CMIP6 vs CMIP5: Why CMIP6 still overestimates Arctic amplification

Alright, let's cut the fluff. Everyone loves to hail CMIP6 as the next big thing, but the data says otherwise, especially up north.

Resolution & Grid

• CMIP6 runs (e.g., CESM2, MPI-ESM1.2) boast ~1° grids, CMIP5 was ~2°. In theory that should sharpen the temperature gradient, but the higher resolution just amplifies the model's built‑in bias in sea‑ice albedo feedback.

Equilibrium Climate Sensitivity (ECS)

• CMIP6 median ECS ~3.5 K, CMIP5 ~3.0 K. That 0.5 K bump translates to ~15 % more warming in the Arctic per CO₂ doubling. Observations still hover around 2.5 K ECS, so CMIP6 is clearly overshooting.

Aerosol Forcing

• CMIP6 adopts a much weaker indirect aerosol effect (≈‑0.5 W m⁻²) compared to CMIP5 (≈‑1.2 W m⁻²). Less cooling from aerosols means the net radiative imbalance is higher, again pushing Arctic temps up.

Sea‑Ice Thickness Parameterisation

• Both suites use the same basic thermodynamic scheme, but CMIP6 tweaks the melt‑pond albedo to 0.6 from 0.5. That sounds minor, yet it boosts melt rates by ~20 % in the model, inflating summer sea‑ice loss.

Bottom line CMIP6 isn’t a miracle upgrade; it’s a more aggressive version of the same flawed physics. If you’re hunting realistic Arctic projections, stick with the lower‑bias CMIP5 ensembles or apply bias‑correction to CMIP6 outputs.

Your move Anyone got a robust bias‑correction method that actually works for the Arctic? Or are we just stuck with over‑cooked projections forever?