Human Causes of Climate Change

Human activities increase greenhouse gas levels in the atmosphere. Adding greenhouse gases to the atmosphere is like throwing another blanket on the Earth. The dominant man-made source of greenhouse gases is the burning of fossil fuels and biomass (living matter). This chapter also discusses other human activities that alter Earth's climate, including deforestation (the removal of trees), urbanization (the replacement of natural surfaces with impermeable man-made surfaces), and air pollution.


Like natural processes, human activities remove carbon from reservoirs where it has long been sequestered and send it into the atmosphere. Fossil fuel burning, the most dominant among these activities, releases CO2 that had been stored in the Earth for millions of years. Burning rain forest to create agricultural or ranch lands, a technique known as slash-and-burn agriculture that occurs in the tropics, releases the

CO2 stored in forests. Deforestation also decreases the amount of CO2 that is absorbed from the atmosphere by plants. Other types of vegetation, such as crops and grassland, are much less efficient at removing CO2 from the atmosphere.

Like CO2 levels, methane levels have been rising precipitously for the past century as human populations have exploded. About 60% of the methane that enters the atmosphere now comes from human activities. Rice production, which feeds a large percentage of the Earth's people, contributes the largest share of methane production. Another source, the gas passed by farm animals, may seem laughable, but its impact is highly significant due to the enormous increases in meat production in recent decades. Storing liquid manure in tanks at massive livestock factory farms causes more methane to enter the atmosphere (although dry manure sitting in a field does not). Methane also comes from decomposition in landfills, waste treatment, and the incomplete burning of rain forest materials.

Other greenhouse gas levels are also increasing because of human activities. Concentrations of ozone in the troposphere, where it is a pollutant and greenhouse gas, have more than doubled since 1976. Tropospheric ozone is created by the action of sunlight on nitrogen oxide and hydrocarbon pollutants such as the carbon- and hydrogen-based emissions from car exhaust. Nitrous oxides (NO and N2O) are themselves greenhouse gases that come from the burning of fossil fuels, forests, and crop wastes, and also from the manufacture and use of fertilizers.

Chlorofluorocarbons (CFCs) are extremely potent greenhouse gases in the troposphere. They also destroy the ozone layer in the stratosphere. CFCs are primarily responsible for the springtime ozone hole over Antarctica. Although CFCs were once widely used, production peaked in 1986, and the chemicals are now being phased out. Nevertheless, they will continue to act as greenhouse gases for several decades until solar radiation breaks them down in the atmosphere. Their current substitutes, HFCs, are also greenhouse gases, but they have less heat-trapping ability. Sulfur hexachloride (SF6) is an extremely potent greenhouse gas that is manufactured for industrial uses.


When people change the way they use the land, they may inadvertently alter their climate. The most dramatic example of this is the urban heat island effect, the phenomenon whereby urban areas are hotter than the surrounding countryside during the day and especially at night. There are two causes of urban heat island effect: The first is excess heat generated by the running of engines and given off by buildings, among many other sources. The second is the lower albedo of man-made surfaces, such as concrete and asphalt, when compared to natural surfaces. Man-made surfaces store the solar energy that strikes during the day and rerelease it into the atmosphere at night. This is why nighttime temperatures over cities have risen dramatically in the past few decades. For example, in the desert city of Phoenix, Arizona, the nighttime low temperature rose more than 10°F (6.5°C) between 1948 and 2005. The temperature differences between urban areas and the surrounding countryside also make the weather over cities more variable, with more storms and more dry spells.

Land use changes can alter climate in other ways. When forests are converted to farms and ranches, the rates of albedo and evapotranspiration are lower, and the balance of water and heat are thereby altered. Climate patterns are changed: Often the region becomes drier. Urbanization also changes albedo and lowers evaporation in the urbanized area; it reduces precipitation over the city but increases it downwind.


Air pollution influences climate in a number of ways. Particulates from fossil fuel burning are smaller and more abundant than natural particles in the atmosphere. Clouds are made when water vapor condenses around natural particles to form tiny water droplets. The droplets coalesce and, when they become large enough, fall from the sky as rain. Water vapor condenses around pollutant particles, but the particles are too small to coalesce and therefore remain scattered throughout the cloud. A polluted cloud may contain up to six times as many droplets as an unpolluted cloud, thereby reflecting incoming sunlight away from Earth. This phenomenon is called global dimming, which brings about a decrease in temperature.

