Stratospheric chemistry and climate change

2.2.3 The Montreal protocol

AfterMolina and Rowland(1974) pointed out the possible ozone destruction by man-made CFCs in the atmosphere, the United Nations Environmental Pro-gramme (UNEP) strengthened the efforts for the protection of the ozone layer. In 1985 the Vienna Convention was signed by 28 countries who agreed to cooperate in research and monitoring of the ozone layer and its destroying substances by sharing information on CFC production and emission and controlling the further development. Beyond this agreement on passive controlling further negotiations on the active protection of the ozone layer were continued. The discovery of the ozone hole in the 1980s pushed these negotiations. On 16th September 1987, 46 countries signed the Montreal Protocol. The Protocol required parties to cut the production and the consumption of the main five CFCs by 50% on the levels at 1986. Production and consumption of the three main halons was frozen at 1986 levels starting from 1993. An important feature of the Montreal Protocol was the flexibility designed into it to allow for adaption considering the growth of scientific knowledge and technological developments. Several adjustments and amendments have been added so far. By May 2000 175 countries had ratified the 1987 Montreal Protocol. By the end of 1998 the production of the originally controlled CFCs had fallen by 95% in industrialised countries. The production of the controlled halons had fallen by 99%. Since the Montreal Protocol permits longer phase-out periods for development countries, the overall world produc-tion has declined by about 88% (CFCs) and 84% (halons) from the base year 1986. The increase of the atmospheric concentration of the major ozone deplet-ing chemicals has clearly slowed down (see Figure 2.8). The ozone loss due to ozone depleting substances is expected to decrease in the future. But the total ozone depletion may behave different with a raising impact of a changing climate on ozone chemistry.

2.3 Stratospheric chemistry and climate change

The observed stratospheric ozone depletion over the past two decades was largely due to the described chemical depletion by chlorine- and bromine containing substances (CFCs). These ozone changes have a subsequent impact on climate.

The climate change, induced by the enhanced concentration of greenhouse gases (GHG), will vice-versa have an impact on the future ozone situation. As pointed out in theScientific Assessment of Ozone Depletion (2002) of the World Meteo-rological Organisation (WMO), theThird Assessment Report on Climate Change (2001)of the International Panel of Climate Change (IPCC), andHassol(2004) the closed coupling between ozone and climate happens through many interacting chemical and dynamical processes, operating in either direction. The complex-ity of the feedback processes makes it difficult to quantify the impact of single processes. Some of the areas where strong coupling between climate change and ozone can be found are summarised here.

The enhanced abundances of GHGs like CO2, CH4, and others, cause a cool-ing in the stratosphere, in contrary to the induced warmcool-ing of the troposphere.

Additionally, the ozone depletion by itself contributes to a cooling effect in the stratosphere, since ozone is the major heat source in the stratosphere. Further-more, the indirect effect of changes in the dynamical structure of the stratosphere, also related to enhanced concentrations of GHGs, may also have an impact on the stratospheric temperatures. Lower temperatures in the lower polar stratosphere will increase the occurance of PSCs, thus leading to enhanced ozone depletion.

Rex et al. (2004) quantified the relation between winter-spring loss of Arctic ozone and changes in stratospheric temperatures. They expect ≈ 15 DU addi-tional ozone loss per Kelvin cooling of the Arctic lower stratosphere. In the upper stratosphere the colder temperatures are expected to work against the ozone de-pletion due to a slowdown of the gas-phase chemical loss reactions.

The observed warming of the troposphere influences the dynamics in the tropo-sphere and in the stratotropo-sphere. That effects the atmospheric transport and the stratosphere-troposphere exchange of trace gases. One example is the dynamic interaction between tropospheric weather systems and the ozone of the lower stratosphere. Low values of ozone are related to tropospheric anticyclones and a high tropopause. Measurements at the meteorological observatory Hohenpeißen-berg, Germany show that the tropopause height has increased about 120 m per decade and was strongly correlated to the column ozone (Steinbrecht et al.

1998).

Besides their direct and indirect radiative effects some of the GHGs, such as CH₄ and N₂O, have an additional direct influence on atmospheric chemistry.

Unclear is the effect of water vapor in the lower stratosphere. There are some hints about a recent water vapor increase in the stratosphere (Rosenlof et al.

(2001) andOltmans et al. (1995)). Changes of water vapor in the stratosphere are related to CH4-increase and to changed dynamics. For Polar regions higher water concentrations may decrease the treshold temperatures for PSC-formation.

The effects of aerosols are difficult to predict. Aerosols have a positive and a negative effect on radiation and of course their chemical composition will also effect the chemical processes, Dameris (2005).

2.3 Stratospheric chemistry and climate change 33

Figure 2.8: Past and Future Abun-dances of Atmospheric Halogen Source Gases. The total combined abundance of ozone depleting species peaked before 2000 and now slowly declines. Effective chlorine values combine the abundances of chlorine-containing gases with those of bromine-containing gases. The atmo-spheric increases of the CFC have slowed down, and CFC-11, and CFC-113 abun-dances have decreased slightly. Because of longer lifetimes, CFC abundances de-crease more slowly than methyl chlo-roform (CH₃CCl₃), which showed the largest reduction. The Montreal Proto-col allows for the use of hydrochlorofluro-carbons (HCFCs) as shortterm substi-tutes for CFCs. As a result their abun-dances continue to grow. The shown halons are bromine-containing source gases. Although their production in de-veloped countries is nearly zero since 1994, they continue to grow. This is due to the fact that substantial amounts are currently stored in fireextinguishing equipment and are only gradually re-leased. Furthermore, the consumption and production are still allowed in de-veloping nations. Methyl bromide and methyl chloride have substantial natu-ral sources. Methyl chloride is not regu-lated and will remain constant, whereas methyl bromide will decrease because of the regulation of the Montreal Protocoll and will then remain constant in bal-ance with its natural sources. From the Scientific Assessment of Ozone Deple-tion, 2002