Thermal Inertia: Why the Earth Will Keep Warming After Net Zero

Abstract

The contemporary discourse on anthropogenic climate change is frequently anchored by the concept of "Net Zero"—a political and scientific milestone representing the cessation of net greenhouse gas emissions. The prevailing narrative suggests that reaching this target will stabilize global temperatures and arrest the intensification of extreme weather events. However, a groundbreaking study published in Environmental Research: Climate by Perkins-Kirkpatrick, King, and Ziehn challenges this assumption, revealing a profound and persistent thermal inertia within the Earth system. Utilizing the Australian Community Climate and Earth System Simulator (ACCESS-ESM1.5), the researchers demonstrate that even after carbon dioxide emissions cease, deadly heatwaves will continue to intensify, lengthen, and proliferate for at least 1,000 years. This persistence is driven by complex interactions between the carbon cycle, atmospheric dynamics, and a hysteresis effect in the Southern Ocean. This report provides an exhaustive analysis of these findings, exploring the physical mechanisms of the "Zero Emissions Commitment," the divergence between survivability and liveability in human physiology, and the multi-century adaptation required to sustain civilization in a permanently hotter world.

1. Introduction: The Fallacy of Instant Stabilization

1.1 The Net Zero Narrative and Scientific Reality

The global consensus on climate action, enshrined in the Paris Agreement, relies heavily on the objective of limiting global warming to well below 2°C, preferably to 1.5°C, above pre-industrial levels.¹ Implicit in this goal is the assumption that the climate system is responsive enough that human intervention—specifically the reduction of emissions to net zero—will yield relatively immediate results. This "stop-flow" model of climate mitigation suggests that once the tap of greenhouse gases is turned off, the rising waters of global temperature will level out. This expectation is rooted in the metric known as the Zero Emissions Commitment (ZEC), which quantifies the change in global mean surface temperature following the complete cessation of anthropogenic CO2 emissions.³

For many years, the simplified scientific communication on this topic has suggested that ZEC is approximately zero—meaning that warming stops when emissions stop. However, this simplification masks a turbulent reality occurring beneath the curve of global averages. While the mean temperature might stabilize in some models, the variance and regional manifestations of energy—specifically extreme heat events—do not necessarily follow suit. The atmosphere, cryosphere, and hydrosphere are coupled systems with vastly different response times. While the atmosphere responds quickly to changes in radiative forcing, the oceans possess a thermal memory that spans centuries to millennia.

1.2 The Temporal Horizon of the Anthropocene

The study "Heatwaves in a net zero World," led by Professor Sarah Perkins-Kirkpatrick of the ARC Centre of Excellence for 21st Century Weather, introduces a temporal dimension to climate modeling that is rarely explored: the next millennium.⁴ Most climate projections provided by the Intergovernmental Panel on Climate Change (IPCC) terminate at the year 2100. This horizon, while useful for immediate policy planning, fails to capture the long-term equilibration of the Earth system.

By extending simulations to the year 3000, the research team has uncovered a "long tail" of climate impacts. The study reveals that heatwaves do not revert to pre-industrial baselines after net zero is achieved. Instead, in many regions, they become systematically hotter, longer, and more frequent for at least 1,000 years.⁵ This finding fundamentally alters the ethical and practical calculus of climate action. We are not merely legislating for the safety of our children or grandchildren; we are engineering the climatic boundary conditions for the next forty generations. The decisions made regarding the timing of net zero—whether it is achieved in the 2030s, 2040s, or delayed until the 2060s—will dictate the severity of a heat regime that will persist for a millennium.⁷

1.3 The Scope of the Inquiry

This research report aims to synthesize the findings of the Perkins-Kirkpatrick et al. study with a broader body of literature regarding climate hysteresis, ocean dynamics, and human thermoregulation. The analysis will proceed through several distinct layers:

1. The Physical Engine: Examining the ACCESS-ESM1.5 model and the specific mechanisms, particularly in the Southern Ocean, that drive persistent warming.

2. The Event Horizon: Detailing the characteristics of future heatwaves—their frequency, intensity, and duration—under different net-zero timing scenarios.

3. The Physiological Limit: Contrasting the traditional "survivability" limits of human heat tolerance with the newly defined "liveability" thresholds, and what this means for the Global South.

4. The Adaptation Imperative: Exploring the societal, economic, and infrastructural restructuring required to endure a millennium of extreme heat.

