1pctCO2 is an idealised ramp experiment. Starting from the pre-industrial control climate, the carbon dioxide concentration is increased by 1% every year, compounding, so it doubles after about 70 years and quadruples after roughly 140.

What it does

Like abrupt-4xCO2, this run is not meant to be realistic — no real-world scenario raises CO2 on such a smooth, steady schedule. Instead of one instant shock, though, 1pctCO2 applies the CO2 as a slow, clean ramp. That lets us watch the climate respond to a gradually strengthening forcing, which is much closer to how the real world warms over time.

Why we care

The steady ramp is what makes this experiment so useful:

• Transient Climate Response (TCR) — the warming at the moment CO2 doubles (around year 70) tells us how much the planet heats up while the ocean is still catching up.
• TCRE — because emissions accumulate smoothly, this run is the backbone for relating cumulative CO2 to warming, which underpins carbon budgets.
• It bridges the gap between the instantaneous abrupt-4xCO2 shock and the messier, forcing-rich historical run.

1pctCO2 is one of the core DECK experiments that every CMIP model runs, so its results can be compared cleanly from one model to the next.