Is the Earth going to evolve towards Mars' fate or Venus' fate?
Upon reading on this site (and many others), one can think that Mars might have supported life in a distant past (discovery of liquid water, valleys, mountains...).
On the other hand, Venus is the perfect example of the consequences of the greenhouse effect. A planet where life is simply impossible because no heat can escape the planet's ultra-thick, CO2 filled atmosphere.
So, with a lot of exageration, one can say that both Mars and Venus could be the Earth's distant future.
Since the Earth is actually facing global warming and greenhouse effect, I was wondering if the Earth would one day be like Venus ? Or if it is going to be like Mars, with no more magnetosphere and almost no atmosphere left ?
Or if it is not going anywhere near those two planets' fate ?
If you are asking about short-term effects related to human's effect on the climate, the answer is (obviously) unclear. But in the very long term, Earth is likely to evolve to a more Venus-like state.
Over the next billion years or so, the Sun's luminosity will slowly increase, which will heat Earth's surface. As a result, more water vapor will evaporate into the atmosphere. Since water vapor is a greenhouse gas, this will compound the heating. It's not entirely clear what the new equilibrium temperature will be after this runaway greenhouse effect runs its course. But it will clearly be more Venus-like than Mars-like.
This is essentially what I'd have said. A Mars outcome is unlikely because the Earth's gravity and magnetic field makes losing it's atmosphere and becoming a Mars very unlikley. Becoming a Venus, over a billion or two years, is entirely possible, but likely with more water than Venus, but closer to Venus than Mars certainly. The hotter sun and water being a greenhouse gas suggests a hot future in 1-2 billion years is highly possible. A repeat of "snowball earth" is also possible if CO2 drops low enough, but . . . I think that's unlikely. Technology could prevent either outcome too.
So the water vapor won't ever leave the earth's gravity until the earth itself is physically destroyed by the sun?