NYTimes | OUR
galaxy, the Milky Way, is home to almost 300 billion stars, and over
the last decade, astronomers have made a startling discovery — almost
all those stars have planets. The fact that nearly every pinprick of
light you see in the night sky hosts a family of worlds raises a
powerful but simple question: “Where is everybody?” Hundreds of billions
of planets translate into a lot of chances for evolving intelligent,
technologically sophisticated species. So why don’t we see evidence for
E.T.s everywhere?
The physicist Enrico Fermi first formulated this question, now called the
Fermi paradox,
in 1950. But in the intervening decades, humanity has recognized that
our own climb up the ladder of technological sophistication comes with a
heavy price. From climate change to resource depletion, our evolution
into a globe-spanning industrial culture is forcing us through the
narrow bottleneck of a sustainability crisis. In the wake of this
realization, new and sobering answers to Fermi’s question now seem
possible.
Maybe
we’re not the only ones to hit a sustainability bottleneck. Maybe not
everyone — maybe no one — makes it to the other side.
Since
Fermi’s day, scientists have gained a new perspective on life in its
planetary context. From the vantage point of this relatively new field,
astrobiology, our current sustainability crisis may be neither
politically contingent nor unique, but a natural consequence of laws
governing how planets and life of any kind, anywhere, must interact.
The
defining feature of a technological civilization is the capacity to
intensively “harvest” energy. But the basic physics of energy, heat and
work known as thermodynamics tell us that waste, or what we physicists
call entropy, must be generated and dumped back into the environment in
the process. Human civilization currently harvests around 100 billion
megawatt hours of energy each year and dumps 36 billion tons of carbon
dioxide into the planetary system, which is why the atmosphere is
holding more heat and the oceans are acidifying. As hard as it is for
some to believe, we humans are now steering the planet, however poorly.
Can
we generalize this kind of planetary hijacking to other worlds? The
long history of Earth provides a clue. The oxygen you are breathing
right now was not part of our original atmosphere. It was the so-called
Great Oxidation Event, two billion years after the formation of the
planet, that drove Earth’s atmospheric content of oxygen up by a factor
of 10,000. What cosmic force could so drastically change an entire
planet’s atmosphere? Nothing more than the respiratory excretions of
anaerobic bacteria then dominating our world. The one gas we most need
to survive originated as deadly pollution to our planet’s then-leading
species: a simple bacterium.
The
Great Oxidation Event alone shows that when life (intelligent or
otherwise) becomes highly successful, it can dramatically change its
host planet. And what is true here is likely to be true on other planets
as well.
But
can we predict how an alien industrial civilization might alter its
world? From a half-century of exploring our own solar system we’ve
learned a lot about planets and how they work. We know that Mars was
once a habitable world with water rushing across its surface. And Venus,
a planet that might have been much like Earth, was instead transformed
by a runaway greenhouse effect into a hellish world of 800-degree days.
By
studying these nearby planets, we’ve discovered general rules for both
climate and climate change. These rules, based in physics and chemistry,
must apply to any species, anywhere, taking up energy-harvesting and
civilization-building in a big way. For example, any species climbing up
the technological ladder by harvesting energy through combustion must
alter the chemical makeup of its atmosphere to some degree. Combustion
always produces chemical byproducts, and those byproducts can’t just
disappear. As astronomers at Penn State recently discovered, if
planetary conditions are right (like the size of a planet’s orbit), even
relatively small changes in atmospheric chemistry can have significant
climate effects. That means that for some civilization-building species,
the sustainability crises can hit earlier rather than later.