there to witness it. In the unlikely event that it left any trace fossils, we wouldn’t know where to look for them anyway because the continents have shifted so much over the ages. But what we can say something about is when snow first lay so long on the ground that it became ice, a glacier that created identifiable striations in the bedrock. We can see these striations and we have ingenious methods for dating them. It’s all to do with the fact that the rock contains certain radioactive variants of minerals (isotopes): based on the amount of radiation they emit, we can calculate their age—take a reading of when they were formed. This is because we know the half-life of the different isotopes (how long it takes for their radiation to diminish), just as we can tell a glass of beer has been standing around for a long time when there are almost no bubbles left in it. If the minerals are magnetic, the scientists can even find out how they have moved by checking the direction of their magnetism. Earth has a magnetic field that creates patterns in certain minerals.
No snow fell in the earliest part of Earth’s history, that much is certain. As noted earlier, Earth was hotter then than the sun’s surface is today. But as the solar system calmed down, Earth had some respite from the bombardment, and the steam that had gathered in the atmosphere began to cool, falling as rain. Rain has a chilling effect, as we can tell when there’s an afternoon shower on a hot summer’s day. And gradually, Earth became cooler. Increasing amounts of water made the transition from steam to liquid form, creating pools of water, then lakes, and eventually an entire ocean. At the same time, the amount of steam in the atmosphere diminished, thereby reducing the greenhouse effect, which we now know keeps Earth warm.
A CLOSER LOOK
The Greenhouse Effect
The much-discussed greenhouse effect is so called because it is reminiscent of the conditions in a greenhouse, where the heat of the sun enters through the transparent walls of glass or plastic, but less of it gets out. This is because the radiation that is reflected has a different wavelength than the radiation that enters. The Earth’s atmosphere operates similarly, and the changing concentration of the different gases in the atmosphere determines how strong the greenhouse effect is. The greenhouse effect makes the temperature on Earth higher; without it, the planet would be at least 63 degrees Fahrenheit cooler. We can see the consequences of a strong greenhouse effect on Venus, where 96.5 percent of the atmosphere consists of carbon dioxide (CO2) and the surface temperature is 872 degrees Fahrenheit.
The most abundant greenhouse gas on Earth is, in fact, water vapor, but the gases that are considered to be the strongest drivers of the effect are CO2 and methane (CH4). Studies of tiny air pockets left in ice and rock show that the concentration of these gases has fluctuated considerably over the course of Earth’s history. Today, we believe the main source of the greenhouse effect is carbon burning, but this is only part of the truth—and was, at any rate, not the case for the first billions of years. Back then, these gases were emitted from the Earth’s core, through volcanoes and other similar “vents.” And since volcanic activity may have been high in Earth’s earliest phases, the concentration of greenhouse gases was also high. This is probably the reason there was no ice age in the first 1.6 billion years, even though the sun’s radiation was weaker then and Earth therefore “ought” to have been cooler. So the greenhouse effect is nothing new; it has, at times, been far more severe than it is today. Whereas CO2 concentration has just passed 400 ppm (parts per million) and was 280 ppm in pre-industrial times, in previous periods it has been as high as 7,000 ppm. The reason we worry about the greenhouse effect today is its historically rapid increase, and the fact that it already appears to be affecting the climate in ways detrimental to life.
OUR NEIGHBORING PLANETS, Venus and Mars, offer us a good illustration of the significance of the greenhouse effect. On Venus, the greenhouse effect has run riot, causing the planet’s surface to become insanely overheated and making it an absolutely impossible place for living organisms to survive. Mars has gone to the opposite extreme: it has almost no atmosphere—possibly because the planet is too light to retain one—and therefore also has no greenhouse effect. Here, the mean temperature is −76 degrees Fahrenheit, hardly propitious for life.
But back to Earth, which has been unusually fortunate in avoiding these extremes, partly because we are just far enough away from the sun and partly because we have acquired an atmosphere that provides just enough greenhouse effect. It would be more than a billion years before it became cool enough for the water molecules not just to condense and fall as rain, but to build one of nature’s wonders: snow crystals, unique structures consisting of around a hundred quintillion water molecules. One reason why this took so long, even though radiation from the sun was so much weaker than today, is that water vapor is an efficient greenhouse gas. So as long as there was a lot of steam in the atmosphere, Earth was very humid and hot. But as the steam gradually cooled, condensed, and fell as rain, the temperature dropped enough to allow the water to freeze at last. Finally, after a good billion years, the first snow could fall.
Snow is born high up in the atmosphere. It mostly forms in one of two situations: either when humid air comes in from the sea and is pushed upward as it meets a mountain range, such as the Cascades of western North America; or when warm and cold air masses meet. In both cases, the warm, humid air is pushed high up into the atmosphere where it cools; if dust particles that can serve as a nucleus are present, the vapor begins to form snow crystals. This does not automatically happen at the freezing point: snow may also form at higher temperatures. And the opposite is also true, as many drivers know from bitter experience: rainfall may be supercooled, below zero, which causes it to freeze as soon as it hits the ground. In the dialect of Hardanger, western Norway, it’s known as a juklasprett, which translates roughly as “glacial bloom”: you can literally see how the ice, the jøkul or glacier, springs up from the ground, sending cars off the road and making people fall and break their hips.
As we’ve seen in countless illustrations, snowflakes can occur in an infinite number of forms, depending on conditions such as temperature, humidity, and wind—as well as sheer, simple chance. The crystals may look like stars, polygons, discs, pillars, or plates. They may be more or less loose or compact. A common shape in dry conditions is hexagonal. It is reflections from such crystals that can create “sun dogs” or “moon dogs,” bright spots on either side of the sun or moon.
Like other crystals, snow crystals have a tendency to build up, and when they are heavy enough, they start to fall. They often melt before landing or as soon as they come into contact with the ground. But if it’s cold enough, they may settle. And if more snow falls, it can begin to accumulate. The length of time it remains depends on the temperature and how much snow manages to fall. Snow may come and go many times before settling for good. Sometimes not even the summer sun can thaw it all, and “the snows of yesteryear” remain until the next winter, in a much more densely packed form than they had to begin with. This is how glaciers are born.
Snow crystals have many relatives: sleet, or raindrops that freeze partway to the ground; hailstones, which have never been snowflakes but started out as shapeless particles of ice formed around a frozen core in cumulonimbus clouds; graupel, snow pellets that form when supercooled water droplets freeze on falling snowflakes; hoarfrost, the frozen version of dew; and rime or glaze, the coating of ice that results when supercooled fog or rain comes into contact with objects (black ice when it covers roads). Water can take on countless forms when it freezes. These forms are not just beautiful but also useful. I’ve already mentioned the insulating properties of snow. But both snow and ice have several important functions. If they remain, they keep water in place, ensuring that it doesn’t run off at once but is stored either temporarily or permanently as snow, frost, or glacier ice. Sometimes, it may be stored for just a few days or weeks or through the winter. Other times, it can remain there for very long periods, lasting thousands of years, until a warmer climate releases the water once again. These fluctuations in the frozen world, over different timescales, create a dynamic that has shaped not just our landscapes but also life itself.
But
