The Big Ratchet: How Humanity Thrives in the Face of Natural Crisis (5 page)

A planet too close to the sun suffers a fate like Venus, with its boiling surface. Once water vaporizes, it exacerbates the warming, leading to a runaway greenhouse effect that spirals the planet further into an over-heated hell. Life has no chance. This is what happens to a planet closer to the sun than the inner edge of its Habitable Zone. Beyond the outer edge, even if a planet has a greenhouse atmosphere, the distance from the sun is just too great to keep the surface above the freezing point.

There’s no guarantee that a planet’s atmosphere will have enough greenhouse gases to protect against an icy demise. Size, a second crucial feature for planets, as for real estate, can make the difference. At slightly more than half the diameter of Earth and one-tenth the mass, our neighbor Mars could not hang onto its atmosphere. Energy from the impacts of streaking asteroids and comets during the planet’s early years propelled much of the atmosphere’s gases into space. Mars’s weak gravitational pull could not overcome the force. Any gases that were left after these catastrophes have been seeping away ever since, helping to seal
Mars’s frozen fate.

Streams of particles with electric charges emanating from the sun—the solar wind—further undermined life’s prospects on Mars. The solar wind whisks away charged particles from the top of a planet’s atmosphere if there is no way for those particles to stay in the planet’s grasp. The Earth has a magnetic shield to ward off the solar wind’s hazard. Mars has no such protection.

The haunting streaks of glowing greenish light from the aurora borealis in the Arctic’s nighttime sky, and its counterpart, the aurora australis in the Southern Hemisphere, signal that Earth acts as a big magnet. Particles from the upper atmosphere, energized through collision with solar winds, funnel into the Earth’s magnetic field and give off light as they encounter gases in the atmosphere. Like a bar magnet attracting iron filings around its poles, the Earth’s internal magnet attracts the charged particles. Without this magnet, over time the particles would
scatter into space in a process that astronomers call “sputtering.” This is what happened to Mars.
It sputtered.

Mars’s small size is to blame for its lack of an internal magnet, as it is for the weak gravitational pull. A planet needs circulating metallic molten fluid in its interior to create electric currents and a magnetic field. An internal source for heat keeps fluid metal circulating, just as heat from a stove keeps a pot of soup churning. Mars once had such a source, but it shut down billions of years ago. Because the planet is so small, its internal energy dissipated out into space. The magnetic field was lost without a source of heat to churn molten metal in the planet’s interior. The solar winds could steal the atmosphere. Water vapor—and the prospects for a blanketing greenhouse effect—blasted off into space. The magnetic field of Earth, driven by its internal heat source at the core, can guard its atmosphere against the threat of solar winds.

Our planet has another feature that protects against outside dangers. Unlike a teetering Mars, Earth has a tilt that is roughly constant at an
angle of 23.5 degrees. The ice ages wax and wane on 100,000-year cycles as the tilt deviates from this roughly constant value. The tilt causes the sun’s energy to spread unevenly over the planet, which gives us seasons in the course of the year as our North and South Poles alternately lean toward our star. If the planet were less steady, the seasons could be more extreme, with one season scorching hot and the other freezing cold—perhaps extreme enough to prevent the evolution of life.

The moon keeps our planet’s wobble in check, stabilizing it against the pull of gravity from other planets. A lucky chance event created our moon. Soon after the planet first formed, when collisions between celestial bodies were common hazards, a Mars-sized body happened to crash into the Earth. The collision, known as the Great Splash, created so much energy that it melted Earth’s surface and sent vaporized rock into space. The debris coalesced and solidified into the moon, which remained trapped in the Earth’s gravitational orbit. Our planetary
neighbors had no such lucky encounters. The tilts of their axes are more extreme and their wobbles more chaotic with the pull from
other celestial bodies. In a few billion years, the moon will spin away from Earth’s orbit, and the planet will start swinging back and forth with a wild wobble like a slowing top. As this takes place and the sun gets brighter and hotter, life on this planet will not be able to persist.

What Goes Around Comes Around

It’s logical to surmise that at some point in their billions of years of history, Venus and Mars were suitable for life, with internal magnets to shield life from the solar winds and just-right temperatures to keep water in a liquid state. But even if our neighboring planets had those features, they probably never had the single, most essential feature for a planet to support life: the ability to regulate itself.

The regulatory machinery that makes it possible for a planet to recycle water, carbon, and much more is the truly distinguishing feature of our planet. More than anything else, it has saved the planet from the scorching fate of Venus and the frozen fate of Mars. It has kept nutrients for plants and animals cycling from land to ocean to deep beneath the surface to the atmosphere and back. It is the most precious, and the least appreciated, foundation for human civilization. As with Earth’s other planetary-scale features, this recycling machinery is not subject to humanity’s control.

So far as is known, ours is the only planet where an atom of carbon can find itself cycling from one form to another on time scales as short as seconds and as long as millions of years. Carbon easily bonds with other elements and forms the backbone of all known life. Carbon is ubiquitous and promiscuous. At different times the same atom can reside in the leaves of a plant, in the cells of an animal’s body, in hard rocks, dissolved in the ocean, or as gas in the atmosphere.

