2. Global Warming

Heating by the greenhouse effect

Our planet’s surface is now kept at a comfortable temperature because the atmosphere traps some of the radiant heat from the Sun and keeps it near the surface, warming the planet and sustaining living creatures. Jean Baptise Joseph Fourier (1768-1830) first conceived the mechanism in the 1820s, while wondering how the Sun’s heat could be retained to keep the Earth hot. Fourier’s idea, still accepted today, is that the atmosphere lets some of the Sun’s radiation in, but it doesn’t let all of the radiation back out. Visible sunlight passes through our transparent atmosphere to warm the Earth’s land and oceans, and some of this heat is reradiated in infrared form. The longer infrared rays are less energetic than visible ones and do not slice through the atmosphere as easily as visible light.

So our atmosphere absorbs some of the infrared heat radiation, and some of the trapped heat is reradiated downward to warm the planet’s surface and the air immediately above it. Fourier likened the thin atmospheric blanket to a huge glass bell jar, made out of clouds and gases, that holds the Earth’s heat close to its surface.

The warming by heat-trapping gases in the air is now known as the “greenhouse effect”, but this is a misnomer. The air inside a garden greenhouse is heated because it is enclosed, preventing the circulation of air currents that would carry away heat and cool the interior. Nevertheless, the term is now so common that we will also sometimes designate the heat-trapping gases as greenhouse gases, and let greenhouse effect designate the process by which an atmosphere traps heat near a planet’s surface.

Right now, the warming influence is literally a matter of life and death. It keeps the average surface temperature of the planet at 288 degrees kelvin (15 degrees Celsius or 59 degrees Fahrenheit). Without this greenhouse effect, the average surface temperature would be 255 degrees kelvin (-18 degrees Celsius or 0 degrees Fahrenheit); a temperature so low that all water on Earth would freeze, the oceans would turn into ice and life, as we know it, would not exist.

The gases that absorb the most infrared heat radiation are minor ingredients of our atmosphere. They are water vapor and carbon dioxide, with water vapor absorbing the most. Sixty to seventy percent of the Earth’s greenhouse warming is now caused by water vapor and carbon dioxide provides just a few degrees.

The main constituents of the atmosphere, nitrogen (77 percent) and oxygen (21 percent) play no part in the greenhouse effect. The two atoms in these diatomic molecules are bound tightly together and are therefore incapable of absorbing significant infrared radiation. In contrast, water vapor and carbon dioxide molecules consist of three atoms that are less constrained in their motion, so they absorb the heat radiation.

Why doesn’t the atmosphere just keep heating up until it explodes? The greenhouse warming rises to a fixed temperature that balances the heat input from sunlight and the heat radiated into space. The level of water in a pond similarly remains much the same even though water is running in one end and out the other.

Humans are pumping increasing amounts of carbon dioxide into the air

Fig. .. 

For hundreds of years, humans have been filling the sky with carbon dioxide. The invisible waste gas is dumped into the air by burning fossil fuels – coal, oil and natural gas. When these materials are burned, their carbon atoms, denoted C, enter the air and combine with oxygen atoms, O, or oxygen molecules, O2, to make carbon dioxide, abbreviated CO2.

Every time we drive a car, use electricity from coal-fired power plants, or heat our homes with oil or natural gas, we release carbon into the lower atmosphere. The burning of forests, whose trees hold much carbon dioxide, has also contributed.

Just a few decades ago, no one knew if any of the carbon dioxide stayed in the atmosphere or if it was all being absorbed in the oceans. Then in 1958 Charles D. Keeling (1928-) began measurements of its abundance in the clean air at the Mauna Loa Observatory in Hawaii. It is located at a remote high-altitude site in the midst of a barren lava field, far from cars and people that produce carbon dioxide and from nearby plants that might absorb it.

The sensitive measurements showed that the amount of carbon dioxide in the atmosphere increases and decreases in an annual cycle. Every spring plants bloom, sucking CO2 out of the air, and every fall CO2 is released back into the air as plants either decay or lose their leaves. The measurements had recorded the breathing of the plants all over the Northern Hemisphere.

But more importantly, Keeling’s measurements showed that humans are also changing the composition of the atmosphere. Superimposed on the annual fluctuations, there was a systematic increase over the entire period of observation, continuing nonstop since 1958 (Fig. GW.1). Year by year the total measured concentration of carbon dioxide grew, as inexorably as the expansion of the world’s population and human industry.

Since 1958, atmospheric concentrations of CO2 have increased from 315 parts per million (106), abbreviated 315 ppm, to 365 ppm at the turn of the century.

Studies of ice deposits in Antarctica indicate that the amount of CO2 has been increasing at an exponential rate ever since the beginning of the industrial revolution in the mid-18th century. Air bubbles that are trapped in the ice act like time capsules, conserving the atmosphere of the past. The air has been sealed off in the bubbles when the ice was laid down, and extracted from cores drilled deep within the layered ice deposits. The ice cores show that the concentrations of the gas averaged 280 ppm just before the industrial era. In the succeeding two and a half centuries, a mere blink in the eye of cosmic time, the atmospheric concentration of carbon dioxide has increased 31 percent.

The atmosphere now contains almost 800 billion tons of carbon dioxide. Humans continue to release about 7 billion tons of it each year. In other words, each person on Earth is, on average, dumping about a ton of carbon dioxide into the air every year, and there is no end in sight. (The world population in January 2002 was 6.202 billion, increasing at the rate of about 6 million people every month.)

Once added to the air, carbon dioxide spreads throughout the entire atmosphere. And it remains in the air for a long time, taking decades and even centuries to disappear. So future generations will have to contend with our present activities.

Since the oceans cover three-quarters of the Earth’s surface, they can absorb great quantities of the carbon dioxide, eventually taking up about half of the amount that is released into the atmosphere by burning coal and oil. In the meantime, not all of the gas stays in the air. Some of it circulates through the atmosphere in one of nature’s grand, known as the carbon cycle.

During the spring and summer, trees and other vegetation take in carbon dioxide from the atmosphere, incorporating some of the carbon into the plant tissue and releasing oxygen into the air. Animals breathe the oxygen and return carbon dioxide to the atmosphere. Plants release some carbon when their leaves fall and decay in the autumn, and when the plants die they also sequester carbon in the soil.

