Global Climate Change is one of the most complex science-derived issues to wind up at the center of political discourse. In the landmark Second Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the section dealing with the science of climate change is over 500 pages long by itself, containing 75 pages of references. Documentation from outside the IPCC is considerably more voluminous.

The terrific complexity of the issue of climate change and its potential environmental and societal consequences poses a challenge for the decision-making apparatus in democratic societies. It is unrealistic to expect the public, policymakers, or the media to read and understand the full body of climate-change literature. Instead, they must rely on publications published by pressure groups with a position for or against climate change, or on the verbal summation of a small number of high-profile experts. But 30-second sound bites do not leave much room for qualifications, and, just as the devil is in the details, the quality of science is in the qualifications. The first thing to be lost in science-policy discussion is a clear representation of the complexity of the issue that accurately depicts both the certainties and the uncertainties involved.

The importance of having accurate portrayals of the nature, magnitude, certainty, and imminence of environmental hazards is hard to overstate. As a society, we have a limited amount of resources with which to address a broad array of hazards, whether we address them through environmental improvement programs such as air quality controls, or whether we address them through other public-health improvement efforts. Wasting resources by ranking problems poorly costs lives and diminishes quality of life.

This guide translates the major lines of evidence regarding global climate change from the arcane language of science into the mainstream language of English, so that the weighing of evidence can be put back into a public debate that too often weighs only the sound bites of pundits, politicians, industry representatives, and celebrity scientists.

This guide is not intended to provide a comprehensive treatment of every facet of global climate change science. Rather, this guide is offered to provide a basic foundation in the policy-relevant elements of climate science in order to enhance the quality of public-policy debate.

Plain English Guide No. 3

Table of Contents

Why a Plain English Guide??*

Climate-change Theory?*

A. In a Greenhouse?*

B. From Greenhouse to Globe?*

C. Human Action and Climate Change?*

Warming and Cooling Forces?*

A. Carbon Dioxide?*

B. Methane?*

C. Nitrous Oxide?*

D. Halocarbons?*

E. Aerosols?*

F. Water Vapor?*

G. Solar Activity?*

H. Ozone?*

I. Section Summary?*

Observed Climate Changes?*

A. Temperature Trends?*

B. Rainfall Trends?*

C. Sea-level Trends?*

D. Surface-water Trends?*

E. Snow Trends?*

F. Ice-mass Trends?*

G. Weather Intensity and Variability Trends?*

H. Section Summary?*

The Impacts of Climate Change?*

 

Uncertainty and Future Research Needs?*

A. The Natural Variability of Climate?*

B. The Role of Solar Activity?*

C. The Impact of Clouds and Water Vapor?*

Conclusion?*

About the Author?*

Acknowledgements?*

Other RPPI Studies?*

Part 2

Climate-change Theory

o some, climate change simply refers to physical changes in climate. The rise and fall of ice ages, triggered by changes in the Earth's orbital dynamics would be one such change. Trends in global average temperature would be another, and there are still many others areas of study, such as storm frequency and relative humidity that all fall into the category of "climate change."

In the way most people use the term today, however, climate change refers to the body of theory that explains how certain gases&emdash;regardless of origin&emdash;influence the climate. Still others focus on the human aspect of the question and consider climate change only in regard to the way that human activities and atmospheric impacts influence the climate.

Humans have studied the climate for thousands of years, building devices of great ingenuity such as Stonehenge to help them predict the changing of the seasons. A better understanding of climate can provide knowledge crucial to agriculture and help humans survive catastrophic weather events. But in recent years, the focus of climate research has centered on the potential for human activity to affect the climate.

The theory of human-caused, or "anthropogenic," climate change may be simple to state, but it is a deceptive simplicity. Unlike Einstein's "E=MC²," the theory of anthropogenic climate change is not a single, mathematical theory. The theory of anthropogenic climate change is actually a cluster of interlocking theories, some well defined, others less well defined.

A. In a Greenhouse

At the heart of the theory of climate change (both of human origin and non-human) is a well-established theory called the greenhouse effect. This theory, first quantified by a mathematician named Joseph Fourier in 1824, is based on well defined and tested laws of thermodynamics and has been repeatedly validated by not only laboratory experiments, but by millions of greenhouse owners.

The greenhouse effect is relatively simple. When energy from the sun reaches the surface of the 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 and subsequently re-emitted in the form of heat. Over a bare patch of ground, with a clear sky overhead, there would be no net increase in the temperature over time because the heat absorbed during the day would be re-radiated toward space overnight, and air masses, warmed by the re-emission would be free to shed that energy by rising, and mixing with the rest of the atmosphere.

But if there is a greenhouse located over that patch of ground, the dynamic changes. The sunlight enters as usual, and some of it is reflected back out as usual. But part of the incoming solar energy that was absorbed by things inside the greenhouse and subsequently re-emitted as heat does not pass back out through the glass, warming the greenhouse up. If there are water-bearing plants or materials inside the greenhouse, water vapor concentration will increase as it warms. And since water vapor is also capable of trapping heat, the greenhouse warms still further. Air masses, warmed by the incoming energy are prevented from rising or mixing with the rest of the atmosphere, keeping the heat in the greenhouse. Eventually, the system reaches temperature equilibrium.

B. From Greenhouse to Globe

Scientists have known for a long time that the greenhouse effect applies not only to greenhouses but also to the Earth as a whole, with certain gases playing part of the role of the glass in the example above. When applied to the whole planet, this relationship between gases in the atmosphere and the temperature of the atmosphere produces "global warming." Global 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 global warming that goes with it, the Earth would be a much colder planet. The greenhouse effect has also been seen to maintain warmer planetary atmospheres on Mars and Venus. The Earth's greenhouse effect warms the surface from an average of -18° C (0° F) to about 15° C (59° F).

