Each element of the cosmos is positively woven from all the others…It is impossible to cut into this network, to isolate a portion without it becoming frayed and unravelled at all its edges.
— Pierre Teilhard de Chardin
The earth’s climate is created by the complex relationships that exist between the sun, atmosphere, water and land. Their interactions determine everything from temperature and precipitation to wind, humidity and atmospheric pressure — whether it is hot or cold, windy or calm, wet or dry.
Everything begins with the sun. When sunlight hits the earth, some of it is reflected back into space. But some is absorbed by the earth’s surface and turned into heat — heat that sets both the atmosphere and oceans in motion, triggering all of the earth’s climate systems.
Although the atmosphere has often been described as a blanket of air surrounding the earth, it is an extremely thin blanket — as thin, say, as the skin on an apple. It contains mostly nitrogen (78 percent) and oxygen (21 percent). Many other gases make up the remaining 1 percent. Some of these gases are only present in tiny amounts — parts per million (ppm) or even parts per trillion — but together they create a natural phenomenon called the Greenhouse Effect.
In a greenhouse, sunlight passes through the glass and is absorbed by the plants and soil, which then give off heat. The heated air would normally rise and be replaced by the cold air from above, but the glass prevents this, keeping the air inside the greenhouse warm.
Similarly, sunlight passes through the earth’s transparent atmosphere, where it is absorbed by the earth’s surface, converted into heat and emitted back into the atmosphere. Greenhouse gases trap some of this heat, and this heat warms the earth.
If there were no greenhouse gases trapping the outgoing heat, the earth’s surface temperature would be -18°C (0.4°F) — too cold for life as we know it.
When the atmosphere at the equator is warmed by the sun, the hot air rises and moves part way toward the poles. As the air rises and moves, it cools, becomes denser and sinks, eventually moving back toward the equator.
This movement of cold and hot air does not happen smoothly or evenly. The size and shape of land masses, the rotation of the earth on its axis, and countless other influences all affect the speed and density of the moving air masses and how they meet each other. When two air masses meet, changes in the weather occur.
The sun’s energy also heats the oceans, which cover 70 percent of the earth’s surface. The heated water sets ocean currents in motion the same way heated air creates the winds. Currents carry warm ocean surface water from the tropics toward the poles, while denser, colder deep water moves in to take its place. Contrasting temperature and salinity (which together determine density) keep the water masses separate.
The ocean currents move like a giant conveyor belt that circles the globe, carrying warm water from the equator up the east coast of North America and across the Atlantic as the Gulf Stream. When this warm water reaches the mid North Atlantic, it releases its heat into the atmosphere, cools, becomes denser and sinks. The deep cold water eventually flows back south, around the southern tip of Africa and into the Pacific, where it warms again and finally makes the return trip west and back to the Atlantic.
The giant loop is known as the thermohaline circulation (THC) — a massive, slow, steady movement of water that has a powerful influence on the world’s climate. Thanks to the Gulf Stream, for example, palm trees grow in Cornwall and southern Ireland, which lie almost at the same latitude as the southern tip of James Bay.
The temperature of the ocean waters also affects sea levels. In warm periods, sea levels rise because water expands as it warms and takes up more room than cool water. During cold periods, sea levels also go down because more of the planet’s water is frozen in glaciers and ice caps.
There is also a close and complex interaction between the earth’s bodies of water and the atmosphere. This interaction is called the hydrologic cycle.
Most of the earth’s water (97 percent) is held in the oceans. Three-quarters of the rest lies frozen in the polar ice caps and in mountain glaciers; most of the remainder is buried underground. Together, freshwater lakes and rivers and water vapor in the air make up only about one hundredth of one percent (.01 percent) of all the water on earth.
When the sun heats the oceans, lakes and rivers, the water evaporates and rises into the air as water vapor. As the air rises and cools, the water vapor condenses, releasing heat as it does. As it condenses it also collects on small particles (aerosols) that are suspended in the atmosphere. If there are enough drops of water vapor, clouds form. If the water drops become large enough, clouds become saturated, and the water falls to the earth as rain or snow, replenishing the world’s lakes, rivers and oceans, groundwater and ice caps.
At any given moment, clouds cover more than half the planet, and they affect the earth’s climate in many different ways. Some low gray clouds produce precipitation. Wispy, high clouds let in light but reduce the amount of heat that can get back out. And low-lying clouds have a cooling effect, because they trap essentially no heat, and they bounce the sun’s light energy back into space before it even reaches the earth.
