Did the Montreal Protocol contribute to the surface warming slowdown?
A ban on gases that created a hole in the ozone layer may have contributed to the slowdown in surface warming, if new research is right. An international team of scientists says cuts in CFC gases agreed under the Montreal Protocol have lowered surface temperatures since the 1990s.
CFCs and the slowdown
A few decades ago chlorofluorocarbons (CFCs) were found in everyday objects like spray cans and fridges. That was until scientists discovered the gases were causing a hole in the ozone layer. In 1987 countries from around the world signed the Montreal Protocol, an international agreement to phase out CFCs.
According to a new study in the journal Nature Geoscience, the international agreement had an unintended consequence for earth’s climate. The authors of the study spotted it when looking at the links between the rate of global warming and the speed at which greenhouse gas emissions were increasing.
Their analysis shows that emissions and temperatures have been rising and falling in sync since 1880. In the 1960s there was a jump in both emissions and warming, marking the start of a period of more sustained global warming. Then in the 1990s that warming slowed.
Figures from the latest report from the Intergovernmental Panel on Climate Change (IPCC) show that over the last 15 years, earth’s surface has warmed at a rate of 0.05 degrees Celsius (°C) per decade. That’s considerably slower than the trend since 1951 of 0.12°C rise per decade.
The authors say the slowdown could have been prompted by the phasing-out of CFCs. Scientists have long known that CFCs are greenhouse gases, so cutting them would have the added benefit of limiting manmade warming. But this new study is the first to suggest the treaty limiting CFCs may have contributed to the current slowdown in surface warming.
Alternative agents
This isn’t the only explanation scientists have for the slowdown in surface warming since the 90s. The IPCC report puts it down to two factors in roughly equal measure.
For one, it says, natural fluctuations in the climate system have cooled surface temperatures to some extent by redistributing heat within the ocean. And second, the amount of sunlight reaching earth’s surface has declined – partly due to natural cycles in the sun’s orbit and partly due to the release of volcanic ash which reflects incoming sunlight.
The authors of the new study acknowledge these as possible causes, but say the effects of the Montreal Protocol and changes in agricultural practices in Asia, which have reduced methane emissions, have been “large enough” to change the long-run warming trend
A tenth of a degree cooler
So just how big is the ‘Montreal’ effect? An accompanying news piece by Felix Pretis and Professor Myles Allen estimates temperatures today could have been almost 0.1°C warmer had CFCs not been cut.
It’s a “small but not negligible” difference, they say – enough to support the idea that the Montreal Protocol contributed to the surface warming slowdown over the last 15 years. However it can’t account for all the missing warming over that period, they argue.
The slightly unsatisfying conclusion for Pretis and Allen is that it’s unlikely any single cause can explain the surface warming slowdown. The Montreal Protocol may well be one contributing factor. There’s also lots of evidence, summed up neatly in the IPCC report, suggesting ocean heating, low solar activity and volcanic aerosols are playing a part.
The research does have two clear outcomes though. For one, it’s another piece of evidence linking human activities and rising temperatures. And two, it highlights emissions cuts are an effective way of tackling climate change.
Estrada et al. (2013) Statistically derived contributions of diverse human in�uences to twentieth-century temperature changes. Nature Geoscience. DOI: 10.1038/NGEO1999