Tipping points are real: Gradual changes in carbon dioxide levels can induce abrupt climate changes

Previous glacial periods were characterised by several abrupt climate changes in the high latitudes of the Northern Hemisphere. However, the cause of these past phenomena remains unclear. In an attempt to better grasp the role of CO₂ in this context, scientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) recently conducted a series of experiments using a coupled atmosphere-ocean-sea ice model.

First author Xu Zhang explains: "With this study, we've managed to show for the first time how gradual increases of CO₂ triggered rapid warming." This temperature rise is the result of interactions between ocean currents and the atmosphere, which the scientists used the climate model to explore. According to their findings, the increased CO₂ intensifies the trade winds over Central America, as the eastern Pacific is warmed more than the western Atlantic. This is turn produces increased moisture transport from the Atlantic, and with it, an increase in the salinity and density of the surface water. Finally, these changes lead to an abrupt amplification of the large-scale overturning circulation in the Atlantic. "Our simulations indicate that even small changes in the CO₂ concentration suffice to change the circulation pattern, which can end in sudden temperature increases," says Zhang.

Further, the study's authors reveal that rising CO₂ levels are the dominant cause of changed ocean currents during the transitions between glacial and interglacial periods. As climate researcher Gerrit Lohmann explains, "We can't say for certain whether rising CO₂ levels will produce similar effects in the future, because the framework conditions today differ from those in a glacial period. That being said, we've now confirmed that there have definitely been abrupt climate changes in the Earth's past that were the result of continually rising CO₂ concentrations."