Science Matters (29 page)

The thin upper skin of the sea, called the mixed layer, is special. Heated by the sun and well mixed with atmospheric gases, it teems with life, from microscopic plankton and algae to giant fish and sea mammals. In contrast, the deep ocean is dark and dense, subject to tremendous pressure and chilled to only a few degrees above freezing. This is why shallow waters like those of the continental shelves, the famous fishing grounds of Georges Bank in the North Atlantic, and Chesapeake Bay produce so much seafood. The deep ocean, on the other hand, is a kind of desert, inhospitable and spare of life.

The deepest ocean waters circulate slowly and can spend many thousands of years in the dark void. Much of the densest, deepest ocean water comes from melting Antarctic ice; that ice water descends to the bottom, spreads out, and slowly rolls across the ocean floor to places as far away as the Bering Strait of the North Pacific.

Virtually all interactions between the oceans and other parts of the water cycle take place at the ocean surface. Rivers and rain add to the top layer, while surface evaporation returns water to the air. Surface waters also provide coastal areas with a thermal buffer, moderating air temperatures during the coldest and hottest months.

At times in its history Earth has had as much as 5 percent of its water budget tied up in ice caps and glaciers, at other times as little as a fraction of a percent. The amount of ice depends in a complicated way on the positions of the continents, and in a regular and predictable way on variations in Earth’s orbit around the sun and the direction of Earth’s axis of rotation. At present, during a period of moderate temperatures, about 2 percent of the globe’s water (about three-quarters of all freshwater) is frozen.

The largest concentration of ice on Earth today is in the thick glaciers that cover the south pole. Such large ice caps can accumulate only if a continent covers one of the poles, giving a base of solid ground to support thick ice. Otherwise, we have a situation like the one at the north pole today. As the northern ice cap starts to build up, the weight of new ice pushes old ice deeper into the water, where it melts because of the higher pressure. Thus, without a solid support, an ocean ice cap can never be more than a few hundred yards thick, and can never contain more than a tiny fraction of Earth’s water. For three-quarters of Earth’s history, there were no continents at the poles, and hence no large ice caps. Occasionally there have been continents at both poles, a condition which probably led to more water being locked up in glaciers than at present.

Scientists have trouble predicting continental wandering, not to mention the effects of those movements on glaciation, but purely astronomical effects on ice caps are easy to predict. The most important effect involves the tilt of Earth’s axis of rotation, which now leans 23 degrees off the axis of Earth’s orbit. Northern and southern hemispheres have opposite seasons at present because when the northern hemisphere is tilted toward the sun
(in summer), the southern hemisphere is tilted away. In general, any effect that makes summers cooler contributes to glaciation. The reason is simple: If summers are cooler, ice and snow will stay on the ground longer in Canada and Siberia. This ice and snow will reflect sunlight, further lowering the temperature and allowing still more ice and snow to remain on the ground the next year. The result: over a period of a few thousand years, large sheets of ice spread out from the pole and the high mountains and cover large parts of Europe and North America. When this happens, we say there is an ice age.

The most recent ice age was in full swing 20,000 years ago, when glaciers extended as far south as Chicago. Today we live in what geologists call an interglacial, a term we find chilling in every sense of the word. On a shorter time scale, chance events also affect the glacial cycle. Major volcanoes can spew dark matter into the atmosphere, blocking sunlight and cooling the planet for a year or two, which creates brief periods of growing glaciers. Increased atmospheric concentrations of carbon dioxide and other greenhouse gases, whether man-made or natural, may have the opposite effect. The best prediction available now is that we are (or should be) heading into another period of glaciation, although the advent of global warming by the greenhouse effect may introduce a temporary glitch in the grand cycle of freezing and melting.

