CLIMATIC EFFECTS

Earth's climate has fluctuated over the course of several billion years. During that span, species have arisen, gone extinct, and been supplanted by others. Humankind and our immediate ancestral species have survived the sometimes drastic fluctuations of the past several million years of our evolving climate quite handily. Although human beings and their ancestors have always changed the environment in which they have lived, human actions since the advent of the Industrial Revolution may be producing impacts that are not only local but also global in scope. Ever-growing fossil fuel use and other activities associated with industrialization have altered the concentrations of several atmospheric gases, including carbon dioxide. Theoretically, these alterations can cause alterations in the overall heat retention of Earth's atmosphere. Scientists around the world are engaged in studies to determine whether these changes in the atmosphere pose a risk of significant, negative environmental and human impacts.

CLIMATE CHANGE THEORY

Climate change means different things to different people. To some, climate change refers to physical changes in climate and little more. To others, climate change is a theory of how certain gases in the atmosphere influence the climate. Still others focus on the human aspect, and consider climate change only in regard to the way human activity influences the climate. Climate change theory is quite complex. Unlike Albert Einstein's E = mc², for example, the theory of human-driven climate change is not a singular theory but consists of several interlocking theories, some better defined than others.

At the heart of climate change theory is a more humble theory called the greenhouse effect. This theory, first quantified by mathematician Joseph Fourier in 1824, has been repeatedly validated by laboratory experiments and by millions of greenhouse owners. The greenhouse effect is simple. When sunlight reaches the surface of Earth, some of its energy is absorbed by the surface (or objects on the surface), some is reflected back toward space unchanged, and some is first absorbed by objects or the surface of Earth and then reemitted in the form of heat. Over a bare patch of ground, this dynamic would cause no net increase in the temperature over time because the heat absorbed during the day would be reradiated toward space overnight.

But if there is a greenhouse on that patch of ground, things are different. The sunlight enters as usual, and some of it is reflected back out as usual, but part of the incoming solar energy that was held by the surface and reemitted as heat is prevented from passing back out by the glass, and the greenhouse warms up a bit. If there are water-beating plants or materials inside the greenhouse, water vapor concentration will increase as evaporation increases. Since water vapor also can trap heat, the greenhouse warms still further. Eventually the air reaches maximum humidity, and the system reaches temperature equilibrium.

Scientists have known for a long time that the greenhouse effect applies not only to greenhouses but to Earth as a whole, with certain gases (called greenhouse gases) playing the role of the glass in the example above. The primary greenhouse gases are water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), ozone, and chlorofluorocarbons (CFC). When scaled up to the entire planet, the natural greenhouse effect produces a pronounced warming effect. This warming is a natural aspect of Earth's environment, crucial for the maintenance of life on Earth. In fact, without Earth's natural greenhouse effect, and the warming that goes with it, Earth would be a much colder planet, inhospitable to life as we know it. The greenhouse effect has also been seen to maintain warmer planetary atmospheres on Mars and Venus. While the effect is mild on Mars, the high carbon dioxide level on Venus (78,000 times that of Earth) keeps the atmosphere about 500°C (900°F) higher than it would otherwise be.

Against the backdrop of an Earth warmed by its own greenhouse effect, other forces operate that can increase or decrease the retention of heat by the atmosphere. Some of these forces are of human origin, some are produced by nature, and some are produced by mutual feedback reactions.

Which forces have dominated in the recently observed warming of the climate is still an open question. The last landmark report of the United Nations' Intergovernmental Panel on Climate Change (IPPC) concludes:

Although these global mean results suggest that there is some anthropogenic [of human origin] component in the observed temperature record, they cannot be considered as compelling evidence of a clear cause-and-effect link between anthropogenic forcing and changes in the Earth's surface temperature. It is difficult to achieve attribution of all or part of a climate change to a specific cause or causes using global mean changes only. The difficulties arise due to uncertainties in natural internal variability and in the histories and magnitudes of natural and human-induced climate forcings, so that many possible forcing combinations could yield the same curve of observed global mean temperature change [IPCC, 1995, p. 411].

Nonetheless, the IPCC's chapter on the attribution of climate change concludes:

The body of statistical evidence. . . when examined in the context of our physical understanding of the climate system, now points towards a discernible human influence on global climate. Our ability to quantify the magnitude of this effect is currently limited by uncertainties in key factors, including the magnitude and patterns of longer-term natural variability and the time-evolving patterns of forcing by (and response to) greenhouse gases and aerosols [IPCC, 1995, p. 439].

