A Guide to Global Warming – Questions and Answers on Climate Change

Introduction

For well over a decade, the George C. Marshall Institute has been addressing the studies, the speculation on hazards, the many efforts to predict, and the theories supporting fear of human-made global climate change.

The George C. Marshall Institute believes that public discussion of human-made global warming as a world encompassing problem may be a clear example of multiple “detail solutions” being proposed before a sound scientific definition of the problem, or even a recognition of the problem’s existeence, has been established.  Accordingly, the Institute has compiled a list of the questions commonly posed about global warming.  The Institute, using well founded knowledge of the present state of the art, has composed a response to these questions that summarizes today’s knowledge and assesses the task ahead to provide an answer.  It is the Institute’s hope that this effort will help the United States determine how best to acquire a valid scientific foundation that can then support sound public policy.

One note of caution.  Concerns about global climate change are particularly complex as they touch on climate science, economics, societal response to climate change, and other issues.  In order to prepare a guide that is accessible to a general audience, some simplification is inevitable.  For this reason, references to the technical literature have been provided in the endnotes for those interested in further study.

 The Board of Directors

1. What is the greenhouse effect?

 

The greenhouse effect is natural and necessary for life on Earth.

 

2. What will be the effect on global climate of increasing amounts of human-made greenhouse gases such as carbon dioxide in the air?

 

Nobody knows for sure.

 

3. Has the earth’s temperature increased over the last 100 years in step with increasing atmospheric CO2?

 

No.

 

4. Has the earth’s temperature increased over the past two decades?

 

Very little, by 0.05°C per decade in the lower atmosphere.

 

5. How precise are measurements of the earth’s temperature?

 

Temperatures taken by satellites are very precise; those taken from ground stations are less so.

 

6. Is it possible that human-made global warming is occurring, but is masked by the cooling effect of air pollution?

 

This is not likely.

 

7. What does the record of past climates show about the relationship between changes in atmospheric CO2 levels and changes in global temperature?

 

The records show that changes in global temperature do not always follow changes in atmospheric CO2 levels.

 

8. How much does the global climate vary naturally?

 

Natural climate changes can occur rapidly.

 

9. What influence does the sun have on global climate change?

 

Scientists are studying this question with great care. It now seems that the sun has a significant influence on climate.

 

10. What is the greenhouse “fingerprint”?

 

It is a unique pattern of changing climate whose detection would suggest that recent warming is caused by greenhouse gases from human activities, as opposed to natural processes.

 

11. Are computer simulations of the earth’s climate accurate?

 

No.

 

12. How much will the earth’s temperature rise if the amount of human-made CO2 in the atmosphere continues to increase in the next 100 years?

 

There is no definitive answer to this question at present.

 

13. Will the earth’s climate change in other ways?

 

Predictions of changes in other aspects of climate, such as precipitation and sea-level rise, are even more uncertain than the projections of global temperature.

 

14. Will global warming produce a rise in sea level and cause major flooding?

 

As computer simulations have become more sophisticated, projections of rising sea levels have become much smaller.

 

15. Will global warming produce more violent storms?

 

This is not likely.

 

16. Will global warming cause the spread of infectious diseases?

 

This is not likely.

 

17. Is there a consensus among climate scientists that greenhouse warming from human activities is a major threat?

 

No. Science is not aimed at building a politically potent consensus over questions of public policy.

 

18. Does the threat of major climate change justify drastic reductions in CO2 emissions by the United States and other nations?

 

No.

 

19. Is there a clear policy on climate change and CO2 emissions which makes sense based on our current knowledge?

 

We have at least 25 years to research this issue before CO2 emission cuts need to be considered.

1. What is the greenhouse effect?

The greenhouse effect is natural and necessary for life on Earth.

The sun supplies the energy to warm the earth. The atmosphere, which is mostly transparent to the incoming sunlight, absorbs outgoing reflected or internal thermal radiation to keep the earth warmer than it would be otherwise.

This absorbing property of the atmosphere is the greenhouse effect. Gases in the atmosphere that absorb infrared radiation, thereby preventing some of the outgoing energy from returning to space, are called greenhouse gases.

Not all gases in the atmosphere absorb outgoing infrared radiation. Nitrogen and oxygen, which make up most of the earth’s atmosphere, have no blocking effect. The gases that absorb the infrared radiation and create the greenhouse effect are mainly water vapor, carbon dioxide, methane, and nitrous oxide. Water vapor and water in clouds absorb nearly 90% of the infrared radiation, while carbon dioxide, methane, and the other minor greenhouse gases together absorb little more than 10% of the infrared radiation.

