Temporal changes in associations between high temperature and hospitalizations by greenspace: Analysis in the Medicare population in 40 U.S. northeast counties

•Trends in heat-hospitalization associations over time were compared by greenness.•Hospitalizations from heat stroke and cardiovascular diseases were examined.•Higher overall risks of heat stroke were found in regions with less greenspace.•Risks for heat stroke decreased over time in regions with less greenspace.•Adaptation to heat-hospitalization associations differed by amount of greenspace.