The news media commonly presents data about the incidence of COVID-19 using two graphical displays: daily cases and cumulative cases. How well do ordinary Americans understand the relationship between these graphs? We carried out a multi-phase study to investigate this. We demonstrated that 594 Amazon Mechanical Turk Workers (MTurkers) had a poor understanding of the link between new daily and cumulative cases and that this misunderstanding was linked to weaker support for social distancing policies. Critically, we found that a brief training intervention improved understanding of accumulation in contexts other than COVID-19. In a pre-registered study, we replicated these effects in 1,435 MTurkers and show the training also benefits understanding of relevant environmental issues including accumulation of annual CO2 emissions, coral-reef degradation, and plastic production. Finally, we also show that these benefits persist for as long as 10 months after the initial training.