What lessons we can learn from the geological record of marine ecosystems about the impact of future global change?  We compare models of the future oceans with marine ecosystem structure during past high pCO2 climates of the “greenhouse Earth” and with transient climate events such as the Paleocene-Eocene Thermal Maximum (56 Ma).  We find that past greenhouse climates supported few coral-algal reefs, generally longer food chains founded in small phytoplankton, and much warmer, and more poorly oxygenated oceans, even in the high latitudes. Past oceans were about as productive as those today, but most  ‘charismatic’ top predators–seals, whales, sea birds–evolved only when short food chains became widespread in the cooler climates of the “icehouse” world. Food chain length is important here, because warmer oceans support generally smaller phytoplankton, that are less efficient at transferring energy to top predators. The future oceans will have some of these same “greenhouse world”  traits, but will also experience ocean acidification and thermal stratification (that further suppresses biological production) produced by the rapid production of greenhouse gases by humans. Both models and past analogs suggest that the ~300 year-long fossil fuel economy  will throw marine ecosystems into a state of nearly continuous change for the next 100,000 years. In the near future, we will have to get used to increasing unpredictability, phase shifts in marine ecosystems, and an ocean unlike the one we grew up with.