Ozone chemicals ban linked to global warming 'pause'
A new study suggests that the ban on ozone depleting chemicals may have also impacted the rise in global temperatures.
CFC gases were responsible for a massive hole in the ozone layer but they also had a powerful greenhouse effect.
The authors link a ban on their use to a "pause" or slowdown in temperature increases since the mid 1990s.
The research is published in the journal Nature Geoscience.
The subject of a hiatus or standstill in global temperatures rises since 1998 has been the subject of intense debate among scientists, and it has been used as a key argument by some to show that the impacts of global warming have been exaggerated.Competing arguments
There have been a number of theories as to why the rise in emissions from CO2 and other gases has not been mirrored in temperatures since the late 1990s.
These include increases in China's use of coal, changes in solar output, and the impact of the El Nino weather cycle.
One report earlier this year suggested that it was caused by long-term changes in the warming of waters in the eastern Pacific.
Now this latest piece of research says that it has been caused by attempts to protect the ozone layer.
A team of researchers carried out a statistical analysis on the connection between rising temperatures and rates of increase in concentrations of greenhouse gases in the atmosphere between 1880 and 2010.
They concluded that changes in the warming rate can be attributed to specific human actions that affected greenhouse gas concentrations.
- The onset of the current pause coincides with a spike in upper ocean heat uptake around 2002 (lower graph).
- It may have begun when energy trapped by greenhouse gases was buried below the surface of the ocean
- However, the continuation of the pause in global surface warming beyond 2004 coincides with a decline in upper ocean heat uptake
- Understanding the cause of this decline in upper ocean heat content is crucial for explaining the continuation of the pause in surface warming
They were able to show that when emissions were reduced during both world wars and the Great Depression, temperature rises also stalled.
They also argue that the introduction of the Montreal Protocol, originally signed in 1987 by 46 countries, had an impact on global temperatures as well.
The treaty phased out the use of chlorofluorocarbons (CFCs). These chemicals, used as spray can propellants and in refrigeration, had helped thin the ozone layer over Antarctica.
But CFCs were not just damaging the ozone layer, they were also having a warming impact, as they are 10,000 times more powerful than carbon dioxide and can last up to 100 years in the atmosphere.
Their removal, say the authors, was a critical factor in the slowdown.
"Our analysis suggests that the reduction in the emissions of ozone-depleting substances under the Montreal Protocol, as well as a reduction in methane emissions, contributed to the lower rate of warming since the 1990s," the authors write.
In a commentary on the research, Felix Pretis and Prof Myles Allen from Oxford University suggest that the CFC ban is "unlikely to be the whole story", but they acknowledge it did make a difference.
"The impact of this change is small but not negligible: without the reduction in CFC emissions, temperatures today could have been almost 0.1C warmer than they actually are."
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