WARMING AND COOLING FORCES

Human activities (as well as nonhuman biological, chemical, or geological processes) release a variety of chemicals into the atmosphere, some of which, according to climate change theory, could exert a warming or a cooling effect on Earth's climate. In climate change literature, these are referred to as "climate forcings." Some of these forcings are actually secondhand responses to changes in the climate caused by others. In such a case, a forcing might be referred to as a feedback.

Carbon Dioxide

Carbon dioxide, considered a warming gas, comprises about 0.036 percent of the atmosphere by volume. As Figure 1 shows, carbon dioxide levels have increased as a component of the atmosphere by nearly 30 percent from the late eighteenth century to the end of the twentieth century, when the level was close to 365 parts per million by volume. Prior to the period of industrialization, carbon dioxide levels were largely stable, at about 280 parts per million, though fluctuations as low as 200 parts per million or as high as 300 parts per million have been observed through analysis of air bubbles trapped in arctic ice cores.

Carbon dioxide is released into the environment by human activities such as fuel burning, cement production, and land use.

Since highly accurate, direct measurement of carbon dioxide levels began only in the late 1950s, most of our understanding of carbon dioxide's historical patterns of fluctuation come from indirect measurements, such as the analysis of gas bubbles trapped in glaciers and polar ice caps. Though indirect measurements carry greater uncertainty than direct measurements of carbon dioxide levels, indirect measurements have contributed to our understanding of Earth's carbon cycle. Still, significant gaps in our understanding remain, specifically involving questions of time lag, the impact of world vegetation on atmospheric carbon dioxide levels, other processes that might lock carbon dioxide away from the atmosphere, and the role of carbon dioxide as a causal agent of climate change.

Methane

Methane is a greenhouse gas up to fifty-six times as powerful a warming agent as carbon dioxide, depending on the time scale one considers. In a twenty year time frame, for example, a given quantity of methane molecules would have fifty-six times the impact of the same quantity of carbon dioxide molecules, but since carbon dioxide has a longer lifespan, this ratio declines over time. As an atmospheric component, methane is considered a trace gas, comprising approximately 0.00017 percent of the atmosphere by volume. As Figure 2 shows, methane levels in the atmosphere increased nearly 120 percent from the middle of the nineteenth century to the end of the twentieth century, when the levels were the highest ever recorded, though the pattern of methane emissions has been highly irregular and actually showed downturns toward the end of the twentieth century for reasons that are not clear. Studies of methane concentrations in the distant past show that methane concentrations have fluctuated significantly, from as few as 400 parts per billion to as many as 700 parts per billion, due to changes in wetlands and other natural sources of methane. Today methane comes from a variety of sources, some of human origin, others of nonhuman origin.

Nitrous Oxide

Nitrous oxide is a long-lived warming gas with a relative warming strength 170 to 310 times that of carbon dioxide, depending on the time scale one considers. Nitrous oxide, like methane, is considered a trace gas in the atmosphere, but at considerably lower levels, about 0.00003 percent of the atmosphere by volume. As Figure 3 shows, nitrous oxide concentrations have increased significantly from the middle of the nineteenth century to the end of the twentieth century. Prior to the industrial period, nitrous oxide concentrations fluctuated at an average of 270 parts per billion by volume, though fluctuations as low as 200 parts per billion by volume have been measured from the distant past.

Nitrogen oxides (of which nitrous oxide is the major component) come from a variety of sources, only some of which are of human origin.

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons (CFCs) are man-made compounds used as cooling agents and propellants in a broad range of applications. There are many different species of CFCs, some of which have been banned. CFCs are very powerful warming gases. Some species are more than ten thousand times more capable of trapping heat than is CO₂. Of course, CFCs also are found at much lower concentrations than the other greenhouse gases. Whereas carbon dioxide is measured in parts per million, and methane in parts per billion, CFCs are measured in parts per trillion. Figure 4 shows the concentration of the three major CFCs from 1929 to 1999.

Some CFCs also can break down ozone. Ozone-depleting CFCs can exert either warming or cooling effects, depending on where they are found. In the lower atmosphere, ozone-depleting CFCs exert a warming effect through the absorption of heat reradiated from Earth's surface. In the upper atmosphere, ozone destruction exerts a cooling effect by destroying some of the high-altitude ozone that can either warm or cool the surface in different circumstances. On a net basis, our current understanding is that the ozone-depleting CFCs (banned by the Montreal Protocol in 1987) exerted a cooling effect. Replacement chemicals for the ozone-depleting CFCs are considered pure warming gases, but with a considerably lower warming potential than the chemicals they replaced. Because of the complexities of ozone chemistry in the atmosphere and uncertainties regarding the warming or cooling potential of remaining ozone-depleting CFCs and replacement compounds, the ultimate impact of CFCs on climate change is highly uncertain.

