by Daniel Brouse and Sidd Mukherjee
September 1, 2025
I said: Our climate model — incorporating complex social-ecological feedback loops within a dynamic, non-linear system — projects that global temperatures could rise by up to 9°C (16.2°F) within this century. This far exceeds earlier estimates, which predicted a 4°C rise over the next thousand years, and signals a dramatic acceleration of warming.
Sidd replied: 9C this century is a stretch… so far consensus is 3 C or so.
I ask:
What do you make of the physics given the observable data? We're already at 1.5°C, which makes 3°C this century seem almost unavoidable. I used to think 9°C was a worst-case scenario -- now I fear it's shifting toward the "highly likely" range. In just the last couple of years, I've seen with my own eyes at least nine tipping points trigger feedback loops.
What strikes me most is not just that each loop accelerates warming on its own, but that they're now amplifying each other. The interactions are compounding in ways that exceed the models.
So my question is: what do you think about the physics of reaching 9°C this century when we consider the observed interactions of these feedback loops and tipping points, including:
Greenland and East Antarctic ice sheet tipping
Slowing of the Atlantic Meridional Overturning Circulation (AMOC)
Mountain glacier loss
Amazon rainforest dieback
Arctic sea ice loss
Boreal forest degradation and wildfire
Permafrost thawing and burning
Warm-water coral bleaching
West Antarctic Ice Sheet instability
Sidd responded: I think 3 C is likely this century and that estimate includes tipping points. But of course, I could be wrong. The oceans have the last word, and they take more than centuries to change to a 9C plus state.
I ask: What do you think about this math?
The Arctic is the fastest-warming region on Earth, and future projections are sobering. Here's what current science says:
Baseline: Over the past century, the Arctic has already warmed ~2-3 °C, roughly 3-4 times the global average (the IPCC's "Arctic amplification").
Projections (depending on emissions):
Low-emissions scenario (Paris goals ~1.5-2 °C global warming): The Arctic could still warm by 3-5 °C above pre-industrial levels by 2100, because amplification doesn't go away even with mitigation.
High-emissions scenario (business-as-usual, ~3-4 °C global warming): The Arctic could warm by 7-10 °C by 2100, with local peaks higher in winter and over land.
Worst-case runaway feedbacks (multiple tipping points reinforcing each other): Some studies suggest the Arctic could hit upwards of 12 °C warming this century, especially if permafrost carbon release, ice-albedo collapse, and ocean circulation disruptions all compound.
Consequences of a 7-10 °C Arctic warming:
Seasonal ice-free Arctic Ocean within decades (likely before 2050).
Permafrost thaw and fires releasing massive CO₂ and methane.
AMOC disruption from Greenland meltwater, destabilizing weather globally.
Accelerated sea-level rise from Greenland and West Antarctic ice sheet tipping.
So, within this century, the Arctic could plausibly warm anywhere from ~5 °C (with strong global action) to ~10-12 °C (with high emissions and interacting tipping points).
The worst-case runaway feedbacks for the entire Earth go beyond just linear warming -- they involve tipping points that reinforce each other, creating a self-perpetuating heating cycle that humanity cannot reverse once triggered. Here's how it could unfold:
1. Ice-Albedo Collapse
Trigger: Loss of Arctic summer sea ice + collapse of Greenland and parts of Antarctica.
Effect: Dark ocean and land absorb far more heat, locking in warming even if emissions stop.
2. Permafrost Thaw + Boreal Fires
Trigger: Permafrost in Siberia, Alaska, and Canada thaws; peatlands ignite.
Effect: Release of hundreds of gigatons of CO₂ and methane (CH₄ is >80x more powerful than CO₂ over 20 years). This alone could add several °C of warming.
3. Amazon & Tropical Rainforest Dieback
Trigger: Drought, fire, and deforestation push the Amazon and Congo toward savannah transition.
Effect: Instead of absorbing carbon, they become carbon sources, releasing billions of tons of CO₂.
4. Ocean Circulation Breakdown (AMOC, Southern Ocean overturning)
Trigger: Greenland meltwater and Antarctic ice disrupt global heat transport.
Effect: Jet stream chaos, monsoon collapse, global food shocks, and further polar warming.
