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Climate Issues & Questions

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The Marshall Institute — Science for Better Public Policy Climate Issues & Questions updated and revised edition Copyright © 2006

Climate Issues & Questions

The debate over the state of climate science and what it tells us about past and future climate has been going on for more than fifteen years. It is not close to resolution, in spite of assertions to the contrary. What is often referred to as a “consensus” is anything but. Many of those making this claim hold a particular point of view that is based on their “expert judgment,” not established scientific fact. For others, especially those engaged in advocacy, the claim of consensus is used to advance their agenda. Although humanity has been interested in climate since prehistoric times, climate science is, in fact, a relatively new field. It is only since the 1970s, when models were developed to connect atmospheric and oceanic climate processes, that scientists have had the tools to study climate as a system.

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Concerns about climate change have resulted in some scientists entering the policy debate because of alarm about either the potential impacts of climate change or the economic impact of ill-conceived policies. Others, unfortunately, have entered the debate to advance political or economic agendas, gain funding for research, or enhance their personal reputations. To the extent that the debate is carried out in the public policy arena or media, the rigors of the scientific process are short-circuited. This state of affairs creates misunderstandings and confusion over what we know about the climate system, past climate changes and their causes, human impacts on the climate system and how human activities may affect future climate. Policy needs are better served by clarity and accuracy.

The purpose of this document is to address a set of fundamental questions about climate change by summarizing the best available scientific information. The information provided is not intended to rebut claims about human impacts on climate or the potential for adverse impacts later this century. It is intended to separate fact from speculation and to demonstrate that while concerns are legitimate, there is not a robust scientific basis for drawing definitive and objective conclusions about the extent of human influence on future climate. The presentation moves from what is well established, to what is not certain, to what is unknown, and may be unknowable. This is the second edition of Climate Issues and Questions.

  1. How is the atmospheric concentration of carbon dioxide (CO2) determined and how accurate are the measurements?

    Atmospheric concentrations of CO2 have been measured directly since 1958. The CO2 concentration in air bubbles trapped in ice sheets is used to determine atmospheric concentration for earlier times. The measurements are consistent and accurate.

  2. How much of today’s atmosphere is CO2?

    The atmosphere is comprised of many gases. CO2, a greenhouse gas, represents 0.038% of today’s atmosphere, while the concentration of water vapor, the most important of the greenhouse gases, varies from near zero in cold, dry polar air to more than 6% in humid, tropical air.

  3. What has been the history of atmospheric CO2 concentrations?

    Atmospheric concentration of CO2 has varied greatly over time, from a high of more than 380 parts-per-million (ppm) 25 million years ago, to a low of about 180 ppm during several periods of glaciation over the past 400,000 years. The atmospheric concentration of CO2 was relatively constant at about 280 ppm for 1,000 years before 1750. Since 1750, CO2 concentration has risen, reaching about 380 ppm in 2004.

  4. Do we know why CO2 concentrations are rising?

    The increase in CO2 concentration appears to be the result of human activities, though only about half of the CO2 emissions that result from human activity accumulate in the atmosphere. The rest accumulates in the oceans or is stored in the biosphere.

  5. What do we know about the relation between increases in the atmospheric concentrations of CO2 and other greenhouse gases and temperature?

    During the 20th century atmospheric concentrations of CO2 and other greenhouse gases rose steadily, but global average surface temperature rose, then fell, then rose again in a pattern that showed no relationship to greenhouse gas concentration. CO2 and other greenhouse gas concentrations were relatively constant from 1000 to 1750, but the Earth experienced a warm period from 800 to 1200, followed by a cold period from 1400 to about 1850.

  6. If temperature changes cannot be correlated with the increase in atmospheric concentrations of CO2 and other greenhouse gases, what is causing them?

    The climate system is a complex set of interactions between solar energy, clouds, particulates, water vapor and other greenhouse gases, and the absorption and reflection of solar radiation at the Earth’s surface. The general nature of these interactions is understood by climate scientists, but their details are highly uncertain.

  7. Is the Arctic warming faster than the rest of the Earth?

    Like the rest of the Earth, the Arctic is warming. The best available evidence suggests that over the 20th century, it warmed at a somewhat faster rate than the global average, but less than would be projected by climate models and less than claimed by the 2004 Arctic Climate Impact Assessment (ACIA). Understanding temperature trends in the Arctic is complicated by limited data and the fact that conditions in the Arctic can change much more rapidly than over the rest of the Earth.

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  8. Do satellites and surface temperature measurements give different results?

    Differences between temperature trends in the lower atmosphere measured by satellites and temperature trends from surface weather stations have been narrowed, but the differences still exist. There are several estimates of both satellite and surface temperature trends. Most temperature trends measured by satellites still show less warming than most temperature trends measured at the surface, but the estimates overlap. In addition, the range of model projections of temperature trends overlaps both sets of measurements. Both sets of measurements are subject to error, as are the model results, and further data and analyses are needed to resolve the remaining differences.