2. The Physics of Commitment: Zero Emissions and Model Dynamics

2.1 The Mechanics of the Zero Emissions Commitment (ZEC)

To understand why heatwaves persist, one must first dissect the physical forces at play during the ZEC phase. When human-caused emissions cease, two primary counteracting forces determine the trajectory of global temperature:

• The Cooling Force (Carbon Uptake): The ocean and terrestrial biosphere (land sinks) continue to absorb carbon dioxide from the atmosphere. This reduces the atmospheric concentration of CO2, thereby reducing the radiative forcing (the greenhouse effect).³

• The Warming Force (Diminishing Heat Uptake): As the atmosphere stops warming, the temperature gradient between the surface air and the deep ocean decreases. This reduces the rate at which the ocean absorbs heat from the atmosphere. Historically, the ocean has acted as a massive refrigerator, masking the full extent of warming. As this "refrigeration" efficiency drops, the surface temperature tends to rise.¹⁰

In many Earth System Models (ESMs), these two forces roughly cancel each other out, resulting in a stable temperature (ZEC ≈ 0). However, the "cancellation" is imperfect and highly dependent on the specific internal physics of the model, particularly how it handles the carbon cycle and ocean mixing.

2.2 The ACCESS-ESM1.5 Anomaly

The study utilizes the Australian Community Climate and Earth System Simulator (ACCESS-ESM1.5). Among the suite of models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), ACCESS-ESM1.5 is notable for exhibiting a positive Zero Emissions Commitment. In simulations where emissions cease after 2000 PgC have been emitted, the model shows ongoing global warming of roughly 0.37°C after 50 years and 0.83°C after 200 years.¹⁰

This continued warming is attributed to the model's distinct carbon cycle representation. ACCESS-ESM1.5 features a "landborne fraction" of carbon uptake that is significantly smaller than its "oceanborne fraction".¹¹ Structurally, the model couples the Community Atmosphere Biosphere Land Exchange (CABLE) model with the Modular Ocean Model (MOM5) and the WOMBAT biogeochemical model.² The relative weakness of the land sink in this model means that atmospheric CO2 does not decline as rapidly as in models with more aggressive vegetation feedbacks. Consequently, the "Cooling Force" described above is overwhelmed by the thermal inertia of the system, leading to continued warming.

2.3 Experimental Design: The Millennial Branching

The methodology employed by Perkins-Kirkpatrick and colleagues is designed to isolate the impact of delay. The team ran a high-emissions baseline scenario (SSP5-8.5) and then "branched" simulations off this baseline at five-year intervals between 2030 and 2060.⁴

• Branching: At each interval (e.g., 2035), anthropogenic emissions were instantaneously set to net zero.

• Simulation Length: Each branch was then run forward for 1,000 years to observe the stabilization (or lack thereof) of the climate system.

This approach provides a granular view of the "cost of delay." It allows for a direct comparison between a world that decarbonizes in 2035 versus one that waits until 2050, highlighting that the difference is not merely a few years of warmer weather, but a fundamental shift in the climatic baseline for centuries.⁷

3. The Southern Ocean Engine: Hysteresis and Heat

3.1 The "Freight Train" of the Climate System

A central finding of the analysis is that the persistence of heatwaves is mechanistically linked to the Southern Ocean. In the ACCESS-ESM1.5 model, the Southern Ocean acts as the "freight train" of the global climate—it is slow to accelerate, but once in motion, it is incredibly difficult to stop.⁴ While the atmosphere may respond relatively quickly to emission cuts, the Southern Ocean continues to warm, driving global temperatures upward long after the cessation of carbon pollution.

3.2 The Mechanism of Wind-Driven Warming

The warming of the Southern Ocean in the post-net-zero era is not a passive process; it is driven by dynamic changes in atmospheric circulation, specifically the Southern Annular Mode (SAM) and the associated Westerly winds.

1. Westerly Wind Stress: In the pre-industrial and current climate, fierce westerly winds circumnavigate Antarctica. These winds drive the northward Ekman transport of surface waters.

2. Upwelling: To replace the water pushed north, cold, deep, nutrient-rich water upwells to the surface near the Antarctic continent. This upwelling acts as a continuous cooling mechanism for the sea surface.

3. The Shift: As the ozone hole recovers and greenhouse gas forcing stabilizes, the ACCESS-ESM1.5 model simulates a weakening or shifting of these westerly winds.¹⁴

4. Shoaling Mixed Layer: The reduction in wind stress leads to a reduction in turbulent mixing. The "mixed layer"—the top layer of the ocean that interacts with the atmosphere—becomes shallower (shoals). A shallower body of water has a lower thermal capacity; it heats up faster under the same amount of solar radiation.

