1pctCO2-rad is the mirror image of 1pctCO2-bgc. The CO2 concentration rises by 1% every year, but this time only the radiation code is allowed to see it, so the climate warms as usual. The carbon cycle, meanwhile, is told CO2 is still at its pre-industrial level.

What it does

The trick is reversed. Here the planet heats up from the rising forcing, but plants and the ocean never "know" that atmospheric CO2 has changed — they only feel the warming. That isolates a single effect: how a warming climate on its own changes the amount of carbon the land and ocean hold. "rad" stands for radiatively coupled.

Why we care

1pctCO2-rad measures the climate–carbon feedback: warming tends to weaken natural carbon sinks — warmer soils respire more, and a warmer ocean holds less CO2. It is the other half of the pair:

• 1pctCO2-bgc — more CO2 in the air means more carbon taken up.
• This run — the warming that CO2 causes pushes some of that carbon back out.

Adding the two idealised halves back together should approximately reproduce the fully coupled 1pctCO2. That decomposition is how models pin down carbon–climate feedbacks, a major source of spread in the remaining carbon budget.