  9. Is evidence of increased ocean heat storage a “smoking gun” indicating climate change?

    Media reports of a paper by James Hansen and 14 co-authors that appeared in the June 3, 2005 issue of Science 1 claimed that it represented the “smoking gun” evidence for climate change. The smoking gun claim is surprising, since there can be no doubt that climate has been changing. While there is a debate over the amount of change, and an even greater debate over the causes of that change, there is no evidence to argue that the world as a whole is not warmer than it was a century ago. In light of this warming, the authors’ conclusion that the Earth is absorbing more energy than it is emitting is obvious. As discussed below, the authors used indirect evidence to test their model, rather than the direct satellite measurements of the Earth’s energy balance. Finally, their finding that the Earth is committed to additional warming is also not surprising, since this concept has been well understood since at least 1990.

  10. What influence does the Sun have on global climate?

    The Sun provides the energy that drives the climate system. Long-term variations in the intensity of solar energy reaching the Earth are believed to cause climate change on geological time-scales. New studies indicate that changes in the Sun’s magnetic field may be responsible for shorter-term changes in climate, including much of the climate of the 20th century.

  11. What is known with a high degree of certainty about the climate system and human influence on it? 

    We know, with a high degree of certainty, that:
    -the surface of the Earth warmed over the past century;

    -the increases in the atmospheric concentrations of CO2 and other greenhouse gases will have a warming effect;

    -the human emissions of CO2 and other greenhouse gases are responsible for much of the increase in atmospheric concentrations of these gases; and

    -the economic growth trends, particularly in the developing nations, will increase human emissions of CO2, at least over the next few decades because economic growth requires energy use and the dominant source of energy will remain fossil fuels. These facts are the basis for concern about potential human impacts on the climate system.

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  12. What major climate processes are uncertain and how important are these processes to understanding future climate?

    Key uncertainties in our understanding of the climate system include the details of ocean circulation, the hydrological (water) cycle, and the properties of aerosols. The cumulative effect of these and other uncertainties in our understanding of the climate system is an inability to accurately model the climate system. Since models are the only way to project future climate, our lack of understanding of key climate processes means we lack the ability to accurately project future climate.

  13. What tools are available to separate the effects of the different drivers that contribute to climate change?

    Climate scientists use general circulation models (GCMs) to try to separate the effects of the different drivers that affect the climate system. These models use mathematical equations to describe the different processes known to occur in the climate system. GCMs are extremely complex because they must try to model all of the processes occurring in both the atmosphere and the oceans, neither of which are homogeneous, by dividing them into small grid boxes, then modeling change in small time increments. The resulting computational demand exceeds the capacity of even the best super-computers.

  14. How accurate are climate models?

    Current climate models have many shortcomings. They cannot accurately model the atmosphere’s vertical temperature profile, their estimates of natural climate variability are highly uncertain, and there are large differences in the response of different models to the same forcing. No climate model has been scientifically validated.

  15. What is the basis for forecasts of large temperature increases and adverse climate impacts between 1990 and 2100?

    Forecasts of large temperature increases and adverse climate impacts between 1990 and 2100 are based on the output of climate models using the IPCC SRES (Special Report on Emissions Scenarios) Scenarios as input. Concerns about the quality of climate model output have been discussed in Question 11. Large increases in temperature depend on three assumptions, none of which are likely: a. No overt action is taken to control greenhouse gas emissions. However, a variety of actions, some voluntary, some mandatory, are currently being taken to control greenhouse gas emissions. b. Greenhouse gas emissions grow at the high end of the range of the IPCC emissions scenarios, i.e., CO2 emissions in 2100 that were over five times current CO2 emissions. These high emission scenarios have been broadly criticized as unrealistic.2 c. The climate system shows a high sensitivity to changes in greenhouse gas concentrations. Reports from a recent IPCC workshop indicate that while there is still a great deal of uncertainty, climate modelers now believe that the climate system is less responsive to greenhouse gas concentrations than would be required for a 5.8°C temperature rise.3

  16. How accurate are the parameters used in climate models?

    The scientific level of understanding of the direct effects of greenhouse gases is high, but the scientific understanding of the other drivers of the climate system is low or very low.

  17. How well have models done in “back-casting” past climate?

    Model results that match global average surface temperature for the past 140 years have been published, but they are suspect because of: (1) the quality of the surface temperature data used to determine global average surface temperature; and (2) the quality of the models themselves.