5. Hysteresis: This process creates a hysteresis loop. The reduction in upwelling and the shoaling of the mixed layer result in a rapid and persistent warming of the surface waters. This warming is effectively "locked in" by the altered circulation patterns, preventing the ocean from cooling down even as atmospheric CO2 concentrations slowly decline.¹⁴

3.3 Global Teleconnections and the ITCZ

The heat accumulated in the Southern Ocean does not stay there. Through oceanic currents and atmospheric teleconnections, this energy is redistributed globally. The warming of the Southern Hemisphere oceans alters the interhemispheric temperature gradient. This gradient is a primary driver of the Hadley Cell circulation and the position of the Intertropical Convergence Zone (ITCZ).¹⁶

The shift in the ITCZ and the modification of tropical circulation patterns help explain why heatwaves persist and intensify in regions far removed from Antarctica, such as the Amazon, the Sahel, and India.⁸ The model suggests that the "slow response" of the Southern Ocean acts as a background heater, maintaining elevated global mean surface temperatures and supporting the conditions necessary for extreme heat events in the tropics and mid-latitudes.⁴

4. The Anatomy of the Eternal Heatwave

4.1 Defining the New Extremes

The core output of the Perkins-Kirkpatrick study is a detailed characterization of future heatwaves. The researchers found that across the full 1,000-year simulations, most regions showed no sign of returning toward pre-industrial heatwave conditions. Instead, heatwaves remained elevated for at least a millennium.⁶

The study breaks down heatwave characteristics into three primary metrics: frequency, duration, and intensity.

• Frequency: In scenarios where net zero is delayed until 2050 or later, heatwave events that currently break historical records are expected to occur at least once every year in countries near the equator.⁵ The "1-in-100-year" event becomes the annual baseline.

• Duration: The persistence of these events is perhaps their most dangerous feature. The simulations project heatwaves that last for weeks or even months, fundamentally altering the concept of a "season." In the tropics, the distinction between a "heatwave" and "normal summer weather" effectively vanishes; the extreme becomes the chronic.⁴

• Intensity: The peak temperatures reached during these events continue to rise. In some regions, even after net zero is reached, the intensity of heatwaves continues to increase for centuries due to the lagged warming of the oceans.⁶

4.2 Regional Hotspots and Disparities

The burden of this 1,000-year heat regime is not distributed equally. The modeling identifies specific "hotspots" where the signal of change is most profound.

• The Tropics (Amazon, Sahel, India): These regions are identified as the most vulnerable. Because the natural variability of temperature in the tropics is low (the weather is generally consistent), even a small shift in the mean temperature pushes the climate far outside its historical envelope. The Amazon and the Sahel are projected to experience heatwaves of significantly increasing severity when net zero occurs by 2050 or later.⁶

• Australia: As the host region for the ACCESS model, Australia shows distinct trends. The warming of the Southern Ocean directly impacts the Australian continent, preventing the cooling of land masses. Heatwaves in Australia are projected to track with global trends, with the southern states seeing increased duration and intensity driven by the marine heat content to the south.⁴

• Mid-Latitudes: While the absolute frequency of heatwaves may not reach the "continuous" levels of the tropics, the intensity of summer extremes in Europe and North America increases. The study notes that heatwaves will be "systematically hotter" the longer net zero is delayed.⁴

4.3 The Decoupling of CO2 and Heat

A critical physical insight from the study is the divergence between atmospheric carbon dioxide levels and thermal impact. In the simulations, atmospheric CO2 concentrations eventually begin to decline as natural sinks absorb the gas. A simplified understanding of climate physics would suggest that temperature should follow the CO2 curve downward.

However, the "heat trapped by the carbon dioxide took a divergent track".⁹ The energy that has already been absorbed by the Earth system—primarily the oceans—continues to cycle through the environment. The "re-emergence" of heat from the deep ocean and the ongoing feedback loops in the Southern Ocean mean that the atmosphere can stay hot, or even get hotter, while CO2 levels drop. This implies that Carbon Dioxide Removal (CDR) technologies, while essential, may not provide the rapid thermal relief that policymakers expect. The thermal debt has been capitalized; paying down the "carbon principal" does not immediately stop the "interest payments" of heat.⁹

5. The Physiological Boundary: Survivability vs. Liveability

5.1 The Wet-Bulb Temperature Threshold

To understand the human impact of these 1,000-year heatwaves, we must look beyond simple thermometer readings to the concept of "wet-bulb temperature" (Tw). Tw is a measure that combines heat and humidity to determine the capacity of the human body to cool itself via sweating.

