run-off and precipitation
It can be reflected off the surface and out to space, and it can be reflected off clouds back to space, for example.
When the cells of plants and animals break down food to produce energy and when fossil fuels (Such as coal and gasoline) are burned.
transpiration and evaporation. duhh
Radio-carbon dating is one, carbon 14 changes to carbon 13 over time, scientists measure how much carbon 14 is left in a fossil to determine how old it is.