Therefore, most of the greenhouse effect is natural and caused by the different forms of water in the atmosphere. However, human activities over the last 100 years, like burning wood, coal, oil, and natural gas, have increased the concentration of greenhouse gases in the atmosphere by an amount equivalent to a fifty percent increase in carbon dioxide alone. According to some projections, the amount of greenhouse gases in the atmosphere from human activities will be effectively equal to a doubling of CO2 in the next 100 years.

1 The term “greenhouse effect,” coined nearly two centuries ago, is scientifically inaccurate. A greenhouse stays warm because the closed windows prevent the inside air from cooling by circulation; the glass does not absorb outgoing infrared radiation.

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2. What will be the effect on global climate of increasing amounts of human-made greenhouse gases such as carbon dioxide in the air?

Nobody knows for sure.

Scientists can calculate how much energy is added to the atmosphere by increases in greenhouse gases. Then, using sophisticated computer models, they attempt to simulate how the climate responds to the added energy. But no one knows how to calculate correctly the climate’s response to the added energy. Two of the many questions in this regard are the impacts of water vapor and clouds in the climate response. Scientists need to learn more about whether changes in atmospheric water vapor and clouds amplify or diminish the effects of human-made greenhouse gases on the earth’s climate.

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3. Has the earth’s temperature increased over the last 100 years in step with increasing atmospheric CO2?

No.

The average global temperature of the earth has increased roughly 0.5°C over the last 100 years. The instrumental surface record of temperature sampled worldwide and going back over 100 years is shown in Figure 1. The amount of CO2 in the atmosphere has also increased in that period.

 

 

Figure 1. Changes in yearly average surface-temperature sampled worldwide, reconstructed for land and sea surface (solid line, University of East Anglia Climate Research Unit [CRU]), and land only (dashed line, NASA-Goddard Institute for Space Studies [GISS]). The temporary warming in 1997-1998 is a natural and unusually strong El Niño event and biases the estimated warming trend upward.

However, these two increases did not take place together. Much of the observed temperature rise of 0.5°C occurred before 1940, whereas most of the additional carbon dioxide (over 80%) entered the atmosphere after 1940. Increased greenhouse gases cannot explain a temperature rise that occurred before the major increases in these gases existed in the atmosphere.

Furthermore, from 1940 to 1970, carbon dioxide built up rapidly in the atmosphere, and according to the computer projections of climate, the temperature of the earth should also have risen rapidly. Instead, as the chart shows, the temperature dropped.

The increase in greenhouse gases cannot explain the rapid rise in temperature prior to 1940, and it cannot explain the drop in temperature from 1940 to 1970. The climate record over the last 100 years provides no support for the idea that human activities, such as burning coal and oil for energy, caused the early 20th century global warming. Natural factors must have caused most of that warming.

2 D. E. Parker et al., Interdecadal changes of surface temperature since the late nineteenth century, Journal of Geophysical Research 99, 14373 (1994), plus updates from the CRU website; J. Hansen and S. Lebedeff, Global trends of measured surface air temperature, Journal of Geophysical Research 92, 13345 (1987), plus updates from the GISS website. Note that the good agreement between the two different records – land alone and land plus ocean – is not understood.

 

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4. Has the earth’s temperature increased over the past two decades?

Very little, by 0.05°C per decade in the lower atmosphere.

Starting in 1979, the temperature of the lower atmosphere (from the surface up to roughly 8 km) has been measured by satellites (See Figure 2). Instruments (Microwave Sounding Units [MSUs]) mounted on satellites take daily temperature readings by measuring the microwave radiation emitted by atmospheric oxygen molecules. The precision of the satellite temperature measurements is confirmed by the fact that they are in excellent agreement with temperatures measured independently by a worldwide network of radiosonde instruments carried aloft by balloons. Compared to the spotty coverage of the surface measurements, especially over the oceans, the satellite record provides essentially global coverage.

Figure 2. Changes in monthly average lower tropospheric temperature between latitudes 82°N and 82°S through August 1999, from satellite Microwave Sounding Unit (MSU) measurements. These data have been corrected for the recently reported effects of, for example, satellite orbit decay. The temporary warming in 1997-1998 is a natural and unusually strong El Niño event.

The combined land and ocean surface record (Figure 1) shows a warming trend of about 0.11°C per decade (1958-1998) and 0.19°C per decade (1979-1998), while the satellite temperature records (Figure 2) since 1979 show a warming trend of 0.05°C per decade. The contrast in temperature trends between the surface and satellite measurements of the lower atmosphere is puzzling because the computer models say the CO2-produced warming trend in the lower atmosphere should be larger than at the surface, but it is not. This important disagreement remains unexplained.

3 J. R. Christy, R. W. Spencer, and E. S. Lobl, Analysis of the merging procedure for the MSU daily temperature time series, Journal of Climate 11, 2016 (1998); F. J. Wentz and M. Schabel, Effects of orbital decay on satellite-derived lower-tropospheric temperature trends, Nature 394, 661 (1998); J. R. Christy, R. Spencer, and W. D. Braswell, MSU tropospheric temperatures: Dataset construction and radiosonde comparisons, Journal of Atmospheric and Oceanic Technology, in press (2000).