  18. Is the global warming over the past century unique in the past 1,000 years or longer?

    The IPCC Third Assessment Report conclusion that the warming of the 20th century unique in at least 1,000 years was based on a study (by Mann, et al.) that has been shown to be incorrect by three studies recently published in the peer-reviewed literature. These studies show that many parts of the world have experienced warmer temperatures at some time during the last 1,000 years than they did during the second half of the 20th century and that climate variability is much greater than indicated by the IPCC.

  19. How much does the global climate vary naturally?

    Climate scientists don’t know the answer to this question, but the available data suggest that there is considerable natural variation on a time-scale of decades to centuries.

  20. What do we know about the extent of human influence on climate? To what extent has the temperature increase since 1975 been the result of human activities?

    The best answer to these questions is “We don’t know.” Human activities have a number of potential impacts on climate. Greenhouse gas emissions contribute to warming, as do some particulate emissions. Other particulate emissions produce cooling. Land-use changes can produce either warming or cooling, depending on the change. The direct effects of greenhouse gas emissions are relatively easy to determine, but their indirect effects, through water vapor and other feedbacks, are poorly understood. The impacts of other human activities—particulate emissions and land-use changes—are poorly understood.

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  21. Could climate change abruptly?

    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 dramatic climatic shifts occurred near the end of the last major ice age, about 15,000 years ago. First, a brief warming occurred, and then the ice age returned for roughly a thousand years. Finally, by 11,500 years ago, the climate quickly warmed again.4 Ice core data indicate that temperatures in central Greenland rose by 7°C or more in a few decades. Other proxy measurements indicate that broad regions of the world warmed in 30 years or less.5 Recently attention has focused on the potential for climate to change abruptly as the result of human activities. A common scenario is the onset of an ice age as the result of human greenhouse gas emissions.

    It is now generally agreed that changes in the Earth’s orbit, which result in changes in the amount of solar energy reaching the Earth’s surface, are responsible for both ice ages and the warm interglacial periods between them. This theory was first popularized in the 1920s by Milutin Milankovitch, a Serbian astrophysicist. He theorized that three factors controlled the amount of solar energy reaching the Earth’s surface:
    - the eccentricity, or shape, of the Earth’s orbit, which varies on a cycle of about 100,000 years;

    - the tilt of the Earth’s axis, which varies on a cycle of about 41,000 years; and

    - the precession of the equinoxes, which varies on a cycle of about 22,000 years.

  22. Will sea level rise abruptly?

    There currently is no scientific evidence to support concern about rapid sea level rise during this century. Longer term, the dynamics of glacier and ice sheet melting are too poorly understood to make reasonable projections.

  23. Will the number of tropical cyclones (hurricanes, typhoons) increase and will they become more intense?

    It is well established that tropical cyclones will not form unless the sea surface temperature in 26°C (79°F) or higher. However, tropical cyclone formation depends on a parameter known as Convective Available Potential Energy (CAPE), which is a function of both sea surface temperature and atmospheric circulation. The atmosphere can either collect the energy available from the warm ocean, leading to cyclone formation, or dissipate it, in which case a cyclone will not form. Since sea surface temperatures are often above 26°C, but tropical cyclones are relatively rare events, dissipative conditions predominate. The same parameter controls tropical cyclone intensity.

  24. Will other extreme weather events, such as heat waves, increase?

    If the Earth warms, some types of extreme weather events will increase, others will decrease, and still others will remain unchanged. The occurrence of what is now defined as extreme heat will increase, while extreme cold will decrease.

1 Hansen, J., et al (2005): Earth’s energy imbalance: Confirmation and implications. Science, 308, 1431-1434.

2 See, for example: Ausubel, J. (2002): Does Energy Policy Matter? George Marshall Institute (, copies of Castles and Henderson’s letters to the Chair of IPCC and presentations at IPCC technical experts meetings, available at, and the report of the Select Committee of the UK House of Lords: The Economics of Climate Change, published on 6 July 2005.

3 Kerr, R.A. (2004): Three Degrees of Consensus. Science, 305: 932-934.

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

5 J.T. Houghton, et al. (2001): op cit., Pg. 140.


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The Marshall Institute - Science for Better Public Policy

Climate Issues & Questions updated and revised edition Copyright © 2006 All rights reserved. No part of this book may be reproduced or transmitted in any form without permission from the George Marshall Institute.

The George C. Marshall Institute, a nonprofit research group founded in 1984, is dedicated to fostering and preserving the integrity of science in the policy process. The Institute conducts technical assessments of scientific developments with a major impact on public policy and communicates the results of its analyses to the press, Congress and the public in clear, readily understandable language. The Institute differs from other think tanks in its exclusive focus on areas of scientific importance, as well as a Board whose composition reflects a high level of scientific credibility and technical expertise. Its emphasis is public policy and national security issues primarily involving the physical sciences, in particular the areas of missile defense and global climate change. The views expressed in this document do not necessarily represent the views and policies of the George C. Marshall Institute, but its publication is an important contribution to the debate.

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