Climate inertia is the idea that the Earth's climate takes a long time to change once something happens to it. Think of it like a big ball on the ground- it takes a lot of force to get it moving, but once it does start moving, it's hard to stop.
In the same way, when something changes in the Earth's climate, like burning a lot of fossil fuels or cutting down a lot of trees, the effects don't happen right away. It takes a while for things to start happening. But once they do start happening, they keep happening for a long time.
This is because the Earth's climate is made up of lots of different parts, like the air, the land, and the water. And all these different parts take time to interact with each other and change. So if something happens to one part of the climate, it can take decades or even centuries for all the other parts to catch up and start showing the effects.
For example, if we keep burning a lot of fossil fuels, the Earth's temperature will slowly start to rise because the heat from those fuels gets trapped in the atmosphere. But it might take a long time for the temperature to rise enough for us to really notice, because it has to affect lots of different parts of the climate system.
So climate inertia is a really important concept to understand when we talk about climate change. It means that even if we stop doing things today that are making the climate change, we may still see the effects for a long time to come.