4 W. Soon et al., Environmental effects of increased atmospheric carbon dioxide, Climate Research, 13, 149 (1999). The 20-year periods can yield biased trends because of the relative shortness of the records. For example, a warm bias in the calculated trends results from the presence of the 1997-1998 El Niño event.

5 L. Bengtsson, E. Roeckner, and M. Stendel, Why is the global warming proceeding much slower than expected, Journal of Geophysical Research 104, 3865 (1999); J. T. Houghton et al., eds., Climate Change 1995 – The Science of Climate Change: Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge: Cambridge University Press, 1996), 527 pp. Hereafter, IPCC 1995.

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5. How precise are measurements of the earth’s temperature?

Temperatures taken by satellites are very precise; those taken from ground stations are less so.

Different estimates of recent trends in the average temperature of the earth vary widely in precision, depending on how and when they were made. Calculating the average surface temperature for a single year requires averaging local temperatures recorded at thousands of points all around the globe at different times of day and night throughout the year. Estimating trends in temperature in order to separate human-made from natural temperature effects requires precise record keeping over many decades. But precise land-based instrumental records are available only for a limited part of the earth’s surface and only since the middle of the last century, a comparatively short time span from which to assess natural climate changes.

Most surface temperature measurements have been made over land, and primarily in inhabited regions. Measurements for the ocean, which covers three-quarters of the area of the globe, are sparse. Measurements from a single station are sometimes used to represent an area as large as a million square miles. Temperature records also have other biases that require correction. One important bias arises from the fact that recording sites in and near cities are subject to the urban-heat-island effect. These sites were originally in rural areas, but as more people moved in and the surrounding areas were developed, the local temperature may have increased for reasons unrelated to changes in atmospheric greenhouse gases.

The most precise records of global temperature began in 1979, when continuous satellite observations started recording temperatures at different heights in the atmosphere. The globally averaged temperatures, derived from the satellites and balloons, of the layer of the atmosphere from the surface to a height of a few kilometers do not show the increasing warming trend over the last twenty years expected by the computer model projections of the effects of added CO2 in the air (see Figure 2).

6 P. J. Michaels, R. C. Balling, Jr., R. S. Vose, and P. C. Knappenberger, Analysis of trends in the variability of daily and monthly historical temperature measurements, Climate Research 10, 27 (1998).

7 “… the global time series since 1990 reveals that the full data set has warmed more than the rural during recent years and this coincides with a decreasing percentage of the rural stations in the full data set. – T. C. Peterson et al., Global rural temperature trends, Geophysical Research Letters 26, 329 (1999).

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6. Is it possible that human-made global warming is occurring, but is masked by the cooling effect of air pollution?

This is not likely.

In attempting to explain the major discrepancy between computer projections of warming expected from increased greenhouse gases and the changes actually observed in the earth’s temperature, some researchers have suggested that the human-made warming might have been offset by a cooling effect of atmospheric pollution by sulfate aerosols – clouds of small particles released into the atmosphere when fossil fuels are burned. These aerosols reflect sunlight back to space and cause a cooling. When the models are modified to take this cooling into account, the predicted temperature increase lessens and moves closer to the observed trend for the past twenty years. However, a careful look at the patterns of temperature change shows that this cannot be correct. For example, the computer models predict a substantial warming trend in the Southern Hemisphere, while the satellite temperature measurements show a statistically significant cooling trend there. Aerosols cannot explain this discrepancy, because the concentration of aerosols in the Southern Hemisphere is very low; the source of the aerosols is mainly the heavily industrialized Northern Hemisphere, and the lifetime of the particles in the air is too short for them to accumulate over the Southern Hemisphere. Unlike CO2, which remains in the air for years and does build up over both hemispheres, the aerosols do not and so the Southern Hemisphere is relatively free of aerosol loading. Adding aerosol forcing to the models may bring the predicted global warming trend closer to the observed temperature trend, but then the observed hemispheric trends are incorrect.

8 B. D. Santer, K. E. Taylor, T. M. L. Wigley, J. E. Penner, P. D. Jones, and U. Cubasch, Towards the detection and attribution of an anthropogenic effect on climate, Climate Dynamics 12, 77 (1995); S. F. B. Tett, J. F. B. Mitchell, D. E. Parker, and M. R. Allen, Human influence on the atmospheric vertical temperature structure: Detection and observations, Science 274, 1170 (1996).

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7. What does the record of past climates show about the relationship between changes in atmospheric CO2 levels and changes in global temperature?

The records show that changes in global temperature do not always follow changes in atmospheric CO2 levels.

