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Large icebergs float away as the sun rises near Kulusuk, Greenland, on Aug. 16, 2019.

Felipe Dana/The Associated Press

Ozone-depleting substances such as chlorofluorocarbons (CFCs) have played an important and underappreciated role in the rapid warming of the Arctic, a group of U.S. and Canadian researchers has found.

The team’s analysis suggests the effect is so pronounced that climate change would, by now, be a far more urgent and dire matter in the North had it not been for the Montreal Protocol – the 1987 treaty that bans the release of those substances because of their harmful effect on the ozone layer.

“There are added benefits of this protocol that we keep discovering as we look at the data,” said Karen Smith, a climate scientist at the University of Toronto and a co-author of the study, published Monday in the journal Nature Climate Change.

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While the Montreal Protocol was not implemented with climate change in mind, it has effectively reined in a key contributor to Arctic warming, she added.

The study suggests that over the past three decades the protocol was buying time for policy-makers as they began to take seriously the cumulative effect of greenhouse gasses in Earth’s atmosphere.

LEGACY OF THE MONTREAL PROTOCOL

Relative amounts of heat trapped by various green-

house gases from 1955 to 2005 show that ozone

depleting substances played a significant and

growing role in climate change until about 1990.

1.2

Radiative forcing (Watt per sq. m)

Carbon

dioxide

1.0

0.8

0.6

0.4

ODS*

Methane

0.2

Nitrous

oxide

0

1960

1970

1980

1990

2000

*Ozone depleting substances including chlorofluorocarbons

JOHN SOPINSKI/THE GLOBE AND MAIL

SOURCE: nature climate change

LEGACY OF THE MONTREAL PROTOCOL

Relative amounts of heat trapped by various greenhouse

gases from 1955 to 2005 show that ozone depleting

substances played a significant and growing role in

climate change until about 1990.

1.2

Radiative forcing (Watt per sq. m)

Carbon

dioxide

1.0

0.8

0.6

0.4

ODS*

Methane

0.2

Nitrous

oxide

0

1960

1970

1980

1990

2000

*Ozone depleting substances including chlorofluorocarbons

JOHN SOPINSKI/THE GLOBE AND MAIL

SOURCE: nature climate change

LEGACY OF THE MONTREAL PROTOCOL

Relative amounts of heat trapped by various greenhouse gases from 1955 to 2005

show that ozone depleting substances played a significant and growing role in

climate change until about 1990.

1.2

Carbon

dioxide

1.0

Radiative forcing (Watt per sq. m)

0.8

0.6

0.4

ODS*

Methane

0.2

Nitrous

oxide

0

1960

1970

1980

1990

2000

*Ozone depleting substances including chlorofluorocarbons

JOHN SOPINSKI/THE GLOBE AND MAIL, SOURCE: nature climate change

“It’s only after decades that we’ve come to determine how it’s made a difference,” said John Fyfe, a research scientist at the Canadian Centre for Climate Modelling and Analysis in Victoria, who was not involved in the study.

However, he added, it is a benefit “that we’re only going to realize for a period of time” because of rising emissions of carbon dioxide from fossil fuels, which remains the largest driver of climate change.

Dr. Fyfe called the new result “very solid” and said that it quantifies a known effect that has long been a challenge to measure.

While CFCs and related chemicals are far more potent than carbon dioxide at trapping heat, they are present in the atmosphere in such low concentrations – in the parts per trillion range – that is it difficult to pin down their influence on Earth’s climate.

To overcome this barrier, Dr. Smith and her colleagues ran a series of computer models that show how the Arctic climate would have unfolded had the ozone-depleting gasses never been invented. They focused on the second half of the 20th century, corresponding to the time when emissions of CFCs were growing because of their widespread use in spray cans and refrigerators.

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“If you want to understand the importance of these gasses, you’ve got to focus on the decades when they were being emitted and when their concentrations were increasing,” said Lorenzo Polvani, a Columbia University climate scientist who led the study.

The simulations showed that the rate of Arctic warming in a world without CFCs would be cut in half – a striking effect for a category of substances that are only present in small quantities to begin with. To ensure the result was not simply a quirk of the simulation, the researchers ran the experiment using two different climate models and arrived at a similar outcome.

The results closely match an Australian study published last fall, which simulated what the global climate would be like had the Montreal Protocol never been put into effect. Worldwide, that study found the protocol can be credited with a 25-per-cent reduction in global warming, “a remarkable success in climate-change mitigation,” Rishav Goyal and colleagues from the University of New South Wales wrote in the journal Environmental Research Letters.

In reality, the protocol was implemented within a few years of scientists discovering that CFCs were linked to an alarming degradation of the ozone layer – a region of the atmosphere that blocks cancer-causing ultraviolet radiation from the sun. The speed of the response stands in stark contrast to decades of delayed action on CO2 emissions, which require changes to the way the world does business that are far more sweeping and politically contentious.

Notwithstanding some recent rogue emissions of CFCs that were last year narrowed down to two provinces in China, the atmospheric concentration of ozone-depleting substances is holding steady. The chemicals continue to play a role in climate change and will do so until they break down – a process that can take from 60 to 330 years.

Dr. Fyfe said that the influence of ozone-depleting gasses on climate, as indicated by the two studies, is large enough that it should finally be possible to spot the effect of the Montreal Protocol directly from observational data.

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“What we’re looking for are trends against a huge amount of variability,” he said. “It took a certain amount of time for nature to play itself out.”

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