Earth’s glacial cycles are of more than abstract interest. If more water is tied up in glaciers, less remains to fill the oceans and sea levels will fall. During the last glaciation, for example, the eastern coast of North America was 150 miles farther east than it is today, and on the west coast an ice and land bridge existed between Siberia and Alaska. Anthropologists believe this bridge allowed the ancestors of the Native American Indians to reach this continent. Conversely, geological evidence also points
to warm interglacial periods, some within the past hundred thousand years, when oceans were 100 feet higher than today. To see what such a sea level change would mean, imagine the present New York and Los Angeles waterfronts under 100 feet of seawater.

The underlying principle of the science of hydrology, which concerns itself with the study of the water cycle, is that water is a mobile resource. It flows by gravity from high to low, thus shaping the land and playing a central role in the rock cycle. It evaporates from land and sea into air, forming clouds and playing a major part in the weather cycle. It falls as rain on the land, filling reservoirs with fresh water and providing the chemical medium for life.

Only a small fraction of Earth’s liquid resources are available to land plants and animals as freshwater. Rainwater that falls to land can follow many different paths. Some water penetrates the soil, some enters lakes, ponds, and streams, some evaporates quickly and returns to the clouds, but most (as much as
99
percent of the total) becomes part of the vast underground reservoir of groundwater. Groundwater accumulates in aquifers—porous rocks such as sandstone—where continuous networks of tiny spaces between mineral grains form huge reservoirs. Water soaks in wherever the aquifer is exposed at the surface, and the reservoir fills by the force of gravity. Aquifers, bounded top and bottom by impervious rock layers, can be tapped by deep water wells. It can take many thousands of years for an aquifer to fill up with water, so tapping that water is analogous to mining a mineral deposit. Throughout the American West, wells are going dry as the supply of stored water is drained. The fact that they
will be replenished in a few thousand years is of scant comfort to ranchers and farmers.

Humans, who depend on the natural water cycle for their survival, affect the cycle in many ways. We divert streams for crop irrigation, the largest single use of water. We dam rivers for hydropower. We create artificial lakes for water storage and recreation. We use flowing water to purge unwanted chemicals from our factories and sewage from our homes.

Until fairly recently, humans regarded freshwater as an inexhaustible resource, one that could be used with little regard for long-term consequences. To some extent this attitude was justifiable, since some aspects of the cycle are resilient. Evaporation can purify water in a relatively short time. We can clean up polluted rivers, streams, ponds, and coastlines in a few years, as newly evaporated water replaces the old, polluted fluid. But other problems are less amenable to short-term solutions. Contaminated groundwater may remain polluted for decades, thus diminishing our stock of potable fresh water at the same time that growing populations demand more. Politicians, as well as scientists, now realize that humans are an important part of Earth’s water cycle.

People instinctively distinguish three principal atmospheric cycles. Weather is the short-term cycle, somewhat unpredictable and almost always different from the “average” weather for a particular day or month. The longer-term cycle related to Earth’s movement around the sun we call the seasons, and we use them to number human lives and accomplishments. On a much longer time scale we recognize the cycle of climate. The climate of a
region changes much more slowly, though many scientists think we are entering a period of relatively rapid climate change as a result of human activities (see Chapter 19). But even under normal circumstances, the passage of several generations may be sufficient to alter the severity of winters, to turn productive farmland into desert, or to transform swamps into firm ground.

Weather, seasons, and climate all involve Earth’s atmosphere, an envelope of gas surrounding our planet that is in its way as complex as the oceans and the crustal rocks. To understand the weather you must understand how the atmosphere is constructed. The atmosphere behaves in many ways like the solid Earth and its oceans. Like hot rocks in the mantle or currents in the sea, air circulates. And like the solid Earth and the oceans, the atmospheric system has layers that differ in temperature and pressure.

The troposphere is the warm layer of air next to Earth’s surface. It extends up about 40,000 feet and provides the expressway for commercial jet travel. The troposphere, high enough to cover Mount Everest, contains most of the cloud systems we see from Earth, but large thunderstorms often produce clouds that stick out above it. Above the troposphere lie successive layers called the stratosphere (to 150,000 feet), mesosphere (to 260,000 feet), and the ionosphere, each of which plays a role in the overall behavior of the ocean of air in which we live.

