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A number of the scientific elements that are the basis of conclusions concerning future climate change do not stem directly from climate system models. In many cases, these elements are subject to little or no debate because of their high level of certainty. As statements they may even appear to some as almost trivial because of the frequency with which they are stated. Yet, in concert with the response to the Forum's specific charge, these statements provide both context and conclusions on the credibility of climate models for developing and considering national policy options. Seven statements that provide this context follow:

(1) Greenhouse gases absorb and re-emit infrared radiation, which includes the wavelengths of radiation emitted by atmospheric gases and clouds and by the Earth's land and oceans.

Basis-Laboratory experiments with greenhouse gases and spectrally resolved studies of radiation absorption and transmission in the atmosphere indicate that a number of gases that are present in the atmosphere are capable of absorbing and emitting infrared radiation. The most important of these so-called greenhouse gases is water vapor. Other important natural greenhouse gases include carbon dioxide, ozone, methane, and nitrous oxide.

(2) Atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and halocarbons (most importantly chlorofluorocarbons), collectively labeled the greenhouse gases, are significantly increased above preindustrial levels, and the increase is due to anthropogenic activities.

Basis: Carbon Dioxide (CO2)—The observed
atmospheric concentration is 30% above
preindustrial levels as determined from air
trapped in ice cores and direct measure-
ments since 1957. The concentration is con-
tinuing to increase. The measured (and esti-
mated) anthropogenic sources (from fossil
fuel consumption, deforestation, and agricul-
turally induced oxidation of humus) are sig-
nificantly larger than the anthropogenic
sinks (reforestation). Changes in carbon iso-
topic composition of atmospheric carbon
dioxide indicate that fossil carbon and bio-
mass reduction have contributed significantly
to the increase in the atmospheric concentra-

CO, Concentration (ppmv)

CO, Concentration (ppmv)

PART 1.
INTRODUCTION

Figure 2a. Concentrations
of atmospheric carbon
dioxide from the 18th cen-
tury to the present. Top:
Multi-year average concen-
trations from ice core data
at Siple Station in West
Antarctica for the period
1744-1953 [Source: A.
Neftel et al.] supplemented
by annual mean concentra-
tions at the Mauna Loa
Observatory for the period
1959-93 [Source: Scripps
Institution of Oceanography
for 1959-74 data and the
Carbon Cycle Group of
CMDL/NOAA for 1974-92
data]. Bottom: Monthly mean

concentrations at Mauna Loa

for the period 1958-92 [Source: As above].

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fact that fossil fuel consumption and land-use change are the contributors
to the increased concentration of atmospheric carbon dioxide.

Basis: Methane (CH4)-The observed atmospheric concentration is more
than 100% above preindustrial levels as determined from air trapped in
ice cores, and direct measurements over the past 2 decades. The con-
centration has been increasing over recent decades. E-timated changes
in anthropogenic sources (e.g., agriculture, energy resource production
and use) are broadly consistent with measured increases in atmospheric
concentrations, and are large compared to anthropogenic sinks or
anthropogenically-induced reductions in emissions (e.g., reduced wetlands).

Basis: Nitrous Oxide (N20)-The observed atmospheric concentration is
about 10% above preindustrial levels as determined from air trapped in
ice cores and direct measurements over the past 2 decades. Estimates of
anthropogenic sources (e.g., nylon production, agriculture) are broadly

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consistent with measured increases in atmospheric concentrations; no anthropogenic sinks are recognized.

Basis: Halocarbons-Preindustrial concentrations of most of these compounds were virtually zero, because there are no natural sources. Thus, the observed increases in atmospheric concentrations are due solely to human activities. Anthropogenic sources (e.g., refrigeration, industrial) have been large; no anthropogenic sinks are recognized and natural removal processes for most chlorofluorocarbons (CFCs) have a time constant of order a century. The rise in atmospheric concentrations of regulated gases has slowed and nearly stopped due to recent reductions in

emissions.

Basis: Chemically Active Gases-The concentrations of CO, nitrogen oxides, and non-methane hydrocarbons are higher than pre-industrial values over large regions. These gases can, through a series of chemical interactions, induce changes in the lifetimes-thus the concentrations-of radiatively active gases, including ozone and methane. As an example, the concentration of tropospheric ozone in some regions is significantly above its level in the 19th century.

There is evidence that the concentrations of carbon dioxide, ozone, methane, and nitrous oxide have also changed over geological time. In many cases these changes have been a driving factor in the different climates that are associated with time periods of altered concentrations. Based on straightforward physics and thermodynamics, the global concentrations of water vapor must also have changed over geologic time, acting primarily in response to the values and distribution of temperature, continental geography, and orography.

(3) Because of their infrared absorption, increased concentrations of greenhouse gases exert a global warming influence. As discussed in Part 2, the magnitude and timing of the resulting warming is less certain.

Basis-Observations and measurements of the radiative effects of greenhouse gases in the present atmosphere and the association of changes in greenhouse gas concentrations with climate changes in the geological past indicate that the resulting changes in the radiation balance from increasing greenhouse gas concentrations will, in the absence of other factors changing the climate, induce global warming. The extent of the warming will be affected particularly by the strength of water vapor and cloud feedback processes, which are major factors in controlling the natural greenhouse effect and which would be expected to respond to the

dioxide and other greenhouse gases being affected by human activities. Feedbacks change the magnitude of the response, as amplifying or moderating influences, but do not change the sign of the response.

(4) The drawdown of the augmented CO2 concentration, which is now about 30% above its preindustrial level, to near its preindustrial level would take centuries, even if emissions were to be very substantially reduced in the near future. Further, because a substantial reduction in global CO2 emissions below current levels is unlikely to occur within the next few decades, the atmospheric CO2 concentration is expected to continue to increase. The drawdown of CFCs and nitrous oxide to their preindustrial levels would also take more than a century, even with a halt in human emissions; however, because of chemical decomposition in the atmosphere, the drawdown of the excess methane concentration to near its preindustrial level would take only several decades if emissions were to be significantly reduced.

Basis-Many of the sinks of CO2 operate on long time scales. For example, while the mixed layer of the ocean (upper 75-200 m) comes into near equilibrium with the changes in atmospheric CO2 relatively quickly, it takes hundreds of years to mix carbon throughout the deep ocean. A further basis for this statement comes from the evaluation of the plausible sinks of emitted carbon dioxide and other gases in comparison to the projected growth of world population, the dependence of the world on the use of fossil fuels for energy, present trends in agriculture and deforestation, and the expected transportation, commercial, residential, and industrial use and emission of these gases. In addition to the importance of the sources, many of the sinks of CO2 operate on longer time scales.

(5) Anthropogenic aerosol concentrations are significantly increased in source regions (near and downwind of aerosol and aerosol precursor emissions).

Basis-The global trend in tropospheric aerosol concentrations is uncertain and not well-documented by global monitoring programs, but the human-influenced emission of aerosol precursors has generally increased in several major regions over the last decades to a century, and regional aerosol concentrations are believed to increase with increasing emissions. Natural sources of tropospheric aerosols include windblown dust, hydrocarbons from vegetation and forests, and soot and other products of forest and grassland fires. Increased concentrations of tropospheric aerosols are measured in and downwind of regions

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