The hydrologic cycle is important because it is how water reaches plants, animals and us! Besides providing people, animals and plants with water, it also moves things like nutrients, pathogens and sediment in and out of aquatic ecosystems. Carbon is in carbon dioxide, which is a greenhouse gas that traps heat close to Earth. It helps Earth hold some of the heat it receives from the Sun so it doesn't all escape back into space. But CO2 is only good up to a point – beyond that point, Earth's temperature warms up too much.