Investigating Climate-Chemistry Interactions on Multiple Spatiotemporal Scales

Abstract

Our ability to assess the impacts of human-caused climate change relies in part on our understanding of the interactions between the climate system and atmospheric composition. In this thesis, I investigate the coupling between climate change and short-lived atmospheric pollutants on global and regional scales, and on timeframes ranging from glacial-interglacial transitions to the late-21st century. The oxidative capacity of the troposphere is central to climate-chemistry interactions. Multiple factors govern the abundance of oxidants, including isoprene, a major volatile organic compound emitted mainly by vegetation. We use a climate-biosphere-chemistry modeling framework to evaluate the sensitivity of global estimates of the oxidative capacity to known uncertainties in the emissions and photochemistry of isoprene during two climate transitions: from the Last Glacial Maximum (LGM, 19,000-23,000 years ago) to the preindustrial (1770s), and from the preindustrial to the present-day (1990s). We find that inadequacies in our current understanding of isoprene emissions and photochemistry impede our ability to constrain the oxidative capacities of the present and past atmospheres and the radiative forcing of some short-lived species over time. For example, we find that the modeled change in global mean OH for the LGM-to-preindustrial transition ranges between −29 and +7%, depending on the assumptions applied. Fine dust is a significant component of regional fine particulate matter (PM2.5) air pollution in the western U.S., especially during spring, and human-caused climate change could increase dust mobilization and transport from arid and semi-arid lands. We perform a systematic statistical analysis to investigate the role of meteorology in controlling the interannual variability of fine dust concentrations in the western U.S. during 2002-2015 March-May. We identify regional precipitation, temperature, and soil moisture, and trans-Pacific transport of Asian dust as key controlling factors. These are in turn mostly driven by large-scale fluctuations in sea surface temperature and/or atmospheric circulation patterns, including the El Nino-Southern Oscillation and the Pacific Decadal Oscillation. In a follow-up study, we quantify the present-day sensitivity of fine dust in the U.S. Southwest to regional drought conditions on interannual timescales. These relationships are used to project future (2076-2095) changes in fine dust concentrations, using multi-model ensemble output following two Representative Concentration Pathways (RCP2.6 and RCP8.5). Together with projections of future population and baseline incidence rates, and results from epidemiological studies of health risks due to PM2.5 exposure, we estimate the resulting excess premature mortality and morbidity. We project future drought-driven increases in annual mean fine dust of 0.10 ± 0.05 μg m−3 (10%) under RCP2.6 and 0.58 ± 0.08 μg m−3 (57%) under RCP8.5. Such increases could lead to 220 (40%, RCP2.6) or 1,200 (200%, RCP8.5) excess all-cause premature mortalities for adults aged ≥30 years, and 260 (90%, RCP2.6) or 1,400 (480%, RCP8.5) excess hospitalizations due to cardiovascular and respiratory illnesses for adults aged ≥65 years, per year in the Southwest relative to the present-day.