Did the Montreal Protocol contribute to the surface warming slowdown?
- 10 Nov 2013, 21:00
- Freya Roberts
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
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.
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
For one, it says, natural fluctuations in the
climate system have cooled surface temperatures to some extent
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
A tenth of a degree cooler
So just how big is the 'Montreal' effect? An
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
Estrada et al. (2013) Statistically derived contributions of
diverse human inﬂuences to twentieth-century temperature changes.
Nature Geoscience. DOI: