"It is open,but........."见补充说明，翻译地对不对？

1. This scenario could apply approximately to Venus, though it is not certain that Venus ever had an ocean.

这种情形大概和金星相似，尽管不能确定金星曾经是否有一个海。

- scenario指的是“情境”，应该说：这种情境可以近似地套用在金星上

- ocean是海洋不是海（范围更大）

2. If it didn't, the greenhouse effect presumably would prevent oceans from ever forming.
如果没有，可能是温室效应阻止了海的形成。

- 严格说应该是：如果没有，据推测，温室效应将使海洋永远无法生成。

3. The Earth certainly has a greenhouse effect; otherwise it would be colder than it is by perhaps 20 degrees C.
地球上则（肯定）存在温室效应，否则地球将比现在要冷（约20摄氏度）。

4. It is an open, but very important question as to whether increased carbon dioxide concentration in the Earth's atmosphere could trigger a runaway greenhouse effect.
大气中二氧化碳（浓度）的增加是否真会使温室效应失控是一个（未知但很）关键的问题。

5. Even a rise in temperature high enough to melt the polar icecaps would have catastrophic consequences for us.
当气温升高到足以融化掉两极冰壳的时候，人类的灾难就已来临了。

- 原文的意思是：甚至连温度仅仅升高到能使两极冰壳