How long do greenhouse gases stay in the atmosphere? Do they eventually drift away or come back to the Earth? Do they stay in the atmosphere forever? Can greenhouse gasses naturally get absorbed by plants when gasses are so high in Earth’s atmosphere?

The reason greenhouse gases stay in the atmosphere is gravity. Earth's gravity is strong enough to hold onto its atmosphere and keep it from drifting into space. And while there's some (very slow) escape of gas, that depends on how heavy a particular gas is. So while hydrogen & helium escape relatively easy, the main greenhouse gas, carbon dioxide, is relatively heavy and so it stays in the atmosphere.

Carbon dioxide can persist in the atmosphere for thousands of years. However other greenhouse gases like methane are gradually removed from the atmosphere by chemical reactions over a period of about 12 years.

Greenhouse gases are in the layer of the layer of the atmosphere that is closest to the surface of the earth (it’s called the troposphere). Gases get mixed around in the troposphere through a process called diffusion, which means they can be absorbed by plants growing on the Earth’s surface or in the ocean. Absorption by plants and also diffusion of carbon dioxide into and out of the ocean means that it’s not always the SAME carbon dioxide that’s in the atmosphere, even though the total levels are increasing because of emissions resulting from human activities.

It’s interesting to note that carbon dioxide is not evenly distributed over the globe; it is patchy with higher concentrations in some places and lower concentrations in others. This is because large-scale weather systems and patterns of circulation in the atmosphere influence the way that gas gets moved about in the troposphere.