The Eastern Seaboard as seen from the International Space Station in 2012.    (NASA)

In 1950, while working at Los Alamos National Laboratory,  renowned physicist Enrico Fermi was lunching with colleagues including Edward Teller, Herbert York an Emil Konopinski.  The group talked and laughed about a spate of recent UFO reports during the meal, as well as a cartoon about who might be stealing garbage can tops.  Was it aliens?

A bit later in the meal Fermi famously asked more seriously, “Where are they?”  Sure, there were many bogus reports back then about alien flying saucers, but Fermi was asking what has turned out to be a significant and long-lasting question.

If there are billions of exoplanets out there — as speculated back then but proven now — why have there been no bona fide reports of advanced extraterrestrials visiting Earth, or perhaps leaving behind their handiwork?

Many answers have been offered in the following decades — that we are alone in the universe, that the distances between solar systems are too great to travel, that Earth became home to life early in the galaxy’s history and other planets are only now catching up, that life might be common in the universe but intelligent life is not.

I would like to focus on another response, however, one that came to mind often while reading a new book by the former holder of the astrobiology chair at the Library of Congress, planetary scientist David Grinspoon.

This potential explanation is among the most unsettling:  that intelligent and technologically advanced beings are likely to ultimately destroy themselves.  Along with the creativity, the prowess and the gumption, intelligence brings with it an inherent instinct for unsustainable expansion and unintentional self destruction.

I should say right off that this is not a view shared by Grinspoon.  His “Earth in Human Hands,” in fact, argues with data and conviction that humans are more likely than not to ultimately find ways to work together and avoid looming global threats from climate change, incoming asteroids, depleting the ozone layer and myriad other potential sources of mass extinction.

But his larger point is the sobering one:  that the fate of Earth is, indeed, in our hands.  We humans are a force shaping the planet that is as powerful as a ring of volcanoes, a giant impactor from space, the long-ago rise of lifeforms that could, and did, dramatically change our atmosphere and along the way caused near global extinction.

It may sound odd, but as he sees it we are now the planet’s most powerful and consequential force of nature.

Since the Industrial Revolution and the spread of technology over the past 200 years, humans have become the dominant force on the planet, says David Grinspoon, the first Chair in Astrobiology at the Library of Congress.  (Credit:  Tony Steele)

“What I’ve sought to do is describe what is reality on our planet,” Grinspoon told me.  “Some people have been hostile and told me it’s arrogant to say humans have so much control over the fate of the planet, and I agree that it’s a sobering thing.”

But the Earth has been and will be dramatically changed by us.  The big question for the future is whether change can be for the better, or will it be unsustainable and for the worse.”

While Grinspoon’s major themes involve competing paths for the future of our planet, they consistently are based on and informed by knowledge gained in recent decades about planets in our solar system and those very far away.  The logic and track record of the search for intelligent life beyond Earth (SETI) also plays a role, as does the author’s relationships  — initially via family in childhood — with Carl Sagan and some of the scientists he mentored.

For instance, Grinspoon has studied Venus and the evolution of its atmosphere. He says that an understanding of its carbon dioxide-based runaway greenhouse effect, which has created surface temperatures of 800 degrees Fm  has been instrumental in the study of climate change on Earth.

David Grinspoon is a senior scientist at the Planetary Science Institute, and the author of “Earth in Human Hands.”

Similarly, the disappearance of much of the Martian atmosphere left the once warmer planet frigid and likely lifeless.  Sagan’s work on the dust storms of Mars, which have the effect of making the planet colder still, was an early scientific foray into understanding the importance of atmosphere and climate on a potential biosphere.  So was Sagan’s work on the possible effects of atomic war — the globally life-destroying “nuclear winter.”

The clear inference:  Planetary atmospheres can change substantially, as ours is doing now with major buildups in carbon dioxide.  Atmospheres can protect and nurture, or they can destroy.

And Exhibit A is the three rocky solar system planets in what is a slightly expanded habitable zone.  But only one supports life.

The buildup of carbon dioxide in the atmosphere and oceans since the onset of the industrial revolution, Grinspoon writes, is a prime example of how intelligent people and their technology can unintentionally have a huge impact on nature and the planet.  The jury remains out as to how humanity will respond.

