1pctCO2-bgc is a companion to 1pctCO2 that splits the climate system in two. The CO2 still rises by 1% every year, but only the biogeochemistry — the carbon cycle — is allowed to notice it. The radiation code is told CO2 is still at its pre-industrial level, so the climate never actually warms.

What it does

This is a deliberately artificial trick. By letting plants and the ocean respond to rising CO2 while keeping the physical climate fixed, we isolate a single effect: how much extra carbon the land and ocean draw down purely because there is more CO2 in the air, with no warming to complicate it. "bgc" stands for biogeochemically coupled.

Why we care

On its own, 1pctCO2-bgc measures the concentration–carbon feedback: the appetite of natural carbon sinks as CO2 climbs. Paired with its radiatively coupled twin 1pctCO2-rad, it lets scientists cleanly separate the two ways CO2 affects the carbon cycle:

• This run — more CO2 means more uptake by land and ocean.
• 1pctCO2-rad — a warmer climate weakens that uptake.

Comparing the fully coupled 1pctCO2 against these two idealised halves is how models quantify carbon–climate feedbacks, which shape the remaining carbon budget.