Every year, roughly half of the heat-trapping gas remains in the air, building up over the decades and centuries. But why should adding such small amounts of an invisible, nontoxic gas be a cause of concern? The total amounts are miniscule, but their consequences are significant. Even relatively small amounts of the gas can warm the Earth by the greenhouse effect, perhaps affecting the climate.

Roger Revelle (1909-1991) and Hans E. Suess (1909-1993) realized the threat decades ago. They argued that the oceans might not readily absorb all of the carbon diode being released into the air, and that the amount of atmospheric CO2 would steadily increase as the fuel and power requirements of our worldwide civilization continued to rise. With prophetic insight, they wrote in 1957 that:

“Human beings are now carrying out a large-scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within only a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years. This experiment, if adequately documented, may yield a far-reaching insight into the processes determining weather and climate.”

As subsequently documented by Charles Keeling, we are indeed altering the composition of our air in a unique global experiment.

Many scientists now think that humans are not only altering the composition of the atmosphere, but that the greenhouse effect of the increased amounts of CO2 will alter the climate and weather. They also now realize that other greenhouse gases are being released into the atmosphere as the result of human activity, and that they additionally contribute to global warming and possible climate change.

New heat trapping gases in our atmosphere

During the past few decades, several heat-trapping gases have been accumulating in the atmosphere as the result of human activity (Table GW.1). Although less common than carbon dioxide and water vapor, each molecule is far more powerful and potentially as significant for global warming. These other man-made greenhouse gases include methane, abbreviated CH4, nitrous oxide, or N2O for short, and the chlorofluorocarbons, known as the CFCs. Even though the total emissions of these molecules are quite small when compared with those of carbon dioxide, they are much more efficient at trapping infrared heat radiation. As a result, they can together contribute about as much global warming as carbon dioxide alone.

Table GW.1. Greenhouse gases produced by human activitya




Greenhouse gas Pre-industrial Recent GWPc Lifetime Sources
concentrationb concentrationb
(1860) (2001) (years)
Carbon Dioxide (abbreviated CO2)266 ppm369.4 ppm1120Burning coal, oil, and natural gas, deforestation, cement manufacture
Methane (abbreviated CH4)848 ppb1782.5 ppb2312Livestock, rice growing, natural gas and oil production, coal mining
Nitrous Oxide (abbreviated N2O)285 ppb314.5 ppb296114Nitrogen fertilizers, chemical manufacturing, waste treatment
ChlorofluorocarbonsRefrigeration,
CFC-11zero262.5 ppt3,80050aerosol cans,
CFC-12zero540.5 ppt 8,100102foam insulation, industrial solvents
Sulfur hexafluoride(abbreviated SF6)zero 4.0 ppt22,2003,200Electrical equipment insulation, magnesium production, medical treatments



a From http://cdiac.esd.ornl.gov/pns/current_ghg.htm. Water vapor produces sixty to seventy percent of the Earth’s global warming, but it is not included here because water vapor is not directly produced by human activity.

b Averages of the measured amounts; ppm = parts per million (106), ppb = parts per billion (109), ppt = parts per trillion (1012).

c The GWP, or the Global Warming Potential, is used to contrast the radiative effects of different greenhouse gases relative to carbon dioxide. Values are the ratio of global warming from one unit mass of a gas to that of one unit mass of carbon dioxide over 100 years.

Methane is the same natural gas that we use at home for cooking and heating. Most of the atmospheric methane does not, however, come from gas wells. It is produced by agricultural activities such as growing rice and raising cattle. The gas is emitted by bacteria that thrive in oxygen-free places like rice paddies and the stomachs of cows.

Since pre-industrial times, the atmospheric concentration of methane has increased more than 110 percent. Over the same period atmospheric carbon dioxide has risen about 30 percent. Although methane is about 200 times less abundant than carbon dioxide, each incremental molecule of methane has about 20 times the heat-trapping power as each additional molecule of carbon dioxide.

When found in swamps, methane is known as marsh gas; it sometimes ignites spontaneously, producing flickering blue flares called will-o’-the-wisps. Some of it also escapes from coal mines, natural gas wells and leaky pipelines.

Nitrous oxide, or laughing gas, is also building up in the air, although not as rapidly as methane. The current rate of increase is about 0.2 percent a year, primarily as the result of nitrogen-based fertilizers but also from burning of fossil fuels in cars and power plants.

The chlorofluorocarbons, abbreviated CFCs, are very effective heat-trapping molecules. The addition of one CFC molecule to the air can have the same greenhouse effect as the addition of 10,000 molecules of carbon dioxide to the present atmosphere. Fortunately, the warming effect of these industrial chemicals may soon be leveling off since they have been banned on the basis of their ozone-destroying capability.

Contrary to popular misconception, however, ozone depletion and global warming are not the same thing. The CFC molecules that destroy ozone also trap heat, but the thinning of the ozone layer does not by itself make the Earth’s surface hotter.

Signs of global warming

The term “global warming” has both a general and specific meaning. It is used in a general sense to indicate that the Earth is getting hotter. Global warming can also specifically imply that a warmer planet is the result of an increase of heat-trapping gases in the atmosphere, resulting from human activity. There is still serious scientific disagreement on both the magnitude and timing of this type of global warming.

It is likely, but not certain, that some of the recent rise of the Earth’s surface temperature is due to such an increase in greenhouse gases. However, much, if not all of the warming could be caused by natural temperature variations that are not related to humankind. Scientists are currently struggling to determine how much of the warming is caused by natural factors or human beings.

Over the past century, the planet as a whole has warmed up in fits and starts, fluctuating between warm and cool periods. For example, the world heated up by about 0.5 degrees Celsius, or 0.9 degrees Fahrenheit, between 1920 and 1940; but the average temperatures dropped by about half that amount between 1940 and 1970, leading some experts to predict a coming ice age. Then the heat turned on again, rising by another 0.5 degrees Celsius in the last three decades of the 20th century. By the 1990s, the world had become hotter than any time in recorded history.