C. Human Action and Climate Change

Against the backdrop of an Earth warmed by its own greenhouse effect, other forces operate that can alter the retention of heat by the atmosphere. Some of these forces are of human origin, some are produced solely by nature, and some are produced by feedback reactions or secondary interactions of one atmospheric component with another.

Theories about anthropogenic climate change, the target of much political discourse today, focus on the role that human action has in changing the climate and the consequences of that change. Further, much discussion of anthropogenic climate change is generally limited to the potentially negative impacts that might occur.

While greenhouse-effect theory is a relatively uncontroversial issue in the scientific sense, the theory of global, anthropogenic climate change is at a much younger stage of development. Very few articles appearing in science journals contradict either the overall theory or details of the greenhouse effect, or recent indications of a warming climate. However, the same cannot be said for the question of causation. Indeed, studies jockey back and forth about potential causes of climate change nearly every month on the pages of leading science journals including America's premier science journal, Science. Even the last landmark report of the prestigious Intergovernmental Panel on Climate Change (IPCC) leaves open the question of causation. The chapter of the IPCC report examining the role of mankind's contribution to climate change begins with a summary that says:

At the end of the chapter, the situation is summarized, thus:

Nonetheless, the chapter on the detection and attribution of climate change concludes that:

Part 3

Warming and Cooling Forces

uman activities (as well as non-human 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 effect, and others which, according to the same theory, could exert a cooling effect. In climate-change literature, these are referred to as "climate forcings." Some forcings can trigger still other forcings, of either a warming or cooling variety. These induced forcings are generally referred to as feedbacks.

A. 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 18^th century to the present and are now at 365 parts per million by volume and rising. Before industrialization, carbon dioxide levels fluctuated near 280 parts per million, though dips as low as 200 parts per million or surges into the mid-300 parts per million have been observed through analysis of air bubbles trapped in polar ice cores.

Carbon dioxide is released into the atmosphere, and back into carbon cycle by both human and non-human processes. Human activities such as fuel burning, cement production, and land-use patterns change the carbon dioxide concentration of the atmosphere, as do changes in ocean currents, volcanic eruptions, changes in atmospheric humidity, and so on. Table 1 shows the major flows of carbon in the environment. Because of the long lifetime, carbon emitted but not absorbed in a given year builds up over time.

Figure 1: Historical Trends in Carbon Dioxide Concentration

Source data: NASA Goddard Institute for Space Studies, http://www.giss.nasa.gov/data/si99/ghgases/

Since highly accurate, direct measurement of carbon dioxide levels only began in the late 1950s, most of our understanding of carbon dioxide's historical patterns of fluctuation comes from indirect measurements, such as the analysis of gas bubbles trapped in Antarctic glaciers. Though such indirect measurements carry greater uncertainty than direct measurements of carbon dioxide levels, they have contributed to rapid growth in our understanding of the Earth's carbon cycle in recent years. 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.

B. Methane

Methane is a greenhouse gas between 5.4 and 56 times more powerful a warming agent as carbon dioxide. Methane has a lifetime in the atmosphere of about 12 years. 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 have increased nearly 150 percent since the beginning of the 19^th century, with current levels being the highest ever recorded, though the pattern of methane emissions is highly irregular and has recently leveled off for reasons that are unclear.

Figure 2: Historical Trends in Methane Concentration

Source data: NASA Goddard Institute for Space Studies, http://www.giss.nasa.gov/data/si99/ghgases/

Studies of methane concentrations in the distant past show that methane concentrations fluctuated significantly, from as little as 0.4 parts per million during the last ice age, to as much as 0.7 parts per million during the industrial period.

Methane comes from a variety of sources, some of human origin, some of non-human origin. Table 2 shows the sources of methane found in the atmosphere.

Source: Adapted from R.T. Watson, L.G. Meira Filho, E. Sanhueza, and A. Janetos, "Greenhouse Gases: Sources and Sinks," Climate Change 1992, eds. J.T. Houghton, B.A. Callander, and S.K. Varney (Cambridge, MA: University Press, 1992), p. 35.

Notes:

1) A Tg, or teragram, is one billion kilograms. 2) Calculations of percent contribution by author, using data from table 14.1, in Barbara J. Finlayson-Pitts and James N. Pitts, Jr., Chemistry of the Upper and Lower Atmosphere (NY: Academic Press, 1999), p. 777.

Since highly accurate, direct measurement of carbon dioxide levels only began in the late 1950s, most of our understanding of carbon dioxide's historical patterns of fluctuation comes from indirect measurements, such as the analysis of gas bubbles trapped in Antarctic glaciers.

C. Nitrous Oxide

Nitrous oxide is a long-lived warming gas with a relative warming strength of 170 to 210 times that of carbon dioxide, depending on the time scale one considers. Nitrous oxide persists in the atmosphere for about 120 years. Nitrous oxide is, like methane, considered a trace gas in the atmosphere, but at considerably lower levels, around 0.3 parts per million of the atmosphere by volume. As Figure 3 shows, nitrous oxide concentrations have increased in recent years. Prior to the industrial period, concentrations of nitrous oxide fluctuated at around an average of 279 parts per billion by volume, though fluctuations as low as 200 parts per billion by volume were seen in the distant past.

Figure 3: Historical Trends in Nitrogen Dioxide Concentration by Year

Source data: NASA Goddard Institute for Space Studies, http://www.giss.nasa.gov/data/si99/ghgases/

Nitrous oxide comes from a variety of sources, some of human origin, some of non-human origin. Table 3 shows the sources of nitrogen oxides found in the atmosphere.

