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.