As greenhouse gas concentrations increase, a warmer climate is expected, but large-scale internal climate processes can affect the rate and amount of our planet’s warming. New research supported by CPO’s Climate Variability and Predictability (CVP) Program shows that the strength of the Atlantic Meridional Overturning Circulation (AMOC), a large system of ocean currents that transports heat and salt, is likely to play an important role in determining Earth’s climate response to rising greenhouse gas concentrations on the centennial timescale. The authors examined AMOC’s role using two models from the Coupled Model Intercomparison Project phase 6 (CMIP6) — the Community Earth System Model version 2 (CESM2) and the Energy Exascale Earth System Model version 1 (E3SM1) — which simulate different AMOC strengths. These models show nearly identical equilibrium climate sensitivity — the amount of warming that occurs after thousands of years once Earth’s climate system has adjusted to changes in CO₂ concentration. However, the authors found AMOC’s strength can critically affect Earth’s transient climate response — the amount of warming on shorter timescales before Earth’s climate system has reached equilibrium. A weaker AMOC in E3SM1 contributes, in part, to a higher transient climate response due to a much faster warming in the upper ocean and less warming in the subsurface ocean. Alternatively, a stronger AMOC in CESM2, which matches observations well, leads to a smaller transient climate response due to a slower warming of the upper ocean. Their findings have important implications for changes we are likely to see in the 21st century.