Everything happening around the poles, that is where the biggest increases in average temperature have happened, should be worrying. When you replace ice/snow (that is good reflecting light and heat from the sun) with something darker like deep ocean water (for floating ice, mostly in the north) or plants (for this case) you are empowering another positive feedback loop towards global warming, accelerating the rate at which the planet as a whole heats up.
What is now covered by vegetation is a hint of everything that is happening all around.