Water from Heaven

On our planet, today both warm and icy, the oceans contain some 1.37 billion cubic kilometers (324 million cu. mi.) of salty liquid water, over 96 percent of the total amount of Earth’s water (see table 6.1).¹ Much of the remaining 3.7 percent—that’s still 29 million cu. km—lies frozen in the ice caps of Antarctica and Greenland, floating in the ice pack, icebergs, and scattered sea ice of the polar oceans or inching downward in mountain glaciers. The shares of frozen and liquid water change with time, and when the last ice age came to an end between 15,000 and 10,000 years ago, more than 40 million cu. km of ice melted. Most of that water finished in the oceans, and sea level rose by about 120 meters, on average nearly an inch a year for some fifty to sixty centuries. Finally, after perhaps a small overshoot several thousand years ago, the seas reached their present level defining the modern map, with for example the Channel separating England from France, the Strait of Malacca separating Malaysia and Singapore from Sumatra and the rest of Indonesia. Sea level would rise still another 70 to 80 meters (more than 200 ft.) if all the remaining ice on Earth were to melt, if our planet were to be completely transformed from icehouse to greenhouse; and such a sea-level rise would flood huge areas, including many of today’s densely populated coastal plains, river deltas, and valleys. During the Tertiary period, “only” a few tens of millions of years ago, such shallow seas covered a significant fraction of today’s dry land. No such extensive flooding is expected for the next few ten thousand years at least. As for the 21st century, even with strong greenhouse warming, sea-level rise by the year 2100 will likely be about two feet (less than a meter in any event), mostly due to thermal expansion of water in the sea’s surface layers, and not to melting ice.

TABLE 6.1 EARTH’S WATER

Estimates given in column 2 are in millions of cubic kilometers (MCkm), where a cubic kilometer is a billion cubic meters. LGM stands for the Last Glacial Maximum, 18,000 years ago. Equivalent depths (column 4) are obtained by dividing the volumes of water by the areas concerned. Rivers, streams, and the biosphere contain minute percentages of the total water, given in column 3 in parts per million (1 ppm = 0.0001%). Estimates of the amount of water underground are very uncertain. The entries correspond to layers from the surface down to 4,000 meters depth. Shallow fresh groundwater, from the surface down to 750 meters depth, amounts to about 4 MCkm, and has shorter residence time. Estimates for fresh and saline water in layers more than 4,000 meters deep, not included in the table, range from 50 to 320 MCkm.

Setting the stage for our present interglacial interlude, the grand ice debacle literally put into circulation vast amounts of fresh water, giving a last rinse to the lands sculpted by ice streams and glaciers. Some of that fresh water percolated underground, constituting a reserve of “fossil” water exploited, indeed overexploited, in many areas of the world. This nonrenewable resource is often treated today as though it were inexhaustible. Apart from such infiltration, most of the fresh water from yesteryear’s long-gone glaciers flowed to the seas and entered the water cycle with its atmospheric phases. Meltwater from mountain glaciers feeds many rivers—for example, the Rhone River in Switzerland and France—and that water can be tapped for supplying cities, for irrigation, and for cooling nuclear or fossil-fuel-burning power plants. But the Rhone glacier would have disappeared long ago were it not regularly renewed by snowfalls continuing (and perhaps intensifying) after the retreat of ice from the Valais region of Switzerland. Life on the land depends on water from the sky.

In the present-day climate, about 40 inches (1,000 mm) of rain fall—on average—on the Earth, somewhat more over the oceans, less (about 30 in. on average) over land. But these are averages, and the range of rainfall is enormous. Decades can go by in the desert without a single drop of water, but in Assam, on the slopes of the Himalayas, over 11 meters (430 in.) of rain fall in the year, most in the two months of the summer monsoon. In South America, annual average rainfall exceeds 60 inches, and over the Amazon basin it approaches 80 inches. In Hilo, on the windward side of the big island of Hawaii, annual rainfall tops 120 inches. But in Antarctica, average annual snowfall amounts to less than 7 inches of water.

