Bill Bryson - A short history of nearly everything

Howard divided clouds into three groups: stratus for the layered clouds, cumulus for the fluffy ones (the word means “heaped” in Latin), and cirrus (meaning “curled”) for the high, thin feathery formations that generally presage colder weather. To these he subsequently added a fourth term, nimbus (from the Latin for “cloud”), for a rain cloud. The beauty of Howard’s system was that the basic components could be freely recombined to describe every shape and size of passing cloud-stratocumulus, cirrostratus, cumulocongestus, and so on. It was an immediate hit, and not just in England. The poet Johann von Goethe in Germany was so taken with the system that he dedicated four poems to Howard.

Howard’s system has been much added to over the years, so much so that the encyclopedic if little read International Cloud Atlas runs to two volumes, but interestingly virtually all the post-Howard cloud types-mammatus, pileus, nebulosis, spissatus, floccus, and mediocris are a sampling-have never caught on with anyone outside meteorology and not terribly much there, I’m told. Incidentally, the first, much thinner edition of that atlas, produced in 1896, divided clouds into ten basic types, of which the plumpest and most cushiony-looking was number nine, cumulonimbus. [32]That seems to have been the source of the expression “to be on cloud nine.”

For all the heft and fury of the occasional anvil-headed storm cloud, the average cloud is actually a benign and surprisingly insubstantial thing. A fluffy summer cumulus several hundred yards to a side may contain no more than twenty-five or thirty gallons of water-“about enough to fill a bathtub,” as James Trefil has noted. You can get some sense of the immaterial quality of clouds by strolling through fog-which is, after all, nothing more than a cloud that lacks the will to fly. To quote Trefil again: “If you walk 100 yards through a typical fog, you will come into contact with only about half a cubic inch of water-not enough to give you a decent drink.” In consequence, clouds are not great reservoirs of water. Only about 0.035 percent of the Earth’s fresh water is floating around above us at any moment.

Depending on where it falls, the prognosis for a water molecule varies widely. If it lands in fertile soil it will be soaked up by plants or reevaporated directly within hours or days. If it finds its way down to the groundwater, however, it may not see sunlight again for many years-thousands if it gets really deep. When you look at a lake, you are looking at a collection of molecules that have been there on average for about a decade. In the ocean the residence time is thought to be more like a hundred years. Altogether about 60 percent of water molecules in a rainfall are returned to the atmosphere within a day or two. Once evaporated, they spend no more than a week or so-Drury says twelve days-in the sky before falling again as rain.

Evaporation is a swift process, as you can easily gauge by the fate of a puddle on a summer’s day. Even something as large as the Mediterranean would dry out in a thousand years if it were not continually replenished. Such an event occurred a little under six million years ago and provoked what is known to science as the Messinian Salinity Crisis. What happened was that continental movement closed the Strait of Gibraltar. As the Mediterranean dried, its evaporated contents fell as freshwater rain into other seas, mildly diluting their saltiness-indeed, making them just dilute enough to freeze over larger areas than normal. The enlarged area of ice bounced back more of the Sun’s heat and pushed Earth into an ice age. So at least the theory goes.

What is certainly true, as far as we can tell, is that a little change in the Earth’s dynamics can have repercussions beyond our imagining. Such an event, as we shall see a little further on, may even have created us.

Oceans are the real powerhouse of the planet’s surface behavior. Indeed, meteorologists increasingly treat oceans and atmosphere as a single system, which is why we must give them a little of our attention here. Water is marvelous at holding and transporting heat. Every day, the Gulf Stream carries an amount of heat to Europe equivalent to the world’s output of coal for ten years, which is why Britain and Ireland have such mild winters compared with Canada and Russia.

But water also warms slowly, which is why lakes and swimming pools are cold even on the hottest days. For that reason there tends to be a lag in the official, astronomical start of a season and the actual feeling that that season has started. So spring may officially start in the northern hemisphere in March, but it doesn’t feel like it in most places until April at the very earliest.

The oceans are not one uniform mass of water. Their differences in temperature, salinity, depth, density, and so on have huge effects on how they move heat around, which in turn affects climate. The Atlantic, for instance, is saltier than the Pacific, and a good thing too. The saltier water is the denser it is, and dense water sinks. Without its extra burden of salt, the Atlantic currents would proceed up to the Arctic, warming the North Pole but depriving Europe of all that kindly warmth. The main agent of heat transfer on Earth is what is known as thermohaline circulation, which originates in slow, deep currents far below the surface-a process first detected by the scientist-adventurer Count von Rumford in 1797. [33]What happens is that surface waters, as they get to the vicinity of Europe, grow dense and sink to great depths and begin a slow trip back to the southern hemisphere. When they reach Antarctica, they are caught up in the Antarctic Circumpolar Current, where they are driven onward into the Pacific. The process is very slow-it can take 1,500 years for water to travel from the North Atlantic to the mid-Pacific-but the volumes of heat and water they move are very considerable, and the influence on the climate is enormous.

(As for the question of how anyone could possibly figure out how long it takes a drop of water to get from one ocean to another, the answer is that scientists can measure compounds in the water like chlorofluorocarbons and work out how long it has been since they were last in the air. By comparing a lot of measurements from different depths and locations they can reasonably chart the water’s movement.)

Thermohaline circulation not only moves heat around, but also helps to stir up nutrients as the currents rise and fall, making greater volumes of the ocean habitable for fish and other marine creatures. Unfortunately, it appears the circulation may also be very sensitive to change. According to computer simulations, even a modest dilution of the ocean’s salt content-from increased melting of the Greenland ice sheet, for instance-could disrupt the cycle disastrously.

The seas do one other great favor for us. They soak up tremendous volumes of carbon and provide a means for it to be safely locked away. One of the oddities of our solar system is that the Sun burns about 25 percent more brightly now than when the solar system was young. This should have resulted in a much warmer Earth. Indeed, as the English geologist Aubrey Manning has put it, “This colossal change should have had an absolutely catastrophic effect on the Earth and yet it appears that our world has hardly been affected.”

So what keeps the world stable and cool?

Life does. Trillions upon trillions of tiny marine organisms that most of us have never heard of-foraminiferans and coccoliths and calcareous algae-capture atmospheric carbon, in the form of carbon dioxide, when it falls as rain and use it (in combination with other things) to make their tiny shells. By locking the carbon up in their shells, they keep it from being reevaporated into the atmosphere, where it would build up dangerously as a greenhouse gas. Eventually all the tiny foraminiferans and coccoliths and so on die and fall to the bottom of the sea, where they are compressed into limestone. It is remarkable, when you behold an extraordinary natural feature like the White Cliffs of Dover in England, to reflect that it is made up of nothing but tiny deceased marine organisms, but even more remarkable when you realize how much carbon they cumulatively sequester. A six-inch cube of Dover chalk will contain well over a thousand liters of compressed carbon dioxide that would otherwise be doing us no good at all. Altogether there is about twenty thousand times as much carbon locked away in the Earth’s rocks as in the atmosphere. Eventually much of that limestone will end up feeding volcanoes, and the carbon will return to the atmosphere and fall to the Earth in rain, which is why the whole is called the long-term carbon cycle. The process takes a very long time-about half a million years for a typical carbon atom-but in the absence of any other disturbance it works remarkably well at keeping the climate stable.