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.

Some of the friendly fire comes from fellow scientists who have gone down the path of doomism or at least what we might call "soft doomism," that is, emissions reductions alone are not adequate to prevent catastrophic warming.

Even revered climate scientist James Hansen, whose early predictions of warming proved prophetic, has gotten sucked into the vortex of soft doomism. The scientific consensus is that we can still avert a catastrophic planetary warming of 1.5 degrees Celsius (3 degrees Fahrenheit) if we rapidly reduce carbon emissions this decade.
(https://www.livescience.com/planet-earth/climate-change/action-on-climate-change-faces-new-threat-the-doomers-who-think-its-too-late-to-act)

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.