But Grinspoon also points to the way that nations around the globe responded to the discovery that the ozone layer was being depleted as an example of how humanity can repair unintentional yet potentially extinction-threatening challenges.

It took a while, but the artificial refrigerants — chlorofluorocarbons (CFCs) — causing the damage were ultimately curtailed and then banned, and there are signs that the worrisome holes in the ozone layer are if not shrinking, at least no longer growing.

The Drake equation, created by astronomer Frank Drake in 1961, assesses the probability of how many planets  in our galaxy might have civilizations that can communicate. The last factor — the “L” for longevity — is considered key. Drake was one of the founders of SETI, and its effort to detect signals from intelligent life beyond Earth.

This brings us back to the Fermi paradox, and the apparent absence of signs of extraterrestrial intelligence.

Fermi, and many others, posited that successful, technological civilizations elsewhere would have the desire and ultimately know-how to expand beyond their original planet and colonize others. Indeed, early SETI gatherings here and in the former Soviet Union took that drive to expand for granted, a reflection of attitudes of the times.

This presumed drive to colonize was often discussed as either a kind of biological imperative or an acknowledgement that these “intelligent” civilizations are likely to have seriously damaged their own planets through unsustainable and hazardous growth. Either way, they would be on the move.

Yet after more than a half century of listening for signals from these presumed intelligent and mobile beings, the SETI effort to detect such life via radio telescopes has come up empty.  There are many potential reasons why, but let’s focus on the one introduced earlier.

The pioneering Drake equation, first put forward in 1961, attempts to assess the probability of finding intelligent civilizations beyond Earth based on factors such as rate of star formation in the galaxy, the number of planets formed and then the percentage with life, then the number with complex life and finally intelligent and technologically-sophisticated life.  But it’s the “L” at the end of the equations, says Grinspoon, that is widely considered the most important.

The SETI radio telescope array in Hat Creek, California.

The “L” is for the longevity of a potentially civilized, intelligent world, or “the length of time over which such civilizations release detectable signals.”

Of all the components of the Drake equation, which is filled with unknowns and partially known estimates, L is no doubt the least well defined.  After all, no extraterrestrial life, and certainly no intelligent life, has ever be detected.

Yet as describe by Grinspoon, “L” — which for Earth is about 200 years now — is the key.

“Let’s say that it’s impossible for a civilization with very powerful technology to last for 10,000 years, or even 1,000 years.  That makes the likelihood of ever making contact with them vanishingly small even if life and intelligence are out there.  The chances of them being close enough to detect and communicate with are pretty much nil.”

If the opposite is true, if it’s possible for a civilization to get over their technological adolescence, then they ought to be detectable.   Actually, they could last for millions of years using their technology to enhance and protect the planet.”

Planets face all kinds of dire threats; over long time periods catastrophes and extinctions are the rule.  But if technology can be used intentionally for the benefit the planet — like protecting it from an asteroid or avoiding the next Ice Age – longevity would clearly improve greatly.”

This interstellar view, he says, helps to see more clearly what is happening on Earth.  Now that through our technologies we have become the prime movers regarding the planet’s health and safety, it is really up to us as a species to choose between allowing these “advances” to knowingly or unintentionally harm the planet, or to consciously use technology to make it better.

Grinspoon does not see our current century as one when the effects of technology are likely to be intentionally positive.  But he does see the movement towards a more sustainable planet to be irreversible, whatever blips might come our way.  What’s more, he said, fossil fuels will be largely gone by 2100 and there’s reason to believe the world’s human population will have stabilized — two enormous changes that favor a longer-lived human civilization.

“The long-held view that humans will always expand, that they will maintain that biologically primitive imperative, that growth is always good — it’s interesting to wonder if those assumptions aren’t inherently wrong,” he said.

“I suggest that true ‘intelligence’ able to sustain itself involves an inherent questioning of those values, and that a more measured and strategic growth pattern, or even material stasis might be values that come with a more universal intelligence.”

Whether that intelligence might be on Earth or might be many hundreds of light years away.