D. Halocarbons

Halocarbons are compounds of human origin used as cooling agents and propellants in a broad range of applications. There are many different species of halocarbon, some of which have been banned because of concerns about their adverse impacts upon the stratospheric ozone layer. One such banned halocarbon that many would be familiar with is Freon, used in household air conditioners and refrigerators. Halocarbons are thought to be very powerful warming gases. Some species are over 10,000 times more capable of trapping heat than is carbon dioxide. Halocarbons can also be very long-lived, persisting for many hundreds of years in the atmosphere after release. One group, called the perfluorocarbons, are virtually "immortal," persisting for up to 50,000 years in the case of perfluoromethane. Offsetting their greater heat-trapping ability is the fact that halocarbons 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, halocarbons are measured in parts per trillion. Figure 4 shows the concentration of the major halocarbon species over the last century.

Figure 4: Historical Trends in CFC Concentration by Year

Source data: NASA Goddard Institute for Space Studies, http://www.giss.nasa.gov/data/si99/ghgases/

Halocarbons can exert a variety of impacts on the climate, depending on the chemical species and where it is found. As discussed previously, greenhouse gases warm the atmosphere by absorbing heat radiated from the surface or lower altitudes. Halocarbons are no different, and function as warming gases at lower altitudes. In the upper atmosphere, however, halocarbons are broken apart by ultraviolet radiation, and the fragments no longer function as warming gases. Rather, the fragments can exert a cooling impact through their interaction with high altitude ozone. If the halocarbon is one that liberates chlorine on being broken up, the chlorine acts as a catalyst that causes ozone destruction. Since ozone is itself a global warming gas, the removal of the ozone exerts a cooling impact on the climate. Ozone destruction particularly cools the upper atmosphere, but also cools all layers below it by allowing more heat to escape into space.

On a net basis, our current understanding is that the ozone-depleting halocarbons (the production of which has been banned by the Montreal Protocol) exert a cooling effect. Replacement chemicals for the ozone-depleting halocarbons are considered pure warming gases but with a considerably lower warming potential than the chemicals they replaced. Because of the incredible complexities of ozone chemistry in the atmosphere and uncertainties regarding the warming or cooling potential of remaining ozone-depleting halocarbons and replacement compounds, the ultimate impact of halocarbons on climate change is highly uncertain.

E. Aerosols

Aerosols are not gases but are liquid or solid particles small enough to stay suspended in the air. Both human and non-human processes generate aerosols. Different aerosol types have different impacts upon the climate. 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. Aerosols do not remain in the atmosphere for long periods of time, tending to be "rained out" regularly. Table 4 shows the types and flux rates for aerosol particulates in the atmosphere.

Source: Intergovernmental Panel on Climate Change, Climate Change 1995: The Science of Climate Change (Cambridge, MA: Cambridge University Press, 1996), Table 2.6, p. 104.

Notes:

1) An Mton, or megaton, is one million metric tons.

2) Percentage calculations by author. Subtotal percentages differ from line-item percentages due to rounding.

Most aerosols of human origin exert a cooling effect on the climate. On a global basis, models suggest, this cooling effect offsets about 20 percent (and possibly more) of the predicted warming from the combined greenhouse warming gases, but the cooling is not uniform: the offsetting impact varies geographically depending on local aerosol concentrations.

Figure 5 shows the fluctuation in aerosol concentrations over time, in the Northern and Southern Hemispheres, measured by the ability of the atmosphere to absorb and scatter light in certain wavelengths, also known as "optical depth."

The omission of aerosol considerations in earlier climate models led to considerable over-prediction of projected global warming and predicted regional impacts, though newer models have done much to internalize the cooling effect of aerosols.

Annual Mean Optical Depth

Graphic source: NASA Goddard Institute for Space Studies, http://www.giss.nasa.gov/data/strataer/

The omission of aerosol considerations in earlier climate models led to considerable over-prediction 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 particulate matter also increases the reflectivity, formation, and lifetime of clouds, affecting the reflection of incoming solar radiation back to space.

F. Water Vapor

Water vapor is the most abundant of the greenhouse gases and the dominant contributor to the natural greenhouse effect. About 0.4 percent of all the molecules in the air are water vapor, about 10 times the abundance of carbon dioxide. Glaciers and ice caps contain about 1,900 times as much water as the total atmosphere does, while ground and surface waters hold about 660 times as much water as the total atmosphere does. The oceans hold over 100,000 times as much water as the entire Earth's atmosphere, at any given time.

Besides the total amount of water vapor in the atmosphere, climate studies must also consider its distribution at different altitudes and locations because small changes in water vapor can have significant local effects on climate. Almost all water vapor enters the atmosphere by evaporation and about 90 percent of the Earth's water vapor is in the half of the atmosphere closest to the Earth's surface. The water-holding ability of the air roughly doubles for every 10° C in temperature, so the water vapor content near the surface can range from slightly above 8 percent in very moist cases in the Persian Gulf to under 0.00002 (0.2 parts per million) percent at the world record coldest temperature in Antarctica. At typical United States temperatures, 1 to 2 percent of the air molecules near the surface are water vapor. Typical stratospheric water vapor concentrations (at altitudes from 15 to 30 kilometers) are around two to three parts per million based on limited observations. About 73 percent of the Earth's water vapor is over the "tropical half" of the Earth, between 30 N and 30 S.

Water vapor plays multiple roles in influencing climate, functioning as either a climate-warming force or a climate-cooling force. When it enters the atmosphere via evaporation, water vapor cools the surface from which it evaporates and is a cooling force. Another way that water vapor can cool the climate when it takes the form of clouds that reflect incoming solar energy away from the Earth. Low-level clouds are thicker and tend to cool the Earth by reflecting energy away. Thin cirrus clouds, by contrast, let incoming energy pass through, but hinder re-radiated energy from passing back into space. Water vapor can also warm the climate because it traps heat even more effectively than carbon dioxide.

