Montreal Protocol on industrial gases 'slowed global warming'
1987 Montreal Protocol explains smaller rise in temperatures that sceptics seized on, study says
A slowdown in global warming that climate sceptics cite in favour of their cause was partly induced by one of the world's most successful environment treaties, according to a study.
The UN's Montreal Protocol, designed to phase out industrial gases that destroy earth's protective ozone layer, coincidentally applied a small brake to the planet's warming, it said.
Without this treaty, earth's surface temperature would be roughly 0.1 degrees Celsius higher today, according to its authors.
"Paradoxically, the recent decrease in warming, presented by global warming sceptics as proof that humankind cannot affect the climate system, is shown to have a direct human origin," according to the paper, published on Sunday in the journal Nature Geoscience.
Signed in 1987, the Montreal Protocol committed signatories to scrapping a group of chlorine- and bromine-containing chemicals. Used in aerosol sprays, solvents and refrigerants, these substances destroy ozone molecules in the stratosphere that filter out cancer-causing ultraviolet light.
Some of the chemicals also happen to be major greenhouse gases, with a powerful ability to trap the sun's heat.
So their phase-out, which began to hit its stride in the 1990s, was also a small but perceptible gain in the fight against climate change, the scientists said.
From 1998 to 2012, earth's mean global temperature rose by an average of 0.05 degrees per decade.
This is far less than the average decadal increase over half a century of 0.12 degrees, and is not in line with the rise in greenhouse-gas emissions.
As a result, sceptics cite the 15-year "pause" as proof that climate change has natural causes, showing that green calls to reduce fossil-fuel emissions are flawed or a scam.
The paper, led by Francisco Estrada, an atmospheric physicist at the Autonomous National University of Mexico, is a statistical comparison of carbon emissions and warming during the 20th century.
Overall, temperatures rose last century by 0.8 degrees.
Two world wars contributed to cooling, as did the Great Depression - massively so. From 1929 to 1932, annual emissions of carbon dioxide fell by 26 per cent. It took until 1937 for carbon dioxide emissions to return to their pre-1929 levels.
The boom after the second world war led to a surge in emissions that, from 1960, began to be perceived in a clear signature of sustained warming, according to the investigation.