For example, when scientists analyzed the relationship between atmospheric CO2 levels and temperatures dating back 250,000 years, inferred from ice cores drilled in Greenland and the Antarctic, they found that sometimes the concentration of CO2 was high when the temperature was low, and sometimes the CO2 was low when the temperature was high. Moreover, a careful analysis showed that some of the atmospheric CO2 changes did not precede the temperature changes, as the greenhouse warming theory would predict. Instead, changes in atmospheric carbon dioxide followed the temperature changes. The atmospheric CO2 changes were not the cause of the temperature changes; the CO2 changes were likely driven by vegetation changes in response to natural variations in air and sea-surface temperatures.

9 H. Fischer et al, Ice core records of atmospheric CO2 around the last three glacial terminations, Science 283, 1712 (1999); A. Indermühle et al, Holocene carbon-cycle dynamic based on CO2 trapped in ice at Taylor Dome, Antarctica, Nature 398, 121 (1999).

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8. How much does the global climate vary naturally?

Natural climate changes can occur rapidly.

Over the last million years, the earth’s climate has shifted dramatically between ice ages and warmer periods like the present one, called the Holocene. The glacial periods, with major advances of ice sheets, have generally lasted about 100,000 years, while the interglacial periods have lasted about 10,000 years. The transition between glacial and interglacial conditions can take place in less than a thousand years – sometimes in as little as decades. Such a dramatic climatic shift occurred near the end of the last major ice age, about 15,000 years ago. First, a brief warming occurred, then the ice age returned for roughly 1,000 years. Finally, by 11,000 years ago, the climate was quickly warming again.10 

During the last 10,000 years the climate has remained relatively warm and stable, allowing humans to advance and prosper. But even during this generally warm period the temperature has fluctuated significantly. The climate was warmer than it is today about 6,500 years ago, during the Holocene Climate Optimum. There is evidence that roughly 1,000 years ago regions of the earth again were substantially warmer than they are today, during the period called the Medieval Climate Optimum. By the 14th century, a cold period called the Little Ice Age had begun. The warming begun in the late 19th and early 20th centuries seems to be a natural recovery from the Little Ice Age.11 

As stated earlier, the average surface temperature over the last hundred years has increased by about 0.5°C. Much of the 0.5°C warming occurred early in the 20th century, before the major increase in greenhouse gases in the atmosphere, so most of the 0.5°C warming must have resulted from natural factors of climate change. The early 20th century warming could not have been caused by human-made greenhouse gases.

Closer to the present, some researchers believe that the 1980s were the hottest decade in 100 years and that some years in the 1990s may have been hotter still. However, the trend in surface temperature is one or two tenths of a degree occurring over a decade or two. Such a change is well within the range of the climate’s natural variations whose mechanisms are not all understood. It is safe to conclude “the natural variability of climate adds confusion to the effort to diagnose human-induced climate change. Apparent long-term trends can be artificially amplified or damped by the contaminating effects of undiagnosed natural variations.”12

10 R. B. Alley et al., Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event, Nature 362, 527 (1993); K. C. Taylor et al., The Holocene – Younger Dryas transition recorded at Summit, Greenland, Science 278, 825 (1997).

11 H. H. Lamb, Climate, History and the Modern World (London and New York: Methuen, 1985), 387 pp.

12 J. D. Mahlman, Uncertainties in projections of human-caused climate warming, Science 278, 1416 (1997).

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9. What influence does the sun have on global climate change?

Scientists are studying this question with great care. It now seems that the sun has a significant influence on climate.

One natural factor in climate change may be variations in the brightness of the sun, over decades to centuries, that are in step with changes in the sun’s magnetism, which has a period of roughly 11 years and is known as the sunspot cycle. Brightness changes have only been recorded over two decades of satellite observations. If brightness changes were to continue to occur over decades to centuries, they might cause terrestrial temperature change.

The climate record indicates a solar effect of this kind. One example is the record of the sun’s magnetism and reconstructed land temperatures of the Northern Hemisphere over 240 years (Figure 3).13  If changes in the sun’s magnetism track changes in the sun’s brightness, over these long time scales, they might explain the high correlation of solar magnetism with terrestrial temperature change. Climate models suggest that changes of roughly 0.5% in the sun’s brightness would produce global average temperature changes of about 0.5°C over a century or so.14 

Figure 3. Changes in the sun’s magnetism (represented by the length of the 22-year Hale cycle, solid line) and changes in the Northern Hemisphere land temperature (dotted line). Shorter magnetic cycles are more intense and suggest a brighter sun. (See Endnote 13)

13 S. Baliunas and W. Soon, Are variations in the length of the activity cycle related to changes in brightness in solar-type stars? Astrophysical Journal 450, 896 (1995). The reason Northern Hemisphere land temperatures are plotted instead of global temperatures is that the Northern Hemisphere record extends back further than the global. Where the two records overlap, there is good agreement.