There are now many other signs that indicate a warming atmosphere. The oceans are rising; mountain glaciers are retreating; polar sea ice is melting; the very timing of the seasons is changing, and erratic and severe weather is more common than just a few years ago. Taken alone, these events are no proof of global warming, but in combination they provide song evidence for a warmer climate.

Rising seas provide additional evidence for a hotter world. As the water in the sea gets warmer, it will expand as most substances do when heated. The sea will then ascend to higher levels, in much the same way that heating the fluid in a thermometer causes the fluid to expand and rise up. This is because warm water or other fluids occupy a greater volume than cold ones. Measurements indicate that the global sea level increased somewhere between 10 and 25 centimeters during the 20th century. However, you could not have noticed the change, for the sea level was only rising between 1.0 and 2.5 millimeters per year.

The melting of ice that now covers land, such as mountain glaciers, also contributes to the sea-level rise and most likely results from global warming. By the end of the 20th century, glaciers were retreating throughout the world, and those in Alaska were typically becoming about a meter thinner every 4 or 5 years.

As it melts and shrinks, the glacial ice releases water into streams and rivers that add to the sea. Such melt waters from mountain glaciers boosted the sea level between 2 and 5 centimeters in the 20th century.
Contrary to popular belief, the melting of floating icebergs will not raise the level of surrounding sea. When ice cubes in your drink at home melt, they similarly do not cause any change in its level, for the melted ice produces the same volume of water as it displaces.

Because most of Antarctica’s ice now covers land, it would add to the ocean’s volume if it melted or broke off in blocks, in a process known as calving. Since the average surface temperature of the Antarctic Peninsula rose by 2.8 degrees Celsius, or 5.0 degrees Fahrenheit, in the last two decades of the 20th century, people are worried about the future meltdown of the Antarctica ice pack.

Long hot summers and warmer nights provide another mark of the warming trend. In the Northern Hemisphere, winter shortened by about a week on both ends during the last two decades of the 20th century. So spring arrives earlier than it used to, autumn arrives a bit longer, and the land stays greener. The longer, warmer summers are beginning to thaw the northern tundra, or permafrost, releasing methane that might further exasperate the build up of heat-trapping gases.
Something strange, it’s claimed, is going on with the weather, providing other unwelcome signs that the heat is on, affecting our everyday lives. In fact, such events were predicted as an early symptom of a rise in the average global temperature, and now they are here. These warming signs include record rainfalls, and severe snowstorms; extraordinarily destructive hurricanes and tornadoes; heat waves unique in weather annals; widespread droughts and devastating forest fires; unseasonable warmth and cold; and some of the worst floods in recorded weather history.

A hotter ocean surface produces such weather extremes by accelerating the water cycle. The cycle begins when some of the ocean evaporates, releasing warm fresh-water moisture into the air. This water vapor rises high into the colder atmosphere where clouds are formed. Winds drive the clouds for great distances over sea and land. Rain or snow from the clouds can then fall to land, refreshing streams, lakes or underground reservoirs. The water then runs down to the sea, where the cycle begins once more.

The rising temperatures heat the oceans more than they used to be, causing more water to evaporate. In addition, a warmed atmosphere holds more moisture than a cool one. As a result, there is more water in the air. When this extra water condenses, heavy rains, severe thunderstorms, blizzard-like snowstorms, and damaging floods become more frequent, intense and severe.

While the oceans are being heated, so is the land. It can become highly parched in dry areas, resulting in droughts that are more severe and persist longer than they used to. The rising temperatures also enlarge heat and pressure differences across the land, causing winds to develop and leading to more tornadoes and hurricanes.

Thus there are many indications that the Earth is getting hotter. Climate change has definitely arrived and the future is bound to be different from today. But it is still unclear how much of this warming and extreme weather is attributable to our accelerated release of greenhouse gases, and how much too natural causes. Many scientists think that both are involved and that human activity must be at least party responsible. There is also scientific uncertainty about the future consequences of global warming.

How hot will it get in the future, and how Fast?

There is almost no doubt that the heat is on, and that the climate is changing. The remaining scientific debates are over why the climate is changing, how fast it will change in the future, and the likely consequences of the change. Sophisticated computer models, known as general circulation models, try to answer these questions by analyzing the climate system and forecasting its future.

To assess the effects of global warming, one assumes that the dominant cause of climate change during the next century will be the release of greenhouse gases into the atmosphere by humans, and that the increasing rate of atmospheric build up of the waste gases will continue unabated. The supercomputer models then evaluate all the factors that push and pull the climate, setting the global thermostat.

The models tell of possible futures when atmospheric carbon dioxide levels are about double the pre-industrial concentrations. Although we are almost one third of the way toward reaching this state, it is going to take another 100 years to get there. And since the climate is very complex, the scientific pronouncements about the future climate vary widely. Some of the climate forecasts for the next century are scary and others are not. No one knows which one is true! And it’s far enough in the future that none of the climate experts will be around to check the reliability of their forecasts.

A prestigious group of international scientists has been working on the problem for years. Known as the Intergovernmental Panel on Climate Change, or IPCC for short, it includes hundreds of climate experts. They evaluate the supercomputer model results and develop a coordinated assessment of future global warming under the auspices of the United Nations, including extensive review by both individual scientists and governments. In their report, entitled Climate Change 2001: Impacts, Adaptation, and Vulnerability, the group concluded that the burning of fossil fuels and other human activities are responsible for most of the rise in global temperatures during the last half of the 20th century.

If present trends in the emission of greenhouse gases continue for 100 years, the group concludes, then resultant human-induced global warming will raise the Earth’s average surface temperature between 1.4 and 5.8 degrees Celsius (2.5 and 10.4 degrees Fahrenheit).

The worst-case scenario predicts a huge temperature increase a century from now, comparable to the temperature rise since the last Ice Age, and there may also be dire consequences if the warming takes a middle course. Even a 2-degree Celsius warming over the next century, near the bottom of the predicted range, will probably be the fastest warming in the history of civilization. As we shall see, some scientists predict apocalyptic consequences if it should occur, but others say these are exaggerated fears.

