The air you just inhaled may have traveled around the world the last few years. In the atmosphere movements are quick, the wind rushes around, and the air you breathe out will soon have mixed with the air around you. The upper part of the oceans also have strong currents that make water flow fast. But in the deep ocean, everything is slow. If something changes on the surface of the earth, it can take thousands of year before balance and a steady state is regained.
You need that steady state to calculate changes in climate. Ingjald Pilskog is a postdoc at Uni Research and the Bjerknes Centre. He is part of the project ORGANIC, where he studies how trace substances like ozone, iron and nitrogen spread around the globe. In this podcast he explains why a climate model must be run for a hundred or several thousand years before you can use the results. You have to spin up the ocean.