A meter of rainfall over the entire globe—the annual average value—constitutes half a million cubic kilometers of water, enough to renew the oceans completely in a little less than 3,000 years. Water from heaven—from the skies of meteorology, not of astronomy—total rainfall over the continents (112,000 cu. km per year; see fig. 6.1) amounts to almost three times total river runoff. But at any given moment, the atmosphere contains only about 13,000 cu. km of water, most of it in fact as water vapor, i.e., H₂O molecules as a gas mixed with the rest of the air. The atmosphere loses about 1,000 mm of water in the course of the year (close to 3 mm on average per day), mostly in the form of rain. Such precipitation depends on the continual resupply of the atmosphere by way of evaporation of water from land and sea surfaces. The ratio of the two numbers (1,000 to 26) shows that the total atmospheric water content must be renewed some forty times over the year; on average, a water molecule spends only nine days in the atmosphere. Water passes rapidly and frequently through the atmospheric part of the water cycle, but this passage is essential to bring water to the land. By contrast, a drop of water in the ocean has a good chance of staying there for centuries, or even for thousands of years if dragged down to the deep sea.

FIGURE 6.1 Earth’s water, the water cycle, and elements of the water budget. Water stocks (see also table 6.1) in the different reservoirs are given in millions of cubic kilometers of liquid water equivalent. Fluxes of water (in italics) are given in thousands of cubic kilometers per year (1 cubic km per year is equivalent to 31.7 metric tons per second). The fluxes include evaporation, precipitation, runoff, and infiltration (not quantified here). The excess of precipitation relative to evaporation over the oceans makes possible the atmospheric transport of water from the oceans to the continents.

Clouds cover more than 60 percent of the surface of the globe, but at any given moment, compared with the amount of water vapor in the atmosphere, the total amount of water they contain in the form of liquid water droplets and/or solid ice crystals is minute. Nevertheless, practically all precipitation comes from clouds, mostly from a small part of the 60 percent cloud cover. Renewal of cloud water proceeds at an even faster pace than renewal of atmospheric water as a whole, with clouds continually forming, growing, and dissipating. Over the subtropical oceans, off the western coasts of the continents—Africa (Morocco, Angola), North and South America (California, Peru), and Australia—low clouds (stratus and stratocumulus) develop every night, disappearing in the course of the day, often without a single drop of rain. Atmospheric water vapor condenses, forming water droplets at the cloud’s bottom, but other droplets are continually evaporating at the cloud’s top. At night, condensation dominates and the clouds thicken, but by day, with the warming effect of the Sun’s rays, evaporation takes over. In France, from the Atlantic coast inland as far as Paris, morning cloud cover often dissipates in the course of the day. In the summer, further away from the ocean over midlatitude continental areas, over much of North America, and in Europe from Germany to Russia, fair-weather cumulus clouds develop in midday and afternoon as a result of land evaporation, and tend to dissipate (i.e., the water droplets evaporate) in the evening, again without bestowing any rain. These relatively thin clouds continually exchange water with the atmosphere but never contain more than a minuscule amount (less than a tenth of a millimeter water equivalent) at any instant. To produce rain, 3 millimeters per day on average, sometimes as much as a few tens or even hundreds of millimeters in a few hours, water must be supplied to form clouds carrying much larger quantities of water in liquid or solid form.

Maintenance of atmospheric moisture requires evaporation of about 3 millimeters of water per day on average over the globe. Obviously, such evaporation can only proceed where water is available—from the surfaces of lakes, rivers, and sea, from wetlands, vegetation, and, to a much smaller extent, from snow and ice. However, to extract an H₂O molecule from liquid water so as to make it a free molecule in air requires energy, to extract a molecule from the crystalline structure of ice or snow still more energy—energy generally in the form of heat: to transform a gram of liquid water into water vapor requires 540 calories (2,500 J or joules); and to vaporize (sublimate) a gram of snow or ice, 620 calories are needed. Evaporation of water from a surface uses up heat that would otherwise warm that surface, so in effect it keeps the surface cool. Thus, evaporation of our perspiration helps keep us warm-blooded animals from overheating when the temperature of the air rises. However, this only works if the air accepts the additional water vapor; when relative humidity gets close to 100 percent (in other words, when the air is nearly saturated with water vapor), evaporation becomes very weak, or rather, there is nearly as much condensation of water vapor on our skin as there is evaporation of perspiration from our skin. As a result, without air conditioning, sleep comes hard during the hot humid nights of the Tropics or of New York summers, when relative humidity sticks close to 100 percent and temperatures remain above 30°C (86°F). In general, evaporation from a surface is highest when that surface is moist and hot, the air dry, and when movement of the air (wind, convection, turbulence) removes water vapor from the immediate vicinity of the surface, mixing it with drier air farther off.

