Southern Ocean Heat Burp in a Cooling World

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Ivy Frenger, Svenja Frey, Andreas Oschlies, Julia Getzlaff, Torge Martin, Wolfgang Koeve are the authors of a study about how Southern Ocean Heat will burp in a cooling world, published in AGU Advances

Global ocean storage of (a) heat and (b) carbon under net-positive CO2 emissions, vertically integrated, at peak atmospheric CO2 concentrations (at CO2 doubling, after 70 years of 1% per year increase); anomaly relative to preindustrial conditions in the Earth system model UVic. It illustrates that the model shows similar patterns as observations and other models, with highest concentrations of heat and carbon storage in the Southern Ocean and the north Atlantic. Image credit: Frenger, Frey et al

In the abstract Frenger, Frey et al. explained that “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 -they added-, 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.”

They explained that “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.”

Finally, they “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.”

In the paper, Frenger, Frey et al. said that “in a net-negative emission scenario more CO2 is extracted from the atmosphere than emitted, and one expects global cooling. We use an Earth system model which is of intermediate complexity in that its ocean is comparatively coarsely resolved and its atmosphere comparatively simple, with the advantage that it can be used for multi-centennial scale climate simulations. We expose the model to an idealized climate change scenario, with first increasing atmospheric CO2 concentration, followed by decreasing atmospheric CO2 that implies sustained net-negative CO2 emissions.”

The researchers found that “after several centuries of global cooling under negative CO2 emissions, global atmospheric warming that is unrelated to CO2 emissions and is caused by ocean heat release. The rate of warming is comparable to average historical anthropogenic warming rates and lasts for more than a century. The ocean heat loss originates from the deep Southern Ocean. We call for assessing the robustness of how models simulate durability of ocean storage of heat and CO2, and pathways of loss to the atmosphere.”

Key Points Warning
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Under net-negative CO2 emissions and global cooling ocean heat release causes substantial centennial scale atmospheric warming
The ocean heat release originates from Southern Ocean deep convection, releasing heat at rates comparable to contemporary ocean heat uptake
Loss of CO2 from carbon-rich deep waters is muted compared to heat loss

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