The Changing Composition of the Global Atmosphere

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The changing composition of the global atmosphere

The levels of many trace gases and aerosol particles are changing in the atmosphere, largely as a result of human activities. Although totalling less than 0.1 per cent of the mass of the atmosphere, these constituents determine much of its chemical and physical state. Changes in atmospheric composition have significant impacts, often a long way from the origins of the emissions (for example, stratospheric ozone depletion, volcanic aerosols, global warming). As a result, the issues are broadening from urban or regional concerns to being at the centre of global environmental protocols. In the case of ozone-depleting substances, we are gaining some control over global atmospheric levels and the ozone layer is expected to respond. Whether control can be extended to carbon dioxide and other greenhouse gases depends on our ability to understand the atmosphere and its ocean and biosphere partners, which are extremely complex systems. It also depends on scientists engaging policymakers to pursue new technological alternatives to the carbon-based economy. In return, science will need to improve ways of verifying the locations and causes of emissions and uptake. The multiple roles of each constituent in the atmosphere will need to be considered if we are to successfully manage the global atmospheric environment. In a landmark paper, the 19th century Swedish scientist, Arrhenius, raised the possibility of carbon dioxide influencing the temperature of the globe. In 1974, Nature published a paper by Molina and Rowland on the impact of chlorofluorocarbons on ozone in the stratosphere. Despite their different eras, both these theoretical works had some similar aspects that are relevant to many global atmospheric composition issues of today. In each case it was some time before the changes predicted by theory were detected in the atmosphere: almost a century for Arrhenius, 10 years for Molina and Rowland. Only after changes were detected did the issues cause concern at the policy level, resulting in controls over emissions of ozone-depleting substances and greenhouse gases: the Montreal protocol and Kyoto protocol, respectively. In the case of ozone, a surprising finding was that accelerated depletion was occurring in the Antarctic stratosphere because of heterogeneous chemical processes. Atmospheric trace gases

Both these studies acknowledged some important characteristics of the atmosphere. The atmosphere is a closed chemical system. Its mixing time is short, about 1 year, so change is not restricted to the area near the source of an emission. Changes in long-lived gases (lifetimes of 1 year or more) have global extent. Some 'trace' compounds in the atmosphere are naturally occurring for example carbon dioxide, methane and nitrous oxide. Their atmospheric concentrations have been significantly increased in the last few centuries by anthropogenic sources. Other trace compounds such as the chlorofluorocarbons, their replacements, and sulfur hexafluoride are synthetic, recently appearing in the atmosphere for the first time. Trace gas concentrations are low (parts per million or much less), and as such they have been are not a direct threat to human health. But many trace gases have important chemical and physical characteristics, even at these low concentrations. They intercept electromagnetic radiation from the sun and from the Earth and have large effects on climate. They can react with other components of the atmosphere, in some cases very effectively, such as the role of chlorine atoms in stratospheric ozone destruction. Furthermore, the atmosphere’s composition is changing.

Evidence of changes
We presently have several means to determine the changes in global atmospheric composition, and the causes: Measurements at baseline stations, for example Cape Grim in Tasmania and other stations in 'networks', such as the CSIRO global network, are monitoring the changes...
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