The H₂O molecules evaporated from a moist surface carry with them the “latent heat of evaporation,” the energy used to evaporate them. It is latent because the energy thus removed from the surface cannot disappear. When and at the place where the water vapor condenses, whether it be a cold surface, such as my eyeglasses in winter, or a cloud in the sky, that latent heat reappears as warming. Corresponding to the evaporation of 3 millimeters of water (on average) every day from the surface of the Earth, a flux of latent heat is removed from the surface and transferred to the atmosphere. In terms of energy, the flux amounts to 90 watts per square meter on average, a total of 45 million billion watts for the entire Earth, significant compared to other energy fluxes in the Earth’s atmosphere and between the Earth and space (see fig. 6.2). Apart from dew or frost forming on cold surfaces mostly at night, apart from fog or mist, the latent heat is generally released as part of condensation of water vapor in clouds, forming where moist air has ascended to higher cooler layers of the atmosphere. In principle, condensation occurs precisely at the level where the air temperature goes below the dew point, where the air becomes saturated and relative humidity reaches 100 percent. In reality, condensation usually occurs under “supersaturated” conditions, with relative humidity above 100 percent, i.e., at temperatures somewhat below the dew point. In any case, the latent heat released heats up the air, i.e., it intensifies the agitation of the molecules making up the air, made up mostly (99%) of nitrogen (N₂) and oxygen (O₂) molecules. Molecules moving at higher speeds effectively take up more space, i.e., the heated air expands and becomes less dense, and by buoyancy it tends to rise as does a montgolfière or hot-air balloon.

Evaporation and condensation recycle the atmosphere’s water forty times a year. Throughout the year, evaporation removes heat from the Earth’s surface, but on average, it does not freeze. Condensation deposits heat in the atmosphere, but on average, it does not indefinitely warm. Global warming or, on the contrary, ice age onset, generally require imbalance over decades. The water cycle accounts for the fact that the seas do not overflow, neither do they empty out, and that overall, condensation in and precipitation from the atmosphere make up the losses by evaporation, not locally but globally. The water cycle operates practically entirely within what may be called the “Earth system,” with neither significant inputs of water from outer space, nor significant losses. But the water cycle requires energy to make it go round, and to compensate the latent heat loss by the Earth’s surface. Energy is needed to keep the Earth warm, in particular its oceans. Closely coupled to the water cycle’s turning with a constant stock of water, Earth’s energy cycle involves a roughly constant flow of energy through the system, coming from the Sun and ultimately escaping to outer space.

FIGURE 6.2 The energy budget of the Earth-atmosphere system, with global annual mean energy fluxes given in watts per square meter. The horizontal (left to right) arrows correspond to conversion of solar radiation into heat in the stratosphere, troposphere, and at the Earth’s surface. Evaporation of water at the surface and its condensation in clouds gives the latent heat flux of 90 W/m² from surface to atmosphere.

Common sense tells us that the Sun keeps the world warm; and on this point, common sense is right. Without the Sun’s radiation, the world would not stay warm enough to keep water from freezing. The flux of heat from the interior of the Earth, only 0.09 watts per square meter on average, is nowhere near enough. The Sun sends our planet an average energy flux of 342 watts per square meter (W/m²), quite enough to supply the 90 W/m² needed to evaporate almost 3 mm of water per day. However, to keep the water cycle going in this way, the Sun’s radiation must reach the surface of the planet, without too much being lost in the atmosphere. Solar radiation consists mostly of visible light (and some near ultraviolet radiation) at wavelengths going from 0.3 to 0.8 micrometers (μm), and a fair amount of near infrared radiation (wavelengths from 0.8 to 4 μm), invisible but warming. In the absence of clouds, much of the solar radiation passes freely through the atmosphere, apart from some bands in the near infrared, absorbed by water vapor, and ultraviolet radiation near 0.3 μm absorbed by ozone. Scattering by the nitrogen and oxygen molecules that make up 99 percent of air does affect the shorter visible wavelengths, giving us the blue sky. As for clouds, they reflect and scatter back to space a significant fraction of the incident solar radiation, while at the same time allowing transmission of some sunlight down to the surface. Even for a completely cloud-covered sky, it rarely gets really dark when the Sun is up. Considering the effects of clouds as well as scattering and absorption by the atmosphere, not quite half of the incident solar radiation, about 150 W/m² (the exact figure being uncertain and a matter of lively debate among specialists) reaches the surface. Is that enough to account for the average surface loss by evaporation of 90 W/m² of latent heat flux?

