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Study examines hiatus in lower-stratosphere cooling rates

A reduction in harmful human emissions that caused a depletion of the ozone layer has led to a hiatus in lower-stratosphere cooling, a new study has indicated.

Experts from the University of Exeter have studied why the lower stratosphere – the layer of the Earth’s atmosphere found above 10km – has stopped cooling since the turn of the 21st Century.

The study queries why this flattening of the cooling trend is taking place at a time of increasing carbon dioxide concentrations - which should enhance the cooling of the stratosphere by increasing infrared emission.

However, the team of scientists have shown that recent changes to ozone concentrations in the lower stratosphere, primarily brought about by a decrease in the use of harmful CFC’s, has led to a pause in the stratospheric cooling.

The findings are particularly important as this stratospheric hiatus is happening at the same time as a pause in global surface warming. Scientists believe that global warming will resume in the coming decades, and has no correlation with the phenomenon observed in the lower stratosphere.

The correspondence study, by Dr Angus Ferraro, Professor Mat Collins and Dr Hugo Lambert, is published in Nature Climate Change.

Dr Angus Ferraro, an Associate Research Fellow at the University of Exeter and one of the co-authors said: “Although most public attention is on surface temperature changes, there has been a whole lot of great scientific work in recent years studying temperature changes throughout the whole atmosphere, and we wanted to highlight that.

“The reasons the stratosphere has also seen a 'hiatus' in temperature change over the past 10-15 years are different to the reasons the observed surface temperature hiatus. Although carbon dioxide warms the surface it acts to cool the stratosphere.

“The future of the temperature of the lower stratosphere, then, depends on the balance between the cooling effects of carbon dioxide and the warming effects of the recovering ozone layer. ”

The study was formulated following discussions between co-author, Professor Mat Collins, and a group of undergraduate students at the University of Exeter. During a seminar, the students asked a series of questions about the apparent 'stratospheric hiatus', to which there were no immediately obvious answers.

Dr Ferraro added; “After a bit of research we decided these issues warranted further clarification in the broader climate science community so we decided to write this piece setting things in order. "

Date: 22 May 2015

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