By Robert Hazen, Ph.D., George Mason University
The Earth is a magnificent physical system. It’s a great collection of atoms constantly changing, reorganizing themselves, as the globe itself undergoes change. In a very broad sense, there are three significant geochemical cycles on the Earth. The primary cycles are the water cycle, the atmospheric cycle, and the rock cycle.
Though quite different in detail, the water cycle, the atmospheric cycle, and the rock cycle share four common characteristics.
First, all of these cycles involve the movement of matter between reservoirs. Second, all of these cycles are driven by energy: either from the Earth’s deep interior or from the Sun. Third, all these cycles can be altered by human activities; and finally, all these cycles are interrelated. They actually interact with each other.
The Water Cycle
Water has played, and continues to play, a unique role in the geological and the biological history of planet Earth. Earth’s water cycle—or the hydrological cycle, as it’s often called—deals with the reservoirs and the movement of water molecules, whether they’re in the liquid state, the solid state, or their gaseous state.
The Earth’s store of water is essentially unchanging: the same molecules are recycled over and over again.
We experience this cycle on different time scales. Part of the water cycle is experienced on a daily basis: weather, rain, evaporation off the ground, and so forth. But there are other parts of the cycle that take thousands of years, or millions of years. Let’s start with the oceans and think about the movement of water.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Moving Water in the Ocean
The principal movement of water out of the oceans is through surface evaporation: water evaporates off the surface into the atmosphere. Since the salts in the water don’t evaporate, the process then distills fresh water from the ocean’s saltwater. The Sun provides the energy for evaporation, which is especially rapid in the warm equatorial regions.
Now, ocean waters are layered. The deeper layers are generally colder, saltier and denser, and only on the surface, the shallower layer on the surface, do you get most of the evaporation. The deep water of the ocean is more or less locked in, so it’s the surface layer of the ocean that acts like a separate repository, a separate reservoir, isolated from the deep ocean.
Ocean waters can move very large distances around the globe through currents. There are surface currents that typically move warm water from the equatorial regions to the north, such as the Gulf Stream, which starts near the equator, but moves all the way up to the north and keeps Great Britain and that part of Europe much, much warmer than it would be otherwise. Deep currents take the cooler water from the north and circulate it deep within the oceans, back down toward the equator.
In this way, the oceans play a very major role in redistributing the heat of the Earth.
Learn more about ocean exploration.
The Importance of Rain
Now we have fresh water evaporating off the ocean’s surface. It’s transferred then, primarily by rain and snow, from the oceans to the land. Even though the atmosphere holds only 0.001 percent of the Earth’s water, it transfers most of the water in these kinds of processes.
The average residence time of water in the atmosphere is just a few days, so most of the rain falls back on the ocean; however, a significant amount falls on the land, and this is how we stock our lakes, rivers, and streams with water. Rain replenishes these repositories, and it may take hundreds of thousands of years, then, for that rainwater to circulate down and fill a groundwater repository.
So we have different time scales here: rain falls quickly to fill the lakes, but then to fill the underground repositories is a much slower period of percolation, the water gradually easing its way down through the porous rock layers.
Indeed, the system of streams and rivers continuously pours fresh water back into the ocean, and the cycle continues.
Learn more about the Earth’s surface.
Role of Ice in the Earth’s Water Cycle
Icecaps and glaciers also participate in a very important way in the water cycle. You have snowfall adding to the net budget of ice and snow, and you have melting and sublimation, which helps take away. The breakup of ice sheets, the formation of icebergs that then melt: all of this reduces the amount of ice, and there seems to be a balance in most periods of time.
These processes are slow on human time scales, but there can be major changes in the proportion of ice to oceans in geological history, and we sometimes can see changes that look rather significant.
In 1998, for example, there was an ice raft the size of Rhode Island that broke off the Antarctic ice shelf. That’s a pretty dramatic change; it doesn’t change the percentages very much, but still we notice that over geological time, you could have rather significant changes in the ratio of ice to ocean. In this regard, ice ages are particularly important. They represent part of the water cycle that spans millions of years.
Common Questions about How the Earth’s Water Cycle Works
First, the water, atmospheric, and rock cycles involve the movement of matter between reservoirs. Second, all of these cycles are driven by energy: either from the Earth’s deep interior or from the Sun. Third, all these cycles can be altered by human activities; and finally, all these cycles are interrelated as they interact with each other.
Ocean waters can move very large distances around the globe through currents. Surface currents usually move warm waters towards the North. In contrast, deeper currents take warmer water from the North and move it towards the equator. In this way, the oceans play a very major role in redistributing the heat of the Earth.
In Earth’s water cycle, when the water of the oceans evaporates, it moves into the atmosphere, which is responsible for transferring most water in the cycle. Falling rain then replenishes many water reservoirs like rivers, lakes, streams, and groundwater reservoirs.