At the very least, the surface temperature has to be kept above the freezing point of water. Indeed, the average temperature at the Earth’s surface is +15°C (59°F), or 288 K (degrees Kelvin on the absolute scale). Now any body warmer than absolute zero (0 K, −273°C, −459°F) emits radiation, and the warmer the body, the shorter the wavelengths of emission. At 500 K or 227°C (441°F), a poker heated in the fire is visibly red-hot, although in fact emitting mostly at longer wavelengths (0.8 to 6 μm) in the near infrared. The white-hot Sun, with temperatures ranging from 4,000 to 8,000 K in its photospheric layers, radiates strongly between 0.3 and 0.8 μm. We humans, warm-blooded animals with normal body temperature 98.6°F (37°C, 310 K), radiate in the medium infrared at wavelengths ranging from 3 to 30 μm. The Earth’s surface, not quite so warm at an average temperature of 288 K (+15°C), emits about 390 W/m² of infrared radiation upward, mostly at wavelengths from 4 to 40 μm.

How can the energy budget be balanced? It would seem (fig. 6.2) that the Sun supplies only 150 W/m² to the surface of the Earth, while the surface loses at least 480 W/m² by evaporation and upward radiation. What keeps the world warm? The answer is in the atmosphere, in that perfectly natural phenomenon, the greenhouse effect. The surface receives some 330 W/m² of infrared radiation coming downward from the atmosphere, absorbed and reemitted by the polyatomic molecules (molecules made of three or more atoms) in the more or less humid air. The most important of the “greenhouse gases,” water vapor (H₂O), absorbs and emits at infrared wavelengths between 5 and 7 μm and beyond 16 μm; carbon dioxide (CO₂) between 13 and 17 μm; and ozone (O₃) at 9.6 μm. Air is relatively transparent at “short” wavelengths (but not too short, certainly not in the ultraviolet), but it blocks a large part of the “long-wave” thermal infrared radiation emitted by the Earth. Air consists mostly of nitrogen (78%) and oxygen (21%), but the structure of these N₂ and O₂ molecules is much too simple for them to interact effectively with infrared radiation, and they contribute virtually nothing to the greenhouse effect.

Atmospheric water vapor plays the major role in the natural greenhouse effect that keeps the Earth’s surface warm enough for water to stay liquid. Absorbing a large part of the infrared radiation emitted upward by the surface, the atmosphere sends enough energy back down, again in infrared form, for the surface to continue to lose energy by radiation and evaporation. Surface evaporation in turn supplies water vapor to the atmosphere. Balancing the surface energy budget, the Earth’s surface loses on average another 20 W/m² in the form of “sensible” heat. What happens is that because the surface and the air immediately in contact with it are a bit warmer here, a bit cooler there, convection can set in, removing heat from the surface even when conditions are perfectly dry. Where the air is warmer and so less dense than average, it rises, while cooler less dense air descends to take its place. Even in the absence of wind, free convection can cool the surface, and when the wind blows, especially when it blows cold on a warm surface, convection and surface cooling are further strengthened (“wind chill”). When temperatures fall, we humans lose relatively little in the way of latent heat because our body mechanisms essentially turn off perspiration so that hardly any water evaporates from our skin. Of course, our breath contains water vapor, and when I’m out in bitter cold, the vapor freezes to form icicles on my moustache, and condenses as fog on my glasses. That can be a nuisance, but loss of sensible heat is a real danger when a strong cold wind blows away the layer of warmer air next to exposed skin. And when temperatures drop below −30°C (−22°F), as frequently occurs in winter in Alaska, Canada, or northern Russia, the air can sting your face, even without wind!

