ANNUAL SYMPOSIUM

Australia's science future 3-4 May 2000
Full listing of papers

Dr David Etheridge studied physics and Earth sciences at the University of Melbourne, with an honours thesis in the atmospheric attenuation of solar radiation and a PhD in the changing composition of the atmosphere over the last 1000 years. He has worked at the Australian Antarctic Division, the CNRS Glaciologie group in France, and the University of Melbourne. He is now with the Global Atmospheric Change program of CSIRO Atmospheric Research. He has led five scientific expeditions in Antarctica, investigating ice sheet changes, atmospheric turbidity and atmospheric composition in ice cores. His research at CSIRO centres on atmospheric greenhouse gases and ozone depleting gases.

The changing composition of the global atmosphere
by David Etheridge
david.etheridge@dar.csiro.au

Abstract
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 in key gas species. By virtue of their locations, these stations measure concentrations in air that has resided for some time above oceans, and are therefore not influenced by regional sources or sinks.

Measurements over space and time can reveal important trends from which the locations and magnitudes of sources and sinks can be inferred. For carbon dioxide, slightly higher levels in the northern hemisphere reflect the greater emissions there. A global increase in concentration over the few decades of available atmospheric measurements shows the imbalance of emissions over sinks, and a clear annual cycle results from the 'pulsing' of the biosphere with the seasons.

The carbon dioxide exchanges can be estimated from measurements of surface processes, scaled up to large geographical regions. Uncertainties in these exchanges can be significantly reduced by analysis of the atmospheric concentrations and isotopic ratios of trace species. However, the frequency and coverage of the atmospheric measurements needs to be dramatically increased if scientists are to determine exchanges with sufficient detail and precision for emissions accounting at regional scales. CSIRO is presently developing an instrument intended for such measurements of CO₂.

Atmospheric measurements show how composition has changed over recent decades. To place these changes in perspective, the air preserved in polar ice is measured. Air in bubbles in ice cores goes back for thousands of years. Changes from natural causes climate variability, biospheric activity, ocean mixing are observed in atmospheric composition in the past. The recent changes in carbon dioxide and methane, however, are unprecedented in both the rate of increase and the levels reached, for hundreds of thousands of years.

Another tool we can apply to understand atmospheric composition changes is the use of isotopes. For example, organic carbon is depleted in the¹³C isotope, compared to carbon in other reservoirs such as the ocean and atmosphere. The CO₂ increase from carbon released from fossil fuel and land clearing is therefore accompanied by a decrease in the ¹³C/¹²C ratio.

Precise measurements of oxygen concentration provide another powerful tracer. The concentration of oxygen in the atmosphere is decreasing, because oxygen is being combined stoichiometrically with carbon from fossil fuels and from forest burning to form carbon dioxide. The increase in carbon dioxide is well known, but the related decrease in oxygen is very small compared to its approximately 20 per cent atmospheric abundance. Measurement of the oxygen change has presented a major measurement challenge. Although oxygen’s decrease could be viewed with concern, it will not have a noticeable effect for thousands of years. The global oxygen trend, measured over the last 20 years or so in air archived at Cape Grim, could be explained quantitatively by the amount of fossil fuel carbon known to have been burnt over this period. However, we know that large areas of forests have also been cleared and combusted, so the oxygen decrease from this cause must have been approximately balanced by the release of oxygen from a greater rate of photosynthesis of the remaining terrestrial biosphere. This is possibly a result of higher plant growth rates caused by higher CO₂ levels, or climate change. The biosphere over this period is said to be in mass balance.

Measurements of trace gases are also possible from satellites. Satellite images show quite clearly the ozone depletion in the stratosphere over recent decades. For this type of applicationn coverage of remote locations and measurement of large concentration changes satellites are well suited. However, satellite trace gas measurement is presently too imprecise to determine global exchanges, where regional differences are only 1 per cent or less. Satellite measurements may well be suited to detection of emissions 'hot spots', such as carbon monoxide from cities, or methane from biomass burning.

Management of future atmospheric composition

Observations like these, combined with theoretical and process studies, contribute to our ability to manage the planet’s atmospheric composition.

For a trace gas species, we can consider the following process:

• determine whether the species ('X') matters
  
• identify the processes affecting the abundance of X
  
• monitor the atmosphere for change
  
• predict the effects of emissions change
  
• control emissions through policy
  
• monitor the atmospheric response

Science is required at each step, with a major engagement with policy at several of the steps. A similar process has already enabled some control over the global levels of ozone-destroying chlorofluorocarbons.

Control of other gases, especially carbon dioxide, is also theoretically possible but could be much more difficult. Huge reductions in emissions compared to 'business-as usual' scenarios would be needed to stabilise future CO₂ concentrations at levels for which climate models predict only moderate changes. The perturbation of the carbon cycle by anthropogenic emissions has been in progress since before the time of Arrhenius, and will take some time to restore. And because of the dependence of modern economies on fossil-fuels, reducing emissions will require major policy changes.

In conclusion, we know that the composition of global atmosphere is changing. Human activities are a major cause. Management of our atmosphere’s composition may be possible. The scientific understanding of the causes is improving but not complete. We will need to increase measurements frequency, location, precision and to better understand the exchanges of compounds between the atmosphere and the land and oceans. Then we may have more ability to predict or even manage future changes. We must also keep in mind that the atmosphere is a complex system and there may be surprises in store.

Discussion

Does human respiration contribute to carbon levels?

David Etheridge. Yes, but it is very small probably undetectable. Respiration of plants and trees is more important. Plants are also taking carbon out of the atmosphere through photosynthesis. Animals are not large sources of CO₂. Animals are more important for methane emissions.

What is the amount of carbon dioxide going into the atmosphere from burning native forests, for example, in the Amazon?

David Etheridge. The increase in forest burning is one cause of the increase in carbon dioxide. Land clearing is also a significant, related source. But they are less than the emissions from fossil fuels about a quarter. Over the last 20 years, the contribution of burning and clearing to the atmosphere has been approximately balanced by the remaining biosphere taking out carbon dioxide through photosynthesis. This may be because the biosphere has been stimulated by greater atmospheric CO₂ levels, for example.

Will the problem of carbon dioxide emissions disappear when we run out of fossil fuels?

David Etheridge. We may run out of fossil fuels but we’re talking about a very long time. There are very large amounts of fossil fuel reserves out there. If we burnt all of them, depending on the rate, this would lead to an enormous increase in atmospheric carbon dioxide. Before we could burn it, our actions could be limited by climate change.

Which other components of the atmosphere would be part of the hit list?

David Etheridge. Methane and nitrous oxide both greenhouse gases are increasing. We know fairly well the sources to target if we want to act on them. They may be more amenable to management than carbon dioxide, especially methane, due to its shorter atmospheric lifetime. Synthetic species such as chlorofluorocarbons have seen a change in levels. The new replacement species for CFCs are now
found in the atmosphere and they may not be inert for example, although they do not harm ozone, in some cases they are greenhouse gases, or have an effect on atmospheric chemistry. We need to keep a watch on them.