Well, there are some interesting scenarios, involving or Sun.
It's going to heat up all the time and this will result in the gradual loss of CO2 (as silicates are going to get more and more eroded and collect the CO2.
In 400 Million years, the Sun is 5% brighter than today, the average temperature is 20°C (15° today); more than 60% of vegetals and animals are gone; those remaining being those that don't need much CO2 to thrive.
In 800 Million years, the Sun is 8% brighter than today, the average temperature is 25°C, CO2 levels are 10 to 20 ppm (today it's 370 ppm-parts per million); even less vegetation and a few animals are left.
In 1.4 Billion years., temperature exceeds 50°C.
In 1.6 Billion years, temperature reaches 100°C and there is no more water; water vapor dissapears into space, Earth is completely sterilized.
Life has appeared 3 Billion years ago, multicellular life 700 Million years ago and life will dissapear in less than 1 Billion years form now.
In 7.6 Billion years from now, the Sun gets rid of its hydrogen, inflates.
In 7.67 Billion years, the Red Giant Sun swallows Mercury and Venus, just misses the Earth (according to some, not all).
In 7.68 Billion years, after loosing all its hydrogen, is suddenly deflated and starts burning its helium.
In 7.79 Billion years, it re-inflates again, pushing once again Earth's orbit (by loosing its mass)-it's its swann song.
In 7.80 Billion years (or 12.3 Billion years after its birth), the Sun gets completely rid of all its helium, collapses upon itself and becomes a white dwarf which will feebly shine and then turn off.
(Found in the march 2002 issue of Science et Vie).
Of course, chaos and impactors could play another role, but scientists think the Sun does have the major role in our planet's fate.

Raoul