Abstract: Marine microbial communities sustain ocean food webs and mediate global elemental cycles. These communities will change with climate; such changes can be gradual or foreseeable, but likely have much more substantial consequences when sudden and unpredictable.

In a complex virtual marine microbial ecosystem, we find climate-change-driven shifts over the 21st century are often abrupt, large in amplitude and extent, and unpredictable using standard early warning signals. Phytoplankton with unique resource needs, especially fast-growing species such as diatoms, are more prone to abrupt shifts.

Abrupt shifts in biomass, productivity, and community structure are concentrated in Atlantic and Pacific subtropics. Abrupt changes in environmental variables such as temperature and nutrients rarely precede these ecosystem shifts, indicating that rapid community restructuring can occur in response to gradual environmental changes, particularly in nutrient supply rate ratios.