Aerosols

Aerosols are not gases in the strictest sense of the word, but are actually liquid or solid particles small enough to stay suspended in air. Both human-made and natural processes generate aerosols. Some aerosol particles tend to reflect light or cause clouds to brighten, exerting a cooling effect on the atmosphere. Other aerosol particles tend to absorb light and can exert a warming effect. Most human-made aerosols seem to exert a cooling effect on the climate. On a global basis, some have estimated that this cooling effect offsets about 20 percent of the predicted warming from the combined greenhouse warming gases, but that cooling is not uniform: the offsetting impact varies geographically, depending on local aerosol concentrations.

The omission of aerosol considerations in earlier climate models led to considerable overprediction of projected global warming and predicted regional impacts, though newer models have done much to internalize the cooling effect of aerosols. Aerosols act as cooling agents through several mechanisms, however, some of which are only poorly understood. Besides directly scattering incoming sunlight, most particulates also increase the reflectivity, formation, and lifetime of clouds, affecting the reflection of incoming solar radiation back to space.

Water Vapor

Water vapor is the most abundant of the greenhouse gases and is the dominant contributor to the natural greenhouse effect. About 99 percent of all the moisture in the atmosphere is found in the troposphere, which extends about 10 to 16 kilometers above sea level. Only about one-third of the precipitation that falls on Earth's continents drains to the oceans. The rest goes back into the atmosphere as a result of evaporation and transpiration.

In the lower part of the atmosphere, the water vapor content of the atmosphere varies widely. On a volume basis, the normal range is 1 to 3 percent, though it can vary from as little as 0.1 percent to as much as 5 percent.

Water vapor can be a climate warming force when it traps heat, or can cause either climate warming or cooling when it takes the form of clouds, which reflect incoming solar energy away from Earth.

Most climate models predict that a warming of Earth's atmosphere would be accompanied by an increased level of water vapor in the lower atmosphere, but determining whether this has happened in response to recent climate warming is difficult. Data on water vapor concentrations are limited, and the data suffer from a range of limitations, including changes in instrument type, limited geographic coverage, limited time span, and so on. Data from satellites may offer some relief for these problems, but such data have been gathered only for a few years.

Some researchers have observed what appear to be slight increases in water vapor in various layers of the atmosphere, ranging up to 13 percent. Others have analyzed satellite data, and seen what appears to be a drying of the atmosphere, rather than increased moisture levels.

Solar Activity

Rather than burning with a steady output, the sun burns hotter and cooler over time. Several cycles of increased or decreased solar output have been identified, including cycles at intervals of eleven years, twenty-two years, and eighty-eight years.

Though measurements of solar output have been taken only for the past eighteen years, longer trend patterns can be derived from indirect data sources, such as ice cores and tree rings. Cosmic rays, which fluctuate with the sun's activity, also strike constituents of the atmosphere, creating radioactive versions of certain elements. Beryllium, in particular, is ionized to 10Be by cosmic rays. The 10Be then gets incorporated into trees as they grow, and is trapped in bubbles in ice masses, as is carbon dioxide.

A 1995 reconstruction of historical solar output levels from 1600 to 2000 shows that solar irradiance has risen over time, but with many short-term peaks and troughs in the overall curve of increase, increasing the level of solar output that constitutes the main driver for the climate system's temperature.

Studies suggest that increased solar output may have been responsible for half of the 0.55°C (1°F) increase in temperature from 1900 through 1970, and for one-third of the warming seen since 1970.

Ozone

Ozone is a highly reactive molecule composed of three atoms of oxygen. Ozone concentrations vary by geographical location and by altitude. In addition, ozone exerts a different climate-forcing effect, depending upon altitude.

At lower, tropospheric altitudes, ozone exerts a warming force upon the atmosphere. Tropospheric levels of ozone have been increasing in the Northern Hemisphere since 1970, and may have doubled in that time. Ozone concentrations in the Southern Hemisphere are uncertain, while at the poles, tropospheric ozone concentrations seem to have fallen since the mid-1980s. At higher, or stratospheric altitudes, ozone exerts a cooling force upon the atmosphere. Ozone concentrations in the stratosphere have been declining over most of the globe, though no trend is apparent in the tropics. Much of the decline in stratospheric ozone concentrations has been attributed to the destructive action of the chlorofluorocarbons discussed previously.

