Fast expansion and slow contraction of the ITCZ in response to CO₂ forcing

The Intertropical Convergence Zone (ITCZ) strongly shapes tropical rainfall and global circulation, yet its response to carbon dioxide (CO₂) forcing remains uncertain. Climate models project a long-term narrowing of the ITCZ under warming, but recent decades instead show a poleward expansion linked to eastern tropical Pacific cooling. Using a 30-member ensemble of abruptly quadrupled CO₂ experiments, we identify a two-phase evolution: a fast expansion over the first decade, driven by basin-wide relative cooling in the equatorial Pacific, followed by a slow contraction as equatorial warming strengthens. Applying this framework to a 1%-per-year CO₂ scenario reveals that the slow phase dominates only after ∼80 years, when the CO₂ levels more than double relative to preindustrial, suggesting that present-day conditions are still dominated by the fast response. This potentially reconciles recent ITCZ expansion with long-term projections of contraction, underscoring the importance of accounting for timescale-dependent processes when interpreting past trends and projecting near-term climate change.