Ice ages and solar pulses goldilocks and the three planets

Why Earth is "just right" for life

Our sun has an inner ring of planets, starting with Mercury and moving out to Venus, Earth, and Mars. Right from their birth 5 billion years ago as cosmic debris, these planets have been more than lumps of rock. For one thing, they are hot, with thin solid crusts hiding large molten cores. Turbulent chemistry in their depths releases gases through the crusts. Although Mercury was too small, and its gravity too weak to capture these gases, the other three have held on to at least some of them, creating atmospheres. These atmospheres contain greenhouse gases such as carbon dioxide, water vapor, and methane that trap solar heat and create climates.

The three atmospheres of the three planets were initially probably rather similar. But they have evolved in very different ways. Today, Venus has a thick atmosphere with enough greenhouse gases to hold temperatures at around 8500F. Mars appears once to have had a considerable atmosphere and a climate that supported rainfall. It may have had life, as well. But somewhere along the way, it lost much of its atmosphere and dried up, and any life is now presumed extinguished. The demise of the life-support system on Mars is a conundrum, because the planet has plenty of carbon at its surface. It was probably once floating in the form of carbon dioxide in the atmosphere, where it would have formed a blanket sufficiently warm for liquid water and for life. But most of that carbon has ended up in rocks.

Earth, by contrast, has a rich and chemically very active atmosphere, and a sufficiency of greenhouse gases to maintain equable temperatures and lots of liquid water—and it is very much alive. Some planetary scientists have dubbed Earth the "Goldilocks planet." When, in the children's story, Goldilocks tasted porridge at the house of the three bears, she found one bowl (Venus) too hot, one (Mars) too cold, and one (Earth) just right. At first, this seems the purest chance. Earth must have been just the right distance from the sun. And yet, since in the early days the three planets had very similar atmospheres, the theory has developed that their different fates had as much to do with the fates of those atmospheres as with the planets' distance from the sun.

Earth's atmosphere has certainly endured, and has proved a congenial place for the development of myriad life forms. Things were often difficult in the early days, it is true. At various points, the planet seems to have been entirely covered by ice and snow, with life surviving only in warm crevasses beneath the frozen exterior. The fate of Mars threatened. "It was a close call," says Joe Kirschvink, of the California Institute of Technology, in Pasadena, who coined the term "Snowball Earth" to describe this condition, which last occurred some 600 million years ago. He believes that the planet escaped a fate similar to that of Mars only because of a buildup of carbon dioxide emitted from volcanoes beneath the ice: "If the Earth had been a bit further from the Sun, the temperature at the poles could have dropped enough to freeze the carbon dioxide, robbing us of this greenhouse escape from Snowball Earth."

Despite such difficulties, Earth came through, and for the past half-billion years at least, it has maintained a surprisingly constant temperature. Not, as we shall see, completely constant, but surprisingly so given the cosmic forces being played out around it. In particular there was the sun. It is the main source of most of the energy and warmth at Earth's surface, of course. By comparison, the contribution of the heat from Earth's core is minute. But the sun has changed a great deal over the lifetime of Earth. Back in the early days—for about the first billion years of Earth's existence—it was a weak beast. It emitted about a third less energy than it does today. Even 500 million years ago, it was as much as 10 percent weaker than it is today. Yet, with Snowball Earth a distant memory, the world then seems to have been warmer than it is now, and ice-free. This is because the atmosphere was rich in methane, carbon dioxide, and water vapor, all forming a thick blanket that kept the planet and its growing armies of primitive life warm. Volcanic activity was still strong, so new releases of carbon dioxide topped up any leakage from the atmosphere, keeping concentrations around twenty times higher than they are today.

But as the planet has aged, the emissions from volcanoes have lessened, and carbon dioxide has gradually started to disappear from the atmosphere. Its decline may at various times have threatened a return of Snowball Earth, and a Martian relapse into a cold, lifeless world. But it may ultimately have saved the planet from a fate similar to that of Venus. This raises an interesting question. Did this happy Goldilocks outcome occur entirely by chance? Or could the planet have developed some kind of crude thermostat? The surprising answer is that it seems to have done just that.

