The oceanic thermohaline circulation (THC) carries Light, warm surface water polewards and dense, cold deep water equator-wards, thereby transporting a large amount of heat towards the poles and significantly affecting high latitude climate. The THC has been remarkably stable, and its variability quite low over the Holocene period (the past 10,000 years). The much greater climate instability and high-frequency variability recorded in ice(1) and deep-sea(31) cores throughout the preceding 150,000 years has been linked to greater THC variability(2,3). Here we argue, using a global coupled ocean-atmosphere-ice general circulation model with realistic geography, that there is a wide range of weak mean states of the THC that Cannot be stably sustained by the climate system. When the model THC is forced into a state in the unstable range, the THC may rapidly strengthen, collapse or display strong oscillations. The existence of this unstable regime may account for the greater variability of the THC and climate before the Holocene period.