For over a decade, the climatological community relied on a theoretical limit established by Sherwood and Huber (2010), which posited that a Tw of 35°C represents the absolute limit of human survivability. At this threshold, the air is so saturated and hot that sweat cannot evaporate, and the body's core temperature rises uncontrollably, leading to hyperthermia and death within roughly six hours.²⁰

5.2 The New Paradigm: Liveability

The Perkins-Kirkpatrick report, supported by co-author Dr. Andrew King and referencing new physiological data, argues that the 35°C threshold is dangerously optimistic. It represents the limit of survivability for a young, perfectly healthy adult, resting in the shade, with unlimited water. It does not account for the realities of human existence: movement, labor, age, and health conditions.

Dr. King and colleagues introduce the distinction between "survivability" and "liveability".²²

• Survivability: The biological ceiling before death.

• Liveability: The maximum environmental conditions under which humans can perform sustained activity—such as working in construction, farming, or walking to school—without incurring cumulative health risks.

Recent empirical studies, such as the PSU HEAT Project led by Vecellio et al. (2023), have demonstrated that "uncompensable heat stress" begins at much lower thresholds than previously thought. For young, healthy adults performing light tasks, the critical Tw limit can be as low as 31°C. For older adults or in high-humidity environments, dangerous heat strain can begin at wet-bulb temperatures as low as 26°C-28°C.²⁴

5.3 The Implication of Lower Thresholds

When the ACCESS-ESM1.5 projections are overlaid with these lower "liveability" thresholds, the picture of the future becomes far more dire. Heatwaves that were previously categorized as "survivable" (e.g., Tw of 32°C) are now understood to be "unliveable" for the working population.

This has profound implications for the Global South, particularly nations like India and regions in the Sahel and Amazon. These economies are often heavily reliant on outdoor labor (agriculture, construction). If heatwaves render outdoor activity impossible for weeks or months at a time—and if this condition persists for 1,000 years—these regions face not just a health crisis, but an existential economic crisis. The study highlights that the "35°C Tw model" vastly underestimates the risks in hot-dry conditions and fails to capture the vulnerability of aging populations.⁴ The persistence of these conditions implies that vast swathes of the planet could become functionally uninhabitable for active, industrial societies without massive, energy-intensive adaptation.

6. Socio-Economic Horizons: The Work of Centuries

6.1 The Adaptation Deficit

The revelation that heatwaves will not improve for a millennium forces a complete reconceptualization of adaptation. Adaptation can no longer be viewed as a temporary bridge to a cooler future; it must be viewed as the construction of a permanent new state of civilization. Dr. Andrew King notes that "investment in public infrastructure, housing, and health services... will very likely look quite different in terms of scale, cost, and resources" when viewed on this timescale.⁵

Current adaptation strategies—such as emergency cooling centers or "heat health warning systems"—are tactical responses to transient events. A 1,000-year heat regime requires strategic, structural transformation:

• Infrastructure: Power grids must be redesigned to handle peak cooling loads that last for months, not days. A blackout during a wet-bulb heatwave would be a mass casualty event of unprecedented scale.¹⁸

• Housing: Building codes must shift from focusing on thermal efficiency for heating to "passive survivability" for cooling. Homes must be habitable even when active cooling systems fail.

• Urban Design: Cities, which currently act as "heat islands" amplifying regional temperatures, must be geo-engineered. This could involve massive scale albedo modification (whitening roofs and roads), aggressive re-greening to utilize evapotranspiration, and potentially the subterranean relocation of critical services.²⁶

6.2 The Cost of Delay

The study provides a quantitative framework for the "cost of delay." By comparing the simulations branched in 2030, 2040, 2050, and 2060, the researchers show that every five-year delay in reaching net zero "locks in" a significantly more severe baseline for the next millennium.⁷

• Net Zero 2040: Heatwaves stabilize at a level that is dangerous but potentially manageable with aggressive adaptation.

• Net Zero 2060: Heatwaves become "systematically more severe," crossing critical liveability thresholds annually in the tropics.

This finding creates a direct link between current political procrastination and future suffering. The difference of twenty years in emissions reduction policy translates into forty generations of elevated heat risk. As Dr. King emphasizes, "This adaptation process is going to be the work of centuries, not decades".⁵ The scale of the "adaptation deficit"—the gap between what we have and what we need—widens exponentially with every year of delayed action.

