BACKGROUND: In the past decade urbanization has increased, with over half the world’s population and 83% of the total United States population, residing in urban areas as of 2021. Urbanization has the potential to decrease connectivity between nature and humans, impacting health and exacerbating urban heat islands. Previous research has found that the natural environment (i.e., trees and bluespace) provide vital ecosystem services through both direct and indirect pathways that positively impact health.
Objective: The objective of my dissertation was to assess the current state of the natural environment in urban areas and gain a better understanding of the relationship between exposure to nature, health, and heat.
METHODS: Research aims were addressed by focusing on publicly available data and a range of metrics used in previous literature to characterize exposure to greenness and natural landscapes and their associations with health and land surface temperature. In Chapter 2, I report on a nationwide quantitative health impact assessment to estimate the reduction in all-cause mortality in populations aged 65 and older in the largest metropolitan areas (n=35) in the United States associated with an increase in greenness in selected years across twenty years (2000, 2010, and 2019). Census tract mean seasonal greenness exposure was estimated by using the Normalized Difference Vegetation Index (NDVI) from Landsat 30 m 16-day satellite imagery from April to September. All-cause mortality estimates were downscaled from county to tract-level from the CDC WONDER Portal. Mortality reduction was estimated using a recently published exposure-response function. Chapter 3 focuses on urban census tracts in Massachusetts (n=525), in order to assess the cumulative exposure of the urban natural environment. Thirty candidate, validated measures of the natural landscape space were derived from data published between 2016 and 2019. Principal component analysis was used to reduce the number of candidate items and confirmatory factor analysis was used to test the underlying hypothesized latent structure of three sub-domains: green space, parks and recreation and blue space. Items hypothesized to measure each latent structure were summed using equal weights and each sub-domain was rescaled from 0–100. The tract-level cumulative exposure index was the sum of the three sub-domains. Lastly, in Chapter 4 using generalized linear models, I investigated the relationship between urban heat and nature using the natural landscape index created in Chapter 3 and mean five-year Land Surface Temperature (LST). The City of Boston was then used as a case-study to identify vulnerable tracts with low nature and high heat to inform planning for climate adaptation strategies and public health planning.
RESULTS: In Chapter 2, overall greenness in the largest metropolitan areas in the US increased from 3% between 2000 and 2010 to 11% between 2010 and 2019. Approximately 34,000 to 38,000 all-cause mortality deaths could have been prevented with an increase in 0.1 NDVI units from 2000 to 2019. In developing the cumulative natural landscape index, in Chapter 3, we found that greenness measures (NDVI, tree canopy, percent impervious, etc.) explained the most variance among all of the items hypothesized to measure the cumulative natural landscape index. The three sub-domains explained 69% of the total variance, with greenness accounting more than half the total variance explained. Lastly, in Chapter 4, we report that with an interquartile shift in overall nature, LST decreased by 0.8°C. More specifically, higher overall greenness, higher percent tree canopy, and higher percent impervious surfaces resulted in the largest change in LST. With an increase in greenness and tree canopy we found a decrease in LST by over 1°C. In contrast, an increase in impervious surfaces resulted in an increase in LST by 1.31°C. Within the City of Boston, we found that 9% of the total population resides in tracts that were within the highest quartile of LST and lowest quartile of overall nature.
CONCLUSION: Overall, this work finds that large urban areas in the United States saw a small increase in overall greenness across the past two decades. With a trend of increasing greenness there is also the potential to reduce all-cause mortality in those 65 years and older. Going beyond overall greenness, we created a cumulative natural landscape index to capture simultaneous exposures to the urban natural environment. Greenness measures such as tree canopy, NDVI, open space, and impervious surfaces explained the most variance in the cumulative nature index and was the most associated with LST as compared to parks and recreation and bluespace. We assessed the impacts of the natural landscape on temperature, and found that an increase in nature resulted in a significant decrease in LST with greenness measures having the largest impact. Results from these studies can be used to support climate action plans and greening initiatives to show the co-benefits of increased exposure to the natural environment and help identify areas that are lacking in these exposures to better implement initiatives.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/46505 |
| Date | 26 July 2023 |
| Creators | Brochu, Paige A. |
| Contributors | Lane Jr., Kevin J. |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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