Science Matters (40 page)

Emission of these pollutants can cause serious environmental problems. Urban air pollution occurs when sunlight hitting nitrogen compounds and hydrocarbons in the air triggers chemical reactions that in the end produce ozone. And whereas ozone in the stratosphere is essential to life on Earth, ozone at ground
level is a caustic gas that stings and can damage the human respiratory system. This “bad ozone” is a major product of modern urban air pollution associated with smog—the brownish stuff that you often see over major cities during the summer.

Urban air pollution can be a serious problem, but it is also often a transitory one. If the air quality in a city declines, then people can be alerted about it immediately as part of the weather forecast. And just as with the weather, the intensity of air pollution varies on a daily basis and can change swiftly with the arrival of a thunderstorm or stiff winds.

Acid rain is a much longer-term problem associated with nitrogen and sulfur compounds, which interact with other atmospheric chemicals to form tiny droplets of nitric and sulfuric acid. When it rains, these droplets of acid wash out and they become, in effect, a rain of dilute acid rather than water. You can see dramatic effects of acid rain in many European cities, where great historical monuments made of limestone are particularly susceptible to the effect of acid. Over the years, the acid rain simply dissolves the fabric of the building.

In the mid-twentieth century, local adverse effects of pollution were often dealt with by the construction of tall smokestacks, particularly in the industrial parts of the Midwest. The effect was to put the pollutants high enough in the atmosphere to be taken away by prevailing winds. But that approach didn’t really solve the problem; it merely displaced the acid rain to the forests of New England.

Governments have responded to problems of urban air pollution by regulating tailpipe emission from vehicles and requiring large industrial plants to use available technology to scrub gases before they are released into the atmosphere. It is not unusual for the equipment needed to clean emissions from a coal-burning electrical plant to cost more than the equipment used to generate
the electricity itself. In the future, if plug-in hybrid and (perhaps) all electric cars come into widespread use, the electricity needed to run those cars will come from plants with this sort of equipment. It is, after all, easier to clean one smokestack than tens of thousands of individual tailpipes.

Acid rain and air pollution are moderate environmental problems. We understand the sources and consequences of the pollution, and we know what has to be done to prevent the pollution. The costs of dealing with these problems, however, are much greater than those of reversing the ozone layer depletion. Political and economic questions become very important. How much are we willing to pay for clean air? How many midwestern jobs are we willing to lose to save New England forests? These are not easy questions, nor can they be answered by science alone.

Although it is possible to talk about removing materials that cause acid rain from smokestacks or exhaust pipes, or even converting to fuels that do not produce those materials, one product must inevitably be produced whenever we burn a fossil fuel: carbon dioxide. Whenever you drive a car, cook food, or use an electric light, chances are that you are adding carbon dioxide to the atmosphere.

This addition of carbon dioxide to the atmosphere gives rise to what scientists call the greenhouse effect. In a greenhouse (or even a car left in the open with the windows rolled up), sunlight passes through the glass and is absorbed by materials on the inside. The heated material then gives the energy back in the form of infrared radiation, but the glass is opaque at infrared wavelengths, so the energy remains trapped, warming the interior of the greenhouse or car. Like glass, carbon dioxide transmits
visible light coming in from the sun, but absorbs infrared radiation that rises from the ground and holds this heat in the atmosphere instead of reflecting it back into space. The term “greenhouse effect” as applied to planet Earth refers to the possibility of global warming due to the accumulation of carbon dioxide from the massive burning of fossil fuels that has taken place since the beginning of the industrial revolution.

Several points should be made about the greenhouse effect. First, there has always been carbon dioxide in the atmosphere, so we are not introducing a totally new substance to the environment. In fact, without the greenhouse effect that arises from naturally occurring carbon dioxide and other gases (notably water vapor and methane), the temperature of Earth’s surface would be about 20 degrees below zero! Nevertheless, the dramatic increase in atmospheric carbon dioxide produced by human activities over the past century, and associated changes in global climate, are the main greenhouse concerns today.

Scientists agree on four key points:

1. Carbon dioxide absorbs infrared radiation and acts as a greenhouse gas, as do other molecules such as methane, water, and ozone.
   
2. Burning fossil fuels by human beings has increased the amount of carbon dioxide in Earth’s atmosphere, and recent studies suggest that the rate of increase is accelerating.
   
3. The climate of our planet is changeable. Today we are living in a period of warming that began in about 1850, following a cool period called the Little Ice Age, which in turn followed a centuries-long era known as the Medieval Warm Period. Since 1850, the average global temperature has increased by approximately 0.5°C, and evidence suggests that at least part of this warming is due to human activities.
   
4. Average global temperatures increased significantly during the past several decades, with the 1990s being the warmest decade on record, and twenty of the twenty-five warmest years in recorded history occurring since 1980. Furthermore, 2004 and 2005 were among the four warmest years on record—at least a full degree warmer than the thirty-year average. Several individual months during that period also set historic records, so there can be little doubt that Earth has become warmer in recent years. On the other hand, satellite data shows little increase in warming since the late 1990s.
   

