Well, either the Earth's climate will grow warmer or it will grow colder. One thing is for damn sure - the climate of the Earth does not, will not stay the same. So which would you prefer? Another ice age? Don't forget that, geologically speaking, we're in middle of an ice age.
From a biodiversity perspective - hot is good, cold is bad. Watery is good, icy is bad. If you really cared about biodiversity, you would not be so concerned about global warming. Will the polar bears go extinct if global warming continues? Perhaps a few specimens will be rugged enough to survive in warmer climates and will evolve into new species. Or they'll go extinct, which they would anyway sooner or later. On the other hand warming will do good for many species, and it will open up the polar regions to all kinds of new life.
Think of it this way. Imagine there was a global phenomenon that would shrink the Sahara desert. Will we try to oppose that, merely because it's a change? The polar regions are the fucking desert. Why are we so afraid of losing it? especially environmentalists!
Would environmentalists rather lose the icecaps or the jungles?
It's not like there's anything we can do about it, but once we're already talking about it, let's talk about it intelligently.