A vocal minority argues that no computer can adequately simulate the complexities of the real world. The computer models, they say, are flawed by massive uncertainties, making their predictions of future global warming about as reliable as a crystal ball or an ouija board. After all, the long-range forecasts of your local weather station are not all that reliable. Even with daily satellite images of weather patterns, we cannot reliably predict the local weather beyond a few days in advance. In Boston, for example, major snowstorms often fail to occur when predicted, and threatening hurricanes often veer off the expected course.

All of the models predict that the globe will warm as the result of the unrestrained emission of heat trapping gases, but different temperatures are obtained under the same conditions and both modest and catastrophic climate changes are foreseen. The reason is that scientists don’t understand the precise role of oceans, water vapor and clouds. They can all amplify the future global warming or cool it. So the experts carefully wrap their pronouncements in statistical properties, scientific uncertainties, and a range of possibilities. The most plausible outcome lies somewhere in between the most extreme projections, between the “end of the world” scenarios and the “good for you” ones.

The mighty oceans are one of the biggest uncertainties. Current estimates suggest that their waters are now soaking up about half the carbon dioxide released into the atmosphere by human activity. Water absorbs less carbon dioxide when it is warmer, which is why you should always keep a carbonated beverage cold. So you might expect that the oceans will amplify the greenhouse warming, absorbing less of the waste gas when it gets warmer and leaving more of it in the air to warm the Earth. Yet, a precise knowledge of how carbon dioxide is buried deep within the oceans, and how the gas is released from them, is not available.

Scientists also do not fully comprehend how water behaves in a warming atmosphere and the amount by which it enhances the warming. Increased temperatures will evaporate more water from the oceans, and the additional moisture should increase the greenhouse warming. Water vapor, in fact, is the main heat-trapping gas in the lower atmosphere. Its greenhouse-amplifying effect is built into the supercomputer climate models, and their predicted average global temperature increase would be substantially less without it. Some critics argue that future warming may even dry out he upper atmosphere, tempering the warming effect of water vapor.

Our understanding of future global warming is additionally hampered by an uncertain knowledge of how clouds affect the Earth’s temperature. At any given moment, clouds now cover at least half the area of the planet, and increased warming should produce even more clouds along with increasing water vapor. The highflying clouds can cool the planet by reflecting more incident sunlight back into space. You may have noticed this cooling effect when a large cloud passed overhead. Clouds also produce a warming effect by absorbing some of the infrared heat radiation emitted by the ground. This heat is reradiated downward, keeping the planet warm. That accounts for warmer nights on a cloudy day. The net temperature effect of clouds depends on which effect dominates and how strong it is, but that is the difficulty, for no one seems to know for sure.

To add to the confusion, the existing supercomputer models offer only a blurred, myopic vision of the world, which isn’t focused enough to resolve most clouds. Although they provide a plausible range of warming forecasts for a century from now, the calculations are reliable only on the broadest scales, such as average temperatures or seasonal changes across the entire world. No computer can possibly evaluate climatic changes everywhere on our planet and in the atmosphere over the next 100 years.

The finite capacity and speed of even the most advanced supercomputer limits its climate calculations to widely separated points, typically no better than 100 or 200 thousand meters between adjacent points. This relatively crude resolution can blur distinctions between land and sea or mountains and plains. It also means that the models cannot zero in and pinpoint localized weather sources, such as clouds or even hurricanes, and they blur distinctions between land and sea or mountains and plains. Thus, even the best global warming predictions represent a stripped-down version of the Earth’s real climate, capable of approximating average conditions over entire countries a long time from now, but too crude to reliably forecast conditions within localized regions of the countries.

In addition, many of the predicted temperature changes from human-induced global warming pale in comparison to natural variations, from the annual seasons to the ponderous ice ages. The climate is naturally changing all the time, warming and cooling regardless of what people do. A prudent society should therefore examine if a hotter globe can be solely attributed to natural processes, as distinct from human ones, and determine how much of the recent rise in temperature is attributable to human activity and how much to natural causes.

Using past records to separate the warming effects of the Sun and humans

Fig. .. 

Changes in the amount or distribution of the sunlight illuminating the Earth can produce substantial variations in our climate, and we have proxy records of its variable output over past centuries. This data can be compared to global temperatures during the same period, to determine if the Sun is responsible for most of the warming and cooling of our Earth. Although natural variations in the solar output can explain most of the temperature variations over the past centuries, it appears that global warming by heat-trapping gases, emitted by human activity, is required to explain the sharp rise in global temperatures during the 1990s.

The Sun is, after all, the driving force for all climate and weather on Earth. The annual seasons are, caused by a change in the amount of incident sunlight, due to the tilt of the Earth’s rotational axis. During the Earth’s yearly orbit, a given hemisphere is tilted toward the Sun, producing summer, then away resulting in winter.

The word “climate” comes, in fact, from the Greek word klima, for tilt. Nowadays climate denotes long-term changes, on time scales of years, decades and centuries, while weather usually refers to short times of hours, days, weeks or months.

The annual change in solar radiation produces large seasonal temperature fluctuations in the Northern Hemisphere where most of the world’s land is now found. The difference in the average surface temperature between northern winter and summer is an enormous 15 degrees Celsius (27 degrees Fahrenheit). In the tropics, the temperature changes little year round, since the amount of incoming solar radiation in those latitudes is least affected by the Earth’s tilt. The tropical regions also receive the greatest amount of heat because the Sun’s vertical rays travel to the ground through the least amount of intervening air.

Although the sunlight that illuminates our days provides a seemingly reliable beacon, the Sun’s visible luminosity varies in tandem with the Sun's 11-year magnetic activity cycle and these changes could affect our climate. The colored sunlight passes right through to the ground, providing a direct warming or cooling of the lower atmosphere.

Stable detectors placed aboard satellites above the Earth’s atmosphere have been precisely monitoring the Sun’s total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant. It is almost always changing, in amounts of up to a few tenths of a percent and on time scales from 1 second to 20 years, and probably longer. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.

Comparisons with other stars, that resemble the Sun in mass and age, indicate that the Sun could undergo more substantial variations in brightness than those observed by satellites so far.

In order to say that the climate is getting warmer, we cannot just extrapolate from recent measurements. The case for global warming must instead be based on century-long temperature records over a large part of the globe. We can then determine if the recent rise in temperature is a significant departure from long-term trend, and evaluate how much of the warming is attributable to human activity and how much to natural causes.

