Robert Frost waxed poetically about whether the Earth would end in fire or ice. He wrote that he leaned toward fire, but that ice would suffice. We now know the inner planets will indeed end in fire, billions of years from now, when the sun expands grotesquely and consumes them. We know the Earth certainly began in fire, as countless dust grains, boulders, and mountains solidified into a molten, smoking body so hot that the end cooling stage is called the Hadean.
But in between that hellish beginning and a distant fiery end, there are long stretches when our planet is destined to be ruled by ice. And one of those periods, the most extreme icy phase the surface would ever see, may have kicked the biosphere into a super-heated overdrive and directly given rise to the first complex forms of life. A new study provides some hints about how this may have happened:
This song of ice and fire was a momentous period for life on Earth. According to Jochen Brocks from the Australian National University, it liberated a flood of nutrients that permanently transformed the oceans, from a world that was dominated by bacteria to one where algae were ascendant. The algae, in turn, revolutionized the food webs in the sea, paving the way for the evolution of larger and increasingly complex organisms—like the first animals. If the Age of Algae had never dawned, we wouldn’t be here.
How that happened is a fascinating story, because it begins when all life was in the ocean and microscopic, and it ends with, well, us. It involves our present-day complex world, food chains connecting the smallest specks of life to the largest megafauna, our warm wet blue-green world now teeming with bizarre, giant animals, and enormous plants compared to the itty-bitty microbes that thrived and died here unchallenged for billions of years.
Not to downplay the seriousness of a climate disaster when you’re caught in the middle of it. Just ask residents in the Houston, PortA, and Rockport region dealing with Hurricane Harvey how dreadful that can be. But in the big picture, if you think the K-T extinction that knocked off non-avian dinos, or the P-T great dying that knocked off 90 percent of life on Earth were some bad times, well, those creatures had it easy compared to what happened after the first photosynthetic organisms appeared around 2.5 billion years ago. These clever little biological machines combined sunlight with carbon dioxide to make sugar, and they gave off free oxygen in the process. Which sounds good to us now, as we have a fairly good opinion of oxygen. But back then it sent rates of CO2 down and atmospheric O2 up, which caused a series of climate catastrophes and slowly poisoned the biosphere for a billion years or more. Up until then, that CO2 sucked up by the new solar-powered bacteria and the methane broken down by the oxygen they gave off had helped keep Earth warm for eons by trapping heat. Once that critical greenhouse effect was greatly reduced, it was just a matter of time before plate tectonics and orbital mechanics lined up right, and when they did, temperatures plunged and ice starting creeping from the primeval poles toward the equator. It happened again and again, and then it got much worse. Almost terminal, in fact.
It’s called the Cryogenian Period, circa 720 to 640 million years ago, because there is good evidence that Earth was completely encased in ice more than once during this time. There is debate as to how far the ice got. In some models even the warmer equatorial regions would have had a thin layer of nearly permanent ice. The land was certainly locked in snow and glaciers, not to mention torched by UV, and scoured by howling wind. But those glaciers were quietly doing what all glaciers do: plowing the surface, picking up rocks and minerals, and grinding them into dust on their long, slow trip to lower ground, eventually dumping the dusty till into the nearest ocean. It would have been an easy time to be a weather forecaster: “Folks, it’s going to be freezing cold, crystal clear, and bone dry for at least the next 100 millenniums”.
It might have stayed that way forever, or at least until the sun fell off the main sequence, except for volcanoes. Volcanoes don’t care about ice—they blow their tops and leak gas in cold climes or warm ones. With little in the way of rain and lightning and the land sealed by ice, greenhouse gases would build up until a tipping point was reached and finally, one mild summer day somewhere in what passed for the tropics, the temperature grudgingly ticked up above freezing and stayed there. Once the ice started melting en masse, a runaway feedback loop of Biblical proportions would have to develop. Water vapor is a greenhouse gas too. As more ice melted, more water was exposed, infusing the air with more water vapor. In addition, darker land and water reflect less sunlight than shiny ice, causing the surface temperature to rise. It runs away, fast! Starting with a complete snowball Earth, models show that within as little as 100 to 1,000 years, every last bit of surface ice would melt and the average global temperature would increase to something like 100° F worldwide, maybe even hotter.
Up until then, photosynthetic organisms that depend on sunlight had been limited by how much open water was available and the concentration of important nutrients like phosphorus. Glaciers delivered the minerals during the snowball phase and the subsequent greenhouse on steroids freed every last acre of ocean. That’s when larger photosynthetic critters made their break. More sunlight, more heat, and more minerals meant newer, larger, more complex sun-loving microbes could evolve. And evolve they did—simple free-floating green plants took hold and exploded, the blooms would have painted large sections of ocean turquoise and emerald green, and a whole new food chain opened up for any organism willing and able to take advantage of the solar-powered feast.
For 3 billion years or more, all life had been microbes. But now, at the end of the final snowball phase about 620 million years ago, the first recognizable metazoans, meaning multicelluar creatures, appeared. Some of earliest were tiny, others so weird they defy modern classification. For a long time, during a strange time called the Ediacaran, some looked more like modern-day plants than animals complete with stalks and fronds waving in the shallow sea. But by 550 million years ago, at the start of the Cambrian Period, ancestors of the first worms, snails, bugs, and even what looked to be primitive chordates—the forerunner of vertebrates—were already thriving. The rest is natural history.
What does this tell us? Well, life is persistent and opportunistic: one species’ catastrophe is another one’s big break. Make no mistake, human-induced climate change poses a grave risk to modern agriculture and thus to modern civilization. Storms like Sandy, Katrina, and Harvey will become more common just for starters. But without underground shelters and supplies, we wouldn’t have a prayer of surviving in anything like a snowball Earth phase, or the subsequent warm-up when temperatures may have topped 140 F in the tropics and quasi-permanent category six hyper-canes roamed the seas like terrestrial puffy white analogues to Jupiter’s Great Red Spot. It’s perhaps some comfort to know that life itself can adapt to such extremes, so the Anthropocene won’t kill off life on Earth. The big remaining worry is it’ll just drive thousands of species to extinction—and one of them might even be us.