14 W. H. Soon, E. S. Posmentier, and S. L. Baliunas, Inference of solar irradiance variability from terrestrial temperature changes, 1880-1993: An astrophysical application of the sun-climate connection, Astrophysical Journal 472, 891 (1996). Although change in the sun?s brightness is the simplest proposed mechanism for affecting terrestrial global temperature, the sun?s output comes in many wavelengths; it also emits energetic particles. Both the sun?s light and particles vary in time, space, and frequency. Components of the earth?s climate system may respond to different aspects of the sun?s diverse energy outflows.

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10. What is the greenhouse “fingerprint??

It is a unique pattern of changing climate whose detection would suggest that recent warming is caused by greenhouse gases from human activities, as opposed to natural processes.

According to the computer simulations of climate, three major features of the greenhouse fingerprint are:

– An increasing trend in global surface temperature, currently of 0.2°C to 0.3°C per decade.15 

Surface temperature measurements show an increase of less than 0.2°C per decade since the late 1970s, at the low end of the forecast. However, more precise satellite and balloon measurements find no statistically significant increasing temperature trend.

– An increasing trend in Arctic temperatures about twice that of the global average temperature.16  Over the past fifty years, the Arctic should have warmed by 1°C or more.

The computer models predict that the Arctic will be especially sensitive to greenhouse gas warming. But Arctic surface and balloon measurements indicate no warming trend during the past 40 years. Lower tropospheric temperature measurements made by satellites during the last 20 years also show no warming trend in the Arctic.17 

– A relatively greater warming of tropospheric temperatures (roughly the lower 8 km in the atmosphere) when compared to the surface with a cooling in the stratosphere (roughly 10-25 km).

This common fingerprint is often misrepresented and misunderstood. In addition to the contradiction presented by the observed lack of warming of the troposphere compared to the surface warming over the last 20 years (see question 4), there are important unresolved questions. For example, computer models predict a cooling of the stratosphere from increased CO2, but this expected cooling is difficult to see among other, more dominant effects from volcanic aerosols to changes in stratospheric ozone and solar ultraviolet radiation.

15 IPCC 1995, p. 301.

16 S. Manabe, R. J. Stouffer, M. J. Spelman, and K. Bryan, Transient responses of a coupled ocean-atmosphere model to gradual changes of atmospheric CO2. Part I: Annual mean response, Journal of Climate 4, 785 (1991).

17 The satellite MSU (1979-1998) and balloon radiosonde date (1958-1998) show no net warming in the high (60°-90°N) northern latitudes. See ftp://wind.atmos.uah.edu/msu/t2lt and J. K. Angell, Comparison of surface and tropospheric temperature trends estimated from a 63-station radiosonde network, 1958-1998, Geophysical Research Letters 26, 2761 (1999).

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11. Are computer simulations of the earth’s climate accurate?

No.

As researchers have come to a better understanding of the workings of climate, the projections have changed substantially. For example, the middle-range estimates of global temperature increase over the next 100 years from the United Nations’ Intergovernmental Panel on Climate Change (IPCC) have dropped significantly, from 3.3°C in 1990 to 2°C in the 1995 report.18 

To understand why there are such huge uncertainties in the computer simulations, consider that the additional energy added to the climate system by the doubling of atmospheric CO2 is about 4 watts per square meter (W/m2) – a small amount of energy compared to the amount of the sun’s radiation (342 W/m2) at the top of the atmosphere. But 4 W/m2 is also small compared to the uncertainties in the climate change calculations. For example, knowledge of the amount of energy flowing from equator to poles is uncertain by an amount equivalent to 25-30 W/m2. The amount of sunlight absorbed by the atmosphere or reflected by the surface is also uncertain by as much as 25 W/m2. Some computer models include adjustments to the energy flows of as much as 100 W/m2. Imprecise treatment of clouds may introduce another 25 W/m2 of uncertainty into the basic computations.19 

These uncertainties in modeling climate processes are many times larger than the 4 W/m2 input of energy resulting from a doubling of CO2 concentration in the atmosphere. It is difficult to see how the climate impact of the 4 W/m2 can be accurately calculated in the face of such huge uncertainties. As a consequence, forecasts based on the computer simulations of climate may not even be meaningful at this time. A comparison of nearly all the most sophisticated climate models with actual measurements of current climate conditions found the models in error by about 100% in cloud cover, 50% in precipitation, and 30% in temperature change. In addition, even the best models give temperature change results differing from each other by a factor of two or more.20 

According to the computer simulations, the temperature of the earth should have increased by at least 1°C since the beginning of the century because of increases in atmospheric greenhouse gases. In fact, the temperature actually increased by only 0.5°C, and, as noted above, much of that increase occurred prior to 1940, before some 80% of the CO2 had entered the atmosphere. Only a few tenths of a degree at most could have been caused by increases in atmospheric CO2. That is 3-4 times less than the computer models predicted. If the predictions exaggerated the warming to date by a factor of 3-4, they are unreliable for projecting the future climate change.

