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.

Scenario 1: A Reference Point
The world society proceeds in a traditional manner without any major deviation from the policies pur-
sued during most of the twentieth century. Population and production increase until growth is halted by
increasingly inaccessible nonrenewable resources. Ever more investment is required to maintain resource
flows. Finally, lack of investment funds in the other sectors of the economy leads to declining output of
both industrial goods and services. As they fall, food and health services are reduced, decreasing life
expectancy and raising average death rates.

Scenario 1, show the behavior of World3 when it is run “as is,” with numbers we consider a “realistic” description of the situation as it appeared on average during the latter part of the twentieth century, with no unusual technical or policy assumptions. In 1972 we called it the “standard run.” We did not consider it to be the most probable future, and we certainly didn’t present it as a prediction. It was just a place to start, a base for comparison.

In Scenario 1 the society proceeds along a very traditional path as long as possible without major policy change. It traces the broad outline of history as we know it throughout the twentieth century. The output of food, industrial goods, and social services increases in response to obvious needs and subject to the availability of capital. There is no extraordinary effort,beyond what makes immediate economic sense, to abate pollution, conserve resources, or protect the land.

The population in Scenario 1 rises from 1.6 billion in the simulated year 1900 to 6 billion in the year 2000 and more than 7 billion by 2030. Total industrial output expands by a factor of almost 30 between 1900 and 2000 and then by 10 percent more by 2020.

Then suddenly, a few decades into the twenty-first century, the growth
of the economy stops and reverses rather abruptly. This discontinuation of past growth trends is principally caused by rapidly increasing costs of non-renewable resources.

In the simulated year 2000, the nonrenewable resources remaining in the ground would have lasted 60 years at the year-2000 consumption rate. No serious resource limits are then in evidence. But by 2020 the remaining resources constitute only a 30-year supply

During those two decades in Scenario 1, the growing population and
industrial plant use nearly the same amount of nonrenewable resources as the global economy used in the entire century before!

This scenario portrays a “nonrenewable resource crisis.” It is not a prediction. It is not meant to forecast precise values of any of the model variables, nor the exact timing of events. We do not believe it represents the most likely “real world” outcome.

The strongest statement we can make about Scenario 1 is that it portrays the likely general behavior mode of the system, if the policies that influence economic growth and population growth in the future are similar to those that dominated the last part of the twentieth century, if technologies and values continue to evolve in a manner representative of that era, and if the uncertain numbers in the model are roughly correct.