A basic physics-based view of the response of water vapor to warming from other greenhouse gases suggests that more water will evaporate from a warmer ocean, and warmer air can hold more water vapor. But basic climate processes, because of their mass and momentum, are not expected to change much so the relative humidity will stay nearly the same. If this scenario is true, water held in the atmosphere will increase by about 7 percent for each Centigrade degree of warming. The increased moisture content could trap more heat, amplifying the warming from other greenhouse gases as a positive feedback. An alternative theory, by Richard Lindzen, postulates that the added evaporation due to climate warming would still occur, but greater storm energy levels would force the added moisture back out of the air more quickly, possibly canceling out the potential amplifying effect on warming.

Most climate models predict that a warming of the 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 only been gathered since the late 1970s.

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.

G. 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 11 years, 22 years, and 88 years.

Highly accurate satellite measurements of solar output have only been taken since the early 1980s. Scientists attempt to derive longer trend patterns from indirect data sources such as ice cores and tree rings. For example, 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.

Using such data, scientists have produced plausible "reconstructions" of historical solar output levels. From 1600 C.E. to the present, such reconstructions suggest that the sun has clearly been running hotter, increasing the level of solar output, which constitutes the main natural driver for the Earth's climate system's temperature.

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

H. Ozone

Ozone is a highly reactive molecule composed of three atoms of oxygen. Ozone concentrations vary by geographical location and by altitude. Ozone is a greenhouse warming gas, but in areas of ozone depletion, ozone exerts a climatic cooling simply because its concentration is decreasing.

At lower, tropospheric altitudes, ozone exerts a warming force upon the atmosphere. Tropospheric levels of ozone in the Northern Hemisphere may have doubled since the 1800s. 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, decreasing ozone concentrations exert 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.

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

I. Section Summary

Human action can affect six out of seven of the major climate factors discussed above&emdash;carbon dioxide, methane, nitrogen oxides, ozone, CFCs, and water vapor. Yet human action is not the only factor involved in determining the atmospheric concentrations of these factors.

As Figure 6 shows, there is still substantial uncertainty regarding the actual climate forcing power of the climate factors described above.

Part 4

Observed Climate Changes

art of the concern about global climate change stems from the human tendency to seek meaning in events which may or may not be more than simply random events. A particularly cold winter, a particularly hot summer, an especially rainy season, or especially severe droughts 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 trigger questions about global climate change.

Our ability really to 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 the Earth's climate has been evolving and changing for over four billion years, recordings of the temperature only cover about 150 years, less than 0.000004 percent of the entire pattern of evolving climate. In fact, temperature records are spotty before the 1950s and only cover a tiny portion of the globe, mostly over land. Temperature readings taken from weather balloons became widespread in the 1960s, although observations are sparse over the oceans. Global satellite temperature readings are nearly continuous since the late 1970s. Modern, reliable measurements of greenhouse gases are an even newer source of data, beginning with carbon dioxide measurements at the South Pole in 1957, at Mauna Loa in 1958, and later for methane, nitrous oxide, and halo.

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 such climate trends is extremely short, most evidence regarding non-temperature-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-caused climate change models, is considered far less reliable than direct observational data.

These limitations in 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, are 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, we do have evidence at hand regarding recent changes in both atmospheric composition and global climate trends that suggest that humanity has at least changed the 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.

While the Earth's climate has been evolving and changing for over four billion years, recordings of the temperature only cover about 150 years, less than 0.000004 percent of the entire pattern of evolving climate.

A. 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 the direct measurements of the Earth's recent temperatures, proxy measurements of temperatures from farther in the past can be derived from bore-hole 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 proxy 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 last few hundred years, though there are discrepancies between some of the data sets. According to the IPCC, temperatures recorded at ground-based measuring stations reveal a mean warming trend ranging from 0.3 ° C to 0.6 ° C since about 1850, with 0.2 to 0.3 ° C of this warming occurring since the middle 1970s. The warming is not uniform, either in time or 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 satellites span a much shorter period of time, 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 much less warming in the lower atmosphere than at the surface (about 0.06 ° C/decade in the lower troposphere and 0.20 ° C/decade at the surface from 1979-1998). Others contend that the discrepancy is partly artificial due to more complete satellite than radiosonde data coverage, and partly real but transient since the radiosonde data in the tropics back to 1960 shows greater warming aloft than at the surface until the late 1970s. So this longer record shows little discrepancy between long-term surface and lower tropospheric warming. 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. Recently, a report by the National Research Council of the National Academy of Sciences sought to reconcile the surface temperature readings with those of the satellites. The NRC concluded that while disparities remain, the satellite temperature record does not suggest that the surface temperature record is in error. Rather, the discrepancy suggests that the upper parts of the atmosphere are not warming as quickly as the low-altitude atmosphere, possibly due to the cooling effects of volcanic eruptions or ozone depletion in the stratosphere.

But even here, taking the simplest of physical measurements, 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 extensive data adjustments 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. This makes the comparability of such records questionable. Further, the shortness of the satellite data record, punctuated as it has been by impacts of volcanic eruptions and the El Niño Southern Oscillation, complicates the evaluation of temperature data.

Some people interpret the observed changes of temperature as evidence to support the theory that human action has caused changes in the global climate. Others find the evidence regarding observed changes in temperature insufficient to allow a sound conclusion regarding the validity of that theory because of the historical volatility of climate variations. While the last 10,000 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, at the end of the last ice age, there is evidence that temperatures rose sharply in certain regions, and perhaps globally, over quite short periods of time. In Greenland, temperatures increased by as much as 7 ° C over only a few decades, while sea-surface temperatures in the Norwegian Sea warmed by as much as 5 ° C in less than 40 years. There is also evidence of about 20 rapid temperature fluctuations during the last glaciation period in the central Greenland records. Rapid warmings of between 5 ° C and 7 ° C were followed by slow returns to glacial conditions at intervals of 500 to 2,000 years. Earth's orbital dynamics were somewhat different prior to the last 10,000 years however, which may limit the relevancy of such changes to modern trends. Still, in interpreting current climate changes, the broadest view of past climate changes lends important perspective.