Carbon dioxide, then as now, was removed from Earth's atmosphere largely by being dissolved in rain to form dilute carbonic acid. That acid ate away at rocks on the ground, which were made primarily of calcium silicate, creating calcium carbonate, which ended up as sediment on the ocean floor. This process has a temperature control built in, because the amount of rain depends on the temperature. So erosion rates rise when it is warm, but faster erosion removes more carbon dioxide from the air and lowers temperatures again. If the thermostat overshoots, and temperatures get too cold, then the rate of weathering slows, and temperatures recover. This is a negative feedback operating through the carbon cycle. It won't save us today, because it takes millions of years to have a serious impact. But over geological timescales, it was probably rather good at moderating temperatures and keeping the planet's climate convenient for life.

Very convenient. Suspiciously so, thought the charismatic British chemist and maverick inventor Jim Lovelock, back in the 1980s. Lovelock wondered if life itself might be controlling this process; and soon afterward two of his acolytes, Tyler Volk and David Schwartzman, suggested that he was right by demonstrating that basalt rocks erode a thousand times faster in the presence of organisms such as bacteria. This introduces a new and extremely dynamic negative feedback. More bacteria will keep the planet cool. But if the air gets too cool, the planet becomes covered by ice, the bacteria die, the erosion slows, and the atmosphere warms again. This process is potentially an extremely powerful thermostat for planet Earth, and is one of the foundation stones of Lovelock's grand vision of Earth as a self-regulating system called Gaia. It may also explain why the carbon cycle feedback did not save Mars: perhaps, at some critical moment, the red planet did not have enough life to make it work properly.

Lovelock is a controversial character. Now in his eighties, he first devised his idea of Gaia while working for NASA and trying to think of ways to decide if other planets had life. He figured that the best way was to look for signs of gases that could be made or maintained in the air only by life forms. And he began to realize that life could evolve quite naturally in ways that would maintain an environment that suited it. He argues that since the early days, life on Earth has evolved sophisticated strategies for stabilizing climate over long timescales. For him, the temperature of life on Earth was "just right" because life made it so by taking control of key planetary life-support systems like the carbon cycle.

For many years, Lovelock was virtually cast out of the scientific community, and Gaia was often seen as quasi-religious mumbo-jumbo. Major journals like Nature and Science would not publish his work. He made his living as a freelance inventor of scientific devices. But his idea of Earth as, metaphorically at least, a single living organism has made him the spiritual father of a whole generation of Earth system scientists. Whether or not you buy the notion of a living Earth, his way of thinking about Earth as a single system with its own feedbacks has been extremely influential.

The thermostat, whether run by life or by geology, is pretty crude. For some 400 million years, planet Earth has been getting cooler. Some see this as a refutation of Gaian ideas. But others, like Greg Retallack, a soil scientist at the University of Oregon, argue that the cooling happened because life, or at any rate large parts of it, wanted it that way. Plants in particular, he says, like it cool. And plants have proved extremely efficient at capturing carbon dioxide and burying it permanently where it cannot return to the atmosphere. Some 7 trillion tons of old vegetable carbon has been stored for tens of millions of years in the form of fossil fuels beneath Earth's surface. In addition, probably as much methane is captured in frozen clathrates beneath the ocean bed. That is a lot of warming stored away, as we are currently in danger of discovering the hard way.

The cooling of Earth has been a long, slow, and fitful process. Around 55 million years ago, as we saw earlier, Earth experienced the "biggest fart in history," a vast surge of methane into the atmosphere from the undersea clathrate store, which pushed air temperatures up by around 9°F. That was clearly no part of a Gaian grand plan. But Gaians would argue that life-mediated feedbacks resumed control. The methane eventually decayed to carbon dioxide, which was in turn absorbed back into the oceans. But even after normality had been resumed, levels of carbon dioxide in the atmosphere were still about five times as high as they are today—at around 2,000 parts per million. Within a million years or so, however, those concentrations began to fall sharply. (Sharply, that is, on geological timescales: the average pace of decline was less than one ten-thousandth of the rate of increase in recent decades.) By 40 million years ago, they had subsided to 700 ppm. And by around 24 million years ago, they were below 500 ppm, probably for the first time since the planet's earliest days.

It was around then that an ice sheet spread across Antarctica—the first permanent ice to form on the planet for hundreds of millions of years. And by about 3 million years ago, another surge of cooling had begun, resulting in ice sheets forming in the Northern Hemisphere, too. Explanations for this general cooling range from continental drift in the western Pacific to another turn of the Gaian thermostat. But we can leave that to one side. Because the ice ages themselves—the geologically brief but extremely vicious cold snaps within the general cooling trend—happened on time-scales of much more interest in our current climatic predicament. Unraveling the causes of the ice ages may, many climate scientists believe, provide vital clues to our fate in the coming decades.

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