Section Summary

It is clear that human action can affect seven of eight of the major greenhouse "forcings": carbon dioxide, methane, nitrous oxide, ozone, CFCs, aerosols, and water vapor. As studies of solar variation have shown, it is also clear that human action is not the only factor involved in determining the impact of these forcings. There is still substantial uncertainty regarding the actual climate impact of the climate forcings.

OBSERVED CLIMATE CHANGES

Part of the concern about global climate change stems from the human tendency to seek meaning in events that may or may not be more than simply a random event. A particularly cold winter, a particularly hot summer, an especially rainy season, or an especially severe drought will all send people off on a search for the greater meaning of the phenomenon. Is it a pattern, or a one-time event? Must we build a dike, or has the danger passed? Since the summer of 1988, virtually all unusual weather events seem to have triggered questions about global climate change.

Our ability to really know what the climate is doing is limited by a short observational record, and by the uncertainties involved in trying to figure out what climate was like in the past, or might be like in the future, for comparison with recent climate changes. While Earth's climate has been evolving and changing for more than four billion years, recordings of the temperature cover only about 150 years, less than 0.000004 percent of the entire pattern of evolving climate. In fact, temperature records are spotty before about 1960 and cover only a tiny portion of the globe, mostly over land. In addition to that 150-year conventional surface temperature record, temperature readings taken from weather balloons cover the years since 1970, and satellite temperature readings cover the years since 1982. Modern, reliable measurements of greenhouse gases are also very recent sources of data, beginning with carbon dioxide measurements at the South Pole in 1957, at Mauna Loa in 1958, and later for methane, nitrogen oxides, and chlorofluorocarbons.

Aside from temperature readings, other climate trends proposed as secondary effects of global warming carry information about the state of the climate. Changes in absolute humidity, rainfall levels, snowfall levels, the extent of snowfall, the depth of snowfall, changes in ice caps, ice sheets, sea ice, and the intensity or variability of storms have all been proposed as secondary effects of global warming. But because the history of recording climate trends is extremely short, most evidence regarding nontemperature-related changes in Earth's climate and atmospheric composition prior to the recent history of direct measurements is gathered from indirect sources such as air bubbles trapped in polar ice, or the study of fossils. This evidence, while interesting as a potential "reality check" for global human-made climate change models, is considered far less reliable than direct observational data.

These limitations in our evidence make it difficult to draw hard-and-fast conclusions regarding what changes have actually occurred recently in comparison to past climate conditions. More importantly, these limitations make it difficult to determine whether those changes are beyond the range of previous climate trends, happening at a faster rate than previous climate trends, or are being sustained for longer than previous climate trends. These are all critical questions when evaluating whether humanity is causing changes to Earth's normal climate patterns.

Nevertheless, scientists have evidence at hand regarding recent changes in both atmospheric composition and global climate trends that suggest that humanity has at least changed Earth's atmospheric composition in regard to greenhouse gases and other pollutants. These changes may or may not be contributing to recently observed changes in global warmth. A quick review of the climate changes suggested by the available evidence follows.

Temperature Trends

Besides readings of Earth's surface temperatures taken with standard glass thermometers, direct readings of atmospheric temperatures have been taken with satellites and weather balloons. In addition to direct measurements of Earth's recent temperatures, proxy measurements of temperatures from farther in the past can be derived from borehole temperature measurements, from historical and physical evidence regarding the extent and mass of land and sea ice, and from the bleaching of coral reefs.

This information is in relatively good agreement regarding what seems to be happening to global temperatures, at least in the recent periods of change spanning the past few hundred years, though there are discrepancies among some of the data sets. Temperatures recorded at ground-based measuring stations reveal a mean warming trend ranging from 0.3°C to 0.6°C (0.5°F to 1.1°F) since about 1850, with 0.2°C to 0.3°C (0.4°F to 0.5°F) of this warming occurring since 1960. The warming is not uniform, either in chronology or in distribution. More of the change occurs over land than over water. More of the warming happens at night, resulting in warmer nighttime temperatures rather than hotter daytime temperatures. More of the warming is noticeable as a moderation of wintertime low temperatures rather than as an increase in summertime high temperatures. Temperatures taken from weather balloons (also called radiosondes) and from satellites span a much shorter period of time (though, arguably, a more rigorously standardized measuring technique), and there is controversy over what they indicate, and how much weight should be given to such a short data set. Some analysts contend that the satellite and balloon recordings show a slight cooling trend in the tropics (about 0.1°C [0.2°F] per decade) since 1982. Others contend that the discrepancy is only an artifact caused by a limited data set, and the recent, unrelated increase in the strength of the El Niño southern oscillation.