If the model projections for the last two decades are compared to the temperature trends of the lower atmosphere as measured by satellites and balloons, then the model projections must be lowered by an even larger factor.

The observed surface and lower-atmosphere temperatures do not support predictions of dramatically rising temperatures from increased atmospheric greenhouse gases.

18 See IPCC 1995, p. 6.

19 R. D. Cess et al., Absorption of solar radiation by clouds: Observations versus models, Science 267, 496 (1995); T. P. Charlock and T. L. Alberta, The CERES/ARM/GEWEX Experiment (CAGEX) for the retrieval of radiative fluxes with satellite data, Bulletin of the American Meteorological Society 77, 2673 (1996); R. S. Lindzen, Can increasing carbon dioxide cause climate change? Proceedings of the National Academy of Sciences USA 94, 8335 (1997).

20 T. P. Barnett, Comparison of near-surface air temperature variability in 11 coupled global climate models, Journal of Climate 12, 511 (1999); W. L. Gates et al., An overview of the results of the Atmospheric Model Intercomparison Project (AMIP I), Bulletin of the American Meteorological Society 80, 29 (1999).

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12. How much will the earth’s temperature rise if the amount of human-made CO2 in the atmosphere continues to increase in the next 100 years?

There is no definitive answer to this question at present.

First, it is difficult to project trends in atmospheric CO2 concentration because the gas is absorbed and released by the ocean, soil and vegetation. Second, it is also difficult to calculate the response of the climate to increased atmospheric CO2 as well as to independently occurring natural climate changes. Current computer simulations of the climate suggest that global warming from increased greenhouse gases may increase the global temperature by somewhere between 0.8°C and 4.5°C by the year 2100, a range too wide to be meaningful.21 

Moreover, according to these same computer models, global temperatures should now be increasing at a significant rate – as much as 0.3°C per decade. But this rate is higher than the recent observed trends measured by satellites, balloons, and surface instruments which suggests that the computer simulations are exaggerating the effect of the increases in CO2.

21 IPCC 1995, p. 40 and Chapter 6.

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13. Will the earth’s climate change in other ways?

Predictions of changes in other aspects of climate, such as precipitation and sea-level rise, are even more uncertain than the projections of global temperature.

Average global temperature is a crude measure of the climate of the planet; it is more important to know how different regions will be affected by changes in seasonal temperature averages and ranges, precipitation, wind patterns, the distribution of different plant and animal species, and other climate-related phenomena. But computer models are no more capable of telling us about regional climate change than they are of predicting globally averaged climate change. One example comes from the comparison of the results of eighteen computer simulations of the warmer and wetter conditions that prevailed in Northern Africa around 6,000 years ago. The average of modeled precipitation is too low by a factor of two to three to sustain the lush vegetation known to have been present.22  Until the computer models, and the physical aspects of the climate coded in them, improve substantially, regional projections remain highly speculative.

22 S. Joussaume et al., Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP), Geophysical Research Letters 26, 859 (1999).

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14. Will global warming produce a rise in sea level and cause major flooding?

As computer simulations have become more sophisticated, projections of rising sea levels have become much smaller.

Knowing the response of sea ice to changing patterns of temperature and precipitation at high latitudes is critical to estimating future sea-level change, but the interaction of sea-ice with the climate is not well understood at present. Predictions in 1980 assumed that the polar ice sheets would melt, contributing to a catastrophic 25-foot rise in sea level. That estimate was reduced to three feet by 1985 and then to one foot in the 1995 report by the U.N. IPCC. Satellite, balloon and surface records indicate that the atmosphere above the Arctic has not warmed substantially in the last several decades, and more limited surface regional records even suggest an Arctic cooling over the last 40 years.23  If the arctic air were to warm, it would still be well below freezing but would hold more moisture because the cold air in these regions is normally very dry. With more moisture in the atmosphere, snowfall would increase and the ice sheets may actually grow.

As for records of sea-level change, tide-gauge measurements indicate no acceleration in sea-level rise in the 20th century.24  Records of sea level, based on precise measurements made by the TOPEX/POSEIDON satellites, show a small trend of 2 mm per year in sea-level rise from 1993 to 1997.25  These measurements imply that if greenhouse gases continue to rise and the global temperature increases as much as the computer simulations say, the estimated rise in sea level over the next 100 years will be less than one foot.

