Laboratory experiments with greenhouse gases and spectrally resolved studies of radiation absorption and transmission in the atmosphere indicate that a number of gases 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.

Carbon dioxide: The observed atmospheric concentration is 30% above preindustrial levels as determined from air trapped in ice cores and direct measurements, and the level continues to increase. The measured anthropogenic sources are significantly larger than the anthropogenic sinks. Fossil fuel consumption and land-use change contribute to increased concentration of atmospheric carbon dioxide.

Methane: The observed atmospheric concentration is more than 100% above preindustrial levels as determined from air trapped in ice cores, and the concentration has increased over recent decades. Estimated changes in anthropogenic sources are consistent with measured increases in atmospheric concentrations and are large compared to anthropogenic sinks or anthropogenically induced reductions in emissions.

Nitrous oxide: The observed atmospheric concentration is about 10% above preindustrial levels as determined from air trapped in ice cores. Estimates of anthropogenic sources are consistent with measured increases in atmospheric concentrations; no anthropogenic sinks are recognized.

Chlorofluorocarbons: Preindustrial concentrations were virtually zero. The observed increases in atmospheric concentrations are due solely to human activities. Anthropogenic sources are large, and natural removal processes for most CFCs have a time constant of about a century.

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 and concentrations of radiatively active gases, including ozone and methane.

The magnitude and timing of the resulting warming is less certain. Observations and measurements of past and present radiative effects and concentrations of greenhouse gases indicate that the 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 by the strength of water vapor and cloud feedback processes, which are major factors in controlling the natural greenhouse effect and which would likely respond to the radiative changes described. These feedbacks change the magnitude of the warming, but not the presence of the warming.

The drawdown of CFCs and nitrous oxide to their preindustrial levels would take more than a century, even with a halt in human emissions. 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.

Many of the sinks of CO2 operate on long timescales. A further basis for this projected drawdown time comes from the evaluation of the plausible sinks of emitted carbon dioxide and other gases in comparison to the following factors: 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.

Increased concentrations of tropospheric aerosols have been measured in and downwind of regions of anthropogenic sources. These aerosols include sulfate aerosols from fossil fuel combustion and complex chemical aerosols from biomass burning. The global trend in tropospheric aerosol concentrations is uncertain. 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.

While this effect is well established, estimates of the magnitude, trends, and extent of the induced cooling effects are uncertain. This uncertainty is due to limitations in the observations of aerosol amount and composition, limitations in the models used to simulate the aerosol system, and the potential indirect effects of anthropogenic sulfate aerosols through changes in cloud extent and character. Reducing anthropogenic emissions of aerosols and aerosol precursors would lead, over periods of weeks to months, to reductions in atmospheric concentrations of tropospheric aerosols by relatively effective natural removal processes. However, the rates of emissions by anthropogenic sources are also uncertain.

Laboratory and atmospheric measurements show that aerosols scatter solar radiation, with much of the radiation scattered in the backward direction - that is, 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 into the troposphere over a few years, and that tropospheric aerosols are removed by precipitation scavenging - the process by which rainfall removes aerosol particles from the air - and contact with the surface within weeks to months.

Observational ship and land records 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 and glacier meltback records also suggest 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 changing amounts of anthropogenic aerosols appears to explain some of the differences between observations and model predictions of the warming due to the increased greenhouse gas concentrations alone. The natural variability of climate, on timescales of months to centuries and variously distributed over the globe, contributes to the uncertainty over the interpretation of the record.

Results of Climate Models

The list below is 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.

Very Probable:

Increased emission of infrared radiation is an automatic consequence of the increased infrared optical depth in the stratosphere due to the increased concentration of CO2. Increased emission of infrared radiation will lead to cooling in the upper stratosphere, and satellite observations and other records show that this cooling is occurring. In the lower stratosphere, temperature changes due to increased CO2 concentrations are complicated by the changes in ozone concentration due to volcanic aerosols, chlorofluorocarbons, nitrogen oxides from aircraft, and changes in tropospheric chemistry, which can variously lead to cooling and warming in the lower stratosphere. Volcanic aerosols have a direct radiative effect on the lower stratosphere, producing a warming.

The distribution of this change is less certain. A warming of the surface temperature over the globe will increase global-mean precipitation because of the relationship between evaporation rate and surface temperature. The underlying physics on this is well established and all models confirm this relationship.

Projected changes and their timing in the extent of the Southern Hemisphere sea ice are less certain. Studies of past climates provide evidence for polar amplification of warming and reduced extent of sea ice. Modeling studies also suggest that this will occur, although some have suggested local expansion of sea ice.

