Topic modelling the mobility response to heat and drought

We conducted a systematic literature review of peer-reviewed full text articles on the nexus between human mobility and drought or heat published between 2001 and 2021, inclusive. We identified 387 relevant articles, all of which were analysed descriptively using a dictionary-based approach and by using an unsupervised machine learning–based Latent Dirichlet Allocation (LDA) model. Most articles were in response to droughts (71%), but heat and extreme temperature became more prominent after 2015. The drought-related literature focuses geographically on African and Southern Asian countries, while heat-related research has mainly been conducted in developed countries (mostly in the USA and Australia). For both hazards, European countries are under-represented. The LDA model identified 46 topics which were clustered into five major themes. One cluster (14% of all articles) included literature on heat-related mobility, mostly data-driven models, including amenity migration. The other four clusters included literature on drought, primarily on farming societies and the agricultural sector with three of those clusters making up 63% of all articles, with the common overarching focus on climate migration and food security. One of the four drought clusters focused on social dysfunction in relation to droughts. A sentiment analysis showed articles focusing on voluntary mobility as part of adaptation to drought and heat were more positive than articles focusing on migration triggered by droughts and heat. Based on the topics and the article characterisation, we identified various research gaps, including migration in relation to urban droughts, heat in farming societies and in urban societies of developing countries, planned retreat from hot to cooler places, and the inability or barriers to doing so. More research is also needed to understand the compound effect of drought and heat, and the social and psychological processes that lead to a mobility decision.