23 J. D. Kahl, D. J. Charlevoix, N. A. Zaitseva, R. C. Schnell, and M. C. Serreze, Absence of evidence for greenhouse warming over the Arctic Ocean in the past 40 years, Nature 361, 335 (1993); I. Polyak and G. North, Evaluation of the Geophysical Fluid Dynamic Laboratory general circulation model climate variability. Variances and zonal time series, Journal of Geophysical Research 102, 1921 (1997).

24 B. C. Douglas, Global sea level acceleration, Journal of Geophysical Research 97, 12699 (1992).

25 R. S. Nerem et al., Improved determination of global mean sea level variations using TOPEX/POSEIDON altimeter data, Geophysical Research Letters 24, 1331 (1997).

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15. Will global warming produce more violent storms?

This is not likely.

According to the 1995 IPCC report, “Overall, there is no evidence that extreme weather events, or climate variability, has increased, in a global sense, through the 20th century…”26  For example, reliable records in the North Atlantic show that during the past fifty years the number of severe hurricanes has not increased and the average maximum intensity of all hurricanes has weakened.27  The number of Atlantic hurricanes was above average in both 1995 and 1996, but these followed a period, 1970-1994, of below-average hurricane activity compared to previous years, so the multi-year average of hurricanes has not appreciably changed in several decades.28  A low incidence of tropical storms in 1997 is thought to be linked to the El Niño conditions in the tropical Pacific Ocean, while 1998 had an above-average number of hurricanes. Expectations for 1999, with no El Niño, were for an above-average number of hurricanes. Such decade-to-decade fluctuations in hurricanes are unrelated to increased atmospheric CO2.29 

The increasing dollar cost of storm and other weather-related catastrophic insurance losses, erroneously cited as proof of increases in weather extremes in recent years, can be accounted for by the rise in property values, development and population, especially in hurricane-prone areas because hurricane losses dominate the weather catastrophe costs. A comparison over time of the average economic impact of storms can be made by adjusting property losses to current values. This calculation shows that the cost per large storm, adjusted for current conditions, has varied substantially from one decade to another, but that recent storm damage is no higher than in some earlier decades.30  Recent storm losses have not been exacerbated by increases in atmospheric carbon dioxide.

Also, heavy rainfall has not increased significantly in the U.S. over the past 80 years. While the percentage of rainfalls of 1-2 inches per 24 hours has increased slightly in recent years, it showed a faster rise early in the 20th century.31 

26 IPCC 1995, p. 173.

27 C. W. Landsea et al., Downward trends in the frequency of intense Atlantic hurricanes during the past five decades, Geophysical Research Letters 23, 1697 (1996); M. C. Serreze et al., Icelandic low cyclone activity: Climatological features, linkages with the NAO, and relationships with recent changes in the Northern Hemisphere circulation, Journal of Climate 10, 453 (1997).

28 W. M. Gray et al., Summary of 1998 Atlantic tropical cyclone activity, verification of authors’ seasonal activity prediction, at Colorado State University website http://www.typhoon.atmos.colostate.edu/forecasts/1998/nov98/index.html.

29 W. M. Gray et al., Early August forecast of Atlantic seasonal hurricane activity and U.S. landfall strike probabilities for 1999, at Colorado State University website http://www.typhoon.atmos.colostate.edu/forecasts/1999/aug99/.

30 S. A. Changnon et al., Effects of recent weather extremes on the insurance industry: Major implications for the atmospheric sciences, Bulletin of the American Meteorological Society 78, 425 (1997); R. A. Pielke, Jr. and C. W. Landsea, Normalized hurricane damages in the United States 1925-1995, Weather and Forecasting 13, 621 (1998); K. E. Kunkel, R. A. Pielke, Jr., and S. A. Changnon, Temporal fluctuations in weather and climate extremes that cause economic and human health impacts: A review, Bulletin of the American Meteorological Society 80, 1077 (1999).

31 T. R. Karl, R. W. Knight, and N. Plummer, Trends in high-frequency climate variability in the twentieth century, Nature 377, 217 (1995).

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16. Will global warming cause the spread of infectious diseases?

This is not likely.

The evidence shows that if global warming were to occur, it would not have a major impact on the spread of infectious diseases, like malaria, to countries in high northern latitudes where other factors, such as the level of economic development, the quality of medical care, and lifestyle are major deterrents to mosquito-borne diseases.32 

Malaria and yellow fever were once common in non-tropical regions. During the Little Ice Age when it was much cooler than it is today, malaria was still a major problem in Europe.33 Earlier in this century, the Soviet Union, not known for its balmy weather, experienced a major malaria epidemic. At the same time, Alaska reported many malaria cases.

