what is the means of"were it to do so"

Antarctica receives more solar radiation than does any other place on Earth, yet the temperatures are so cold and the ice cap is reflective, so that little polar ice melts during the summer; otherwise, the water levels of the oceans would rise 250 feet and engulf most of the world’s great cities.

1. is reflective, so that little polar ice melts during the summer; otherwise,
2. is so reflective that little of the polar ice melts during the summer; were it to do so,
3. so reflective that little polar ice melts during the summer, or else
4. reflective, so that little of the polar ice melts during the summer, or
5. reflects so that little of the polar ice melts during the summer; if it did

does it means that if it is not  like this?

=if it were to do so.

虚拟倒装。

1.  A is wrong because of the word - 'would' after "otherwise".  we can read it with "would", it will be following:  otherwise, the water levels of the oceans would rise 250 feet ....

2.  A would be correct if the sentence were "otherwise, the water levels of the oceans (will) rise 250 feet ...

3.   B is the best.           if .. were, .....would rise...

虚拟