Scientists have therefore reconstructed variations in the climate of the past, comparing them to the Sun’s changing output. Data extracted from tree rings and Antarctica ice cores indicate that solar activity has indeed fallen to unusually low levels at least three times during the past one thousand years, each drop corresponding to a long, cold spell of roughly a century in duration. As an example, sunspots virtually disappeared from the face of the Sun for the 70-year period between 1645 and 1715, when Europe experienced one of the coldest periods of the Little Ice Age. During that time, alpine glaciers expanded, the river Thames, England, and the canals of Venice, Italy regularly froze over, and painters depicted unusually harsh winters in Europe. The Sun was then about 0.25 percent dimmer, and the reduction in solar brightness produced an estimated drop of about 0.5 degrees Celsius (0.9 degrees Fahrenheit) in the global mean temperature.

It is very difficult to distinguish any human influence in the observed temperature record of past centuries. The natural ups and downs of the Sun’s brightness or the cooling effect of volcanoes can explain almost all of it. Fine particles created during powerful volcanic eruptions, such as Mount Tambora in 1815, Mount Krakatau in 1883, and Mount Pinatubo in 1991; can spread out high above the ground, forming an invisible, umbrella-like shield that blocks some of the incoming solar radiation and causes temporary global cooling. Similar sulfate aerosols arise from fossil fuels burnt in power plants, factories and automobiles, partially offsetting the full warming effect of their carbon-dioxide emission.

The warning signal of human-induced global warming only rose above the confusing noise of the Sun and other natural effects in the 1990s, when the Earth became exceptionally hot. There is absolutely no evidence for such a decade in the historical temperature records going back 1,000 years (Fig. GW.2). This sharp, unprecedented rise in the average global temperature during the last decade of the 20th century cannot be explained as a temporary swing produced by natural causes alone, and its is very likely that heat-trapping waste gases are at least partly responsible for it. The available evidence suggests that greenhouse gases emitted by industrial economies mainly cause this warming.

Humans have conquered the land, moved mountains, and redirected rivers. Airplanes have provided easy access to almost anywhere in the world, and communications satellites have connected us all in an electronic web. The globe has shrunk, and we might now be altering the entire atmosphere.

For most of history, we believed that climate and weather are governed by outside forces, beyond the influence of humans, but today we are no longer so sure. As the result of rapid, unprecedented population and industrial growth over the past century, humans can collectively alter the course of nature.

Likely consequences of global warming

If current emissions of carbon dioxide and other greenhouse gases go unchecked, temperatures should continue to rise and the climate will inevitably change. A significant warming ought to occur by the end of the 21st century, but the predicted consequences range from an ominous, catastrophic future to a mildly uncomfortable one. Two extreme arguments have therefore been advanced. One side, supported by environmentalists, warns of an imminent apocalypse and global catastrophe caused by our own tinkering with nature. In their perspective, belching smokestacks, gasoline-powered automobiles, power-generating stations and the voracious destruction of forests are turning up the heat on an overburdened environment, pushing it over the edge with raging storms, flooded cities and poisoned air. The alternate view, championed by some industrialists, is that doomsday predictions are exaggerated and hopelessly uncertain. And if the world does get a little warmer, they say, it won’t be that bad. After all, most people like a warmer climate and many of us could use a little more heat in our lives.

There are both positive and negative aspects to even the worst-case forecasts of global warming. Not everyone and every place will be affected the same. Some regions will benefit, and others may suffer unbearable damage. Many aspects of a temperature increase would probably be welcome at high northern latitudes that are now too cold over much of the year. Regions that are near the equator are already hot, and a further increase in heat could be devastating. So there is bound to be a mixed verdict, and the outcome of the debate depends upon which kind of evidence you focus on.

Who should we believe? It is probably unwise to lapse into apocalyptic think, especially in view of the scientific uncertainty and long-time scales, but ostrich-like denial is also imprudent since a lot is at stake, from the wealth of nations to the future of the planet. The best we can do is to examine the full spectrum of potential, long-range consequences, from good to bad.

In the best-case projection, at the low end of the predictions, global warming will, by itself and independently of other influences, raise the average temperature of the world by 1.4 degrees Celsius (2.5 degrees Fahrenheit) over the next 100 years. If the warming takes this gradual, modest course, some parts of the world will benefit, and the normal resilience of society ought to accommodate the climatic change.

The modest increase in temperatures at mid-northern latitudes, where most people live, will be welcome. There will be longer summers, shorter winters and warmer nights. Residents of cities like Boston will suffer fewer colds, experience fewer heart attacks from shoveling snow, and spend less on heating, snowplowing and road salting. On the other hand, summer air conditioning will cost more, the winter ski slopes may turn to slush, and the colorful fall foliage could disappear as trees move away from the heat to the north.

If global warming is at the upper end of the prediction, at 5.8 degrees Celsius (10.4 degrees Fahrenheit) in 100 years, many humans should also be able to adapt without much difficulty. After all, this rise in temperature is less than the average daily temperature difference between New York City and Atlanta, Georgia, or between Paris and Naples, and there is little evidence of greater risk to people who now live in the warmer southern climate. And those who live in the colder, northern locals are already used to a seasonal temperature increase between winter and summer that can be three times greater than the largest predicted heating over the next 100 years.

So, the good thing is that humans are adaptable. But the bad thing is also that humans are adaptable. As long as the climate changes occur slowly, we can adapt without realizing what is happening, but some very uncomfortable things can happen at the top part of the expected warming in 100 years.

There is an applicable proverb about frogs. If you put a frog in boiling water, it will jump out and save itself. But if you gradually increase the heat, with the frog in the water, it will die.

Very hot will be decidedly unwelcome in many places. Within deserts, entire cities will be immobilized under the heat. The wealthy will move out of Palm Springs, and Las Vegas could become a ghost town. Residents in many other large cities should experience severe heat waves, making them feel like the world is melting down in a pool of sweat. As the climate becomes hotter and drier, drought will probably become more severe in areas prone to it, and supplies of freshwater will dwindle. More frequent bouts of extreme weather will also be expected, with widespread flooding and intense hurricanes.

