What would the world have looked like without the Montreal Protocol? Credit: NASA

The Montreal Protocol has not only helped to prevent damage to the Earth's ozone layer — it has also slowed global warming, say climate chemists.

Twenty-one years ago the Montreal Protocol was drawn up to restrict the use of chemicals such as chlorofluorocarbons (CFCs) that contributed to the destruction of the ozone layer, the part of the atmosphere that filters out most of the Sun's potentially cancer-causing ultraviolet rays.

But now, sophisticated climate chemistry models have shown that the protocol has done much more than rescue the planet from sunburn.

Olaf Morgenstern from the Centre for Atmospheric Science, University of Cambridge, UK, and colleagues from the Hadley Centre and the Meteorological Office in Exeter, UK, have run climate-chemistry models to create a future scenario describing what would have happened if the Montreal Protocol had never existed. They call it "the world avoided".

The Montreal Protocol provided a dual protection to ozone and climate. Guus Velders , Netherlands Environmental Assessment Agency, Bilthoven

The team worked out a predicted value for chlorine levels in the stratosphere (the second layer of the atmosphere, from around 10 to 50 kilometres above Earth's surface) in the year 2030, on the basis of how quickly chlorine levels were rising in the late 1960s and early 1970s. "Chlorine is known to be the main driver for atmospheric ozone depletion in the stratosphere," says Morgenstern.

They ran this value of chlorine – 9 parts per billion by volume (ppbv) – through their computer models simulating climate chemistry. For comparison, they also ran the models with a chlorine level of 3.5 ppbv, which was the level in the stratosphere by the late 1990s.

Some of what they found in a world without the Montreal Protocol was expected – massive losses of ozone in all areas of the world, including the tropics. "The chemistry results are not surprising at all," says John Pyle, from the University of Cambridge, who led the team. After all, the Montreal Protocol was all about ozone.

But the model used by Pyle's team also showed what effect this ozone loss would have had on the planet's climate at the surface. And those changes are significant. "What we're seeing here is a big signal," says Pyle. The results are published in Geophysical Research Letters1.

Chlorine runs amok

Morgenstern explains that in their model, greater ozone depletion led to cooling in the north and south polar vortices, the winds that circulate around the poles. This led to stronger winds, increasing coupling between the stratosphere and the troposphere, the part of the atmosphere directly above the Earth's surface. Coupling between the two layers means that changes in one layer have an effect on the other and vice versa.

When the Montreal Protocol was introduced, thoughts about the atmosphere were very different says Mark Baldwin, an atmospheric scientist with research consultants Northwest Research Associates in Redmond, Washington state. "Back in the 1980s they really didn't have a handle on stratospheric/tropospheric coupling if you let chlorine run amok," he says.

The models show that coupling increases as more ozone is lost, and that its effects induce temperature changes at different regions of the planet. These changes were most marked at the poles. In the most southerly latitudes in their spring, for example, the surface temperatures in some places would be 3°C colder and in others 3°C warmer in 2030 if ozone had not been controlled. "This kind of change would have major implications," says Morgenstern. A 3°C rise in temperature, for example, would lead to large areas of thawing ice.

"That's huge regional change," says Guus Velders from the Netherlands Environmental Assessment Agency, Bilthoven. It's clear that "the Montreal Protocol provided a dual protection to ozone and climate."

Velders did a simpler modelling study last year and showed that without the Montreal Protocol the contribution of ozone-depleting substances to radiative forcing — the balance of radiation coming in and going out of the atmosphere — would have been much higher, although he didn't couple this to local surface climate2.

Halocarbon hitlist

The lessons of Montreal might be applied to other aspects of climate change, but would need to be targeted at substances that, like CFCs, are relatively easy to eliminate, says Velders.

Tackling carbon emissions in the same way would be impossible, he says, because these are the by-products of all combustion processes. Baldwin, agrees: "Compared to reducing greenhouse gases, reducing ozone-depleting substances was relatively easy."

But other haloalkanes — hydrofluorocarbons and perfluorocarbons — are also potent greenhouse gases, and would be easier to regulate with a Montreal Protocol-like initiative, Velders says.

The "world avoided" scenario is still a simplified vision – the models only took changes in chlorine into account, other greenhouse gases were kept at a constant level, and the dynamics of the oceans were not included. But Pyle is confident that the team's results are realistic. "We're very confident in saying that had there not been the Montreal Protocol the ozone impact on the polar world would have been significant."