5. Warm-Water Coral & Marine Food Web Collapse
Trigger: Ocean heatwaves and acidification kill reefs and plankton.
Effect: Loss of a major carbon sink + food security collapse for billions.
6. Soil & Crop Failure Feedbacks
Trigger: Heat waves, drought, and shifting rainfall make large parts of the tropics uninhabitable.
Effect: Widespread famine, mass migration, political collapse → more emissions from land-use change.
Conservative physics (linear models): ~4-5 °C global average by 2100 (IPCC's high-end).
With interacting feedbacks: 6-9 °C this century is plausible -- consistent with paleoclimate evidence (the PETM, ~55 million years ago, saw ~6-8 °C warming from runaway carbon release).
Runaway scenario (beyond human control): If permafrost, methane hydrates, and major biosphere collapses all reinforce, Earth could enter a "Hothouse Earth" trajectory of 10+ °C warming over centuries to millennia. That would mean large parts of the planet become uninhabitable for humans.
The "worst-case runaway" isn't just about higher temperatures. It's about crossing thresholds where Earth's system drives itself into a hotter state regardless of human emissions -- a true runaway greenhouse scenario. Humanity would lose control of the climate dial.
A 9 °C rise in global average temperature this century would be catastrophic -- not just "bad" in terms of economics or infrastructure, but existential for organized human civilization. At that level of warming, the Earth system would move into conditions not seen for tens of millions of years, well outside the Holocene climate in which agriculture, cities, and modern society developed.
Here's what it would likely mean for humanity:
Uninhabitable Tropics
Wet-bulb temperatures (heat + humidity) would routinely exceed the threshold (~35 °C wet-bulb) at which humans cannot survive outdoors, even in shade with unlimited water. Large regions of South Asia, the Middle East, Africa, and parts of the Americas would become physiologically uninhabitable.
Sea-Level Rise
Melting of Greenland and large parts of West Antarctica would accelerate. By 2100, multi-meter rise is possible; over subsequent centuries, 20-70+ meters is locked in. Coastal megacities (New York, Shanghai, Mumbai, Lagos, London) would be drowned.
Collapse of Food Systems
Staple crops (wheat, maize, rice) have heat tolerances ~30-35 °C. At +9 °C global warming, yields crash across the tropics and subtropics, and mid-latitudes face heat, drought, and water scarcity. Global famine becomes likely.
Oceanic Collapse
+9 °C means ocean stratification, mass bleaching of all coral reefs, collapse of plankton and fisheries. Billions of people who depend on marine protein lose their food source.
Mass Extinction
Up to 50-70% of species would not adapt fast enough. Biodiversity loss undermines ecosystem services humans rely on (pollination, water purification, soil fertility).
Mass Mortality from Heat Stress
Heatwaves would kill millions each summer in regions without full climate control. Infrastructure (power grids, water supply) would break under demand.
Mass Migration
Billions of people displaced from coasts, tropics, and deserts. Migration on this scale would overwhelm political systems, trigger conflict, and collapse borders.
Pandemics
Climate-stressed ecosystems increase disease spillover (mosquito expansion, permafrost pathogens, etc.). Public health systems overwhelmed by simultaneous climate disasters.
Collapse of Civilization
Agriculture, trade, and finance rely on stability. At +9 °C, economic collapse is almost certain. Wars over water, food, and habitable land become chronic. Nuclear conflict risk rises as states fight for survival.
Loss of Human Habitats
Safe zones shrink to high latitudes and high altitudes (Canada, Scandinavia, Siberia, New Zealand, Patagonia). Even these may face ecological collapse, food scarcity, and climate refugee pressures.
Human Survival vs. Extinction
While humans as a species might persist in isolated refugia, modern civilization would not. Population could decline from 10 billion (projected mid-century peak) to under 1 billion or less.
Locked-in "Hothouse Earth"
With runaway feedbacks (methane, permafrost, Amazon dieback, AMOC collapse), Earth could move into a semi-permanent hot state. Even if emissions ceased, the climate would not stabilize for centuries-millennia.
A 9 °C rise this century is not just "worse than we expected" -- it represents an existential threat to humanity, putting civilization and billions of lives at risk. It would transform Earth into a planet fundamentally hostile to human survival.
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