Southern Ocean Heat Burp in a Cooling World

The ocean accumulates carbon and heat under anthropogenic CO2 emissions and global warming. In net-negative emission scenarios, where more CO2 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 CO2 increase of 1% per year until CO2 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 CO2 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 CO2 concentrations or emissions, it presents a mechanism that introduces a breakdown of the quasi-linear relationship of cumulative CO2 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 CO2 emissions simulate durability of ocean storage of heat and CO2, and pathways of loss to the atmosphere.