Southern Ocean Heat Burp in a Cooling World

The ocean accumulates carbon and heat under anthropogenic CO 2 emissions and global warming. In net‐negative emission scenarios, where more CO 2 is extracted from the atmosphere than emitted, we expect global cooling. Little is known about how the ocean will release heat and carbon under such a scenario. Here we use an Earth system model of intermediate complexity and show results of an idealized climate change scenario that, following global warming forced by an atmospheric CO 2 increase of 1% per year until CO 2 doubling, features subsequent sustained net‐negative emissions. After several hundred years of net‐negative emissions and gradual global cooling, abrupt discharge of heat from the ocean leads to a global mean surface temperature increase of several tenths of degrees that lasts for more than a century. This ocean heat “burp” originates from heat that has previously accumulated under global warming in the deep Southern Ocean, and emerges to the ocean surface via deep convection. Little CO 2 is released along with the heat which is largely due to particularities of sea water carbon chemistry. As the ocean heat loss causes an atmospheric temperature increase independent of atmospheric CO 2 concentrations or emissions, it presents a mechanism that introduces a breakdown of the quasi‐linear relationship of cumulative CO 2 emissions and global surface warming, a metric that underpins political decision‐making. We call for assessing the robustness of how models forced with net‐negative CO 2 emissions simulate durability of ocean storage of heat and CO 2 , and pathways of loss to the atmosphere.