B. Rainfall Trends

Changes in precipitation trends are, potentially, a form of indirect evidence reflecting whether the Earth is currently experiencing climate change. As the IPCC report observes, "an enhanced greenhouse effect may lead to changes in the hydrologic cycle, such as increased evaporation, drought, and precipitation." But the section of the report on precipitation changes as an indirect measure warns "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."

According to the IPCC, the global trend in rainfall has shown a slight increase (about 1 percent) during the 20^th century, though the distribution of this change is 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 dateline) in recent decades, while rainfall farther from the equator has declined slightly.

C. 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 the Earth is currently undergoing 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.

Indeed, recent studies of sea levels indicate a rise of 18 cm over the last 100 years, though there is considerable uncertainty attached to that figure: estimates range from 10&endash;25 cm. And, though the rate of warming has been accelerating, there is little evidence that the rate of sea-level rise has sped up accordingly, as theory would suggest. But thermal expansion of water is only one contributor to sea-level changes. Glaciers, ice sheets, and land-water storage all play a role&emdash;a highly uncertain role. Based on limited data, it is estimated that glacier and ice cap melting may have accounted for two to five centimeters of the observed sea level rise, but the range of uncertainty is high.

D. Surface-water Trends

Global warming would also be expected to influence surface waters such as lakes and streams, through changes induced in the hydrologic cycle. But the last published report of the IPCC reports no clear evidence of widespread change in annual stream flows and peak discharges of rivers in the world. 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.

E. Snow Trends

Snowfall, snow depth, and snow coverage (or extent) would also be affected by global warming, but studies examining changes in such 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 over the past 21 years of study, snowfall levels have actually increased by about 20 percent over northern Canada and by about 11 percent over Alaska. Snowfall over China decreased during the 1950s, but increased during the 1960s and 1970s. Snowfall over the 45 to 55 degree latitude belt has declined slightly. Average snow depth, which responds both to atmospheric temperature and to the ratio of rainfall to snowfall, shows equally mixed changes. Snow-depth measurements of the former Soviet Union over the 20^th century show decreased snow depth of about 14 percent during the winter, mostly in the European portion of the ex-Union, while snow depth in the Asian sectors of the former Soviet Union has increased since the 1960s.

F. Ice-mass Trends

Ice masses include glaciers, ice caps, ice sheets, and sea ice. With regard to glaciers and ice caps, the state of knowledge is quite limited. Many of the world's glaciers have clearly retreated over the last 100 years. However, as the IPCC acknowledges, "…continuous, long term measurements of the mass balances of glaciers and ice caps are very limited."

Data on ice-sheet changes are actually contradictory: there is not enough evidence to know whether the Greenland and Antarctic ice sheets are shrinking or growing. They may even be doing both, growing on top and shrinking at the margins.

Finally, regarding sea ice (floating masses such as icebergs), the last published IPCC report observes that, "neither hemisphere has exhibited significant trends in sea ice extent since 1973 when satellite measurements began."

G. Weather Intensity and Variability Trends

Finally, increases in the intensity or variability of weather are considered another form of indirect evidence reflecting whether the Earth is currently undergoing climate change.

Predictions of increased incidences of extreme temperatures, tornadoes, thunderstorms, dust storms, and fire weather have been drawn from some climate-change models. But evidence has not, so far, borne out these predictions on a global scale. The IPCC concludes that: "…overall, there is no evidence that extreme weather events, or climate variability, has increased, in a global sense, through the 20^th 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."

There is not enough evidence to know whether the Greenland and Antarctic ice sheets are shrinking or growing.

H. Section Summary

As Figure 7 indicates, evidence regarding changes in Earth's climate in the 20^th century is mixed and encompasses a range of uncertainties.

While the IPCC report holds that there is a discernible human influence on climate, this conclusion does not rest primarily on the evidence of actual changes in the Earth's climate, as shown in this figure. On that note, the IPCC 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, the conclusion that humanity is exerting a "discernible influence" on climate is based on mathematical modeling exercises and "reality checked" with what hard evidence exists. The IPCC sums up the question of attributing observed climate changes to human action, thus: "Although these global mean results suggest that there is some anthropogenic 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."

Source: Intergovernmental Panel on Climate Change, Climate Change 1995: The Science of Climate Change (Cambridge, MA: Cambridge University Press, 1996), Figure 3.22, p. 180.

Part 5

The Impacts of Climate Change

lobal warming and the potential climate changes that might accompany such warming are estimated through the use of complex computer models that simulate (with greater or lesser complexity and success) the way that the Earth's climate might change in response to the level of greenhouse gases in the air. Most climate-change commentary acknowledges that the potential temperature changes predicted by global warming theory do not pose a direct threat to human life. Mortality might rise from hotter summers, but this might be balanced by mortality reductions from warmer winters. Rather, the major concerns about climate change focus on the second- and third-hand 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 the Earth, and the possible rise of oceans due to thermal expansion and some melting of sea ice, ice sheets, or polar icecaps. 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 salt-water intrusion, could cause harm to various freshwater estuaries, deltas, or groundwater supplies.

Some have also predicted a series of third-hand 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.

One need not wade far into the IPCC's 880-page volume on the potential impacts of climate change, however, before encountering an admission that uncertainty dominates any discussion of such potential impacts.

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.

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

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.

Part 6

Uncertainty and Future Research Needs

hile 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 human-caused climate change, in fact, is both a search for new discoveries about how climate works and a continuing refinement of our understanding of existing underlying theories.

Many areas of uncertainty remain. 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 to become the tail which wags the dog of climate change. Three of the major remaining uncertainties are discussed below.

