Air quality monitoring offers mixed news

Atmospheric concentrations of carbon dioxide and some other greenhouse gases displayed near record growth rates last year. But there was also good news about the state of the atmosphere in 2005.

Dr Paul Fraser, from CSIRO marine and atmospheric research, says that carbon dioxide grew by two parts per million (0.54%) in 2005, the fourth year in a row of above-average growth.

"To have four years in a row of above-average carbon dioxide growth is unprecedented," Fraser said."In addition, the trend over recent years suggests the growth rate is accelerating."

He says the 30-year record of air collected at the Australian Bureau of Meteorology's observation station in Cape Grim, Tasmania, showed growth rates of just over 1 part per million in the early 1980s, but in recent years carbon dioxide has increased at almost twice this rate. "This is a clear signal that fossil fuels are having an impact on greenhouse gas concentrations in a way we haven't seen in the past."

Synthetic greenhouse gases, including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6), displayed a strong increasing trend. Fraser says that the highest ever growth rate, a 7 parts per trillion or 5.3% increase, was recorded in 2005.

Nitrous oxide also shows an increasing growth rate, growing by about 1 part per billion, or 0.3% in 2005. "We are in a period of high nitrous oxide growth rate," Fraser said. "Only once before have we seen such high growth rates in consecutive years."

He stated the rise in nitrous oxide is driven by agricultural practices including land clearing and the use of nitrogen fertilisers.

The combined impact of these increases in greenhouse gases results in a record level in 2005 of greenhouse gas heating, or radiative forcing, which is the main driver of increasing surface temperatures.

Fraser says there is some good news for the atmosphere. "Concentrations of methane, the second most important gas responsible for the enhanced greenhouse effect, have not grown for six years," he said. "In fact, the past two years have shown slight decreases in methane, the first time we have seen this."

While the reason for decrease is not certain, Fraser suggests it is due to better management of the exploration and use of natural gas, leading to less leakage.

There is also good news for the ozone hole. "Ozone-depleting gases have been decreasing since 1997," Fraser said. "The fall in concentrations has continued in 2005, so we have seen a decline in concentration of ozone-depleting gases for nine years now."

Fraser says that air from Cape Grim is an excellent monitor of global atmospheric trends due to its pristine location. The Cape Grim station, managed by the Australian Bureau of Meteorology, monitors and studies global atmospheric composition in a program led by CSIRO and the Bureau.