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.
Unfortunately, human beings have a careless predilection for disrupting this cycle by putting lots of extra carbon into the atmosphere whether the foraminiferans are ready for it or not. Since 1850, it has been estimated, we have lofted about a hundred billion tons of extra carbon into the air, a total that increases by about seven billion tons each year. Overall, that’s not actually all that much. Nature-mostly through the belchings of volcanoes and the decay of plants-sends about 200 billion tons of carbon dioxide into the atmosphere each year, nearly thirty times as much as we do with our cars and factories. But you have only to look at the haze that hangs over our cities to see what a difference our contribution makes.
We know from samples of very old ice that the “natural” level of carbon dioxide in the atmosphere-that is, before we started inflating it with industrial activity-is about 280 parts per million. By 1958, when people in lab coats started to pay attention to it, it had risen to 315 parts per million. Today it is over 360 parts per million and rising by roughly one-quarter of 1 percent a year. By the end of the twenty-first century it is forecast to rise to about 560 parts per million.
So far, the Earth’s oceans and forests (which also pack away a lot of carbon) have managed to save us from ourselves, but as Peter Cox of the British Meteorological Office puts it: “There is a critical threshold where the natural biosphere stops buffering us from the effects of our emissions and actually starts to amplify them.” The fear is that there would be a runaway increase in the Earth’s warming. Unable to adapt, many trees and other plants would die, releasing their stores of carbon and adding to the problem. Such cycles have occasionally happened in the distant past even without a human contribution. The good news is that even here nature is quite wonderful. It is almost certain that eventually the carbon cycle would reassert itself and return the Earth to a situation of stability and happiness. The last time this happened, it took a mere sixty thousand years.
18 THE BOUNDING MAIN
IMAGINE TRYING TO live in a world dominated by dihydrogen oxide, a compound that has no taste or smell and is so variable in its properties that it is generally benign but at other times swiftly lethal. Depending on its state, it can scald you or freeze you. In the presence of certain organic molecules it can form carbonic acids so nasty that they can strip the leaves from trees and eat the faces off statuary. In bulk, when agitated, it can strike with a fury that no human edifice could withstand. Even for those who have learned to live with it, it is an often murderous substance. We call it water.
Water is everywhere. A potato is 80 percent water, a cow 74 percent, a bacterium 75 percent. A tomato, at 95 percent, is little
Because water is so ubiquitous we tend to overlook what an extraordinary substance it is. Almost nothing about it can be used to make reliable predictions about the properties of other liquids and vice versa. If you knew nothing of water and based your assumptions on the behavior of compounds most chemically akin to it-hydrogen selenide or hydrogen sulphide notably-you would expect it to boil at minus 135 degrees Fahrenheit and to be a gas at room temperature.
Most liquids when chilled contract by about 10 percent. Water does too, but only down to a point. Once it is within whispering distance of freezing, it begins-perversely, beguilingly, extremely improbably-to expand. By the time it is solid, it is almost a tenth more voluminous than it was before. Because it expands, ice floats on water-“an utterly bizarre property,” according to John Gribbin. If it lacked this splendid waywardness, ice would sink, and lakes and oceans would freeze from the bottom up. Without surface ice to hold heat in, the water’s warmth would radiate away, leaving it even chillier and creating yet more ice. Soon even the oceans would freeze and almost certainly stay that way for a very long time, probably forever-hardly the conditions to nurture life. Thankfully for us, water seems unaware of the rules of chemistry or laws of physics.
Everyone knows that water’s chemical formula is H2O, which means that it consists of one largish oxygen atom with two smaller hydrogen atoms attached to it. The hydrogen atoms cling fiercely to their oxygen host, but also make casual bonds with other water molecules. The nature of a water molecule means that it engages in a kind of dance with other water molecules, briefly pairing and then moving on, like the ever-changing partners in a quadrille, to use Robert Kunzig’s nice phrase. A glass of water may not appear terribly lively, but every molecule in it is changing partners billions of times a second. That’s why water molecules stick together to form bodies like puddles and lakes, but not so tightly that they can’t be easily separated as when, for instance, you dive into a pool of them. At any given moment only 15 percent of them are actually touching.
In one sense the bond is very strong-it is why water molecules can flow uphill when siphoned and why water droplets on a car hood show such a singular determination to bead with their partners. It is also why water has surface tension. The molecules at the surface are attracted more powerfully to the like molecules beneath and beside them than to the air molecules above. This creates a sort of membrane strong enough to support insects and skipping stones. It is what gives the sting to a belly flop.
I hardly need point out that we would be lost without it. Deprived of water, the human body rapidly falls apart. Within days, the lips vanish “as if amputated, the gums blacken, the nose withers to half its length, and the skin so contracts around the eyes as to prevent blinking.” Water is so vital to us that it is easy to overlook that all but the smallest fraction of the water on Earth is poisonous to us-deadly poisonous-because of the salts within it.
We need salt to live, but only in very small amounts, and seawater contains way more-about seventy times more-salt than we can safely metabolize. A typical liter of seawater will contain only about 2.5 teaspoons of common salt-the kind we sprinkle on food-but much larger amounts of other elements, compounds, and other dissolved solids, which are collectively known as salts. The proportions of these salts and minerals in our tissues is uncannily similar to seawater-we sweat and cry seawater, as Margulis and Sagan have put it-but curiously we cannot tolerate them as an input. Take a lot of salt into your body and your metabolism very quickly goes into crisis. From every cell, water molecules rush off like so many volunteer firemen to try to dilute and carry off the sudden intake of salt. This leaves the cells dangerously short of the water they need to carry out their normal functions. They become, in a word, dehydrated. In extreme situations, dehydration will lead to seizures, unconsciousness, and brain damage. Meanwhile, the overworked blood cells carry the salt to the kidneys, which eventually become overwhelmed and shut down. Without functioning kidneys you die. That is why we don’t drink seawater.
There are 320 million cubic miles of water on Earth and that is all we’re ever going to get. The system is closed: practically speaking, nothing can be added or subtracted. The water you drink has been around doing its job since the Earth was young. By 3.8 billion years ago, the oceans had (at least more or less) achieved their present volumes.
The water realm is known as the hydrosphere and it is overwhelmingly oceanic. Ninety-seven percent of all the water on Earth is in the seas, the greater part of it in the Pacific, which covers half the planet and is bigger than all the landmasses put together. Altogether the Pacific holds just over half of all the ocean water (51.6 percent to be precise); the Atlantic has 23.6 percent and the Indian Ocean 21.2 percent, leaving just 3.6 percent to be accounted for by all the other seas. The average depth of the ocean is 2.4 miles, with the Pacific on average about a thousand feet deeper than the Atlantic and Indian Oceans. Altogether 60 percent of the planet’s surface is ocean more than a mile deep. As Philip Ball notes, we would better call our planet not Earth but Water.
Of the 3 percent of Earth’s water that is fresh, most exists as ice sheets. Only the tiniest amount-0.036 percent-is found in lakes, rivers, and reservoirs, and an even smaller part-just 0.001 percent-exists in clouds or as vapor. Nearly 90 percent of the planet’s ice is in Antarctica, and most of the rest is in Greenland. Go to the South Pole and you will be standing on nearly two miles of ice, at the North Pole just fifteen feet of it. Antarctica alone has six million cubic miles of ice-enough to raise the oceans by a height of two hundred feet if it all melted. But if all the
