A planet is considered "Class M" if it can sustain human life1. We live on a Class M planet. However, the conditions that make a planet conducive to life may change over time, so it is important to have instrumentation and gauges and stuff that will keep track of important variables.
It is estimated that when about one trillion tonnes of fossil carbon is released into the atmosphere by human activities, we will have reached a critical point, at which time we may have to leave the planet2.
We're nearly half way there.
James Hrynyshyn at the blog Class: M has devised a sort of tricorder that keeps track of the carbon release count-down, and embedded it on the left sidebar of his blog. Go have a look.
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1That is, of course, a reference to Star Trek and is not real.
2We won't, however, be able to.
I am ever reminded of the quote from Larry Niven: ""The dinosaurs became extinct because they didn't have a space program."