Plants take up co2 during the growing season in spring and summer and then release it as they decay in fall and winter. The answer is a combination of co2"s abundance and its residence time in the atmosphere. Co2 accounts for about 0. 1 percent of the atmosphere, substantially more than all other. Other ghgs contribute more to global climate change than co2 on a per-unit basis. The global warming potential (gwp) of a given ghg expresses its estimated climate impact over a specific period of time compared to an equivalent amount by weight of carbon dioxide. Co2 is still the most important greenhouse gas because it is emitted in far larger quantities than other ghgs. Earth"s climate has alternated between periods of warmth and relative cold. Section 3: climate change: what the past tells us during the warmest periods, the polar regions of the world were completely free of ice.