Some research has aimed to understand how OMZs have changed over geological time scales. Throughout the history of Earth’s oceans, OMZs have fluctuated on long time scales, becoming larger or smaller depending on multiple variables. The factors that change OMZs are the amount of oceanic primary production resulting in increased respiration at greater depths, changes in the oxygen supply due to poor ventilation, and amount of oxygen supplied through thermohaline circulation. From recent observations, it is evident that the extent of OMZs has expanded in tropical oceans during the past half century. Vertical expansion of tropical OMZs has reduced the area between the OMZ and surface where oxygen is used by many organisms. Currently, research aims to better understand how OMZ expansion affects food webs in these areas. Studies on OMZ expansion in the tropical Pacific and Atlantic have observed negative effects on fish populations and commercial fisheries that likely occurred from reduced habitat when OMZs shoal.