And even here, taking the simplest of physical measurements, temperature, uncertainties are present. Temperature readings (satellite or ground station) were not taken specifically for the sake of evaluating the climate patterns of the entire Earth. Consequently the readings were taken from a variety of locations, cover only selected parts of the atmosphere, and are not necessarily well placed to be most informative about the climate as a whole. Further, measurement techniques and stations varied over the course of the temperature record, with data adjustments of a full degree occasionally needed to make the different sets of data compatible with each other. Satellites and balloons measure a different part of the atmosphere than ground stations do, making the comparability of such records questionable. In addition, the shortness of the satellite data record, punctuated as it has been by impacts of volcanic eruptions and the El Niño southern oscillation, further complicate the evaluation of temperature data.

Finally, perspective is important. While the past ten thousand years have been abnormally placid as far as climate fluctuations go, evidence of prior climate changes show an Earth that is anything but placid climatically. Some 11,500 years ago, for example, there is evidence that temperatures rose sharply over short periods of time. In Greenland, temperatures increased by as much as 7°C (12.6°F) over only a few decades, while sea surface temperatures in the Norwegian Sea warmed by as much as 5°C (9°F) in fewer than forty years. There is also evidence of about twenty rapid temperature fluctuations during the last glaciation period in the central Greenland records. Rapid warmings of between 5°C and 7°C (9°F to 12.6°F) were followed by slow returns to glacial conditions over the course of 500 to 2,000 years.

Rainfall Trends

Changes in precipitation trends are, potentially, a form of indirect evidence reflecting whether Earth is currently experiencing man-made climate change. Climate change models suggest that an enhanced greenhouse effect could cause changes in the hydrologic cycle such as increased evaporation, drought, and precipitation. But the IPCC warns that "our ability to determine the current state of the global hydrologic cycle, let alone changes in it, is hampered by inadequate spatial coverage, incomplete records, poor data quality, and short record lengths."

The global trend in rainfall showed a slight increase (about 1%) during the twentieth century, though the distribution of this change was not uniform either geographically or over time. Rainfall has increased over land in high latitudes of the Northern Hemisphere, most notably in the fall. Rainfall has decreased since the 1960s over the subtropics and tropics from Africa to Indonesia. In addition, some evidence suggests increased rainfall over the Pacific Ocean (near the equator and the international dateline) in recent decades, while rainfall farther from the equator has declined slightly.

Sea Level Trends

Changes in sea level and the extent of ice sheets, sea ice, and polar ice caps are still another form of indirect evidence reflecting whether Earth is currently undergoing anthropogenic climate change. Climate change theory would suggest that rising global temperatures would cause sea levels to rise due to a combination of the thermal expansion of water and melting of glaciers, ice sheets, ice caps, and sea ice.

Studies of sea leavels considered to reflect our best understanding of sea-level rise, as summarized in the 1995 reports of the United Nations Intergovernmental Panel on Climate Change, indicate a rise of 18 cm during the twentieth century, with a range of uncertainty of 10 to 25 cm. There is little evidence that the rate of sea-level rise has increased during that time period, though in theory the rate of warming has been accelerating. But thermal expansion of water is only one contributor to sea-level changes. Glaciers, ice sheets, and land water storage all play a role—a highly uncertain role.

Surface waters

Global warming would also be expected to influence surface waters such as lakes and streams, through changes induced in the hydrologic cycle. However, the last published report of the IPCC states no clear evidence of widespread change in annual streamflows and peak discharges of rivers in the world (IPCC, 1995, p. 158). While lake and inland sea levels have fluctuated, the IPCC also points out that local effects make it difficult to use lake levels to monitor climate variations.

Snow and Ice Trends

Global warming would also be expected to influence things such as snowfall, snow depth, and snow coverage (or extent), but studies examining changes in these aspects of the climate are quite mixed. Consistent with the indications of slight warming of the global climate, snow cover has declined in recent years, with a higher percentage of precipitation in cold areas coming down as rain rather than snow. But while the annual mean extent of snow cover over the Northern Hemisphere has declined by about 10 percent since 1979, snowfall levels have actually increased by about 20 percent over northern Canada and by about 11 percent over Alaska. Between 1950 and 1990, snowfall over China decreased during the 1950s but increased during the 1960s and the 1970s. Snowfall over the 45–55-degree-latitude belt has declined slightly. Snow depth levels, which respond both to atmospheric temperature and to the ratio of rainfall to snowfall, show equally mixed changes. Snow-depth measurements of the former Soviet Union over the twentieth century show decreased snow depth of about 14 percent during the Soviet winter, mostly in the European portion, while snow depth in the Asian sectors has increased since the 1960s.