These and other diseases thrive in various climates – warm as well as cool. What has made the difference is the level of economic development that provides access to modern medicine, higher living standards, and other means to prevent the spread of illness.34 

32 P. Reiter, Global Warming and vector-borne disease in temperate regions and at high altitude, Lancet 351, 839 (1998).

33 P. Reiter, From Shakespeare to Defoe: Malaria in England in the Little Ice Age, Emerging Infectious Diseases 6, in press (2000).

34 D. J. Gubler, Resurgent vector-borne diseases as a global health problem, Emerging Infectious Diseases 4, 442 (1998).

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17. Is there a consensus among climate scientists that greenhouse warming from human activities is a major threat?

No. Science is not aimed at building a politically potent consensus over questions of public policy.

Advocates of immediate precautionary actions often state there is nearly unanimous agreement among scientists that the globe is warming at an unacceptably rapid rate and that society is to blame. But the claim of a scientific consensus on the causes of global warming is not supportable. Consider these two points.

First, the report by the IPCC states that a human-caused warming 100 years from now might be as high as 4.5°C – a significant increase – or as low as 0.8°C.35  A 0.8°C warming over a century is comparable to the climate changes commonly encountered in nature. Thus, according to the IPCC, the combined projections of the models imply that the human-made warming 100 years from now will be somewhere between important and unimportant, hardly the basis for a meaningful consensus when the scientific uncertainty is so large. Furthermore, the IPCC report states that the human effect was difficult to quantify because it is small and masked by natural variability:

Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. These include the magnitude and patterns of long term natural variability…36 

Second, the idea of seeking a consensus on global warming reveals a misunderstanding of science itself. For science, facts are determined by the scientific method, wherein quantitative predictions resulting from a theory are refuted or confirmed by experiments. The known inadequacies of the present models mean they can tell us very little about the cause of climate change.

Climate scientists say it will take at least ten years of continuing observation and theoretical studies to decide whether increasing CO2 levels are likely to cause significant global warming. T. P. Barnett, co-reviewer of the 1995 IPCC report, states, “The next 10 years will tell; we’re going to have to wait that long to really see.”37  K. Hasselmann of Germany’s Max Planck Institute for Meteorology concurs, “It will take another decade or so [for the CO2-caused global warming] to work up out of the noise.” 38 

35 See IPCC 1995, p. 40.

36 See IPCC 1995, p. 5.

37 R. A. Kerr, Greenhouse Forecasting Still Cloudy, Science 276, 1041 (1997).

38 R. A. Kerr, Greenhouse Forecasting Still Cloudy, Science 276, 1041 (1997). Even these assessments are highly optimistic because the necessary observing systems with demonstrated capabilities to detect the climate effect of increasing anthropogenic CO2 are not yet in place.

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18. Does the threat of major climate change justify drastic reductions in CO2 emissions by the United States and other nations?

No.

Some argue that even if the probability of major climate change is small, the catastrophic consequences are so great as to justify action to avoid it, much as people take out insurance against comparatively remote risks. However, before buying insurance, one weighs the cost of the insurance against the risk of the event. In the case of human-made global warming, the cost of drastic reductions in CO2 emissions would be major economic disruption and diversion of large sums of money that could be spent in far more productive enterprises. What is more, there is no reliable actuarial table for disruptions from climate change. In other words, because we do not know if there will be serious climate change or what its impact might be, this is not the time to asses if the risks justify a great deal of insurance, or none at all. Furthermore, it is unclear how a unilateral reduction by the U.S. will help matters significantly when it is global emissions that affect the air’s concentration of CO2. Emissions from the developing world – China and India, in particular – will be expanding at a rapid rate over the same period and would overwhelm the proposed reductions of U.S. emissions in a few years.39 

39 M. Parry et al., Adapting to the inevitable, Nature 395, 741 (1998).

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19. Is there a clear policy on climate change and CO2 emissions which makes sense based on our current knowledge?

We have at least 25 years to research this issue before CO2 emission cuts need to be considered.

Even assuming the worse-case scenarios, delaying substantial cuts in CO2 emissions for the next 25 years would produce an additional global temperature rise of no more than a few tenths of a degree C by the year 2100.40  That means we have at least 25 years in which to sharpen our understanding of climate and seek valid predictions, without contributing to serious climate change. An incremental warming of a few tenths of a degree, spread over decades, constitutes no hazard while we seek important, additional information to build a foundation for national and worldwide energy policy. Policies made in haste, or based on poor information, are likely to have a destructive impact on the U.S. and world economies and the well-being of its citizens. Rather, we should foster efficient use of our resources so as to meet the world’s needs with minimal environmental impact.

40 T. M. L. Wigley, R. Richels, and J. A. Edmonds, Economic and environmental choices in the stabilization of atmospheric CO2 concentrations, Nature 379, 240 (1996); T. M. L. Wigley, The Kyoto Protocol: CO2, CH4, and climate implications, Geophysical Research Letters 25, 2285 (1998).

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