The most reasonable estimates for the rate of sea level rise are 5 - 40 cm by 2050 as compared to a rise of 5 - 12 cm if the rates of rise over the past century continue. The most tractable part of making the estimate for the next several decades is projecting the component due to sea water expansion, whose rate of change is closely dependent on the atmospheric warming. Reasonable estimates of the retreat of mountain glaciers are also available. The mass balances of the polar ice sheets are highly uncertain, and are likely to be important only on longer timescales. These estimates ignore long-term issues relating to the slow response of the major ice caps, potentially different responses on Antarctica and Greenland, and the continuing rise in sea level as the deep ocean only slowly experiences the warming at the surface. Also, these estimates do not consider potential catastrophic collapse of the west Antarctic ice sheet, which remains a controversial subject.

This projection is based on comparison of the magnitude of the forcing from known levels of solar variability to projected changes in greenhouse gas forcing.

Probable:

Oceanic regions where surface waters mix downward and deeper waters upwell to the surface will have a smaller than average surface temperature response to global warming. The warming will also depend on changes in precipitation and freshwater input at the sites of downward mixing.

Historical records indicate cooling of a few tenths of a degree lasting up to a few years following major volcanic eruptions. The historical frequency of explosive volcanic events large enough to produce substantial increases in stratospheric aerosols suggests that such events could occur over the next several decades.

Uncertain:

As yet, there is no clear evidence that suggests how the character of interannual variability may change due to greenhouse warming, but there is potential for multifaceted and complicated, even counter-intuitive, changes in variability. Many potential changes in variability can be identified, suggesting that some will occur. For example, in all models, standing and transient atmospheric eddy activity in the mid-latitudes decreases with the reduced meridional temperature gradient associated with global warming; this may lead to reduced wintertime variability for warmer climates.

However, there is limited capability to estimate how various regions will respond to global climate change.

There is a significant mismatch of spatial scales between present climate system models and regional climate variations. The best estimates of regional change are based on the large scale characteristics of model simulations. Differences between global and regional changes are uncertain but are expected to be present.

An increase in tropical storm intensity is plausible but uncertain because of potential changes in poleward heat flux, uncertainties in tropical sea surface temperature response, and in the strength of the Hadley circulation in a greenhouse warming condition. The probability of more frequent tropical storms is uncertain in part because global circulation models are currently not run at the appropriate spatial resolution to simulate hurricane formation and other factors that might lead to changes in hurricane generation.

Uncertainties in the factors controlling the natural variability of the climate, in the model simulations, and in the perturbations to atmospheric composition make it extremely difficult to predict or even suggest the details of changes in the global or regional climate on the timescale of only a few decades.

Processes governing changes in the distribution and character of vegetation are not incorporated in climate models. Limited assessments suggest plausible changes in climate may occur as a result of vegetation changes that result from the greenhouse-gas-induced climate changes. These effects could amplify or moderate expected climatic changes.

Opportunities for Reducing Uncertainties

Progress will require significant effort because the problems are complex, improvements in model parameters require a sustained and long-term program of research and observations, and the records of past changes and influences require careful reconstructions to make them more complete and more useful. Although progress may be modest, research on many processes and feedbacks must be sustained because greater understanding of climate systems would benefit science and society.

Improved spatial resolution in atmospheric and oceanic models will improve the representation of the present climate and potential global-scale changes. Climate changes at the surface and as a function of altitude can be better represented by inclusion of significantly more representative parameters of the atmospheric boundary layer and vertical convection processes. Substantially improved observations and improved representations of water vapor in climate models will reduce a major source of uncertainty in model predictions.

Improving the linkages coupling the atmosphere, oceans, and land surface will reduce uncertainties in estimates of the overall climate response by improving the accuracy of the climate simulations, by eliminating the need for ad hoc adjustments to fluxes between components that are used in some models, and by allowing fuller exploration of natural climate variability over all timescales. More explicit representation of land surface processes, including vegetation, soil characteristics, and CO2 and O3 effects on stomatal resistance, will reduce uncertainties in estimates of soil moisture, summertime continental drying, and changes of regional climate. Sub-continental and regional scale features of global climate change can be better represented by focusing on tailoring global predictions to specific regions using both finer-scale models and empirical techniques.

Model comparisons against observations and with other models can accelerate the rate of model improvement: improved reconstruction, simulation, and analysis of preindustrial and paleoclimatic periods can help assess model projections of climate sensitivity and variability and lead to enhanced model credibility. Finally, improved representation of atmospheric chemistry and vegetation and soils is needed to improve prediction of radiative feedbacks and to allow calculation of the effects of other types of global environmental change.

The full version of the report is available from the U.S. Global Change Research Information Office; tel. 517-797-2730; fax 517-797-2622; Internet access through World Wide Web http://www.gcrio.org.

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