by Daniel Brouse and Sidd Mukherjee
September 3, 2025
Our probabilistic, ensemble-based climate model -- incorporating complex socio-economic and ecological feedback loops within a dynamic, nonlinear system -- projects that global temperatures could rise by as much as 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 responded: "9°C this century is a stretch. Consensus so far is closer to ~3°C."
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 at least nine tipping points trigger feedback loops with my own eyes.
What strikes me most is not just that each loop accelerates warming, but that they are now amplifying each other. The interactions are compounding in ways that exceed current models.
So my question is: What do you think about the physics of reaching 9°C this century, given the observed interactions of feedback loops and tipping points, including:
Sidd replied: "I think ~3°C is likely this century, and that estimate already includes tipping points. But of course, I could be wrong. The oceans have the last word -- and they take centuries to drive a 9°C+ shift."
The Arctic is warming far faster than the global average -- ~2-3°C already, about 3-4 times faster than the planet as a whole. Projections vary:
Consequences include seasonal ice-free summers by mid-century, permafrost fires releasing CO2 and methane, and destabilization of AMOC, accelerating sea-level rise and global weather extremes.
If multiple tipping points reinforce each other, the climate may enter a self-perpetuating heating cycle beyond human control. The main candidates include:
Temperature outcomes:
As global mean temperature exceeds 1.5 °C and multiple climate tipping points activate, the critical question is not simply how much warmer the planet becomes, but how quickly feedbacks amplify that warming.
Scientific consensus: Current models suggest that carbon-cycle feedbacks -- permafrost thaw, weakening ocean and land sinks, methane release from wetlands, and fire-driven emissions -- could add ~0.2-1.0 °C of warming by 2100 on top of direct human emissions. This range reflects assumptions that:
Under a high-emissions trajectory, with multiple tipping elements engaged, the upper end of this estimate (or beyond) becomes more plausible.
My concern: These consensus estimates are already lagging reality. Observations suggest that at least nine major tipping points are not only triggered but are now reinforcing each other. Instead of unfolding over centuries or millennia, the pace is measured in years or decades. Models have struggled to keep up with this rapid nonlinearity.
Regardless of the rise in global mean temperature, cascading feedbacks are already reshaping weather extremes.
In just ten days during July 2025, the U.S. experienced:
These events illustrate how tipping feedbacks manifest in human terms -- not only as gradual warming, but as sudden escalations in climate volatility and infrastructure failure.
The permafrost offers one of the clearest examples of the widening gap between theory and reality:
This introduces new scientific uncertainties:
The key point is clear: the pace of greenhouse gas release is orders of magnitude faster than earlier models assumed. These feedbacks are not hypothetical--they are already active.
Permafrost fires are only one piece of the puzzle. Another, less understood feedback arises from tropospheric ozone. While CO2 is essential for photosynthesis, fossil fuel combustion does not just add CO2--it also drives chemical reactions that increase ground-level ozone, a powerful phytotoxin. Unlike protective stratospheric ozone, tropospheric ozone damages living tissues, including crops, forests, and grasslands.
Decades of research show that ozone exposure can reduce plant growth by 10-40%, depending on species and exposure levels. In many cases, ozone exposure doesn't merely stunt growth--it kills plants outright, either through direct poisoning of leaves and roots or by weakening their resilience to drought, heat, pests, and disease. This compounds ecosystem vulnerability, undermining the agricultural and natural systems that sustain humanity.
We are already observing the consequences: much of the world's vegetation has lost up to 40% of its net primary productivity (NPP) due to ozone stress. Even more alarming, in the last two years, global forests have shifted from being net carbon sinks--absorbing CO2--to net sources of emissions. This reversal accelerates global warming and destabilizes the climate system, while simultaneously threatening food and water security.
