Models suggest that marine calcifiers (organisms that precipitate a calcium carbonate exoskeleton) are especially vulnerable to anthropogenic ocean warming and acidification (Cooley and Doney 2009). Short-term experiments using marine calcifiers show that changes in these two stressors can affect physiology (Beniash et al. 2010), shell and soft body growth (Kroeker et al. 2010), and shell function, (i.e., vulnerability of a shell to break under crushing predation (Fitzer et al. 2015b)). However, organism responses don’t always have the same directionality (positive, negative, no change) or intensity (Ries et al. 2009) even when exposed to the same stressor. Furthermore, how short-term experiment results scale to longer time periods and across multiple generations remains poorly known. My research evaluates how these traits associated with shell calcification vary across different climatic and environmental conditions at different temporal and spatial scales and what the functional cost of these trait shifts are. Specifically, I focus on traits associated with shell strength and dissolution prevention, such as mineralogy (Harper 2000), internal shell organics (Lopez et al. 2014, Telesca et al. 2019), and shell structure (Johnson 2020). I assess changes in these traits and their functional consequences across natural pH and temperature gradients both spatially and temporally. I do this in three distinct chapters: Chapter 1 assesses changes in shell mineralogy in response to warming and acidification over a 60 year period along the eastern Pacific in a foundational marine mussel (Bullard et al. 2021); Chapter 2 uses that same species to determine changes in internal shell organics and shell structure along a pH and temperature gradient and how these changing traits influence shell strength and toughness; and Chapter 3 evaluates long-term temporal changes (i.e., Pleistocene to today) in shell calcification of five closely related venerid species. My research fills gaps in our knowledge about long-term responses of marine calcifiers to ocean warming and acidification. Additionally, it integrates multiple fields, such as paleontology and materials engineering, to fully capture trait changes and their functional consequences. Results of this work are useful for creating more accurate predictions about the responses of marine calcifiers to future conditions.