The filtering of incoming solar radiation by polluted clouds has been seen everywhere around the planet. Between the 1950s and the early 1990s, sunlight was reduced 9% in Antarctica, 10% in the United States, 16% in parts of the British Isles, and nearly 30% in Russia. The most extreme reduction was found in Israel, which expe--rienced a 22% decrease in sunlight in a 20-year period that ended in the early 1980s.

Researchers suggest that global dimming has already had dire effects. The horrendous Ethiopian drought that culminated in 1984 and brought about one million deaths and upset 50 million lives, can be explained by global dimming. In Africa, under normal conditions, the summer Sun heats the air north of the equator, generating a low-pressure cell that sucks the summer monsoon rains into the Sahel, the semiarid region south of the Sahara desert. (Monsoons bring warm, moist air from the ocean onto the land, often accompanied by intense rains.) The summer monsoon is crucial because it is the only rain that countries in that part of the world, such as Ethiopia, receive. But for twenty years, in the 1970s and 1980s, the monsoon rains did not come into the Sahel.

Global dimming could have been behind this drought. In those decades, large amounts of pollutants from Western Europe and North America wafted over northern Africa, dimming the incoming solar radiation. A large enough dimming could have kept temperatures in the area from increasing enough for the air to rise and suck the monsoon rains northward. With no monsoon rains, the Sahel became drought-stricken.

The Ethiopian drought eventually abated. This can also be explained by global dimming. In the past two decades or more, Western Europe and North America have decreased their pollutant emissions by burning cleaner fuels and installing pollution control devices on smokestacks and exhaust systems. Because of this, fewer pollutants now drift over northern Africa, so the air there heats enough to rise and bring the monsoon rains into the Sahel. Additional pollution ien the Jets Were Ground

An amazing scientific experiment—one that could not have been performed under any other circumstance—came out of a national tragedy. David Travis of the University of Wisconsin, Whitewater, had been studying the effects of jet plane contrails on climate for 15 years. As shown in the photograph, he could see from satellite photos that contrails sometimes cover as much as 50% to 75% of the sky. He assumed that they had an effect on temperature, but he could not tell what kind and how much. Travis found the opportunity of his research career when, after the terrorist attacks on September 11, 2001, nearly the entire commercial air fleet of the United States was grounded. He knew that by studying the atmosphere in a state that it had never been in during his lifetime-contrail free—he could learn the most about the effects of contrails on climate.

Travis collected data from more than 5,000 weather stations all over the contiguous 48 states. What he discovered was startling: Like clouds, contrails moderate daily temperature swings. Contrails reduce the amount of solar radiation that hits the ground, making days cooler, and trap heat being reemitted from Earth, making nights warmer. During the three-day period in which the planes did not fly, there was an amazing 1.8°F (1°C) increase in temperature across the United States—the great

est single temperature effect ever seen. Travis notes that this large effect was due to just one source—other sources of pollutants were not restricted. Therefore, he reasoned, the effect of global dimming from all sources must be huge.

control measures could decrease or end global dimming regionally and globally. But the flip side of a decrease in this pollution could well be an increase in global warming.


Human activities are releasing greenhouse gases that have been sequestered in fossil fuels and biomass for millions of years back into the atmosphere. Man-made greenhouse gases such as CFCs that have never before existed on Earth are also being added. Changes in how people use the land—as when they change forests to farmlands or cities—also alter the composition of the atmosphere. Human activities cause climate to cool because atmospheric pollutants block sunlight. Still, scientists have found that global dimming has been counteracting some portion of global warming. As air pollution is reduced, the effect may likely be a further increase in global warming.

John Christy, an atmospheric scientist at the University of Alabama, Huntsville, and a former climate skeptic, stated in a 2004 interview with National Public Radio: "It is scientifically inconceivable that after changing forests into cities, turning millions of acres into farmland, putting massive quantities of soot and dust into the atmosphere and sending quantities of greenhouse gases into the air, that the natural course of climate change hasn't been increased in the past century."

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