These feedbacks are deeply interconnected. Fossil fuel combustion increases CO2, which drives warming, while simultaneously producing tropospheric ozone, a potent plant toxin. In fact, all forms of carbon combustion generate ozone precursors -- and less efficient forms, such as ethanol and other plant-based fuels, can produce even more ozone per unit of energy released due to incomplete combustion. Ozone-stressed ecosystems lose resilience, making them more vulnerable to drought, pests, and wildfire. Wildfires then feed back by releasing massive amounts of CO2 and generating additional ozone, compounding the stress on vegetation. These intertwined feedbacks are pushing Earth toward a state of compound, cascading instability, where multiple reinforcing processes accelerate climate disruption beyond linear prediction.
But ozone does not only devastate plant life -- it also strikes directly at human health. Tropospheric ozone is a leading cause of asthma and other respiratory illnesses worldwide, triggering millions of hospital visits and premature deaths every year. Chronic exposure damages lungs, increases cardiovascular stress, and disproportionately harms children, the elderly, and those with preexisting conditions. The emerging runaway ozone feedback, therefore, is expected to have a devastating dual effect: weakening ecosystems that support life while simultaneously worsening human disease and mortality.
The critical question is no longer whether runaway feedbacks will manifest, but how quickly and at what scale. Mounting evidence shows these processes are unfolding at rates orders of magnitude faster than once assumed, compressing what was believed to be a centuries-long trajectory into mere decades -- or even years. Already, ozone pollution, escalating wildfires, and other human-driven disturbances have, for the past two years, transformed the world's forests from carbon sinks into net sources of CO2. Yet, the precise math and physics governing the interactions among permafrost thaw, wildfire combustion, tropospheric ozone, and ecosystem collapse remain deeply complex and poorly constrained. Because these are nonlinear systems, thresholds can be crossed abruptly, triggering sudden and irreversible shifts rather than gradual change.
This is the true frontier of climate science: not simply tracking emissions, but mapping the cascading, self-reinforcing loops already reshaping the Earth system. Our current research is focused here -- on quantifying the pace, scale, and tipping dynamics of these runaway feedbacks -- in order to illuminate how close we are to thresholds that could define the future of human civilization.
At this level, modern civilization likely collapses, with human survival reduced to scattered refugia.
The idea of a global average temperature obscures human experience. For example, while the average may rise 3°C, some regions (like the Arctic) could see 10°C+ increases. What matters more is regional extremes and wet-bulb thresholds -- the real determinants of human survival.
Wet-bulb temperatures >31°C are already being observed in parts of the U.S. Gulf Coast, Florida, and Mississippi River Valley, with lethal consequences. These levels were once thought "impossible" in the U.S., but climate change is shifting baselines.
This metric -- not the global average -- is the clearest signal of existential risk to human health and survival.
The question is not whether Earth will warm -- it is how fast, how far, and how violently feedbacks will accelerate the process. A 9°C rise this century may or may not occur, but even "consensus" outcomes (~3°C) would be catastrophic.
The decisive factor is human action: whether we allow runaway feedbacks to trigger an irreversible "Hothouse Earth," or whether we cut emissions, restore ecosystems, and adapt quickly enough to keep habitable zones intact.
We are not just modeling the future -- we are choosing it.
* Our probabilistic, ensemble-based climate model — which incorporates complex socio-economic and ecological feedback loops within a dynamic, nonlinear system — projects that global temperatures are becoming unsustainable this century. This far exceeds earlier estimates of a 4°C rise over the next thousand years, highlighting a dramatic acceleration in global warming. We are now entering a phase of compound, cascading collapse, where climate, ecological, and societal systems destabilize through interlinked, self-reinforcing feedback loops.
We examine how human activities — such as deforestation, fossil fuel combustion, mass consumption, industrial agriculture, and land development — interact with ecological processes like thermal energy redistribution, carbon cycling, hydrological flow, biodiversity loss, and the spread of disease vectors. These interactions do not follow linear cause-and-effect patterns. Instead, they form complex, self-reinforcing feedback loops that can trigger rapid, system-wide transformations — often abruptly and without warning. Grasping these dynamics is crucial for accurately assessing global risks and developing effective strategies for long-term survival.