Heat-Mortality Demographic Sensitivities in Los Angeles County and Potential Climate Change Impacts

The city of Los Angeles (LA), with 3.858 million residents, currently experiences a high number of hot days each year with a highly variable summer climate. The region is also expected to undergo a significant increase in heat events over the 21st century. Such events have been correlated with reduced air quality and elevated rates of human mortality and morbidity. Climate change is predicted to alter the frequency and intensity of excessively hot days, yet Los Angeles-specific modeled projections of extreme heat events has been minimally studied. Additionally, demographic changes are expected to dramatically increase the number of people vulnerable to these events, in turn affecting public health. A key question concerns vulnerable populations with respect to age and ethnicity in LA, and how various subcategories of the population respond to synoptic-scale extreme heat events, both now and in the future. In the state of California, the ‘oppressive' air masses (found to result in excess mortality and morbidity) are dry tropical (DT) and moist tropical (MT).

Accordingly, the first objective of this project is to evaluate the impact of the most oppressive synoptic air masses (DT and MT+) upon six categories of standardized mortality data in LA, and further develop historical mortality algorithms for those particular air masses. The framework for analyzing heat events is rooted in synoptic climatology using the Spatial Synoptic Classification (SSC) system, which classifies each day into one of seven weather types based on surface conditions. The following demographic designations are used: all cause total mortality, all cause elderly (65 and older) mortality, all cause male and female mortality, all cause non-Hispanic white, Hispanic, and black mortality. Moreover, these are divided into three racial and gender categories within the all-cause 65 and older population group.

The second objective of this study is to estimate future mortality using two climate models (Community Climate System Model 3 (CCSM3) and the Coupled Global Climate Model, (CGCM3)), and three scenarios (A1FI (“higher emissions”), A2 (“mid-high emissions”), and B1 (“lower emissions”)). This is completed for two decades: the 2050s and the 2090s. The demographic groups outlined above are used in the future mortality estimates through application of the predictive mortality algorithms developed in the initial historical modeling.

Results show that there is an approximately 5 percent average increase in summer mortality among all demographic groups in LA during days that oppressive air masses are present. This number varies considerably when we subdivide by population category or by consecutive days of oppressive air masses. For example, for the entire population, after three consecutive oppressive air mass days, the average increase in mortality increases to 12 percent. Blacks, Hispanics, and females appear to be more vulnerable to heat-related death than non-Hispanic white men. The overall findings presented will provide better understanding of the specific city-level subpopulation responses to hot weather in LA, and highlight those groups with vulnerabilities to extreme heat. As of the development of this abstract, we have not yet determined the climate change modeled impacts upon mortality, which will also be presented.

With the expected improved observations and understanding, we can support operational meteorologists and other stakeholders (e.g., LA public health) in their warnings, leading to more targeted adaptation and intervention strategies now, and for projected climate change and city-specific growth. Future work is also planned to address the response at the neighborhood-level, due to vulnerabilities and demographic compositions, to oppressive weather. Further, we plan to examine heat island reduction strategies on lessening frequency of the offensive air masses and their impact on mortality, and involve stakeholder collaboration, intervention, and improving disaster resilience from heat/mortality.