In the distant future, humankind may be able to comfortably live on what is currently a dusty red planet. Mars is farther from the Sun and therefore much colder. Warming up the planet could be the first step in terraforming the climate and surface of Mars. Once water begins to flow and plants begin to grow, organisms can be introduced.
Tropical trees can be planted; insects and some small animals can be introduced. Humans will still need gas masks to provide oxygen and prevent high levels of carbon dioxide in the lungs.
Leonard David, Mars: Our Future on the Red Planet
Right now the Martian atmosphere is approximately 96% carbon dioxide (see en.wikipedia.org/wiki/Atmosphere_of_Mars). It could ...