Climate forcing, positive feedbacks, and planetary tipping points: A synthesis of deep-time proxies, earth system sensitivity, and interventional feasibility

Reconstructing Earth's deep-time paleoclimate record provides a critical framework for evaluating modern anthropogenic climate forcing. Here, we synthesize proxy records from the "Big Five" mass extinction events—Late Ordovician (LOME), Late Devonian, Permian-Triassic (PTME), Triassic-Jurassic (TJME), and Cretaceous-Paleogene (K-Pg)—and evaluate their proxy signals against modern biogeochemical trends. While cumulative carbon emissions during deep-time hyperthermals were larger in absolute magnitude, current annual carbon injection rates ( ∼ 10 GtC/year) and global warming velocities ( ∼ 0.2°C/decade) exceed reconstructed pre-extinction baselines by factors of 10x to 100x. We catalog 17 positive climate feedback loops, 16 of which are currently active, and apply an Earth System Feedback Framework to demonstrate that integrating biogeochemical and cryospheric tipping dynamics elevates standard Equilibrium Climate Sensitivity (ECS ≈ 3.0°C) to an Earth System Sensitivity gain factor of GESS ≈ 4.76 (ESS ≈ 4.0–5.0°C for doubled CO2). Synthesizing this feedback framework with Earth system tipping point literature (e.g., Armstrong McKay et al., 2022; Steffen et al., 2018; Kemp et al., 2022), we use this feedback gain function to model the boundary and evaluate the risk horizon for reaching a "Point of Human Agency Loss"—defined as the biogeochemical threshold where autonomous natural feedback emissions ( ≥ 10.0 Gt CO2e/year) match or exceed maximum technological human mitigation and carbon removal capacity. We establish that crossing the 1.5°C to 2.0°C global warming horizon (projected mid-century) shifts the Earth system into a self-sustaining feedback regime. Driven by coupled biophysical stress vectors (niche thermal limits, crop yield collapses, and hydrological contraction; Xu et al., 2020) and the non-linear carrying-capacity decay function K(T), human support capacity is projected to contract non-linearly from baseline Holocene levels ( ∼ 8.5–9.0 billion) to an equilibrium ceiling of ∼ 1.3–1.5 billion at a stabilized state (+3.3–3.5°C). Finally, we perform a Multi-Criteria Decision Analysis (MCDA) across 16 proposed climate interventions, evaluating scale, cost, geopolitical friction, termination shock, and moral hazard risk. We conclude that while physical variables (temperature, ocean pH) may be geochemically reversible over centuries via solar radiation management and permanent carbon removal, biospheric destruction, species extinction, and sea-level commitment exhibit severe hysteresis (path-dependence where a system fails to return to its initial state even if forcing is removed), making damage containment rather than ecosystem restoration the primary operational objective.