Agriculture in some regions will be better than other regions. The longer growing season and increasing carbon dioxide will foster plant growth, making much of the developed world greener. Agriculture will likely become more productive in Canada, northern Europe, Russia and the northern United States. As droughts turn some mid-American farms to dust, both agricultural production and population centers in the United States will shift north, and the same thing will probably happen in Europe.

The world’s poorest countries are, on the other hand, highly vulnerable to agricultural disaster, for they are already located in arid and semi-arid regions. A further rise in temperature will almost certainly reduce crop yield in south Asia and sub-Sahara Africa, where expending deserts will additionally claim more land.

As environmental conditions change over time, plants and animals will migrate, as they have throughout geological history – moving up and down in latitude as the globe warms and cools. The recent increase in temperatures has already caused some species to move north, and the accelerated heating could wipe out many of them in the future. Some plants and animals might not be able to move fast enough to keep pace with the rapid rate of temperature change, and climate-sensitive habits could be destroyed altogether, hastening the extinction of some species.

No one can see much advantage in the rising seas, which are one of the most certain effects of the warming projected during the coming decades. There is just one indirect advantage – the meltdown of polar ice, that contributes to the sea rising, could result in permanent ice-free passage in the Arctic Ocean, providing a new shipping route between Europe and Asia.

The climate experts predict a rise in sea level of between 0.09 and 0.9 meters (3 inches to 3 feet) over the next 100 years if nothing is done to curtail the emission of greenhouse gases. The resultant flooding will seriously disrupt coastal areas where more than a quarter of the world’s population now lives. In the worst-case increase, Venice and Alexandria will be inundated, as will many cities on the Atlantic and Gulf coasts of the United States, including Boston and New York City. Residents in South Florida will not have to worry about the sweltering heat; their homes will be flooded with seawater.

As with agriculture, the developing nations will get the short end of the stick. Thirty million people in Bangladesh could be displaced by a 0.9-meter increase in sea level, and the rising waters would most likely force the evacuation of 70 million Chinese. Salt water could move several thousand meters inland at the mouths of rivers, invading coastal drinking water systems. The Nile, Yangtse, Mekong and Mississippi deltas are all at risk. Island nations will suffer severe flooding or completely disappear under the rising waters; they include the Bahamas, many of the Caribbean islands, Cyprus and Malta in the Mediterranean, and several Archipelagos around the Pacific Ocean.

Even a modest rise in sea level will wipe many of the world’s beaches out of existence. Flooding isn’t the problem; it’s the removal of sand by waves. Even a 0.3-meter (1-foot) rise in sea level creates wave action that erodes away up to 5 meters (200 feet) of some beaches. So, people who live by the seashore had better sell their homes, and you can forget winter vacations in parts of Florida and the Caribbean islands.

The last factor in our catalog of likely consequences of severe global warming is increased health risk – a topic close to the hearts of most people and nearly every politician. Many diseases might spread dramatically as the temperatures head upward, especially those born by mosquitoes. As the world warms, mosquitoes will move north, into regions where the winter cold used to kill them, carrying malaria, dengue fever, yellow fever and encephalitis with them. Extremely hot weather may also directly terminate the lives of a lot of people, particularly the very old and the very young living in cities.

The list of possible consequences of global warming looks pretty grim, especially if the worst forecasts come true. Some of the threats are immediate and inevitable; others are remote and uncertain. Only time will tell for sure.

But that doesn’t justify inaction. Even if worldwide emission of heat-trapping gases were capped at today’s amounts, their concentrations in the atmosphere would continue to increase. And if we stopped burning fossil fuels altogether, the atmosphere would not immediately recover. The carbon dioxide already released in the air would stay there for at least a century, keeping the planet warm and the global temperatures high. So, its time we acted to correct the problem – we may have waited too long already.

The heated debate about global warming

Whether we like it or not, global warming has become politicized, the subject of a contentious debate. It has entered the arena of world politics, a shadowy realm of diplomacy, economic interests, political alliances, and national security.
No ratified, international global-warming treaty agreement exists, at least so far. Extremists on both sides of the issue have hired lobbyists to influence policy makers and mounted huge public relations campaigns to persuade the average person to accept their views. The conflicting information has confused the general public, often leading to an overall apathy. And since neither side in the debate will compromise, a consensus is impossible.

When you strip away the rhetoric, the experts know little about the future severity of climate change and even less about the future physical impact in particular countries or regions. After decades of research, the model builders cannot say precisely what will happen to the climate as the result of the atmospheric build up of heat-trapping gases. They just don’t know enough about the atmosphere, clouds or the oceans to predict accurately the future global climate.

The uncertainty paralyzes discussion. Scientists have to generate a wide range of possible futures, some very threatening and others less so. Not all of these outcomes are likely to be true, and none is definitive, but people tend to latch onto those that fit their preconceptions. Especially the extremists, who selectively interpret the scientific forecasts to bolster their case – the liberals choose doom and gloom and the conservatives favor good times for all.

The environmental organizations and their allies insist that global warming is here, a harsh and inexorable reality, and that it is due to the rise in carbon dioxide caused by coal and oil burning. The Earth, they say, has entered a resulting widespread climatic disruption that is going to get a lot worse if quick actions are not taken to reverse the accumulation of heat-trapping gases in the atmosphere.

Advocates for this view employ the full range of potential environmental disasters, suggesting that they will almost certainly occur. Deserts will expand, drought will spread, water supplies will evaporate away, rising seas will flood coastal areas and cover island nations, widespread famine will ensue as farms dry up, and heat waves and tropical diseases will threaten out health. The future, some argue, could be apocalyptic as ice caps melt and the sea level rises. The frightened public often isn’t aware that most of these catastrophes will not occur for 100 years, and then only if the upper end of the uncertain scientific predictions applies.

The industries considered most responsible for global warming are the most critical of its scientific validity. The coal and oil companies, as well as the oil-producing nations, argue that the computerized climate models are crude and approximate, incomplete, inconclusive, and so flawed that their predictions of future climate change cannot be used as a basis for taking action. The risks of global warming, they therefore argue, have been widely exaggerated, and there is no proof that anything catastrophic will happen.

Moreover, even if the global temperature is rising, they argue, its cause is solely or mostly natural. The climate is always changing whether or not human beings have anything to do with it, and humans continue to be insignificant when compared to the natural forces that have determined the climate for millions of years.