Energy budgets must balance, not only for the surface but also for the atmosphere. The atmosphere absorbs and so transforms into heat a small but nonnegligible fraction of the energy of solar radiation, a few tens of watts per square meter, the exact amount that gets through the atmosphere being a matter of debate. The Earth’s atmosphere emits 240 W/m² of infrared radiation to space,² and this rate of energy loss almost exactly compensates the rate at which solar energy is absorbed and converted into heat by the system (Earth with its oceans, ice, and atmosphere). Many specialists put a number on the planetary greenhouse effect by taking the difference between the flux of thermal infrared radiation emitted upward by the warmed surface (390 W/m²) and the flux (240 W/m²) of absorbed solar radiation that finally returns to space in the infrared; that gives a greenhouse effect of 150 W/m². The atmosphere also radiates about 330 W/m² of infrared downward to the surface. The natural greenhouse effect constitutes a recycling of absorbed solar energy in the form of infrared radiation downward from the atmosphere to the surface so that the surface can radiate upward more than the actual absorbed solar energy flux. The recycling results from the opacity of the atmosphere at the infrared wavelengths of the Earth’s thermal radiation. Without such recycling, the surface temperature corresponding to the absorbed solar energy would be a frigid −18°C (0°F), known to specialists as the “effective” temperature. Another common way of quantifying the greenhouse effect is to take the difference (33°C, 59°F) between the actual surface temperature and the effective temperature. It should, however, be noted that an Earth without an atmosphere providing a greenhouse effect would be an Earth without clouds, ice, or vegetation. Such an Earth would be as dark as our Moon, absorbing about 93 percent of the incident sunlight, reflecting only 7 percent. Like our Moon, its effective and average surface temperature would be close to 0°C (32°F, 273 K), still pretty cold.

In the Earth system, the water cycle operates in a closed circuit, with a practically constant stock (fig. 6.1). But the energy cycle (fig. 6.2) involves inflow of energy from the Sun, outflow to space. To maintain a living watery Earth despite the continual heat loss to space by way of thermal radiation, continual inflow of energy is essential, and the Sun supplies it. Without the Sun, Earth would be a cold frozen globe, at best with a few volcanic hotspots. But an Earth without water would not be more attractive, a dry Moon-like planet warming to torrid temperatures by day, cooling well below zero by night, with only the same volcanic hotspots to liven things up a bit.

The Sun warms the Earth with its radiation, furnishing a flux of energy worth 342 watts per square meter on average. This average energy flux is just the solar “constant” of 1,368 W/m² divided by four, and this division by four corresponds to the ratio of the Earth’s surface area 4πR² to its cross-section πR², R being the Earth’s radius. In reality, of course, the energy flux reaching the “top” of the atmosphere at any place on Earth varies between zero at night (and at any moment, night prevails over half of the globe), and a maximum value at local noon. That maximum value is the solar constant adjusted for the Sun-Earth distance at those locations where the Sun passes through the zenith. The absolute maximum occurs on January 9, when planet Earth goes through perihelion, i.e., through the point of its elliptical orbit closest to the Sun. On that date, 1,410 W/m² reach the top of the atmosphere at local noon along the parallel at latitude 23°27’ South. However, the Sun only passes through the zenith in the zone between the Tropics, i.e., between latitudes 23°27’ North and South, shifting from north to south from June to December and back again. That includes such places as Honolulu, Havana, Acapulco, San Juan (Puerto Rico), and Darwin (Australia). Outside the tropical zone, whether it be in Paris or generally in Europe, in North America north of the Rio Grande, in New Zealand, or in Melbourne (Australia), the Sun never goes through the zenith. Over the whole year, average solar radiant energy flux reaching the top of the atmosphere ranges from 140 W/m² at the North Pole (150 at the South Pole) to more than 400 W/m² at the equator. At the time of the winter solstice (June 21 in the South, December 21 in the North), no solar radiation at all reaches the zone of polar winter night beyond the Antarctic and Arctic Circles at latitudes 66°33’ South and North, respectively. In the winter hemisphere, average insolation over a 24-hour period ranges from zero to a maximum close to 400 W/m² near the equator, and the Sun does not reach the zenith. In the summer hemisphere, on the other hand, the Sun passes higher in the sky and stays longer above the horizon. Beyond the polar circle, it’s the land (or ice) of the midnight Sun. On the day of summer solstice, average insolation exceeds 400 W/m² over the entire summer hemisphere, and on December 21, at the South Pole, it reaches 560 W/m². Of course, only part of the incident radiation penetrates the atmosphere and is absorbed and converted into heat. Near the poles, ice, snow, and clouds reflect much of the incoming solar radiation back to space, and that magnifies the contrasts between the poles and the equator with respect to the solar energy actually available for keeping the world warm. Near the poles, average net solar energy input over the year is only 40 W/m²; while near the equator, eight times as much solar energy flux—over 320 W/m²—is converted into heat.

