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 nat ural greenhouse effect and which would be expected to respond to the radiative changes induced by the changes in concentrations of carbon

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

of anthropogenic sources, including sulfate aerosols from fossil fuel combustion and complex chemical aerosols from biomass burning. Stratospheric aerosols are largely of natural origin. Large variations in the concentrations of stratospheric aerosols are determined by volcanic eruptions and, in polar regions, by the temperature (i.e., at cold enough temperatures, atmospheric gases can condense to form aerosols).

(6) Sulfate aerosols, both from volcanic injections and from fossil fuel combustion, exert a cooling influence on the climate. While the sign of the effect is well-established, estimates of the magnitude, trends, and extent of the induced cooling effects are uncertain due to limitations in the observations of aerosol amount and composition, limitations in the models used to simulate the aerosol system, and potential indirect effects of anthropogenic sulfate aerosols through changes in cloud extent and character. Reduction of anthropogenic emissions of aerosols and aerosol precursors (e.g., sulfur dioxide) would lead, over periods of weeks to months, to reductions in atmospheric concentrations of tropospheric aerosols due to relatively effective natural removal processes. However, the rates of emissions by anthropogenic sources in some regions are not well-documented, particularly for biomass burning sources.

Basis-Laboratory and atmospheric measurements show that aerosols scatter solar radiation, with much of the radiation scattered in the backward direction (i.e., back out into space). Observations and model simulations indicate that large volcanic eruptions temporarily cool the climate. Observations also indicate that settling and transport move stratospheric aerosols (mostly volcanic) into the troposphere over a few years, and that tropospheric aerosols are removed by precipitation scavenging and contact with the surface within weeks to months.

(7) Global average surface air temperatures are about 0.5°C (1°F) higher than average temperatures in the 19th century.

Basis-Observational records drawn from ship and land measurements since about 1850, while somewhat uncertain due to changes in instrumentation, measurement and observation techniques, and station location, indicate that the world is warmer now than in the 19th century. Limited borehole (ground surface) and glacier meltback records also suggest generally warmer conditions during this century. This change cannot yet be unambiguously ascribed to increased concentrations of greenhouse gases. Over the past century, the cooling influence of

There are significant uncertainties in predicting future climates as a consequence of (a) natural climate variability; (b) the potential for uncertain or unrecognized climatic forcing factors (e.g., explosive volcanism, new or unknown anthropogenic influences, etc.); and (c) inadequate understanding of the climate system (e.g., a recent discovery that clouds absorb significant shortwave radiation which is not included in models, or potential errors in fluxes at the top of the atmosphere associated with changes in relative humidity). These uncertainties may (or may not) lead to re-assessment of model capability. We must expect that new observations or results from process studies may yield information that causes us to re-evaluate and improve the capability of climate models. Our estimates of the credibility of climate system models can, of necessity, be consistent only with known facts and based only on the "best" current knowledge.

The list given below is not a "forecast" but rather a scientific judgment based on the assumption that the concentrations of anthropogenic greenhouse gases will increase. Many of the model experiments on which these points are based have an even more specific assumption, a scenario of a 1% per year increase in CO2 to mimic the radiative forcing of the projected increase in the concentrations of all greenhouse gases.

VIRTUALLY CERTAIN

(1) Large stratospheric cooling will result from the increase in
CO2 concentration and ozone depletion; the start of such cool-
ing has been predicted by models and
observed in the upper stratosphere.

2

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