Two important new findings since the IPCC WGI Second Assessment Report (IPCC, 1996) (hereafter SAR) demonstrate the importance of atmospheric chemistry in controlling greenhouse gases:

Currently, tropospheric ozone (O₃) is the third most important greenhouse gas after carbon dioxide (CO₂) and methane (CH₄). It is a product of photochemistry, and its future abundance is controlled primarily by emissions of CH₄, carbon monoxide (CO), nitrogen oxides (NO_x), and volatile organic compounds (VOC). There is now greater confidence in the model assessment of the increase in tropospheric O₃ since the pre-industrial period, which amounts to 30% when globally averaged, as well as the response to future emissions. For scenarios in which the CH₄ abundance doubles and anthropogenic CO and NO_x emissions triple, the tropospheric O₃ abundance is predicted to increase by an additional 50% above today�s abundance.

CO is identified as an important indirect greenhouse gas. An addition of CO to the atmosphere perturbs the OH-CH₄-O₃ chemistry. Model calculations indicate that the emission of 100 Mt of CO stimulates an atmospheric chemistry perturbation that is equivalent to direct emission of about 5 Mt of CH₄.

A major conclusion of this report is that atmospheric abundances of almost all greenhouse gases reached the highest values in their measurement records during the 1990s:

The atmospheric abundance of CH₄ continues to increase, from about 1,520 ppb in 1978 to 1,745 ppb in 1998. However, the observed annual increase in CH₄ has declined during the last two decades. This increase is highly variable; it was near zero in 1992 and as large as +13 ppb during 1998. There is no clear, quantitative explanation for this variability. Since the SAR, quantification of certain anthropogenic sources of CH₄, such as that from rice production, has improved.

The atmospheric burden of nitrous oxide (N₂O) continues to increase by about 0.25%/yr. New, higher estimates of emissions from agricultural sources improve our understanding of the global N₂O budget.

The atmospheric abundances of major greenhouse gases that deplete stratospheric ozone are decreasing (CFC-11, CFC-113, CH₃CCl₃, CCl₄), or increasing more slowly (CFC-12), in response to the phase-out in their production agreed to under the Montreal Protocol and its Amendments.

HFC-152a and HFC-134a are increasing in the atmosphere. This growth is consistent with the rise in their industrial use. HFC-23, an unintended by-product of HCFC-22 production, is also increasing.

Perfluorocarbon (PFC) e.g., CF₄ (perfluoromethane) appears to have a natural background; however, current anthropogenic emissions exceed natural ones by a factor of 1,000 or more and are responsible for the observed increase.

There is good agreement between the increase in atmospheric abundances of sulphur hexafluoride (SF₆) and emissions estimates based on revised sales and storage data.

There has been little increase in global tropospheric O₃ since the 1980s at the few remote locations where it is regularly measured. Only two of the fourteen stations, one in Japan and one in Europe, had statistically significant increases in tropospheric O₃ between 1980 and 1995. By contrast, the four Canadian stations, all at high latitudes, had significant decreases in tropospheric O₃ for the same time period. However, limited observations from the late 19th and early 20th centuries combined with models suggest that tropospheric O₃ has increased from a global mean value of 25 DU (where 1 DU = 2.710¹⁶ O₃ molecules/cm²) in the pre-industrial era to 34 DU today. While the SAR estimated similar values, the new analysis provides more confidence in this increase of 9 DU.

Changes in atmospheric composition and chemistry over the past century have affected, and those projected into the future will affect, the lifetimes of many greenhouse gases and thus alter the climate forcing of anthropogenic emissions:

The atmospheric lifetime relates emissions of a component to its atmospheric burden. In some cases, for instance for methane, a change in emissions perturbs the chemistry and thus the corresponding lifetime. The CH₄ feedback effect amplifies the climate forcing of an addition of CH₄ to the current atmosphere by lengthening the perturbation lifetime relative to the global atmospheric lifetime of CH₄ by a factor of 1.4. This earlier finding is corroborated here by new model studies that also predict only small changes in this CH₄ feedback for the different scenarios projected to year 2100. Another feedback has been identified for the addition of N₂O to the atmosphere; it is associated with stratospheric O₃ chemistry and shortens the perturbation lifetime relative to the global atmospheric lifetime of N₂O by about 5%.