A. The Natural Variability of Climate

Despite the extensive discussion of climate modeling and knowledge of past climate cycles, only the last 1,000 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 one views climate variability makes a significant difference in the conclusions drawn regarding either the comparative magnitude or rate of climate changes, or the interpretation of those changes. The last published IPCC report summarizes the situation succinctly:

Without knowing which perspective is more reflective of Earth's climate as a whole&emdash;the last 10,000 years, or a longer period of time&emdash;it is difficult to put recent warming trends into perspective. It also remains difficult to relate those trends to potential impacts on the climate and on the Earth's flora and fauna.

B. The Role of Solar Activity

At the front end of the climate cycle is the single largest source of energy entering the system, namely, the sun. And while great attention has been paid to most other aspects of climate, until recently, less attention has been paid to the sun's role in the heating or cooling of the Earth. Several recent studies have highlighted this uncertainty, showing that solar variability may play a far larger role in the Earth's climate than it was previously given credit for by the IPCC. If the sun has been heating up in recent times, some researchers have claimed, the increased solar radiation could be responsible for up to half of the observed climate warming of the past century.

Harvard astrophysicist Sallie L. Baliunas, for example attributes up to 71 percent of the observed climate warming of the past century to increased solar irradiance. Other researchers such as noted climatologist Tom Wigley, however, rank the influence of solar activity on climate warming much lower, at "somewhere between 10 percent and 30 percent of the past warming." But as with satellite measurements of Earth's temperature, the short time line of satellite measurements of solar irradiance introduces significant uncertainty into the picture. Most researchers believe that at least another decade of solar radiation measurement and modeling of irradiance impacts on climate will be needed to clearly define the influence of solar input on the global climate.

C. The Impact of Clouds and Water Vapor

Between the emission of greenhouse gases and changes 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 relationship between clouds, water vapor, and global climate is complicated in and of itself, the situation is further complicated by the fact that aerosols exert a poorly understood influence on clouds. Earlier computer models, which omitted the recently recognized 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 have offset 20 percent of the expected impact of present day warming gases, though this effect is expected to diminish in the future due to other pollution controls on sulfur aerosols. In addition, though some 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.

As the IPCC report acknowledges: "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."

Part 7

Conclusion

he theory of human-caused climate change is extremely complex, a mixture of the certain and the uncertain, the obvious and the subtle, the evidenciary and the theoretical. The complex nature of climate-change theory poses a challenge in the public policy arena, where decisionmakers must grapple with a question that even experts have difficulty in understanding, and where nuanced handling of uncertainty is not only rare, but may be actually discouraged by the demands of the policymaking process.

A considerable body of evidence suggests that the average temperature of the Earth's atmosphere has been warming over the course of the 20^th century. But important questions remain, such as "why has it been warming," "what impacts will warming produce," and "what should we do about it"? On these questions, conclusions are far more tentative. Evidence suggests that the warming seen in the first half of the 20^th century was non-human in origin&emdash;most probably a result of increased solar radiation. The balance of evidence suggests that a significant portion of the warming that has been seen in the latter half of the 20^th century is due to human activity. Specifically, the emission of heat-trapping gases known as greenhouse gases and changes in land-use patterns seem to have caused additional heat retention by the atmosphere.

But questions remain regarding other possible causes of the observed atmospheric warming, and tremendous uncertainties limit the ability of scientists to predict the second- and third-hand impacts of that warming, as well as the steps that can be taken to forestall or ameliorate the negative consequences that may arise.

About the Author

r. Kenneth Green is Director of the Environmental Program at Reason Public Policy Institute. Dr. Green has published several previous peer-reviewed policy studies on climate change including: A Plain English Guide to the Science of Climate Change; Climate Change Policy Options and Impacts; Evaluating the Kyoto Approach to Climate Change; and A Baker's Dozen: 13 Questions People Ask About the Science of Climate Change. Dr. Green recently completed writing an encyclopedia chapter on climate change for the Macmillan Encyclopedia of Energy, and has completed expert reviews for the three volumes of the forthcoming IPCC Third Assessment Report, and the National Assessment report of the United States Global Climate Research Project. Green received his doctorate in environmental science and engineering (D.Env.) from UCLA in 1994.

Acknowledgements

he author wishes to thank the numerous reviewers who have contributed to the accuracy and clarity of this revision to 1997's Plain English Guide to the Science of Climate Change, as well as those who helped me with the original version. The author is particularly indebted to Steven Schroeder, for his help with sections on water vapor sequestration. This Guide also depends heavily on the publications of the United Nations Intergovernmental Panel on Climate Change, whose 1995 three-volume "Second Assessment Report" is still the common "touch stone" reference for discussions of climate change.

Other RPPI Studies

Kenneth Green, Richard McCann, Steve Moss, and Roy Cordato, Climate Change Policy Options and Impacts, Policy Study No. 252 (Los Angeles: Reason Public Policy Institute, February 1999).

Kenneth Green, A Baker's Dozen: 13 Questions People Ask About the Science of Climate Change (Los Angeles: Reason Public Policy Institute, October 1998).

Kenneth Green, A Plain English Guide to the Science of Climate Change, Policy Study No. 237 (Los Angeles: Reason Public Policy Institute, December 1997).

Richard McCann and Steven Moss, Nuts and Bolts: The Implications of Choosing Greenhouse-Gas Emission Reduction Strategies, Policy Study No. 171 (Los Angeles: Reason Public Policy Institute, November 1993).

Steven Moss and Richard McCann, Global Warming, Policy Study No. 167 (Los Angeles: Reason Public Policy Institute, September 1993).