On average, the atmosphere contains the equivalent of a layer of 26 mm (a little over an inch) of water, but that water content, mostly in gaseous form (i.e., as water vapor), depends on continuous resupply by evaporation, ultimately depending on energy supplied by the Sun. The enormous variation of solar energy flux between winter and summer entails strong variations in atmospheric water. In the Northern Hemisphere, the average amount of water in the atmosphere water goes from 20 mm equivalent in winter to nearly 35 mm in the summer; and in the Southern Hemisphere, it goes from 21 mm in July to 28 mm in January. That may seem not very much, but for the Northern Hemisphere it amounts to a relative variation of 75 percent; in different terms, the atmosphere of the Northern Hemisphere contains 15 kilograms water per square meter (about 3 pounds per square foot) more in summer than in winter, altogether some 3,750 billion metric tons more for the entire hemisphere. By contrast, relative variation is only 33 percent in the Southern Hemisphere. The two variations do not compensate exactly, and for the entire globe the atmosphere contains more water in July and August than it does in December and January. The cause of the North-South difference? It’s not the Earth’s elliptical orbit, which brings the Earth closer to the Sun in January, making more radiant energy flux available then for heating. What accounts for the greater humidification of the atmosphere during northern summer, especially considering the greater area covered by ocean in the Southern Hemisphere? This apparent paradox can be resolved, considering that atmospheric layers near land surfaces undergo much stronger heating than over the sea. As a result, the larger land area in the North acts to warm the air more than in the South, and heating of the air determines how much water it can hold. Oceans of the North lose water to the atmosphere between June and September, oceans of the South between December and February. For the world as a whole, the oceans contain a little less water in July than in January, about 3 mm (⅛ in.), but what’s that considering how deep is the sea?

Between evaporation and precipitation, atmospheric water completely renews itself in a little over a week on average. The seesaw of humidity between north and south, driven by the astronomical cycle of insolation, depends both on the strong seasonal temperature changes over the continents and on water exchange between the oceans and the atmosphere. However, changes in air temperature necessarily also entail changes in density: at fixed pressure, higher temperatures imply lower density. Between April and June, air above the Northern Hemisphere’s continents gets progressively warmer and less dense while taking up more humidity; and dry air tends to migrate to the South. The effect is tiny, corresponding to a flow of less than 1.5 mm per second (about 18 ft. per hour, or 2 mi. a month); it reverses between August and October, and does not extend beyond tropical and subtropical latitudes. Even so, the total atmospheric mass above a hemisphere is slightly greater in winter than in summer. For the entire globe, the effect of the Southern Hemisphere’s winter is stronger, and the atmosphere is slightly more massive between June and August than from November to March. The total amount of dry air, nearly entirely nitrogen, oxygen, and argon, stays constant throughout the year, but because of the humidity variations, dry air must flow back and forth between north and south and between winter and summer. Because these atmospheric flows operate on very large scales, they affect the rotation of the Earth. The Earth’s spin slows when the atmosphere piles up above the equator and Tropics because that increases the moment of inertia of the Earth-atmosphere system. It speeds up, on the other hand, when a slight excess of atmospheric mass piles up around Antarctica during southern winter. Tiny as they may be, the corresponding fluctuations of the Earth’s rotation are accurately measured by astronomers.