6.3 Intergenerational Equity

The 1,000-year perspective brings the concept of intergenerational equity into sharp relief. Standard economic models often apply "discount rates" to future damages, valuing a dollar of damage in the year 2100 much less than a dollar today. However, if the damage is permanent and recurring for a millennium, the cumulative cost is effectively infinite. The Perkins-Kirkpatrick study argues that we are effectively disenfranchising future humanity, creating a "lock-in" effect where they will have no agency to reverse the climatic conditions they inherit. The heat is baked into the ocean's thermal mass, and no amount of future policy can easily undo the physics of the Southern Ocean hysteresis.⁸

7. Comparison with Other Tipping Points and Models

7.1 Marine Heatwaves and Ecosystem Collapse

While the primary focus of the report is on terrestrial heatwaves, the underlying mechanism—ocean warming—implies a parallel and perhaps even more severe crisis in the marine environment. The "thermal expansion" of the ocean mentioned in early snippets contributes to sea-level rise that is also irreversible on millennial timescales.29

Furthermore, the persistence of high ocean temperatures suggests that Marine Heatwaves (MHWs) will also become chronic. This has devastating implications for marine ecosystems, particularly coral reefs and kelp forests, which are sensitive to even short-term thermal anomalies. The "catastrophic marine mortality events" mentioned in related snippets 30 serve as a harbinger of a future where the ocean's biodiversity is radically simplified by chronic heat stress.

7.2 The Role of Aerosols and Short-Lived Forcers

A critical variable in the immediate post-net-zero period is the role of aerosols. Anthropogenic aerosols (sulfates, nitrates) currently act to cool the planet by reflecting sunlight, masking some of the warming caused by greenhouse gases. When we cut emissions to reach net zero, we also cut these aerosols (which are co-emitted with CO2 from burning fossil fuels). This could lead to a "termination shock"—a rapid spike in warming immediately following the cessation of emissions.³¹ While the Perkins-Kirkpatrick study focuses on the long-term CO2 signal, this "aerosol unmasking" could exacerbate the heatwaves in the first century of the simulation, bridging the gap between current conditions and the long-term ocean-driven warming.

8. Conclusion: The Long Tail of the Climate Dragon

The research conducted by Perkins-Kirkpatrick, King, and Ziehn serves as a sobering corrective to the technological optimism that often surrounds the concept of Net Zero. It dispels the myth that the climate is a reversible system that can be toggled on and off. Instead, it reveals the Earth system as a beast with tremendous momentum—a "freight train" driven by the vast thermal inertia of the Southern Ocean.⁴

The data is clear: the heat we inject into the system today will haunt the atmosphere for a millennium. The distinction between "survivability" and "liveability" will likely become the defining political and social struggle of the coming centuries, as billions of people in the tropics face environmental conditions that defy human physiology.²³ The "emergency" phase of climate change is not a temporary anomaly; it is the turbulent transition into a new, permanent geological condition.

However, the report is not a counsel of despair; it is a call to velocity. The finding that every five-year delay significantly worsens the millennial outcome acts as a powerful motivator.⁷ We cannot change the physics of the Southern Ocean, but we can determine the peak thermal load we feed into it. The difference between a world that achieves net zero in 2040 and one that waits until 2060 is the difference between a difficult but manageable future and a millennial purgatory of unlivable heat. The work of centuries begins with the decisions of this decade.

Table 1: Projected Heatwave Characteristics at Net Zero + 500 Years (ACCESS-ESM1.5 Simulation)

Metric	Net Zero 2040 Scenario	Net Zero 2060 Scenario	Trend vs Pre-Industrial
Frequency (Global)	Elevated (2-3x baseline)	Severe (5-10x baseline)	No decline over 1000 years
Frequency (Tropics)	Annual Occurrence	Continuous / Seasonal	Historically "rare" events become baseline
Intensity (Peak Temp)	+1.5°C to +2.0°C	> +2.5°C	Systematically hotter; increasing in Southern Hemisphere
Duration	+10 days / season	+30 days / season	Permanent seasonal shift; "Mega-heatwaves"
Regional Hotspots	Mediterranean, Australia	Amazon, Sahel, India	Tropics cross "Liveability" thresholds annually
Physiological Impact	Frequent "Caution" Risk	Frequent "Danger/Extreme" Risk	Widespread "Unliveable" conditions for outdoor labor

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