Southern Ocean Heat Burp in a Cooling World

The ocean accumulates carbon and heat under anthropogenic CO2 emissions and global warming. Little is known about how the ocean will release heat and carbon under potential future “net-negative CO2 emissions.”

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.

We find, 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.

The average warming rate over the decades until the peak warm anomaly is reached is comparable to the average rate of observed global warming since the 19th century, and the maximum decadal warming with 0.14C per decade is analogous to historical warming over the past five decades (Allen et al., 2018).
This anomalous warm period is “non-linear” as compared to the gradually quasi-linearly decreasing temperature trend prior to the warm period.

It lasts for about 200 years, and happens despite linear forcing of continuously decreasing atmospheric pCO2, and under a regime of persistent net-negative CO2-emissions.