generalize the results, it might still be possible to generalize the assessment methods across different contexts. Another factor that should not be overlooked is that many assessment methods require an expertise for both extracting and analyzing the data. Something that might be a limiting factor, at least outside your university. It's therefore, important that researchers do not only develop assessment methods that can only be used by researchers, but also methods that can be used by different types of practitioners working with urban development. In conclusion, assessment methods can be used to evaluate the contribution of nature-based solutions to different kinds of sustainability challenges. But depending on which benefits we assess, we do not assess and which methods and indicators we choose for the assessment. The result will differ. This might influence how we understand and value the benefits provided by nature-based solutions. And in the long run, the decision-making process.