Several chemically reactive gases � CO, NO_x (=NO+NO₂), and VOC � control in part the abundance of O₃ and the oxidising capacity (OH) of the troposphere. These pollutants act as indirect greenhouse gases through their influence on atmospheric chemistry, e.g., formation of tropospheric O₃ or changing the lifetime of CH₄. The emissions of NO_x and CO are dominated by human activities. The abundance of CO in the Northern Hemisphere is about twice that in the Southern Hemisphere and has increased in the second half of the 20th century along with industrialisation and population. The urban and regional abundance of NO_x has generally increased with industrialisation, but the global abundance of this short-lived, highly variable pollutant cannot be derived from measurements. Increased NO_x abundances will in general increase tropospheric O₃ and decrease CH₄. Deposition of NO_x reaction products fertilises the biosphere, stimulates CO₂ uptake, but also provides an input of acidic precipitation.

The IPCC Special Report on Emission Scenarios (SRES) generated six marker/illustrative scenarios (labelled A1B, A1T, A1FI, A2, B1, B2) plus four preliminary marker scenarios (labelled here A1p, A2p, B1p, and B2p). These projected changes in anthropogenic emissions of trace gases from year 2000 to year 2100, making different assumptions on population development, energy use, and technology. Results from both sets of scenarios are discussed here since the preliminary marker scenarios (December 1998) were used in this report:

Model calculations of the abundances of the primary greenhouse gases by year 2100 vary considerably across the SRES scenarios: in general A1B, A1T, and B1 have the smallest increases of emissions and burdens; and A1FI and A2 the largest. CH₄ changes from 1998 to 2100 range from -10 to +115%; and N₂O increases from 13 to 47%. The HFCs � 134a, 143a, and 125 � reach abundances of a few hundred to nearly a thousand ppt from negligible levels today. The PFC CF₄ is projected to increase to between 200 and 400 ppt; and SF₆ to between 35 and 65 ppt.

SRES projected anthropogenic emissions of the indirect greenhouse gases (NO_x, CO and VOC) together with changes in CH₄ are expected to change the global mean abundance of tropo-spheric OH by -20 to +6% over the next century. Comparable, but opposite sign, changes occur in the atmospheric lifetimes of the greenhouse gases, CH₄ and HFCs. This impact depends in large part on the magnitude of, and the balance between, NO_x and CO emissions.

For the SRES scenarios, changes in tropospheric O₃ between years 2000 and 2100 range from -4 to +21 DU. The largest increase predicted for the 21st century (scenarios A1FI and A2) would be more than twice as large as that experienced since the pre-industrial era. These O₃ increases are attributable to the concurrent, large (almost factor of 3) increases in anthropogenic NO_x and CH₄ emissions.

The large growth in emissions of greenhouse gases and other pollutants as projected in some SRES scenarios for the 21st century will degrade the global environment in ways beyond climate change:

Changes projected in the SRES A2 and A1FI scenarios would degrade air quality over much of the globe by increasing background levels of O₃. In northern mid-latitudes during summer, the zonal average increases near the surface are about 30 ppb or more, raising background levels to nearly 80 ppb, threatening attainment of air quality standards over most metropolitan and even rural regions, and compromising crop and forest productivity. This problem reaches across continental boundaries since emissions of NO_x influence photochemistry on a hemispheric scale.

A more complete and accurate assessment of the human impact on greenhouse gases requires greater understanding of sources, processes, and coupling between different parts of the climate system:

The current assessment is notably incomplete in calculating the total impact of individual industrial / agricultural sectors on greenhouse gases and aerosols. The IPCC Special Report on Aviation demonstrates that the total impact of a sector is not represented by (nor scalable to) the direct emissions of primary greenhouse gases alone, but needs to consider a wide range of atmospheric changes.

The ability to hindcast the detailed changes in atmospheric composition over the past decade, particularly the variability of tropospheric O₃ and CO, is limited by the availability of measurements and their integration with models and emissions data. Nevertheless, since the SAR there have been substantial advances in measurement techniques, field campaigns, laboratory studies, global networks, satellite observations, and coupled models that have improved the level of scientific understanding of this assessment. Better simulation of the past two decades, and in due course the upcoming one, would reduce uncertainty ranges and improve the confidence level of our projections of greenhouse gases.

Feedbacks between atmospheric chemistry, climate, and the biosphere were not developed to the stage that they could be included in the projected numbers here. Failure to include such coupling is likely to lead to systematic errors and may substantially alter the projected increases in the major greenhouse gases.

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