6. Oceans and Rapid Climate Change

A key component of global ocean circulation, the Atlantic Meridional Overturning Circulation (AMOC), is believed to play an important role in abrupt climate changes, both in the past and potentially in the future. As a nonlinear system, the AMOC has multiple equilibrium states characterized by different AMOC strengths, and it has been hypothesized that past abrupt climate changes, including the warm Dansgaard-Oeschger and cold Heinrich events, were related to the transition between such states. The question arises whether an abrupt climate change caused by the AMOC shift could also occur in the future as a result of anthropogenic global warming. Answering this question is complicated by the fact that state-of-the-art coupled climate models typically simulate a mono-stable AMOC for modern climate conditions, which contradicts observationally based indicators suggesting that the AMOC may be bi-stable (i.e., having two stable equilibria). This stability bias is largely due to a common model bias in tropical precipitation—the double Intertropical Convergence Zone problem distorting the Atlantic freshwater budget. After correcting this bias, we find that the AMOC can rapidly weaken and then collapse in experiments with CO₂ doubling, which suggests that the risk of AMOC shutdown in the future should not be underestimated.