How much time do we have on Earth?

If we want to explore the possibility of humans colonizing another planet, one of the first questions to ask is how much time we might have to figure out how to do it.  So how much time might we humans have on this planet which at present is our only home?

The most optimistic answer is about two billion years.  At that point, the sun will begin to burn out, and in its death throes will first become much larger.  At that point, Earth will get so hot that we would be incinerated.  Two billion years, though, is a pretty good chunk of time, about as long as multi-cellular life has already colonized Earth.

But we may have a good deal less time than that.  Species last on average about two million years, though simpler organisms tend to last longer, and more complex organisms, like us, often last for shorter periods than that.  How old we humans as a species are is not exactly clear, but the oldest estimate is that homo sapiens cannot be more than a quarter of a million years old.  If homo sapiens survives as long as an average species, though, we should still have 1.75 million years ahead of us.  Not two billion, but nonetheless, still time to achieve a lot.

Earth, however, may not remain habitable for creatures like us humans for that long.  The climate is always changing, moving from extremes of cold to weather that is warm enough to melt away all the ice everywhere.  The atmosphere also changes, including, critically, levels of oxygen.  The availability of water is not constant, so deserts and forests come and go.  Earth even passes through strong fields of radiation about every 250 million years, which could make life difficult for us.

How much of our current climate change is due to human activity, and how much is do to the activity of the Sun, geothermal exchanges, volcanoes, ocean currents and extra-terrestrial bodies is not clear.  However, scientists estimate that if climate change continues at its present rate of change (whatever the reason), and we do not radically alter our life styles on a global scale, we will seriously begin to feel constraints on the way we live by 2050.  At some point we will start fighting for our very lives over water and food. 

Barring a major meteor strike or a series of events leading to vast eruptions of nitrogen from the ocean depths that would destroy our oxygen supply, global warming in itself, while possibly causing millions to die, are not expected to lead to the extinction of all human life.  It is not inconceivable that the current trends could lead to more drastic changes, which could make all human life impossible, but changes on that scale are not imminent.  But even initial changes that occur within the next 40 or 50 years could slow down our economies drastically, which means we will have fewer resources to spend on exotica like space exploration – without which we will never be able to colonize another planet, even assuming we can find one to colonize.

So instead of billions, or millions, or even thousands of years, we may something closer to a century to manage a move to another planet.  The paradox is that the better care we take of the planet we already have, the more time we are apt to have to find another one.

Can we do it in time?  how much time do we need?  how much time do we have?