Glaciers, Ice Caps, and Ice Sheets

With regard to glaciers and ice caps, the state of knowledge is even more limited. Glaciers and ice caps may have accounted for 2 to 5 centimeters of the observed sea-level rise discussed above, but the range of uncertainty is high. With regard to ice sheets, data are contradictory: There is not enough evidence to know whether the Greenland and Antarctic ice sheets are shrinking, hence contributing to sea-level rise, or growing, and hence retarding sea-level rise. They may even be doing both—growing on top and shrinking at the margins.

Weather Intensity and Variability Trends

Finally, increases in the intensity or variability of weather are considered another form of indirect evidence reflecting whether Earth is currently undergoing human-driven climate change. Predictions of increased incidence of extreme temperatures, tornadoes, thunderstorms, dust storms and fire-promoting weather have been drawn from basic global climate change theory. However, evidence has not so far borne out these predictions on a global scale. The IPCC concludes:

[O]verall, there is no evidence that extreme weather events, or climate variability, has increased, in a global sense, through the 20th century, although data and analyses are poor and not comprehensive. On regional scales, there is clear evidence of changes in some extremes and climate variability indicators. Some of these changes have been toward greater variability; some have been toward lower variability [IPCC, 1995, p. 173].

Section Summary

Evidence regarding changes in Earth's climate in the twentieth century is mixed, and encompasses a range of uncertainties. While the most recently published IPCC report holds that there is a discernible human influence on climate, this conclusion is not dependent on the evidence of actual changes in Earth's climate, as shown in this figure. On that note, the IPCC (1995, p. 411) says, "Despite this consistency [in the pattern of change], it should be clear from the earlier parts of this chapter that current data and systems are inadequate for the complete description of climate change." Rather, this conclusion is based on mathematical modeling exercises and "reality checked" with what hard evidence we have.

UNCERTAINTY AND FUTURE RESEARCH NEEDS

While recent studies of climate have contributed a great deal to our understanding of climate dynamics, there is still much to learn. The process of searching for evidence of man-made climate change, in fact, is both a search for new discoveries about how climate works, and continuing refinement of our understanding of the underlying theories we already have.

While greenhouse effect theory is a relatively uncontroversial issue in the scientific sense, the theory of global, human-driven climate change is at a much younger stage of development. Although there are very few articles in science journals that contradict either the overall theory or details of the core greenhouse effect, the same cannot be said for the theory of human-driven climate change and the consequences of that change. Indeed, nearly every month on the pages of leading science journals, studies jockey back and forth about key elements of human-made climate change.

Current climate change models have acknowledged weaknesses in their handling of changes in the sun's output, volcanic aerosols, oceanic processes, and land processes that can influence climate change. Some of those uncertainties are large enough, by themselves, to become the tail that wags the dog of climate change. Three of the major remaining uncertainties are discussed below.

The Natural Variability of Climate

Despite the extensive discussion of climate modeling and knowledge of past climate cycles, only the past thousand years of climate variation are included in the two state-of-the-art climate models referred to by the IPCC. As discussed earlier, however, the framework in which we view climate variability makes a significant difference in the conclusions we draw regarding either the comparative magnitude or rate of climate changes, or the interpretation of those changes as being either inside or outside of the envelope of normal climate change variations. The IPCC report summarizes the situation succinctly:

Large and rapid climatic changes occurred during the last ice age and during the transition towards the present Holocene period. Some of these changes may have occurred on time-scales of a few decades, at least in the North Atlantic where they are best documented. They affected atmospheric and oceanic circulation and temperature, and the hydrologic cycle. There are suggestions that similar rapid changes may have also occurred during the last interglacial period (the Eemian), but this requires confirmation. The recent (20th century) warming needs to be considered in the light of evidence that rapid climatic changes can occur naturally in the climate. However, temperatures have been far less variable during the last 10,000 years (i.e., during the Holocene) [IPCC, 1995, p. 416].

Until we know which perspective is more reflective of Earth's climate as a whole—the last ten thousand years, or a longer period of time—it will be difficult to put recent warming trends in perspective, or to relate those trends to potential impacts on the climate and on Earth's flora and fauna.

The Role of Solar Activity

At the front end of the climate cycle is the single largest source of energy put into the system: the sun. And while great attention has been paid to most other aspects of climate, little attention has been paid to the sun's role in the heating or cooling of Earth. Several studies in the late 1990s have highlighted this uncertainty, showing that solar variability may play a far larger role in Earth's climate than it was previously given credit for by the IPCC. If the sun has been heating up in recent times, researchers observe, the increased solar radiation could be responsible for up to half of the observed climate warming of the past century. But as with satellite measurements of Earth's temperature, the short timeline of satellite measurements of solar irradiance introduces significant uncertainty into the picture. Most researchers believe that at least another decade of solar radiation measurement will be needed to clearly define the influence of solar input on the global climate.

