Further evidence on human global warming

Professor Tim Wigley

Professor Tim Wigley

A team of international climate scientists including University of Adelaide’s Professor Tom Wigley has today reported further strong evidence of the human influence on climate change.

Published today in the Proceedings of the National Academy of Sciences of the USA (PNAS), the researchers have detailed a comprehensive study investigating the causes of temperature changes in Earth’s atmosphere.

They have analysed satellite temperature data over 34 years and compared these data with results from more than 20 different climate models, focussing on the vertical structure of atmospheric temperature change (from the troposphere or lower levels of the atmosphere through to the stratosphere or upper reaches of the atmosphere).

The study was led by scientists from the Lawrence Livermore National Laboratory in the United States and builds on work published in 1996 by the same group. The 1996 Nature paper, ‘A search for human influences on the thermal structure of the atmosphere’, was the first published work to clearly identify the human fingerprint in observed temperature changes.

“With this paper we have built on our earlier work with another 20 years of data that adds further strong evidence for the human impact on our climate,” says Professor Wigley, ARC Fellow with the University of Adelaide’s Environment Institute.

Professor Wigley says the study is more much comprehensive than other published studies and has been able to better define the human ‘signal’ in atmospheric temperature change. There is a clear pattern of warming temperatures in the troposphere and cooling temperatures in the stratosphere, changes that are the characteristic signature of human activity.

“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.

“One of the standard skeptic ‘arguments’ is that all the observed changes are caused by natural variability, and often supposed to be due to solar activity,” says Professor Wigley.

“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.

“There is simply no other way to explain the changes that have occurred since 1979 (when special research satellites were introduced by the US National Oceanic and Atmospheric Administration) other than as a result of human influences – primarily greenhouse gases and related pollutants like sulphur dioxide emissions and gases that affect the atmospheric concentrations of ozone.

“Other published work has already shown a convincing and growing pile of pebbles of evidence for the dominant role of humans in climate change. Our paper adds a huge boulder to that pile.”

Find the paper here.

Read a longer interview with Professor Wigley here.

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