But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
Explanation:

In lines five to nine, the speaker explains how "ice" could also be a potentially destructive element. To summarise, the speaker states that 'ice would be great and sufficient to destroy a world if it had to be destroyed twice'.

Life can be built or sustained only in moderate climates. We had seen previously how life on Mercury wasn't possible because of the heat. Similarly, life on colder planets is improbable. But we don’t have to go out into the space to demonstrate that; we know how winter doesn't support life as much as the other seasons do.

Speaking of how ice could be destructive, it is to be noted that it stands second to “fire”. The speaker is confident that it is the fire that is going to destroy the world. Ice can be harmful, but the speaker feels it may not be as malignant as fire.

Let us think of two scenarios:
• Being trapped in a room that caught fire
• Being trapped in a walk-in refrigerator
Between the two scenarios, the survival rate is better in the second. Fire will spread faster and can kill you instantly. Since ice gives you a slower death, you might be able to find time to save yourselves.

History has shown us how destructive ice can become. Studies show us that during the last ice age, “roughly about $$35$$ different types of large mammals went extinct”, although there is no evidence proving that the ice destroyed the species. But what is evident is the fact that the homo sapiens (early humans) and several other live forms survived and evolved. Hence, ice might have destroyed some species, but it certainly couldn’t destroy the world.

Nevertheless, the speaker is not addressing ice in its literal sense. As seen previously with fire, ice also is used as a metaphor in the poem. The line “I think I know enough of hate” reveals that ice embodies the negative emotion of hate.

As the previous stanza shows, uncontrolled and unreasonable desire can cause an apocalypse. But, if the world somehow survives the catastrophe, this mental venom known as hate could easily poison the world to death.

Hatred can also be considered a highly contagious disease. Hatred is the root of a sizable part of violence around the globe. There are countless reasons why people dislike one another, including their class, gender, position in the community, authority, religion, race, ethnicity, sexual orientation, creed, customs, nationality, political views, and physical or imagined characteristics. And many people who are the focus of hatred turn on their persecutors and commit acts of violence in retaliation. So, hatred is a destructive force strong enough to destroy the world.
Meanings of difficult words:

 S.No Words Meanings 1 Perish To die, especially in a violent or sudden way 2 Suffice Be enough or adequate 3 Improbable Not likely to be true or to happen 4 Homo sapiens The primate species to which modern humans belong 5 Contagious Of an emotion, feeling, or attitude, which is likely to spread to and affect others 6 Catastrophe An event causing great and usually sudden damage or suffering 7 Venom Extreme malice and bitterness shown in someone's attitudes, speech, or actions
Reference:
National Council of Educational Research and Training (2007). First Flight. Fire and Ice-Robert Frost (pp. 15). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.