The amount of water on earth does not really change. Instead, the same water is recycled over and over. But the hydrologic cycle does not work tidily, and the water seldom goes up and down in the same place. Winds can move clouds in unpredictable ways, and the water that evaporates off a lake can fall as rain a half a continent away.
The earth’s land areas also affect climate in different ways. Temperatures are more extreme, for instance, in the middle of large land masses. Mountains force the air that hits them to rise so that the water vapor condenses into clouds, which release their water as rain and snow.
The color of the land affects climate, too. Just as a black car sitting in the sun will feel hotter than a white one, dark-colored bare rock will soak up the sun’s rays and turn the energy into heat; such surfaces are said to have low albedo. Light-colored areas of ice and sand have high albedo. They reflect about 40 percent of sunlight back into space before it can warm the surface. Freshly fallen snow has the highest albedo of all, reflecting 80 percent or more of the light that hits it back into space before it can turn into heat.
The sun’s effect on the air, water and land creates the earth’s climate. But how are humans disturbing this natural system?
Humans are affecting climate by changing the balance of another vital natural cycle called the carbon cycle.
Carbon is a chemical element found in everything from plants and animals (one-fifth of the human body, for example, is carbon) to certain rocks and the air we breathe. Most of the planet’s carbon is buried in the earth’s crust as fossil fuels and embedded in rocks like limestone and chalk. Carbon is also stored in the earth’s oceans, soils and plants.
Carbon constantly circulates between the air, land, water and all the living things on the planet. Carbon (C) is introduced into the atmosphere as carbon dioxide (CO2). This happens naturally in a number of ways. Volcanoes bring up carbon from deep below the earth’s surface when they erupt. Limestone rocks erode slowly over time, releasing carbon into the air.
And all living things contribute to the carbon cycle when they breathe in oxygen, combine it with the carbon in their bodies, and breathe it out again as carbon dioxide. When plants and animals die, their bodies decompose and release their carbon back into the soil, air or water.
Much of the carbon released into the air is naturally recycled through the world’s vegetation and oceans, which pull carbon out of the atmosphere.
Plants take in carbon from the air and use it to make leaves and wood. All types of vegetation take in carbon, but young, growing forests, especially tropical forests (which grow year round), are the most efficient.
The oceans have a much larger capacity to take up carbon; they absorb one-third of the carbon dioxide that humans put into the atmosphere. Waves and other turbulence mix carbon dioxide from the atmosphere into the water. Some of the carbon dioxide is used by tiny marine plants (phytoplankton) for growth and by ocean animals to build their shells. When these plants and animals die, they sink into deep water where the carbon can be sequestered for hundreds or even thousands of years. And if these ocean bottom accumulations are buried in mud, they can be removed from the earth’s surface systems for millions of years.
Like all of the earth’s natural systems and cycles, the carbon cycle has been operating for millions of years. But this system is now being influenced by a new agent. Humans are pulling huge amounts of carbon fuels — oil, coal and natural gas — out of the ground and burning them, releasing large amounts of carbon dioxide into the atmosphere over a very short period. At the same time, we have been rapidly cutting and burning the earth’s forests, adding carbon dioxide to the air while reducing the forests’ ability to remove excess carbon from the air.
Carbon dioxide is now accumulating in the atmosphere twice as fast as the natural sinks can remove it, causing the planet to get warmer. And the effects of this warming are long-term and far-reaching.
29. Yahai Lu and Ralf Conrad, “In Situ Stable Isotope Probing of Methanogenic Archaea in the Rice Rhizosphere,” Science, August 12, 2005.
30. Michael Williams, Deforesting the Earth (Chicago: University of Chicago Press, 2003), 436.
31. Based on IPCC Second Assessment Report (SAR), Climate Change 1995: Impacts. Adaptations and Mitigation of Climate Change (Cambridge: Cambridge University Press, 1996), www.grida.no/climate/vital/32.htm.
32. National Geographic, May 1998; Canadian Centre for Policy Alternatives, CCPA Monitor, October 2004; J Jonathan Weiner, The Next One Hundred Years: Shaping the Fate of Our Living Earth (New York: Bantam, 1990), 182; Intergovernmental Oceanographic Commission, 2004; IPCC, Climate Change 2001: The Scientific Basis (Cambridge: Cambridge University Press, 2001), 188; John Houghton, Global Warming: The Complete Briefing, Third Edition (Cambridge: Cambridge University Press, 2004), 9.