While the most dramatic changes have been the rise of an oxygen atmosphere long ago, a subtler yet ultimately more fateful change has been a long-term drop in atmospheric carbon dioxide. The geological record from Planet Earth tells us the lifeblood of life itselfcarbon, as contained in carbon dioxide found in our atmosphere or dissolved in the oceanshas been diminishing over time. A second realization has come from astronomy, showing us that all stars grow brighter and hotter as they age. Our sun, over timelike all stars in the so-called "main sequence"has already increased its heat output by 30 percent since its origin, some 4.6 billion years ago.
These two trends, of diminishing carbon dioxide, and increasing solar heating, will inevitably conspire to first end plant life, then animal life, and then cause our planet to lose its oceans. Eventually the heat of the enlarging sun will end all life on the planet. Our sun will enter a red giant phase, and either envelope the Earth, or melt it to slag.
We have attempted to predict when these events will happen, based on the best scientific evidence now available. Surely we have made errors, for any first summary of an emerging science will be fraught with error. But in some ways two undeniable trends make predication of the far future easier that predicting nearer term events.
Errors will arise in predicting exactly when this will occur, not if, and not in which sequence.
Just as we know that every human on the planet is doomed to die after some given period of life, so too do we know that our planet, as an abode for life, will eventually change into one unsuitable for life.
The clues and predictions about how the Earth did and will change through time can help us make generalizations about the life cycles of other habitable planets in the cosmos.
All abodes for life surely start with simple, single-celled, bacteria-like life forms that sometimes evolve toward complexity, and sometimes not. In those lucky planets where there is some form of long-term temperature stability, and where complex life does emerge, there must be some finite period before all such complexity succumbs to enlarging suns and diminishing water or carbon availability. All such planets must fall back into microbial ages, and then to no life at alljust as our own will.
The first realization of our own mortality is often a challenging blow. Yet sooner or later we must all come to grips with the realization that this precious life we lead is only lent, never given. So too with planets.
But there is hope. Just as living a more healthy life through diet and exercise can greatly extend the life of a human, so too do we think that a combination of judicious understanding of our planet's life-support systems, coupled with (and informing) engineering on a planetary scale, could extend the lifetime of the Earth as a habitable planet.
It is the challenge for our species to think in time scales virtually unthinkable to we short-lived beings. Yet million-year periods of survival are the norm for most species, and the fossil record gives us numerous examples of species that have lived for hundreds of millions of years.
Such great swaths of time are our biological right.
We can do this if we understand the factors that cause living planets to survive, and begin mediating against these newly understood factors that bring an end to planetary habitability.
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