(1) Air Pollution

Studying a Volcano to Understand Climate Change

We are in the middle of a large, uncontrolled experiment on the only planet we have.

Donald Kennedy

In June of 1991, after 600 years of slumber, Mount Pinatubo in the Philippines exploded. A huge amount of volcanic material blasted out of the mountain, sending a cloud of air pollutants and ash to a height of 35 kilometers (22 miles).

Avalanches of hot gases and ash roared down the sides of the mountain, filling valleys with volcanic deposits. It was the second-largest volcanic eruption of the 20th century. (The largest took place in Alaska in 1912.)

The eruption of Mount Pinatubo killed hundreds of people, destroyed homes and farmland, and caused hundreds of millions of dollars in damage. At the same time, it enabled scientists to test whether they understood the global climate well enough to estimate how the eruption would affect temperatures on the earth. By the late 1980s, most of the world’s climate scientists had become concerned that human actions, especially fossil fuel use, were enhancing the world’s natural greenhouse effect and contributing to a rise in the average temperature of the atmosphere.

Some stated publicly that such global warming was likely to occur and could have disastrous ecological and economic effects. Their concerns were based in part on results from computer models of the global climate. But were these models reliable?

Although their complex global climate models mimicked past and present climates well, Mount Pinatubo provided scientists with an opportunity to perform a more rigorous test of such models. Soon after the volcano erupted, James Hansen, a U.S. National Aeronautics and Space Administration (NASA) scientist, estimated that the Pinatubo explosion would probably cool the average temperature of the earth by 0.5 C° (1 F°) over a 19-month period. The earth would then begin to warm, Hansen said, and by 1995 would return to the temperatures observed before the explosion. His projections turned out to be correct.

To make his forecasts, Hansen added the estimated amount of sulfur dioxide released by the volcano’s eruption to a global climate model and then used the model to forecast how the earth’s temperature would change. His model passed the test with flying colors. Its success helped convince most scientists and policy makers that climate model projections-including the impact of human actions-should be taken seriously.

Hansen’s model and nineteen other climate models indicate that global temperatures are likely to rise several degrees during this century-mostly because of human actions-and affect the earth’s global and regional climates, economies, and human ways of life. To many scientists and a growing number of business executives, global climate change (a broad term referring to changes in any aspects of the earth’s climate, including temperature, precipitation, and storm activity) represents the biggest challenge that humanity faces during this century. The primary question is: What should we do about it?

An enormous cloud of air pollutants and ash rises above Mount Pinatubo in the Philippines on June 12, 1991. The volcano exploded in a catastrophic eruption, killing hundreds. Sulfur dioxide and other gases emitted into the atmosphere by the eruption circled the globe, polluted the air, reduced sunlight reaching the earth’s surface, and cooled the atmosphere for 15 months.

What Are the Major Air Pollution Problems?

Three major outdoor air pollution problems are industrial smog from burning coal, photochemical smog from motor vehicle and industrial emissions, and acid deposition from coal burning and motor vehicle exhaust.

The most threatening indoor air pollutants are in smoke and soot from wood and coal fires (in developing countries) and chemicals used in building materials and products (in developed countries).

The Atmosphere Consists of Several Layers

We live at the bottom of a thin layer of gases surrounding the earth, called the atmosphere. It is divided into several spherical layers, each characterized by abrupt changes in temperature caused by differences in the absorption of incoming solar energy.

We live at the bottom of a thin layer of gases surrounding the earth, called the atmosphere. It is divided into several spherical layers, each characterized by abrupt changes in temperature caused by differences in the absorption of incoming solar energy.

About 75–80% of the earth’s air mass is found in the troposphere, the atmospheric layer closest to the earth’s surface. This layer extends to about 17 kilometers (11 miles) above sea level at the equator and to8 kilometers (5 miles) over the poles. If the earth were the size of an apple, this lower layer containing the air we breathe would be no thicker than the apple’s skin.

Take a deep breath. About 99% of the air you inhaled consists of two gases: nitrogen (78%) and oxygen (21%). The remainder is water vapor (varying from 0.01% at the frigid poles to 4% in the humid tropics), slightly less than 1% argon (Ar), 0.038% carbon dioxide (CO2), and trace amounts of dust and soot particles and other gases including methane (CH4), ozone (O3), and nitrous oxide (N2O).

The troposphere is a dynamic system involved in the chemical cycling of the earth’s vital nutrients. And its rising and falling air currents and winds are largely responsible for the planet’s short-term weather and long-term climate.

The atmosphere’s second layer is the stratosphere, which extends from about 17 to 48 kilometers (11–30 miles) above the earth’s surface. Although the stratosphere contains less matter than the troposphere, its composition is similar, with two notable exceptions: its volume of water vapor is about 1/1,000 that of the troposphere and its concentration of ozone (O3) is much higher.

Much of the atmosphere’s small amount of ozone (O3) is concentrated in a portion of the stratosphere called the ozone layer, found roughly 17–30 kilometers (11–19 miles) above sea level. Stratospheric ozone is produced when oxygen molecules interact with ultraviolet (UV) radiation emitted by the sun. This “global sunscreen” of ozone in the stratosphere keeps about 95% of the sun’s harmful UV radiation from reaching the earth’s surface. The UV filter of “good” ozone in the lower stratosphere allows us and other forms of life to exist on land and helps protect us from sunburn, skin and eye cancer,

cataracts, and damage to our immune systems. It also prevents much of the oxygen in the troposphere from being converted to photochemical ozone, a harmful air pollutant.

The earth’s atmosphere is a dynamic system that consists of four layers. The average temperature of the atmosphere varies with altitude (red line). Most UV radiation from the sun is absorbed by ozone (O3), found primarily in the stratosphere in the ozone layer 17–26 kilometers (10–16 miles) above sea level.

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