Underestimation of Climate Sensitivity and Feedback Processes

by Daniel Brouse
April 2, 2025

In the 1990s, we first hypothesized the non-linear acceleration of climate change. By the early 2000s, this hypothesis had evolved into established climate theory, now widely recognized as scientific fact. My lab partner, a Doctor of Physics from Ohio State, and I collaborated to provide key evidence supporting this theory. Over the years, we have observed a dramatic reduction in the doubling time of climate change impacts — the rate at which these effects intensify. Initially, the doubling time was approximately 100 years, but it has since decreased to 10 years and, more recently, to just 2 years. This trend implies that the damage caused by climate change today is double what it was two years ago. In two years, it could be four times worse; in four years, eight times worse; and within a decade, potentially 64 times worse. These projections are conservative, assuming the doubling period does not continue to shrink further. Alarmingly, this rapid acceleration does not appear to be an anomaly. If this trajectory persists, the consequences will likely be far more catastrophic than previously anticipated.

Dr. James E. Hansen’s paper Global Warming Acceleration: Impact on Sea Ice confirms our theory with an analysis of the acceleration of global warming and its impact on sea ice, ice sheets, and ocean circulation, with a particular focus on the Atlantic Meridional Overturning Circulation (AMOC). Climate sensitivity is higher than previously estimated (ECS = 4.5 +/-0.5°C for doubled CO2), which fundamentally alters expectations for future climate change. IPCC models fail to account for freshwater effects on sea ice cover, leading to an underestimation of climate sensitivity and feedback processes.

AMOC Shutdown Risk: Hanson emphasizes that accelerated warming increases the likelihood of AMOC shutdown by reducing surface water density in deepwater formation regions. A shutdown would trigger severe long-term consequences, including rapid sea level rise.

An AMOC shutdown would have severe and far-reaching impacts worldwide. For instance, on the U.S. East Coast it would lead to rapid sea level rise, stronger storms, and extreme weather shifts. As the AMOC slows or stops, warm water would no longer be transported northward, causing ocean water to pile up along the East Coast, which could raise sea levels by 1-3 feet almost immediately, on top of global sea level rise. This would have particularly devastating effects on the Northeast U.S. (New York, Boston, DC, Miami), where flooding would be most severe.

The weakening or shutdown of the AMOC would also impact hurricane formation. With warmer waters off the East Coast, hurricanes would become more intense and slower-moving, leading to longer-lasting and more destructive storms, like Hurricane Sandy. As the Gulf Stream weakens, hurricanes could linger longer over coastal cities, causing catastrophic flooding. Additionally, the disruption of weather patterns would lead to a stark contrast: the Northeast and Europe could experience much colder winters while the Southeast U.S. (Florida, Georgia, Carolinas) would face more extreme heat and droughts.

Increased rainfall and flooding would also become more common. With the Gulf Stream weakened, storms could stall over cities, dumping massive amounts of rainfall, similar to what occurred during Hurricane Harvey in Texas but along the East Coast. Nor'easters and stronger winter storms would become more frequent, particularly in New England. The disruption of the AMOC would also have catastrophic consequences for marine ecosystems. The circulation helps distribute nutrients in the ocean, and its shutdown could disrupt fish migration patterns and collapse major fisheries, especially in the Northeast U.S., where industries like cod fishing and lobster harvesting would be devastated.

In a Climate Crisis

Since the Earth’s equilibrium climate sensitivity (ECS) is 4.5 +/-0.5°C for doubled CO2, the likely temperature rise this century under current emission levels will be significantly higher than IPCC estimates. Here’s a breakdown of how that translates into global warming:

1. CO2 Projections & Radiative Forcing

  • Pre-industrial CO2 (~280 ppm) to Present (~420 ppm): We’ve already experienced a ~1.25°C rise in global temperature.

  • Business-as-Usual Scenario (High Emissions, ~800-1000 ppm CO2 by 2100): If emissions continue at current levels, CO2 could more than double from pre-industrial levels, reaching or exceeding 560 ppm (the doubling threshold for ECS calculations).

  • Additional non-CO2 greenhouse gases (methane, nitrous oxide, etc.) also contribute to warming.

2. Temperature Increase by 2100

Using ECS = 4.5 +/-0.5°C, and considering:

  • Climate sensitivity refers to long-term equilibrium response (centuries), but transient warming occurs within this century.

  • The Transient Climate Response (TCR), which is lower than ECS, typically ranges ~60-70% of ECS, meaning near-term warming may be slightly less than equilibrium projections.

Projected Warming by 2100 Based on ECS = 4.5°C

  1. Lower-End Case (~550-600 ppm CO2, with significant mitigation efforts)

    • Optimistic warming: ~3.5-4.5°C

    • Comparable to IPCC's worst-case scenarios (RCP 6.0-7.0 range)

  2. High-Emissions Case (~800-1000 ppm CO2, with some mitigation)

    • Potential warming: ~5-7°C

    • This is significantly worse than IPCC's RCP 8.5 projection, which assumes ECS = 3°C.

  3. Business as Usual Case (Feedbacks Accelerate Warming: AMOC Shutdown, Ice-Albedo, Methane Releases, etc.)

    • Expected warming: 7-9°C or more

    • This would resemble past hyperthermal events like the Paleocene-Eocene Thermal Maximum (PETM).

3. Policy & Adaptation Implications

  • If ECS is higher than IPCC estimates, it means current climate policies are underestimating risk.

  • The IPCC's mid-range scenarios (projecting ~3°C by 2100) may be too optimistic if ECS is truly ~4.5°C.

  • Higher ECS implies faster and more extreme tipping points, including AMOC collapse, rapid sea level rise, and uninhabitable regions.

4. Urgency for Rapid Emission Cuts

  • Even immediate aggressive action (net-zero by mid-century) would still result in ~2.5-3°C warming due to existing CO2 levels and slow response time.

  • Avoiding catastrophic warming (>4°C) requires drastic reductions in fossil fuel use this decade.

Bottom Line: At Current Emissions, Warming by 2100 is Likely 7°C or Higher

This level of warming would lead to massive global disruptions, including widespread droughts, stronger hurricanes, agricultural collapse, and displacement of hundreds of millions due to rising sea levels. If ECS is truly 4.5°C, the window to prevent catastrophic warming is far smaller than most policymakers assume.

* Our climate model employs chaos theory to comprehensively consider human impacts and projects a potential global average temperature increase of 9° above pre-industrial levels.

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