Endnotes

1. Intergovernmental Panel on Climate Change (IPCC), Climate Change 1995: The Science of Climate Change (Cambridge, MA: Cambridge University Press, 1996). Because neither the executive summary nor the individual chapter summaries do justice to the full detail of the report, the information in this guide will be drawn from the body of the text whenever possible. Anthropogenic means "created by humans." Though anthropogenic is an unfamiliar term to many, it's far less cumbersome than human-caused, or "of human origin" and avoids the sexist connotations of "manmade."
2. Joseph Fourier, "Remarques Générales sur la Température du Globe Terrêstre et des Espaces Planétaires," Annals de Chimie et de Physique, vol. 27 (1824), pp. 136-167.
3. The primary greenhouse gases are water vapor (H₂O), Carbon Dioxide (CO₂), Methane (CH₄), Nitrous Oxide (NO), and Halocarbons.
4. IPCC, Climate Change 1995: The Science of Climate Change; Stanley E. Manahan, Environmental Chemistry, Fourth Edition (Monterey, CA: Brooks/Cole Publishing Company, 1996), p. 57.
5. Fortunately, the greenhouse extremes on Venus and Mars will not occur on the Earth. The air on Venus and Mars is more than 95 percent carbon dioxide. Venus has about 175,000 times as much carbon dioxide per unit area as on Earth, and Mars has 31 times as much.
6. The Intergovernmental Panel on Climate Change is preparing a new set of reports, which will not be published until sometime late in 2000, or perhaps 2001. While such reports are not without detractors and controversy, they constitute an important touchstone against which virtually all new climate findings are weighed. Until the new reports are released, the 1995 IPCC reports constitute the last published documents that summarized the state of knowledge about climate change in a systematic, peer-reviewed manner. As such, the 1995 reports will be a primary reference source in this Guide.
7. IPCC, Climate Change 1995: The Science of Climate Change; p. 411.
8. Ibid., p. 439.
9. Ibid.
10. IPCC, Climate Change 1995: The Science of Climate Change, p. 311.
11. Friederike Wagner, Sjoerd J.P. Bohncke, David L. Dilcher, et al., "Century-scale Shifts in Early Holocene Atmospheric CO₂ Concentration," Science vol. 284 (June 18, 1999), pp. 1971-1972. See also Barbara J. Finlayson-Pitts and James N. Pitts, Jr., Chemistry of the Upper and Lower Atmosphere (New York, NY: Academic Press, 1999), p. 776.
12. Deep ocean reservoirs are not included, for example, though they can exchange carbon with the atmosphere over long periods of time.
13. IPCC, Climate Change 1995: The Science of Climate Change, pp. 75&endash;87.
14. The determination of relative warming strength depends on the time-span that one is examining, because the different gases have different lifespans as well as different heat-trapping abilities. In a 20-year timeframe, a molecule of methane would have 56 times the impact of a carbon dioxide molecule, but since carbon dioxide has a longer lifespan, this ratio declines over time. In a 500 year framework, a molecule of methane is only 6.5 times as strong a warming gas as a molecule of carbon dioxide is estimated to be.
15. IPCC, Climate Change 1995: The Science of Climate Change, p. 121.
16. Ibid., p. 87.
17. T. Blunier, J. Chappellaz, J. Schwander, et al., "Variations in atmospheric methane concentration during the Holocene Epoch," Nature, vol. 374 (March 2, 1995), pp. 47-48; Edward J. Brook, Todd Sowers, and Joe Orchardo, "Rapid Variations in Atmospheric Methane Concentration During the Past 110,000 years," Science, vol. 273 (August 23, 1996), pp. 1087-1090.
18. IPCC, Climate Change 1995: The Science of Climate Change, p. 88.
19. Ibid., p. 121.
20. Ibid., p. 88.
21. J. Flückiger, A. Dallenbach, T. Blunier, et al., "Variations in Atmospheric N2O Concentration During Abrupt Climatic Changes, Science, vol. 285 (July 9, 1999), pp. 227-229.
22. IPCC, Climate Change 1995, The Science of Climate Change, p. 121.
23. Ibid., pp. 90&endash;93, 119&endash;123.
24. Ibid.
25. Ibid., p. 122.
26. Ibid, p. 118. Total greenhouse gas forcing estimate is 2.45 Wm^-2, total direct aerosol forcing is &endash;0.5 Wm^-2. If one includes indirect effects of particulates, the aerosol forcing reaches &endash;1.3, or about 53 percent of the total greenhouse gas forcing.
27. Ibid., p. 297.
28. Ibid., p. 103.
29. Ibid., p. 161.
30. Calculated from Kevin E. Trenberth and Christian J. Guillemot, "The Total Mass of the Atmosphere," Journal of Geophysical Research, vol. 99, no. D11 (November 20, 1994), pp. 23,079-23,088.
31. Calculated from José Peixoto and Abraham H. Oort, Physics of Climate (New York: American Institute of Physics), 1992, p. 272.
32. Calculated from Rebecca J. Ross and William P. Elliot, "Tropospheric Precipitable Water: A Radiosonde-based Climatology," NOAA Technical Memorandum ERL ARL-219 (Washington D.C.: National Oceanic and Atmospheric Administration, 1996), pp. 83-85.
33. S. J. Oltmans and D. J. Hofmann, "Increase in Lower-Stratospheric Water Vapour At a Mid-Latitude Northern Hemisphere Site from 1981 to 1994," Nature, vol. 374, no. 6518 (March 9, 1995), pp. 146-149.
34. Calculated from Trenberth and Guillemot, "The Total Mass of the Atmosphere."
