Column: Life and Death of Planet Earth

Peter Ward
for National Geographic News

April 21, 2003

Astrobiology is a relatively new science concerned with the frequency
and nature of life in the universe.

So far, of course, we are aware of but a single planet that contains life: our own blue and green Earth. Yet it seems ludicrous to assume that this is the only planet with life in the Universe, given the immense numbers of stars in our Milky Way galaxy, and the immense number of galaxies revealed by humankind's powerful telescopes that relentlessly peer into the night skies.

Surely there are millions of other planets where life has either formed and has perhaps traveled to. The astrobiologists of planet Earth have every confidence that there will be evidence of life either in our own solar system, or in the planetary systems of neighboring stars.

A key to searching for life is the finding of other terrestrial, or "Earthlike" planets.

Yet what exactly is an "Earthlike" planet?

The obvious answer is a planet like our own, with oceans, continents, and temperatures that rarely or never exceed the boiling point, or descend below the freezing point of liquid water.

Yet our definition of "Earthlike" is for the Earth as we know it now, in our current Age of Animals, more than four billion years since our planet's origin.

Much evidence from the Earth's rock and fossil record makes it abundantly clear that our current conditions did not extend back to the time of our planet's formation.

If we go far enough back in time we can find a planet that was, at different times, frozen from pole to pole, or so hot that liquid water could not exist on its surface. Neither of these times would be even remotely "Earthlike" in our experience, and yet both existed for immense periods of time in the past.

In a similar fashion there is a dawning realization that the future of our Earth will certainly witness periods as "un-Earthlike" as these past episodes, and in many ways there will be times when the future recapitulates the cold and heat of the past.

A new discipline within astrobiology is emerging, one that we might dub "Planetary gerontology"—the science of planetary aging. Our Earth is indeed aging, and a result of this will be the eventual end of all life on the planet—and eventually the physical destruction of the planet itself. These are the subjects of our new book, The Life and Death of Planet Earth.

Much of the information that has led us to a hypothesis of what might be called "ends" of the world comes from the fields of geology and astronomy. From our planet's rock record we have learned that the early Earth had a much different atmospheric composition than it does today, and that even the atmosphere of a few million years ago was vastly different than that of now.

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 itself—carbon, as contained in carbon dioxide found in our atmosphere or dissolved in the oceans—has 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 time—like 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 all—just 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.