Seeing the Forest for the Trees

Hakan Sener
Aug 27
3 min read

Updated: Sep 10

New analysis argues ECS ~4.5 °C and stronger past aerosol cooling, explaining recent heat surge and raising urgency for faster climate action.

A new analysis by James Hansen and Pushker Kharecha synthesizes paleoclimate evidence, modern observations, and modeling to argue that equilibrium climate sensitivity (ECS) is ~4.5 °C (±1 °C, 95% CI) for doubled CO₂—significantly above the IPCC’s 3 °C best estimate. The authors also contend that aerosol cooling intensified (became more negative) from 1970–2005, contrary to IPCC assessments, and has weakened since ~2005 as emissions fell—especially from ships—helping explain the sharp recent warming. Their case: relying too heavily on global climate models, without fully integrating paleoclimate and satellite-era constraints, leads to underestimates of both sensitivity and aerosol effects.

Key Findings: Why Climate Sensitivity Is Likely Higher Than the IPCC’s Best Estimate

ECS is high—around 4.5 °C (±1 °C)

Using updated Last Glacial Maximum reconstructions (Tierney/Osman/Seltzer) that show ~6–7 °C global cooling at the LGM, the authors infer ECS ≈ 4.8 °C ± 1.2 °C (95% CI) from paleoclimate alone, which excludes an ECS of 3 °C with >99% confidence. Their overall best estimate, integrating multiple lines of evidence, is 4.5 °C ± 1.0 °C (95% CI).

Earth has darkened—pointing to strong cloud feedback

Satellite data show a ~0.58% albedo decline since the early 2000s, equivalent to ~1.97 W/m² more solar energy absorbed globally; most of this is attributed to cloud changes, not sea ice or water vapor alone. The authors infer a cloud-feedback contribution ~1–1.5 W/m² over the last 25 years, consistent with higher ECS.

Aerosol story: stronger cooling 1970–2005, then weakening after ~2005

They argue aerosol forcing grew more negative by ~0.5–1 W/m² (1970–2005) as emissions spread to cleaner regions where they more effectively seed clouds, and then became less negative as emissions declined—particularly due to shipping rules—boosting recent warming. With a more negative 1970–2005 aerosol forcing, matching observed temperatures requires higher ECS (~4.5–6 °C).

IPCC’s model-centric constraint can mask high sensitivity

Because models are tuned to reproduce 20th–21st century warming with uncertain aerosol forcing, a lower ECS can appear adequate. The authors’ “response-function” experiments show the recent ~1.6 °C global level and the ~0.4 °C jump in just a few years are hard to reconcile with ECS ≈ 3 °C and the IPCC aerosol trajectory, but are explainable with higher ECS plus a realistic aerosol history.

Bottom line

Across paleoclimate, modern satellite/radiation data, and simple physical constraints, the authors find consistent support for high ECS and underestimated aerosol cooling in past assessments.

Approach: Three Lines of Evidence, Equally Weighted

Paleoclimate: LGM temperature reconstructions and energy-balance accounting yield ECS ~4.8 °C ± 1.2 °C. This leverages real-world feedbacks operating together—hence fewer structural uncertainties than in models alone.

Modern observations: CERES satellite records show Earth’s reflectivity fell markedly (albedo −0.58%), implying a large net cloud effect; sea-ice darkening and clear-sky water vapor explain only ~0.15 and ~0.1 W/m², respectively.

Modeling/energy balance: Using a transparent gain framework (ΔTₑq = 1.2 °C / (1−g)), adding a cloud feedback gain g_cl ≈ 0.25 raises ECS from 2.4 °C (water-vapor+albedo only) to ~4.8 °C, consistent with the satellite-inferred cloud contribution.

Implications: Policy, Projections, and Priorities

Mitigation urgency: If ECS is ~4.5 °C, allowable carbon budgets for any temperature goal shrink sharply. The authors also argue the 1.5 °C target is effectively no longer attainable, given current warming and forcing trends.
Aerosol-era whiplash: As aerosol cooling wanes, the system reveals more of the underlying GHG-driven warming—amplifying near-term temperature rises and complicating detection/attribution if models assume weaker historical aerosol cooling.
Model development: Incorporate explicit cloud–aerosol processes and use observed radiation balance changes as constraints, not just surface temperatures. Pair models with paleoclimate benchmarks to avoid underestimating ECS.
Risk communication: The paper calls for frank messaging about high sensitivity and diminishing aerosol masking, and highlights the potential legal/policy ramifications in light of recent ICJ language on climate protection duties.

Don’t Miss the Forest for the Trees

Hansen and Kharecha argue in the analysis that multiple, independent constraints now converge on a higher ECS and a historically stronger aerosol cooling than featured in mainstream assessments. With Earth’s albedo falling and aerosol masking fading, recent warming makes physical sense—but also heightens near-term risk. Their takeaway: policy, models, and communications must reflect a higher-sensitivity climate system to avoid underpreparing for what’s already “in the pipeline”.

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