In spite of these stark facts, uncertainties persist about the rate and extent of global climate change. The primary scientific tools for predicting future climate are enormously complex computer codes called global circulation models (GCM). These models work by splitting the atmosphere and the oceans into boxes a few hundred kilometers on a side, calculating changes in each box, and then adding up the effect of all of the boxes. On a short time scale, these types of programs are used to generate daily weather reports. When they are applied to climate pre dictions (which involve weather a hundred years in the future rather than the weather a few days from now), they require an understanding of literally hundreds of processes on Earth, from the formation of icebergs to the details of ocean chemistry, and can take months to run on the fastest computers available. This is not a criticism; after all, it is what you would expect from a computer model designed to represent a complex and multifaceted system like Earth’s climate. It is important to realize, however, that the complexity of the GCM and our current imperfect understanding of many of the effects we have to put into them give rise to uncertainties in the predictions that come from the models. Just because something
comes from a computer doesn’t mean it’s true, any more than seeing words in print guarantees their veracity.

In particular, we can identify a number of areas that contribute to the uncertainty of GCM predictions.

• Clouds
  . Clouds play an important but complex role in determining climate. Low-lying clouds reflect sunlight, thereby cooling the surface, while high clouds trap heat, which contributes to warming. Thus, it is not only the amount of cloud cover that we have to be able to predict, but the type of cloud as well. At present, we simply do not understand the physics and chemistry of cloud formation well enough to predict these sorts of things from first principles.
  
• Oceans
  . Far more carbon dioxide is dissolved in the world’s oceans and their sediments than in the atmosphere, and there exists a complex and imperfectly understood interaction between these two reservoirs. It is thought that even small changes in ocean currents can have an effect on atmospheric carbon dioxide.
  
• The Sun
  . Since the sun is the primary driver of climate circulation, even small changes in solar output can be expected to have an effect on global warming and cooling. Some scientists estimate, for example, that from 1645 to 1715 the sun’s output was about 1 percent less than it is today, and this period corresponded to the coldest part of the Little Ice Age. GCM calculations suggest that little of the current warming trend can be attributed to direct solar heating, but cannot rule out more complex interactions. Some scientists suggest, for example, that changes in the sun’s magnetic field may affect Earth’s cloud cover by influencing the cosmic rays whose interactions provide condensation nuclei in the atmosphere.
  
• Ice
  . The extent and distribution of ice, as well as its movement in glaciers and ice shelves, has an as yet poorly understood effect on climate. For example, ice and snow reflect the sun’s radiant energy back into space, whereas darker rocks and soils more effectively absorb this energy. Might melting ice—for example, in Greenland—accelerate global warming?
  

Standard practice in the climate change debate is to calculate Earth’s temperature for the case in which atmospheric carbon dioxide doubles. The Intergovernmental Panel on Climate Change (IPCC), which collates the calculations of hundreds of scientists around the world and issues reports every few years, suggested in 2007 that Earth’s average temperature will increase by about 2°C over the next century.

The GCM have not yet progressed to the point where we can make detailed regional predictions about what this sort of warming would entail, but some of the consequences could be coastal flooding, the spread of tropical diseases northward, and major changes in rainfall patterns, which would have profound (and as yet unpredictable) effects on agriculture. In addition, the rapid change in climate can be expected to have a negative impact on many ecosystems and drive many species to extinction.

But how do we reverse the trend? Any effort to reduce drastically the consumption of fossil fuels will cost consumers money and will require significant changes in lifestyle as well. Carbon-based fuels are the energy source for automobiles, jet planes, ships, and most of our electric power plants. Finding ways out of this dilemma is not easy.

A few obvious measures are already being tackled. We can attempt to slow the rapid destruction of rain forests in the Amazon while we are planting new forests elsewhere. Living trees pull carbon out of the air and store it in their tissues, thereby helping
to cancel the effect of burning fossil fuels. We can implement new ways to conserve energy while we increase our reliance on such renewable energy sources as wind, solar, and hydroelectric power. We can develop new, more efficient forms of biofuels that balance the carbon dioxide of burning fuel with new plant growth. Efforts in all of these promising areas are being vigorously pursued around the world.

In spite of such promising efforts, the greenhouse effect is the most difficult and alarming of the many environmental problems that face the global ecosystem. On the one hand, it is the most difficult to model because the effects of adding carbon dioxide to the atmosphere are uncertain, while the cost of doing something about it is high. It’s unrealistic to think that you can take the world economy, which runs almost entirely on fossil fuels, and change it over to other sources of energy in a very short period of time. It typically takes thirty to fifty years for a new fuel to work its way into the economy. If the more disastrous predictions of greenhouse warming are true, in about fifty years the warming will already have occurred, and it will be too late to do anything about it.

What’s more, the best scientific estimates suggest that warming will not cause severe environmental changes for several decades, far beyond the planning horizon of corporations, governments, and society’s other major institutions. The question boils down to this: Are you willing to change your lifestyle now because it’s likely that global warming will adversely affect the lifestyles of your grandchildren?