The slow movement of dozens of plates that carry continents across the Earth’s surface and give rise to mountains powers carbon’s recycling machinery. The churning of the Earth’s mantle, driven by the planet’s internal heat, underlies the plates’ movements. Plate tectonics does not exist on either Venus or Mars. If it ever existed there, that was in the distant past. Our neighboring planets have no mechanism for recycling their carbon. As a result, the carbon on Venus is stuck in the atmosphere with no way to get out. On Mars, it’s the opposite. Carbon has no way to get back to the atmosphere. Without the cycling, carbon cannot be part of life.

The cycling starts deep below the Earth’s surface. Magma rises toward the surface, propelled by the heat from radioactive elements deep in the Earth and pressure from the weight above. As magma rises, expanding bubbles erupt. Lava flows. Volcanic gases spew tens of miles into the atmosphere, and winds entrain them in the atmospheric mix. The greenhouse gas carbon dioxide enters the atmosphere with the volcanic eruption.

Volcanoes are not unique to Earth. Venus’s volcanoes spewed so much carbon dioxide that liquid water didn’t have a chance on the greenhouse world that the volcanoes created. Mars, too, boasts ancient volcanic features and lava flows. But our neighbors lacked linear mountain ranges like the Andes, the Himalayas, and the Rockies, which formed from the wrinkles where plates collided. And they did not have continents, whose puzzle-piece shapes on Earth reveal how the plates’ movements broke apart landmasses. Continents are the telltale signs that plate tectonics are at work.

The discovery of Earth’s recycling machinery dates only to the latter part of the twentieth century. Early claims by Scottish geologist Arthur Holmes in the 1920s were scorned. He pinpointed the continual, churning convection in the Earth’s interior, describing how the rising hot material cools on its way to the surface and sinks after it cools.
Holmes’s mechanism explains why the Earth has land. Sometime within the first few billion years of Earth’s history, the less dense material of the Earth’s crust must have risen to the surface to form continents. The continual heating and cooling process moved continents, broke them apart, and forced them to collide. This thermal convection explains the ridge running along the Atlantic Ocean’s floor where crustal plates form and spread. It also explains linear mountain chains, the places where plates collide, wrinkling at their edges and rattling the ground with earthquakes.

Volcanoes carry carbon from deep in the Earth to the atmosphere. From there it falls back to the Earth dissolved in rainfall to become part of living organisms, whose skeletons eventually carry the carbon back to the Earth’s depths to repeat the cycle over millions of years.

The idea fit with German meteorologist Alfred Wegener’s claim a decade earlier that the continents were once united. He argued that not only do the coastlines of the continents seem to fit together, as anyone can see on a world map, but that the far-flung fossils of plants and animals proved that now-separate landmasses must once have been
joined. Most geologists dismissed these ideas until irrefutable evidence, including actual pictures of deep-sea mountain ridges and the discovery of zebra-like magnetic stripes in the Earth’s crust, confirmed that magma upwells through the crust and forces the plates apart to form the deep-sea ridge.

Plate tectonics provides more than just the machinery by which volcanoes can spew carbon dioxide into the atmosphere. The collisions of the plates thrust mountains upward, exposing bare rocks critical for the next step in carbon’s life-enabling cycle. Carbon dioxide in the atmosphere dissolves in raindrops, making the rain slightly acidic. When the rain falls, the acid weathers rocks. You can see this on the chiseled names and dates on tombstones as they become weathered and blurred with centuries of rainfall. The weathering process pulls carbon dioxide out of the atmosphere on geologic time scales, countering the carbon dioxide that volcanoes spew in.

The cycle continues. Carbon-containing minerals from the weathering process eventually wash into streams, then rivers, and ultimately to the ocean. Plankton and other organisms build their shells with these
calcium- and carbon-containing substances. The organisms die. Some shells sink to the seafloor. The sunken shells become part of the sediment, and the seafloor spreads through plate tectonics, eventually carrying the calcium- and carbon-containing sediment to the edge of another continental plate. The sediment from the sunken shells slips down into the Earth’s interior, taking the carbon with it. With high temperatures and pressures deep below the surface, carbon dioxide dissolved in the magma rises again and enters the atmosphere through volcanic eruptions. The process starts anew. Around it goes. Millions of years go by for a carbon
atom to complete the cycle.

The extraordinary, life-enabling feature of this cycle is that it operates faster or slower depending on the temperature. When times are hot, the acid-producing chemical reactions speed up, eroding more rock. This is what happened during the warm reign of the dinosaurs about 65 million years ago. With faster reactions, more carbon dioxide gets pulled from the atmosphere, and temperatures cool. When times are cool, the process slows down, more carbon dioxide accumulates in the atmosphere, and temperatures warm. The self-correcting cycle oscillates between what geologists call hot houses—times of fervent volcanic activity—and ice houses—times when weathering outpaces volcanoes. Over time scales of millions of years, weathering serves as a thermostat to regulate our planet’s climate. The thermostat prevents a runaway greenhouse like Venus or a frozen state like Mars.

Humanity cannot change the speed of continental plates moving across the Earth’s surface, the collisions between plates that form mountains, or the frequency of volcanoes, just as we cannot control Earth’s other planetary features. But we can put more carbon dioxide in the atmosphere by burning forests and deep stores of carbon for fuels, creating the current global warming problem.