The oil and gas companies insist that any future warming from carbon dioxide emissions will be moderate and that rising levels of the benign gas will be a good thing. Far from being a pollutant, carbon dioxide is a powerful fertilizer that helps plants grow. If you reduced the amount of the gas in the air, the plants would be in real trouble; they could even disappear.

This side of the debate argues that any attempt to get rid of coal and oil will cause an economic disaster; it might even cripple the global economy. With sales exceeding two billion dollars a day and trillions of dollars a year, the oil industry is indeed a powerful economic force. A significant reduction in the use of coal and oil, some say, could eliminate millions of American jobs, reduce the United States’ ability to compete globally, interfere with the free market, and endanger the lifestyle of every American.

The same arguments, they say, apply to most of the rich industrial countries, whose economies are dependent on the fossil-fuel industry. Any regulations to curtail emissions from coal and oil burning could slow economic growth in all of these countries.

With so much money at stake, it is perhaps not surprising that the oil and coal industry have spent millions of dollars to cast doubt on global warming. After all, they are just protecting their interests. Critics argue that they have also been suppressing the true implications of global warming, somewhat as the tobacco industry did about the dangers of cigarettes. Some lawyers have even threatened a class action suit to force a reduction in the emission of heat-trapping gases.

Not every industry supports the unrestrained use of fossil fuels. The insurance industry favors restraints, fearing that their profits will fall as extreme weather increases. Floods, hurricanes and other severe storms, attributed to global warming, have already caused annual insurance losses of billion of dollars, naturally passed on to the customers by higher rates, and led to insurance policy exclusions for those living in storm-prone areas. Now entire countries are entering the fray, attempting international agreements with legally binding limits to the emissions of heat-trapping gases.

Doing something about global warming

Governments can blunt the feared global warming of the future by adopting energy policies that shift from coal and oil to natural gas, and eventually to energy sources that do not generate heat-trapping gases. Every time you turn a light on, the electricity most likely comes from burning coal. It still supplies 56 percent of the electricity in the United States. Yet, for the same energy production, coal burning releases more carbon into the air than burning oil and natural gas releases even less. All that carbon combines with oxygen in the air to create carbon dioxide.

Some sources of electricity emit no carbon into the air and produce no heat-trapping gases. They include hydroelectric power, solar energy, and the power of the wind. In the United States, current subsidies and tax incentives for the development of oil, coal and natural gas amount to about 20 billion dollars a year. If these funds were shifted to the cleaner energy sources, it would make them more competitive. Of course, the largest carbon-free source of energy is nuclear power, which produces 20 percent of the United States requirement, and nuclear energy is subsidized by about 10 billion dollars a year.

Countries can avoid the clear cutting of their forests and plant a lot more trees. Each tree removes about a ton of carbon dioxide from the air, locking the gas into its branches, trunk and leaves. Trees also tend to outlive humans and they prevent erosion. By protecting existing forests and planting new ones, countries could offset 10 to 20 percent of the expected carbon dioxide build up during the next century.

Mandates for limiting fossil-fuel emissions or protecting forests are nevertheless difficult to legislate, partly because the threat is uncertain. Policy makers like black and white issues, but future global warming effects are gray. There are pros and cons to a hotter world, winners and loser.

There is also a lack of immediacy. Very dramatic warming effects occur on a vast time scale, over decades and centuries, so we are unlikely to witness them in our lifetimes. They certainly will not happen before the next re-election campaign of government leaders.

Global climate change is an issue that all countries have to deal with, both the rich industrial nations and the poor developing ones. But there are stark differences between the countries, blocking any substantive international agreement so far.

People in the poorer nations argue that the average person in the rich countries eats more food, consumes more energy and poisons the air more than they do. And the industrial countries became wealthy largely by burning the coal and oil that produced most of the heat-trapping carbon dioxide that is now in the air. They are thus responsible for most of whatever global warming is likely to bring. The rich nations are still responsible for most emissions today, about 80 percent of it. So, the poorer nations say, it is only right that the wealthy countries be the ones to cut back on their emissions.

The developing countries point out that they will endure greater damage from future climate disasters caused by the rich countries’ emissions. Global warming, for example, is expected to increase crop yields in temperate northern regions, where the rich, industrial countries are located, while harming agriculture in the lower, warmer latitudes where most poor nations are.

Rich nations have the resources needed to adapt to climate changes; for some other countries this is not an option. The poorer countries are hard-pressed enough to assure the economic survival of their rapidly growing populations. As the produce more and more people and less and less food, these countries will not want to limit the economic growth required for survival.

On the other hand, the industrial countries insist that warming is a global concern, and that all countries must share in the solution. This is particularly so, they argue, since the developing countries’ emissions are expected to surpass those of the rich nations in 20 or 30 years.

The United States has been cast as the wealthy villain, the most greedy, selfish and irresponsible of all. It is by far the biggest single producer of heat-trapping gases, both in total output and on a per capita basis, contributing 25 percent of the total with just 4 percent of the world’s populations. By way of comparison, the average European consumer, who also lives in an industrial country, consumes about half as much energy as the average American. So a little self-restraint and denial might be appropriate for the Americans, and it might help their strategic vulnerability.

Much of America’s energy comes from oil-producing nations in the Middle East, and the United States spends at least 25 billion dollars each year in their military defense. Yet, most of them have deplorable human rights records and enormous gaps between rich and poor. These conditions have helped breed the terrorism that now threatens the United Stares.

And what about the poor developing nations? They are not about to adopt restraints that might slow their industrial growth just to keep the rich, industrialized nations a little cooler. Yet, if the developing countries take the same path as the wealthy ones, burning coal and oil to fuel their growth, then atmospheric carbon dioxide will soar.

If the poorer nations are forced to accelerate the burning of fossil fuels, to feed and house and employ their expanding populations, then their carbon dioxide production will soon dwarf that of the rich industrialized countries. By 2015 the nations of Asia, led by China and India, will surpass even the unrestrained emissions of the richer nations.

