By Matthew Doran
ABC News Online, 17 September 2013
A report by a team of international scientists concludes there now is no doubt climatic changes are due to humans rather than any other natural factors.
Their report, published in the Proceedings of the National Academy of Sciences, builds on work previously done by scientists in the United States and presents a clear pattern of warming in parts of the atmosphere that is indicative of a human effect.
The only Australian researcher who was part of the team, Professor Tom Wigley of Adelaide University, says it analysed satellite temperature data over 34 years.
He says the team showed there was no other way to explain climatic changes at various atmospheric levels.
Professor Wigley said by looking at temperature changes across atmospheric layers from the Earth's surface to about 20 kilometres skyward the results show clear characteristics of human interference.
"If the sun were the cause of the changes then one would see warming at the surface and in the lower atmosphere and in the upper atmosphere and in fact what we see is the opposite," he said.
"We see warming at the surface and cooling in the upper atmosphere, so that immediately discounts the sun as a causal factor.
"One of the standard sceptic arguments is that all the observed changes are caused by natural variability and often supposed to be due to solar activity. What we have shown beyond a shadow of doubt is that the climate changes we are observing cannot be due to the sun or any other natural factors."
Professor Wigley said the scientific team had concluded there was simply no other way to explain the changes that had occurred since 1979 when weather satellites were introduced.
The team found human influences, primarily greenhouse gases and related pollutants such as sulfur dioxide emissions and gases, had affected the atmospheric concentrations of ozone.
Scientists said study was comprehensive
Professor Wigley said it was probably the most comprehensive study yet done to try to identify the human influence on climate.
"The main thing is that we can identify what is called a human fingerprint, or a distinctive pattern of change in the observational record, and that pattern is derived from climate modelling experiments," he said.
"We look at patterns of change that can be attributed to other things, such as changing output of the sun for example, and we show that those cannot be identified in the observational record.
"We can see the human fingerprint, we can't see the fingerprint of any other cause, and so it's pretty obvious that the only explanation is there's been a very distinctive human influence on the patterns of climate change."
The scientists said more had been done to tackle ozone depletion than the effects of greenhouse gases.
"Greenhouses gases trap the warmth in, they allow radiation from the sun to penetrate to the lower layers of the atmosphere, but they don't allow as much outgoing radiation and that's what's called the greenhouse effect," Professor Wigley said.
"Ozone is a little bit different. The ozone hole which is in the upper part of the atmosphere and primarily, but not exclusively, at high latitudes is caused by a group of chemicals called halocarbons.
"Those chemicals are controlled under the Montreal Protocol, so we're solving that problem but because those gases have very long life times - more than a century for some of them - it takes a long time to heal the ozone hole.
"That's still a distinctive part of the human fingerprint that we search for."
Professor Wigley said the data relied on by scientists had a high degree of accuracy.
"We use satellite data that it highly precise and quite accurate that has been available since the late 1970s," he said.
"The main workers are colleagues of mine that work in the Lawrence Livermore National Laboratory, which is part of the US Department of Energy in California."
Professor Wigley has responded to news reports suggesting there has been a recent pause in average global surface temperature rises.
"The issue is the fact that, since about 15 years ago, the rate of warming observed has been less than models predicted that it would be, so that could imply that the models are wrong," he said.
"But in fact the real answer is that the heat that normally would accumulate in the atmosphere, and we know this from observational data, has gone into the deeper ocean.
"It's an unusual event, but it's just part of the natural variability of the climate system, so we do understand why there has been this slow down in warming and it's certainly nothing to do with the credibility of climate models."