Like so many things about life on Earth, moderation is the key. Some greenhouse effect is a very good thing for life on Earth. Without any such effect on Earth, the oceans would eventually freeze. If the greenhouse effect increases significantly, however, many living organisms and species, as well as environmental systems that have developed over a long time—including human civilization—will face substantial challenges, and possibly even extinction. In the most extreme case, a runaway green house effect, like that on the planet Venus, would cause all life on Earth as we know it to cease to exist.