So what’s being done about the problem? In December 1997 representatives of the world’s nations met in Kyoto, Japan, to establish, for the first time, specific legally binding targets and timetables for the emission of heat-trapping gases (Focus GW.1). The treaty, called the Kyoto Protocol, would require the United States and other industrial countries to reduce emissions by 2012 to an average of 5.2 percent below emission levels in 1990. The accord has been signed by more than 100 countries, but there is no way that is going to cool the planet very soon. In order to take effect, the Kyoto Protocol has to be ratified by a substantial number of industrial nations, but none of them have ratified it, at least by 2001, and they probably won’t.

Focus GW.1. The Kyoto Protocol

In December 1997, at an International Climate Summit in the ancient Japanese capital of Kyoto, more than 100 nations agreed to reduce the emissions of heat-trapping gases that can warm the planet. Known as the Kyoto Protocol to the United Nations Framework Convention on Climate Change, or just the Kyoto Protocol for short, the agreement calls for reductions in the emissions of six greenhouse gases: carbon dioxide, methane, nitrous oxide, two fluorocarbons and sulfur hexafluoride.

The accord established different levels of reductions for individual countries. Thirty-eight industrialized countries would be required to reduce emissions by an average of 5.2 percent by 2012 compared with their 1990 levels. The United States would be committed to a reduction of 7 percent below 1990 levels, the European Union to 8 percent and Japan to 6 percent.

The developing countries are exempt from mandatory emission controls, since the vast majority of the emissions to date have not been caused by them. Even the fast-growing developing countries face no constraints, though the their emissions are expected to surpass those of the industrial nations in two or three decades.

The protocol will take effect once it is ratified by at least 55 nations that collectively account for at least 55 percent of 1990 carbon dioxide emissions. The terms become binding on an individual country only after its government ratifies the treaty. No industrial country had ratified the treaty by late 2001.

The industrial countries cannot easily meet the limits of the Kyoto Protocol even if it is ratified. On the eve of the Kyoto negotiations in 1997, America’s emissions were already up 10 percent from 1990 levels and they have risen about 1.2 percent a year since then. Other industrial nations, too, have recorded rising, not falling, emissions.

Powerful liberal groups have been calling for more ambitious limits to emissions, while more conservative interests argue against the drastic measures already required in the Kyoto Protocol. Both groups incorporate selected evidence about future global warming that agrees with their objectives, avoiding a balanced appraisal of the dangers.

The details of the Kyoto Protocol continue to be discussed in international bargaining sessions. It includes principles of international emissions trading which would allow a country to trade reductions if they fall below the country’s limit. If an industrial nation were to exceed its emission quota, it could purchase the unused rights from lower-emitting countries, but the rulebook for these trades is still being worked out.

Under a joint implementation plan, a wealthy country could get credit towards its targets by investing in specific emissions-saving projects in developing countries, such as more efficient power planets. In another tradeoff, some countries in the former Soviet Union might be able to sell emission credits for reductions that occurred prior to the negotiations, primarily as the result of national economic problems.

Another provision under negotiation counts vast forests towards a country’s emission-reduction credits, since trees absorb carbon dioxide from the air as they grow. Since forests can always be cut down, this is nevertheless just a temporary abatement, and not the same as a permanent prevention of carbon dioxide from leaving vehicles, power plants or factories.

As the diplomats and politicians continue their stately dance, the Kyoto Protocol will probably live or die on the basis of its economic repercussions. No nation will accept the treaty if its causes serious damage to the country’s national economy, and economic incentives will be required to implement the planned reductions.

The United States continues to find the Kyoto Protocol unacceptable because it unfairly requires only the industrialized countries to cut emission. The “one-sided” treaty is not likely to become acceptable until the fast-developing countries, like China and India, also face emission constraints. So the developing countries must eventually be persuaded to participate in any reduction of the emission of heat-trapping gases if the treaty is to survive.

Both the rich and poor nations want an agreement that will not cause serious harm to their national economy. Emission restraints will certainly be costly, most likely r

A global solution needed, in which all nations participate in curtailing the emissions and agree on the best way to achieve it. Whatever the plan, its implementation should include a change in energy consumption habits to slow the inevitable global warming.

Individuals can reduce their consumption of the fossil fuels that electrify and heat their homes, offices and schools, power their vehicles, and fuel their factories. Ordinary people can use energy efficient appliances and lighting, reduce their daily electricity use, drive their cars less, and insulate their homes and offices so they require less heat. Some of this energy conservation has already begun, but not enough is being done.

One third of all greenhouse emissions come from automobiles. Burning a gallon of gasoline in an average car produces 9 kilograms of heat-trapping gases, and over the course of a year that car dumps about 1,000 kilograms of waste gas into the air – which is about equal to the weight of the car. A sports utility vehicle emits twice as much, and so does a pickup truck. So the world’s population should buy fewer cars, at least those that are powered by gasoline, and they should especially avoid the gas-guzzling kind.

Every large automaker is now investing heavily in new engine technology to improve fuel efficiency. We already have hybrid cars that combine internal combustion with battery power, and we may eventually be able to purchase cars that use clean hydrogen gas as fuel.

And many of the fiercest corporate opponents to emission regulations are now voluntarily cutting their emissions of heat-trapping gases, perhaps to head off tougher regulations in the future. Oil giants like Shell and BP are taking steps to end the burning off of natural gas at oil wells – some say to mollify environmentalists in their European markets where there is strong public interest in global warming. Large automakers in Europe, including Ford and General Motors, have reluctantly agreed to improve the fuel efficiency of their automobiles. Other corporate giants like Du Pont are lowering their output of certain chemicals, the CFCs, that contribute to global warming, an action that began years ago because some of the same chemicals damage the ozone layer.

So humans have modified the atmosphere, warming the globe, and we are starting to do something about it. But its bound to be only a temporary fix. In the long run nature will take over the weather and climate once again. A hundred million years ago, when the dinosaurs roamed the Earth, there were no ice caps and tropical plants flourished near the South Pole. Deep cold nearly turned the Earth into a ball of ice about 10 thousand years ago, when the planet was in the depths of an ice age. In just a few million years from now, entire continents and oceans can be destroyed or created new, changing the flow of air and ocean currents and altering global weather patterns. And even if it is pretty warm right now, the die is cast for the next glaciation and the ice will come again.