My world is at the outer edge of the habitable zone. There is enough heat and geological activity to start life, but a snowball earth is looming and threatens to freeze it all. There is some spring thaw at the equator, allowing some simple algae to multiply. The carbon dioxide is not sufficient to make an earth-like warm planet, though.
Then comes the pitch-patch: a pitch-black patch of single-celled algae that thrives in the the pools of molten ice in spring.
The pitch-patch manages to venture out of the pools and populate the thin film of water that makes ice slippery. It has antifreeze ethanol in its membranes. Most importantly, it is black, therefore absorbing enough sunlight and heat to raise the temperature around it. This creates a positive feedback that melts more ice and promotes more growth.
Eventually, the pitch-patch creates large icefields of algae. Its effect is noticeable from outer space, just as we see forests from orbit around Earth.
So, can the pitch-patch, along with carbon dioxide, make the difference and melt larger fields of ice? Can it create a more hospitable environment at-least around the equator?
EDIT: To point this out, there is sufficient carbon dioxide for algal growth and some greenhouse effect. However, the planet is too far to allow liquid water all-over the planet. For now, it is present in liquid form in a narrow band around the equator, and some life thrives there. Polar ice caps are much larger than those on Earth, but not exceedingly so. However, the chances of the snowball-earth to occur, are much higher than those on our planet, because of the distance from its sun.