Nature-Based Solutions (NbS) are increasingly recognized for their potential to address various societal challenges, particularly in the context of the biodiversity and climate crises. The IUCN identifies seven key societal challenges that NbS can address: climate change mitigation and adaptation, disaster risk reduction, economic and social development, human health, food security, water security, and reversing environmental degradation and biodiversity loss. While NbS initially focused on resource management and ecosystem function, their role in addressing a broader range of societal challenges is now gaining recognition. This study uses a systematic analysis of the NbS research landscape from 1990 to 2021 to identify key themes and evaluate their evolution over time and geographical distribution. The goal is to highlight research gaps and propose recommendations to better align NbS research with societal needs, particularly in vulnerable regions.

Previous NbS studies have used various methods, including machine learning algorithms and desktop reviews, to examine specific aspects of the NbS research landscape. However, few reviews have comprehensively assessed the entire landscape, particularly concerning societal challenges and access to NbS. This study addresses this gap by employing a systematic analysis focusing on the seven IUCN-defined societal challenges. This approach enables a detailed examination of how NbS research has been tailored to address these challenges across different time periods and geographic locations, using indices of global vulnerability to identify priority regions for future research.

The study utilized a systematic search strategy within the Web of Science Core Collection database (1990-2021). The search employed a broad range of terms related to NbS, aiming to capture the evolution of terminology within the field. A Document Co-citation Analysis (using CiteSpace software) was employed to identify 17 major research clusters based on frequently co-cited references. These clusters were then manually labeled and categorized based on their dominant themes. The researchers assigned the seven IUCN societal challenges to each research cluster based on a thorough review of the abstracts, introductions, and conclusions of the papers within each cluster. A sensitivity analysis was conducted to assess the impact of various search term choices and assignment of societal challenges. Geographical distribution of author affiliations was analyzed to understand the geographic focus of NbS research and compare this with global vulnerability indices for each societal challenge. Keyword analysis complemented the geographical distribution data. The data sources used for the vulnerability indices include the World Bank's Gini index, the Legatum Prosperity Index, the Global Food Security Index, and the Global Water Security Index.

The analysis revealed that NbS research has primarily focused on climate change impacts and biodiversity loss, particularly before 2015. While research on disaster risk reduction has gained prominence since then, along with increased funding and the UN SDGs, four societal challenges remain underrepresented: economic and social development, human health, food security, and water security. Research on these challenges has been disproportionately skewed toward Europe and North America, despite greater vulnerabilities existing in other regions. Figure 1a illustrates 17 research clusters with yearly and cumulative attention trends, highlighting the societal challenges addressed in each cluster over time. Figure 1b shows the distribution of authors across NbS research, revealing the underrepresentation of the four understudied challenges. Figure 2 contrasts NbS research production with global vulnerability across various regions and societal challenges. It highlights the disproportionate amount of NbS research conducted in Europe and North America compared to regions with higher vulnerability. Supplementary Figure 1 details the top 5 keywords per country for the 20 countries with the highest proportion of author affiliations. The analysis identifies a mismatch between the geographic distribution of research and the regions most vulnerable to the understudied challenges.

The findings highlight the need to address the imbalance in NbS research. The study suggests six research pathways to advance the field: 1) Maintain research on the foundational aspects (climate change, biodiversity loss, disaster risk reduction), linking it to understudied challenges; 2) Expand research to capture interdependencies between societal challenges; 3) Pilot NbS projects in various contexts to test theoretical research; 4) Innovate with new research designs and materials; 5) Re-assign research priorities to address the challenges where countries are most vulnerable; and 6) Connect existing scientific evidence with the NbS concept. Boxes 1-6 provide examples and details for each pathway. These pathways aim to ensure that NbS research better serves vulnerable populations and addresses the multifaceted nature of global challenges.

This study reveals an imbalance in NbS research, with insufficient attention to economic and social development, human health, food security, and water security. The proposed six research pathways offer a framework for future research to address these gaps, promoting interdisciplinary collaboration and a focus on vulnerable regions. Prioritizing research in these areas, along with incorporating local knowledge and expertise, is crucial for the effective implementation of NbS.

The study's limitations include potential biases in the search strategy and the manual assignment of societal challenges to research clusters. The reliance on author affiliations to determine geographical distribution may not perfectly reflect the actual focus of research. Further, the analysis focuses primarily on academic literature and may not fully capture the contributions of grey literature or practitioner experiences.