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.

We are hurtling toward climate chaos. The planet's vital signs are flashing red. The consequences of human-driven alterations of the
climate are no longer future threats but are here now. This unfolding emergency stems from failed foresight, political inaction,
unsustainable economic systems, and misinformation. Almost every corner of the biosphere is reeling from intensifying heat, storms,
floods, droughts, or fires. The window to prevent the worst outcomes is rapidly closing.

Key Highlights
• The year 2024 set a new mean global surface temperature record, signaling an escalation of climate upheaval.

• Currently, 22 of 34 planetary vital signs are at record levels.

• Warming may be accelerating, likely driven by reduced aerosol cooling, strong cloud feedbacks, and a darkening planet.

• The human enterprise is driving ecological overshoot. Population, livestock, meat consumption, and gross domestic
product are all at record highs, with an additional approximately 1.3 million humans and 0.5 million ruminants added weekly.

• In 2024, fossil fuel energy consumption hit a record high, with coal, oil, and gas all at peak levels. Combined solar and
wind consumption also set a new record but was 31 times lower than fossil fuel energy consumption.

• So far, in 2025, atmospheric carbon dioxide is at a record level, likely worsened by a sudden drop in land carbon uptake
partly due to El Niño and intense forest fires.

• Global fire-related tree cover loss reached an all-time high, with fires in tropical primary forest up 370% over 2023,
fueling rising emissions and biodiversity loss.

• Ocean heat content reached a record high, contributing to the largest coral bleaching event ever recorded,
affecting 84% of reef area.

• So far, in 2025, Greenland and Antarctic ice mass are at record lows. The Greenland and West Antarctic ice sheets
may be passing tipping points, potentially committing the planet to meters of sea-level rise.

• Deadly and costly disasters surged, with Texas flooding killing at least 135 people, the California wildfires alone
exceeding US$250 billion in damages, and climate-linked disasters since 2000 globally reaching more than US$18 trillion.

• Climate change is endangering thousands of wild animal species; more than 3500 species are now at risk and
there is new evidence of climate-related animal population collapses.

• The Atlantic meridional overturning circulation is weakening, threatening major climate disruptions.

• Climate change is already affecting water quality and availability, undermining agricultural productivity, sustainable
water management, and increasing the risk of water-related conflict.

• A dangerous hothouse Earth trajectory may now be more likely due to accelerated warming, self-reinforcing
feedbacks, and tipping points.

• Climate change mitigation strategies are available, cost effective, and urgently needed. From forest protection
and renewables to plant-rich diets, we can still limit warming if we act boldly and quickly.

• Social tipping points can drive rapid change. Even small, sustained nonviolent movements can shift public
norms and policy, highlighting a vital path forward amid political gridlock and ecological crisis.

• There is a need for systems change that links individual technical approaches with broader societal
transformation, governance, policies, and social movements.

Figure 1.Time series of climate-related human activities. In panel (f), tree cover loss does not account for forest gain
and includes loss due to any cause. For panel (h), statistics are based on total energy supply (Energy Institute 2025);
hydroelectricity and nuclear energy are shown in supplemental figure S2. Sources and additional details about each
variable are provided in supplemental file S1.

Figure 2.Time series of climate-related responses. For surface temperature anomaly (d), estimates based on a
segmented linear regression model are shown in gray (prior to 2010) and black (beginning in 2010). For area burned
(o), the black horizontal lines show changepoint model estimates, which indicate abrupt shifts (supplemental figure S3).
For other variables with relatively high variability, local regression trendlines are shown in black. The variables were
measured at various frequencies (e.g., annual, monthly, weekly). The labels on the x-axis correspond to midpoints of years.
Sources and additional details about each variable are provided in supplemental file S1.