Clouds and Water Vapor

Between the emission of greenhouse gases and change in the climate are a range of climate and biological cycles that can influence the end result. Such outcome-modifier effects are called "feedbacks" or "indirect effects" in the climate change literature.

One such feedback is the influence of clouds and water vapor. As the climate warms, more water vapor enters the atmosphere. But how much? And which parts of the atmosphere, high or low? And how does the increased humidity affect cloud formation? While the relationships among clouds, water vapor, and global climate are complicated in and of themselves, the situation is further complicated by the fact that aerosols exert a poorly understood influence on clouds.

Earlier computer models, which omitted the recently validated cooling effect of aerosols, overestimated the global warming that we would have expected to see by now, based only on the levels of greenhouse gases that have been emitted. As discussed earlier, aerosols themselves may offset 20 percent of the expected impact of warming gases. In addition, though direct cooling impacts of aerosols are now being taken into account by climate models, aerosol impact on clouds remains a poorly defined effect with broad implications, given a range of additional cooling potential of up to 61 percent of the expected warming impact from the warming greenhouse gases.

The last published report of the IPCC acknowledges that "the single largest uncertainty in determining the climate sensitivity to either natural or anthropogenic changes are clouds and their effects on radiation and their role in the hydrological cycle . . . At the present time, weaknesses in the parameterization of cloud formation and dissipation are probably the main impediment to improvements in the simulation of cloud effects on climate" (IPCC, 1995, p. 346).

THE IMPACTS OF CLIMATE CHANGE

Global warming, and the potential climate changes that might accompany such warming, are estimated using of complex computer models that simulate, with greater or lesser complexity and success, the way Earth's climate would change in response to the level of greenhouse gases in the air. It is widely acknowledged that the potential temperature changes predicted by global warming theory do not pose a direct threat to human life. In fact, since more people die from extremes of cold rather than heat, the actual warming of the atmosphere, on net, could save more lives through warmer winters than it takes through hotter summers.

The major concerns about climate change focus on the second- and thirdhand impacts that would theoretically accompany global warming. Climate change theory suggests that warming of the overall environment could lead to a variety of changes in the patterns of Earth's climate as the natural cycles of air currents, ocean currents, evaporation, plant growth, and so on change in response to the increased energy levels in the total system. The most commonly predicted primary impacts of global warming are increased activity in the hydrologic, or water cycle of Earth, and the possible rise of oceans due to thermal expansion and some melting of sea ice, ice sheets, or polar ice caps. More dynamic activity in the water cycle could lead to increased rainfall in some areas, or, through increased evaporation rates, could cause more severe droughts in other areas. Rising sea levels could inundate some coastal areas (or low-lying islands), and through saltwater intrusion, could cause harm to various freshwater estuaries, deltas, or groundwater supplies.

Some have also predicted a series of thirdhand impacts that might occur if the climate warms and becomes more dynamic. Wildlife populations would be affected (positively and negatively), as would some vegetative growth patterns. The "home range" of various animal and insect populations might shift, exposing people to diseases that were previously uncommon to their area, and so on.

But one need not wade far into the most recently published IPCC report on the potential impacts of climate change before encountering an admission that uncertainty dominates any discussion of such potential impacts:

Impacts are difficult to quantify, and existing studies are limited in scope. While our knowledge has increased significantly during the last decade and qualitative estimates can be developed, quantitative projections of the impacts of climate change on any particular system at any particular location are difficult because regional scale climate change projections are uncertain; our current understanding of many critical processes is limited; and systems are subject to multiple climatic and non-climatic stresses, the interactions of which are not always linear or additive. Most impact studies have assessed how systems would respond to climate changes resulting from an arbitrary doubling of equivalent atmospheric carbon dioxide concentrations. Furthermore, very few studies have considered greenhouse gas concentrations; fewer still have examined the consequences of increases beyond a doubling of equivalent atmospheric carbon dioxide concentrations, or assessed the implications of multiple stress factors [IPCC, 1995, p. 24].

The IPCC report goes on to point out that this extreme uncertainty is likely to persist for some time, since unambiguous detection of human-made climate change hinges on resolving many difficult problems:

Detection will be difficult and unexpected changes cannot be ruled out. Unambiguous detection of climate-induced changes in most ecological and social systems will prove extremely difficult in the coming decades. This is because of the complexity of these systems, their many non-linear feedbacks, and their sensitivity to a large number of climatic and non-climatic factors, all of which are expected to continue to change simultaneously. The development of a base-line projecting future conditions without climate change is crucial, for it is this baseline against which all projected impacts are measured. The more that future climate extends beyond the boundaries of empirical knowledge (i.e., the documented impacts of climate variation in the past), the more likely that actual outcomes will include surprises and unanticipated rapid changes [IPCC, 1995, p. 24].