What we call dry land, continually drained by the rivers that flow to the sea, would indeed dry out completely were it not for the return to the continents, by way of the atmosphere, of water from the oceans. On average, about 120 cm (47 in.) of water evaporate from the sea’s surface in a year, not completely compensated by the 107 cm or 42 inches of annual rainfall there. Most of the water evaporated from the sea’s surface does indeed return by a nearly direct route, and quite quickly, in a matter of days. Still, considering that oceans cover 70 percent of the Earth’s surface, the difference of 12 cm (5 in.) of water per year accounts for the 40,000 cu. km of water that flow in the rivers from land to sea. However, precipitation (rain and snow) over land, on average 75 cm (30 in.) of water per year, adds up to a volume of 112,000 cu. km of water, nearly three times as much (fig. 6.1). This comes about because water falling on the continents has nearly two chances out of three of being reevaporated before reaching the ocean. The atmospheric recycling of rainfall plays a particularly important role far from the sea in the interiors of the continents, and it depends to a large extent on vegetation. Plants use the water they pump from the ground in two different ways. By the process of photosynthesis, they transform some of this water into their own organic matter. However, they return larger amounts of water directly to the atmosphere through their stomata, microscopic openings on their leaf surfaces that serve also as points of entry and exit for carbon dioxide and oxygen in the processes of photosynthesis and respiration. The process of evaporative evacuation of water, analogous to our perspiration and called evapotranspiration, acts to maintain the plant’s temperature within tolerable limits. In the interior of the Amazonian basin, far from the Atlantic and cut off from water of Pacific Ocean origin by the high barrier of the Andes, the rain that falls consists of water several times recycled through vegetation since its evaporation from the Atlantic Ocean off the coast of Brazil. Each time water evaporates, the slightly heavier H₂O molecules containing heavy isotopes of either hydrogen (deuterium: D or H-2 rather than ordinary H-1) or oxygen (O-18 or O-17 rather than ordinary O-16) tend to get left behind. In the Amazonian interior—for example, at Leticia, where Colombia, Brazil, and Peru meet—rainwater contains a measurably lower proportion of the heavier isotopes, having been several times through evapotranspiration by vegetation since its extraction from the Atlantic. The same heavy water deficit occurs wherever the rain that falls is composed of multiply recycled water without direct supply of water evaporated from the sea. And that raises the question of the risk that deforestation, reducing the chances of recycling water by evapotranspiration, may lead to reduction of rainfall in the continental interiors. The answer is not simple. The analogous inverse reasoning (“Rain follows the plow”), more wishful thinking than wisdom, led many to invest their own perspiration in farming on semiarid lands of the western Great Plains in the United States. The topsoil blew off as dust at the first prolonged severe drought. That led to the dust bowl tragedy of the 1930s described in John Steinbeck’s novel The Grapes of Wrath (and the film), and in Woody Guthrie’s song “So Long, It’s Been Good to Know You.”

The planetary water budget may appear simple, but on closer inspection, many complications appear. For the globe as a whole, supply of water to the land depends on the slight excess of evaporation relative to precipitation over the seas, multiplied by the recycling of the rainfall over mostly vegetated land. The inspection of individual ocean basin budgets turns up some surprises. For example, the Mediterranean exports water vapor: rainfall there is quite small, but evaporation removes a meter (39 in.) of water a year. Even taking into account the flow from the Nile River and a few others, without water inflow from the Atlantic by way of the Strait of Gibraltar, the high rate of evaporation would be enough to dry out the Mediterranean completely in a few thousand years. And that did indeed happen six million years ago, when tectonic motions closed the opening to the Atlantic. In fact, only the Atlantic and the Indian Oceans export large amounts of water vapor. Although evaporation from the vast surface of the Pacific Ocean is enormous, still more rain falls there: 1,290 mm (over 50 in.) of precipitation per year against 1,200 mm of evaporation. The supply of water to Asia depends in part on the Indian monsoon, in part on some water of Atlantic origin managing to cross Europe without precipitating. No surprise then to find vast expanses of desert and semiarid land in Asia, especially in those areas of Central Asia where high mountain ranges block the penetration of the water-laden air of the Indian monsoon.