Planet Earth’s Incredible, Shrinking Habitable Zone
published during a waxing gibbous moon.
Incredible, Shrinking Habitable Zone

South Lake Tahoe. This winter, it’s inundated with snow. Without a summer around the corner, no human could survive here. Credit: Joe Mascaro

For humans, Planet Earth is an inviting home. Whether winding our way through lush jungle, negotiating an alpine road or nestling our toes into Kentucky Bluegrass, we love her. We breathe in her oxygen, taste her raindrops and dive into her pounding surf.

But she’s dying. She’s dying a long, slow death.

Mind you, this long shuffling-off doesn’t refer to her acute risk from human meddlings like pollution, strip mining or the Full House sequel. Rather, Earth’s chronic condition is one of vanishing habitability: Earth’s incredible, shrinking habitable zone. On the scale of time that governs planetary birth and death, Earth is past middle age. As human beings that span scarcely a nanosecond in eons, we find it hard to imagine that with every passing day, our planet is becoming less and less habitable. And yet, this is the reality of our home.

Much has changed since Earth first settled into its gorgeous blue marble: oceans and forests teeming with life. The peak in global productivity was probably sometime during the Carboniferous Period, which concluded about 300 million years ago, before the dawn of the dinosaurs. It was then that jungles spanned nearly all latitudes. Carbon dioxide—that troublesome pollutant in the age of human-caused climate change—was give-or-take ten times higher than it is today. Earth was a sweltering place: a massive heat pump that dispersed the sun’s energy to the polar latitudes and ultimately to space.

But since the close of the Cretaceous and demise of the terrible (misidentified) lizards, Earth has been trending colder. Over just the last five million years or so, the position of the continents has slowed the effectiveness of Earth’s heat pump, and led to a lowering of the global thermostat. The planet’s wobbling orbit and axial tilt—which rock back-and-forth at around 100,000 to 30,000-year time scales—have become more noticeable. What we colloquially call the “ice age”, was just a temporary expression of this lasting condition: Earth getting first slightly warmer and then slightly colder.

Blissfully unaware of these multi-millennial rhythms, humans experience modern-day cold times as “winter”. While hearth, family, candy canes and dogsleds distract us, coniferous trees are blanketed in ice crystals, plunged into a deep freeze. The sea turns slushy and then white and then solid. In the far north, the sun hides below the horizon and charged particles irradiate the atmosphere. In a few thousand years, these conditions will linger a bit longer each century. And before long, the whole northern hemisphere will slide into uninhabitability.

In the high altitudes, we see even more terrifying expressions of Earth’s vanishing habitability. On the top of Mauna Loa, Hawaii’s largest shield volcano, black rock is strewn across the horizon. Underfoot, the ropey, pahoehoe lava crunches with each step. Aside from Earth’s persistent microbes inoculating its barren surface, the rock is not unlike that on the moon or Venus: an inert, chemical amalgam, ejected onto an equally vapid surface. No crops grow here. No large animals make their home. The clouds press overhead, slinking around cinder cones and slag piles.

In the ocean depths, there exist creatures that amaze and delight us: tubeworms and giant squid. But these are the most inhuman of beasts, evolved to tolerate the most horrible of environments: places only a handful of humans will ever see. Away from the warmth of ocean vents, across the abyssal plane, conditions are dark, frigid and crushing.

In the not-so-distant future, the environments uninhabitable to humans will grow. As our sun ages, she will increase her energy output and change the quality of energy she throws. Our moon will drift away from our grasp, softening tides and slowing the rotation of the Earth. While there will be seasons in this landscape of change, our home will inevitably hold less Kentucky Bluegrass and fewer tropical jungles. She will produce less rain and fewer waves. Her rich atmosphere will desert us. Her jets of magma will envelop us.

If, by this time, we haven’t made a home elsewhere… we will end.