A short history of nearly everything - Bill Bryson [132]
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 but water. Even humans are 65 percent water, making us more liquid than solid by a margin of almost two to one. Water is strange stuff. It is formless and transparent, and yet we long to be beside it. It has no taste and yet we love the taste of it. We will travel great distances and pay small fortunes to see it in sunshine. And even though we know it is dangerous and drowns tens of thousands of people every year, we can't wait to frolic in it.
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