Most governments, globally, are moving toward targets of net zero emissions by 2050 or earlier, with the aim of meeting the overall aims of the Paris Agreement by 2100. Yet, many governments continue to push climate action into the future, with limited immediate action followed by steep emission cuts after 2040. While this approach can deliver end-of-century global warming levels approaching 1.5ºC, the intervening years will include a period of “overshoot” in which temperature exceeds this warming target for a decade or more. This is particularly important in marine systems, which absorb and release heat more slowly than the atmosphere. Here, we use CMIP6 model projections to explore potential implications for marine biodiversity of pathways that maintain global temperatures at or below 1.5ºC warmer than the preindustrial, against those that vary in magnitude and duration of overshoot. Specifically, we use climate-velocity trajectories to explore comparative effects on range shifts, and we use metrics of marine heatwaves to explore the potential effects of extreme events, including tipping points for key ecosystems. These results, together with process-based understanding, provides us with a first-order estimate of some of the ecological costs of delayed climate action.