Uncertainties of this scale do not imply, as some analysts have asserted, that there is no reason to fear negative change, nor does it imply that we must fear drastic impacts. Rather, uncertainties of this scale indicate the need for a sustained research program aimed at clarifying our understanding of Earth's climate and how human activities might or might not translate into negative environmental impacts.

INTERNATIONAL AGREEMENTS ON RELEASE OF GREENHOUSE GASES

In 1997, a treaty was developed to limit the amount of greenhouse gases released into the atmosphere. Under the auspices of the United Nations Secretariat, the Third Conference of the Parties, held in Kyoto, Japan, produced the Kyoto Protocol (Table 1). This protocol calls for reductions in greenhouse gas emissions by various countries, though developing countries, predicted to become the dominant producers of greenhouse gases in the twenty-first century, were not bound to greenhouse gas reductions. Several countries, such as Australia, Iceland, and Norway, were allowed to increase their levels of greenhouse gas emissions under the treaty. The major reductions in emissions were to come from Europe, where Latvia, Estonia, and Lithuania agreed to an 8 percent reduction in emissions relative to 1990 levels. Japan and Canada agreed to 6 percent reductions from 1990 levels, and the United States agreed to reduce greenhouse gas emissions 7 percent below 1990 levels.

The Kyoto Protocol covers reductions in carbon dioxide, methane, nitrous oxide, and three fluorocarbons: hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. The protocol also included mechanisms for considering greenhouse gas reductions stemming from changes in land use, and enshrined the principle of international emissions trading, though not the mechanism or specifics, which were left for later Conferences of the Parties to resolve. Finally, the Kyoto Protocol created a "clean development mechanism" by which developing countries could develop advance credits for taking actions that would limit the release of greenhouse gases in the future.

Several obstacles stand in the way of the Kyoto Protocol. In 1997, prior to President Clinton's acceptance of the Kyoto Protocol, U.S. Senate Resolution 98, the Byrd-Hegel resolution, which was passed by a vote of ninety-five to zero, imposes specific requirements that must be met before the Kyoto Protocol can be ratified. The resolution calls for a specific timeline and commitments by developing countries to reduce greenhouse gas emissions, and evidence that adoption of the Kyoto Protocol would not result in serious harm to the U.S. economy. In addition, the Fifth Conference of the Parties (1999) failed to resolve numerous outstanding issues held over from the previous conference, and put off critical decision making until the Sixth Conference of the Parties in The Hague, Netherlands, in November 2000.

COST OF REDUCING GREENHOUSE EMISSIONS AND IMPACT ON ENERGY SYSTEMS

Reducing emissions of carbon dioxide and other greenhouse gases is not a trivial problem. Fossil fuels provide the overwhelming majority of energy production globally, and are predicted to do so through 2010. In the United States in 2000, fossil fuels were used to produce 70 percent of all energy generated. Alternative technologies such as nuclear power, solar power, wind power, hydropower, geothermal power, and hydrogen power have promise, but also have significant limitations and are considerably more costly than fossil fuel use. Consider that nearly 25 percent of total U.S. energy consumption in 1996 was for transportation, which is nearly all powered by fossil fuels.

Estimates of the cost of reducing greenhouse gas emissions, and the impact that such reductions would have on energy systems, vary widely. Estimates of greenhouse gas reduction costs are critically dependent on the assumptions used in economic models. Models assuming that greenhouse gas reduction targets will be met using international, multiemission trading systems suggest lower costs than those models that assume less international trading, single-gas approaches, carbon taxes, and so on.

In a comparison of nine economic models, estimated costs to the United States as of the late 1990s ranged from a loss in gross domestic product from $40 billion to $180 billion, with assumptions of no emission trading; from $20 billion to $90 billion with trading only among developed countries; and from $5 billion to $20 billion with assumptions of global trading systems. Studies showing values at the higher end of the ranges outnumbered those showing costs at the lower end of the spectrum. The largest costs in such models stem from accelerated fuel substitution and the adoption of more expensive nonfossil fuel forms of fuel generation.

Kenneth Green

See also:Acid Rain; Air Pollution; Atmosphere; Environmental Problems and Energy Use; Ocean Energy Systems; Pollution and Energy Efficiency.

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