Greenhouse gases are gases which are significantly less transparent in the infrared range than across the overall Solar spectrum. The Earth converts a portion of the incoming Solar radiation to infrared. This allows greenhouse gases to act as a partial one way valve in terms of the atmosphere's heat budget, allowing more of the incident Solar energy in than the reflected and re-radiated energy out.
Greenhouse gases have been required to maintain Earth's climate within a range favorable to life. Gaia's hypothesized regulation of greenhouse gases to achieve this homeostasis is perhaps the best-known example of Gaia in action.
However, overly large or rapid changes in the quantity of greenhouse gases can be harmful to life. For example, a period of low greenhouse-gas concentrations between 850 and 630 million years ago, possibly somehow related to the earlier oxygen crisis, led to the freezing of all or most of Earth's surface, known as Snowball Earth. And since the Industrial Revolution, humans have been adding greenhouse gases (mainly carbon dioxide) to the air quickly enough to trigger a global climate crisis that is bringing the Holocene Epoch of climate stability to a close.
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