Convection, the same mechanism that drives plate tectonics, causes weather in the near surface layers. The lower atmosphere is a giant convecting system, powered by solar energy and wrapped around the turning planet. Most of the sun’s energy arrives near
the equator, where warm air expands and rises. On a nonrotating planet, rising air would circulate from equator to pole in high atmospheric currents, eventually to cool and descend near the poles. The surface flow of air would travel from pole to equator. In the northern hemisphere of such a world, wind and weather would usually come from the north.

Earth’s 24-hour rotation complicates this picture by stretching the simple north-south convection cell into three large east-west convection cells. Surface winds blow west to east in the temperate regions of the northern and southern hemispheres, creating the prevailing westerlies. One of these cycles dominates the west-to-east weather pattern characteristic of most of the United States. In equatorial regions the pattern is reversed with east-to-west easterlies, such as the Atlantic Ocean’s trade winds that provided sailors a speedy voyage from Europe to the Americas. Stagnant air masses, such as the doldrums at the equator, form in between these major air currents.

No Earth system is more carefully monitored than the lower atmosphere. Radio and TV news programs provide frequent updates on present values and probable changes in several key atmospheric properties: in particular temperature, pressure, humidity, wind speed and direction, and pollution. Other terms—wind chill factor and comfort index, for example—combine two or more of these basic variables to indicate how good or bad it feels to be outside.

Temperature variations in space and time are often extremely complex. Not only does the temperature rise and fall on a daily and seasonal basis, but temperature also varies by dozens of
degrees with altitude. Forecasters report surface temperatures, yet much of the weather action—rainfall, for example—depends on the very different temperatures of upper-level air.

Barometric pressure is the weight of all the air overhead—roughly fourteen pounds per square inch (which is the same weight exerted by a column of mercury about 30 inches high at sea level). On a planet with perfectly still air the pressure would hardly vary at all. But on our world large air masses form vast circular currents, which pile up air at their margins (high-pressure zones, or highs) and suck air out of their centers (low-pressure zones, or lows).

Humidity is a relative term, reported as a percentage. The amount of water vapor that air can retain without producing fog or rain varies greatly, and depends on air temperature. On a 90-degree day the atmosphere can hold several percent water by weight, while in midwinter New England air holds less than half a percent water. The relative humidity is a measure of how much water the air actually holds, compared to how much water the air can absorb. When the relative humidity is high, perspiration cannot evaporate easily from your skin and you feel uncomfortable. As the temperature drops, the air’s ability to hold water decreases, and water condenses out in droplets. That’s why water condenses on the outside of your iced drink glass even on relatively dry summer days.

Wind speed and direction in North America depend on many factors that modify the general west-to-east flow. Topography, the location and temperature of large bodies of water and the distribution of pressure highs and lows all play important roles. In recent years it has become fashionable in some television weather circles to provide fancy computer graphics showing the meandering path of the jet stream. Jet streams are high-altitude wind currents that behave like fast-flowing rivers of air about
eight miles high. Roughly speaking, the jet stream divides cold northern air from the warmer air masses in temperate regions. The general jet stream trend is always west to east, but like rivers they adopt sinuous paths, changing position and speed on a daily basis, influencing and being influenced by the location of surface highs and lows. Because of their great speed, often greater than 100 miles per hour, jet streams may affect aircraft operation and scheduling. Flights from the East Coast to California typically take an hour longer than the return because of this effect.

Natural and man-made pollutants have become a fixture of weather reporting. Concentrations of airborne pollens, especially prevalent during spring and summer months, are described by an arbitrary scale of particles per given volume of air. Most city weather reports include similar statistics on smog, a collective term for a variety of man-made pollutants.