35. William P. Elliott, "On Detecting Long-Term Changes in Atmospheric Moisture, Climatic Change, vol. 31 (1995), pp. 349-367.
36. Richard S. Linzen, "The Importance and Nature of the Water Vapor Budget in Nature and Models," in Climate Sensitivity to Radiative Perturbations: Physical Mechanisms and their Validation, NATO ASI Series I: Global Environmental Change, vol. 34, ed. H. Le Treut (Heidelberg, Germany: Springer-Verlag, 1995), pp. 51-66.
37. William P. Elliott, "On Detecting Long-Term Changes in Atmospheric Moisture," Climatic Change, vol. 31 (1995), pp. 349-367; David Rind, "Just Add Water Vapor," Science, vol. 281 (August 21, 1998), p. 1152.
38. William P. Elliott, "On Detecting Long-Term Changes in Atmospheric Moisture," Climatic Change, vol. 31 (1995), pp. 349-367.
39. Ibid.
40. IPCC, Climate Change 1995: The Science of Climate Change, p. 162.
41. David Rind, "Just Add Water Vapor," p. 1152; Steven R. Schroeder and James P. McGuirk, "Widespread Tropical Atmosphere Drying from 1979 to 1995," Geophysical Research Letters, vol. 25, no. 9 (May 1, 1999), pp. 1301-1304.
42. Judith Lean and David Rind, "Climate Forcing by Changing Solar Radiation," Journal of Climate, vol. 11 (December 1998), pp. 3069-3094; Richard Kerr, "A New Dawn for Sun-Climate Links," Science, vol. 271 (March 8, 1996), pp. 1360-1361.
43. Judith Lean and David Rind, "Climate Forcing by Changing Solar Radiation," Journal of Climate, vol. 11 (December 1998), pp. 3069-3094.
44. Ibid.
45. Ibid.
46. IPCC, Climate Change 1995, The Science of Climate Change, p. 78.
47. Discussion of the relative reliability of such data with regard to evaluating current change is from K. Hasselmann, "Climate Change Enhanced: Are We Seeing Global Warming," Perspectives, Science, vol. 276 (May 9. 1997), p. 915.
48. Ibid.
49. Bleaching of coral reefs marks a variety of environmental disturbances, from high-heat episodes, to high-pollution episodes such as the eruption of Mount Pinatubo in June 1991.
50. Benjamin Santer, et al., "Intepreting Differential Temperature Trends at the Surface and in the Lower Troposphere," Science, vol. 287, no. 5456 (February 18, 2000), pp. 1242-1245; Dian Gaffen, et al., "Multidecadal Changes in the Vertical Temperature Structure of the Tropical Troposphere," Science, vol. 287 (February 18, 2000), pp. 1242-1245.
51. R.W. Spencer and J.R. Christy, "Precision and Radiosonde Validation of Satellite Gridpoint Temperature Anomalies, Part II: A Tropospheric Retrieval and Trends During 1979-90," Journal of Climate, vol. 5 (1992), pp. 858&endash;866.
52. National Research Council, "Reconciling Observations of Global Temperature Change" (Washington D.C.: National Academy Press, 2000).
53. Ibid, p. 2
54. IPCC, Climate Change 1995: The Science of Climate Change, p. 181.
55. Michael L. Parsons, Global Warming, the Truth Behind the Myth (New York: Insight Books, Plenum Press, 1995), ch. 5.
56. Richard A. Kerr, "Studies Say&emdash;Tentatively&emdash;That Greenhouse Warming is Here," Research News, Science, volume 268 (June 1995), p. 1567.
57. Parsons, Global Warming, Ch. 5.
58. IPCC, Climate Change 1995: The Science of Climate Change, p. 179.
59. Ibid., p. 178.
60. Ibid.
61. Ibid., pp. 151&endash;152.
62. Ibid.
63. Ibid., pp. 152&endash;154.
64. Ibid., p. 368.
65. Ibid., p. 381.
66. Ibid., p. 158.
67. Ibid., p. 157.
68. Ibid, p. 380.
69. Ibid.
70. Ibid., p. 150.
71. Ibid., p. 173.
72. Ibid, p. 411.
73. This is not a particularly huge logical leap, since virtually all organisms impact their ecosystem, with humankind as no exception.
74. Ibid.
75. IPCC, Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses (Cambridge, MA: Cambridge University Press, 1996), p.12.
76. Ibid.
77. Richard Stone, "If the Mercury Soars, so May Health Hazards," News and Comment, Science, vol. 267 (February 17, 1995), p. 957; Rita R. Colwell, "Global Climate and Infectious Disease: The Cholera Paradigm," Association Affairs, Science, vol, 174 (December 20, 1996), p. 2025; and Gary Taubes, "Apocalypse Not," News and Comment, Science, vol. 278 (November 7, 1997), p. 1004.
78. IPCC, Climate Change 1995: Impacts, Adaptations and Mitigation of Climate Change, p. 24.
79. Ibid.
80. IPCC, Climate Change 1995: The Science of Climate Change, p. 416.
81. Richard C. Willson, "Total Solar Irradiance Trend During Solar Cycles 21 and 22," Reports, Science, volume 277 (September 26, 1997), pp. 1963-1965; and IPCC, Climate Change 1995: The Science of Climate Change, p. 117.
82. Richard A. Kerr, "A New Dawn for Sun-Climate Links," Research News, Science, vol. 271 (March 8, 1996), pp. 803-804.
83. William J. Broad, "Another Possible Climate Culprit: The Sun," New York Times, September 1997.
84. Richard A. Kerr, "A New Dawn for Sun-Climate Links," Research News, Science, vol. 271 (March 8, 1996), pp. 803-804.
85. Richard A. Kerr, "Did Satellites Spot a Brightening Sun," News, Science, Vol. 277 (September 26, 1997), pp. 1923-1924.
86. Potential aerosol impacts on clouds are given a value range from 0-1.5 Wm^-2, compared to the total warming potential of the well-mixed greenhouse gases of 2.45 Wm^-2. IPCC, Climate Change 1995: The Science of Climate Change, p. 118.

IPCC, Climate Change 1995: The Science of Climate Change, p. 346.