5.1 Human-mediated environmental change
Human-mediated climate change is resulting in widespread and uneven changes in global temperature, humidity, and precipitation patterns and more frequent extreme weather events (IPCC 2021). In addition to climate warming, regional changes in humidity and precipitation will result in increased drought in some areas, while others become wetter (Konapalaet al. 2020). If mosquitoes and their transmission cycles are more sensitive to humidity at higher temperatures, then future increases in wet vs. dry heat may have very different implications for mosquito populations and pathogen risk. Regional variation in temperature and relative humidity could have important implications for both the seasonal timing and peak of vector-borne disease (Santos-Vega et al. 2016, 2022) as well as pathogen persistence or emergence. For example, it has been suggested that future temperatures in tropical Africa will exceed the thermal optimum for malaria and result in reduced transmission (Mordecai et al. 2020). However, these tropical regions are characterized by humid heat, and malaria may persist if the maximum temperature for transmission increases at high humidity. Similarly, the potential for arboviruses to expand into warming temperate climates may be greater in regions with increasing humid heat vs. dry heat, which has not been considered in current mechanistic model projections of disease risk with various climate change scenarios [e.g., (Ryan et al. 2020a, b; Caldwell et al. 2021)].
Land-use change is another key human driver affecting mosquito-borne disease transmission (Baeza et al. 2017). For example, urban landscapes are one of the most rapidly growing land cover types across the globe (United Nations 2019), with the proportion of people living in urban environment projected to increase from 55% to 68% between now and 2050. High environmental heterogeneity in urban areas creates substantial variation in the local microclimates mosquitoes experience, through differences in temperature, moisture, and wind speed (Stewart & Oke 2012). These differences are mediated by the extent of impervious surfaces, the distribution of vegetation, and the three-dimensional structure created by buildings and trees. Together, these changes result in urban heat and dry islands (Heaviside et al. 2017) with higher land surface (Yuan & Bauer 2007) and near-surface air temperatures (Coseo & Larsen 2014) and lower relative humidity (Heaviside et al. 2017; Lokoshchenko 2017; Yang et al. 2017; Hao et al.2018) compared to more vegetated landscapes. This fine-scale variation in mosquito microclimate can have significant implications for multiple mosquito species (e.g., Aedes aegypti, Ae. albopictus, Anopheles stephensi ) that drive urban outbreaks of diseases (e.g., dengue, chikungunya, Zika, and malaria) (Beebe et al. 2009; Stoddardet al. 2009; Li et al. 2014; Thomas et al. 2016, 2017; Murdock et al. 2017; Heinisch et al. 2019; Takken & Lindsay 2019).
Small-scale variation in temperature and relative humidity could also have important implications for the spatial distribution of risk in urban environments (Fig 5). Recent studies that combine field experimentation with direct monitoring of urban microclimates and mosquito abundance demonstrate that fine-scale variation (e.g., individual neighborhoods or city blocks) in both temperature and relative humidity can have important implications for mosquito life history, population dynamics, and disease transmission within urban environments (Murdock et al. 2017; Evans et al. 2018b, 2019; Wimberly et al. 2020). Thus, neighborhoods with a high proportion of impervious surfaces that experience mean temperatures near or exceeding the thermal optimum for transmission could experience even higher decreases in vectorial capacity than what models would predict from temperature alone, if drier conditions increase desiccation stress and reduce mosquito survival.
To generalize the effects of changing temperature and humidity across diverse locations and into the future, it will be necessary to develop a conceptual framework that incorporates the psychometrics of temperature and atmospheric moisture with mosquito biology and the natural and built environments in which transmission occurs. Incorporating the effects of humidity into hierarchical models and assessment of mosquito population dynamics and disease transmission will increase the precision of mapping environmental suitability, both globally and regionally with human-mediated environmental change, as well as across heterogeneous human-modified landscapes.