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Put into force in 1989, the Montreal Protocol (and its subsequent amendments) is an international treaty that protects the world’s ozone layer by reducing global emissions of ozone-depleting substances (ODS). To understand the effects of this treaty and the response of the stratosphere to ODS reduction, Rolando Garcia, Douglas Kinnison, and Daniel Marsh, scientists at the National Center for Atmospheric Research (NCAR), used the Whole Atmosphere Community Climate Model (WACCM) to assess the “World Avoided” through implementation of the Protocol. They modeled the effects of uncontrolled emissions of ODS on stratospheric ozone and climate in the 21st century.
“We wanted to build on existing work about what the world has avoided by enacting the Montreal Protocol,” says Rolando Garcia, a senior scientist in NCAR’s Atmospheric Chemistry Division. “We also wanted to see how long it might have taken to reverse the effects of uncontrolled ODS emissions if action to address the problem had not been taken before the mid-2050s.”
Work led by Paul Newman at NASA’s Goddard Space Flight Center motivated the NCAR scientists. Newman and his collaborators simulated the effects of the Montreal Protocol using the Goddard Earth Observing System (GEOS) model, which showed a complete collapse of the protective ozone layer by 2050. Using a newly released version of WACCM, Garcia, Kinnison, and Marsh expanded on the work of Newman and colleagues by looking into the effects of unrestrained ODS emissions on both the chemistry of the atmosphere and the global climate. The latest version of WACCM can now be run coupled to a deep ocean model, which allowed the NCAR scientists to calculate the response of Earth’s climate to the addition of ODS. Greatly increased concentrations of ODS would have a notable effect on global average temperature because these compounds are also potent greenhouse gases (GHG). While much less abundant than carbon dioxide, methane, or nitrous oxide, ODS are very effective GHG because they absorb outgoing infrared radiation in the “atmospheric window,” the range of infrared wavelength where carbon dioxide and other GHGs are largely “transparent,” explains Garcia.
In running the World Avoided experiment, Garcia and his colleagues were able to confirm Newman’s finding that, if ODS emissions had continued to grow at the rate observed prior to the adoption of the Montreal Protocol (3% per year), the ozone layer would have collapsed globally by the mid-21st century.
“This is an important finding, because the agreement of climate-chemistry models running similar experiments indicates that independent model implementations of currently accepted stratospheric chemistry lead to essentially the same results,” Garcia says.
The WACCM calculations also showed that by 2070 ODS emissions alone would have increased global average temperature by an amount equivalent to what is expected to occur under a moderate greenhouse gas emissions warming scenario, the “Representative Concentration Pathway” 4.5 (RCP4.5). This scenario prescribes changes in GHG that would produce a radiative imbalance of 4.5 Watts per square meter (Wm-2) with respect to pre-industrial conditions by the end of the 21st century. By comparison, the radiative imbalance due to uncontrolled growth of ODS calculated with WACCM is about 4 Wm-2 near the end of the 21st century, which is almost as large as thatdue to the change in non-ODS GHG under the RCP4.5 scenario. Consequently, the global temperature increase by the late 21st century would have been about twice as large in the World Avoided scenario as that calculated under RCP4.5 alone.
Finally, the scientists used WACCM to model how quickly the Earth’s protective ozone layer might have recovered if reductions of ODS emissions began only by the mid-2050s. They chose this date because it coincided with the collapse of the ozone layer in their calculations – a catastrophe that would have forced the world to deal immediately with the resulting impacts. The scientists found that the ozone layer in the Tropics and middle latitudes recovered relatively quickly, within a few years of the complete cessation of ODS emissions. However, in the polar regions substantial ozone depletion was sustained into 2070, the end point of the model simulations. In addition, global warming did not abate significantly by 2070 in these calculations.
The reason for this disparity has to do with the atmospheric lifetime of the chemical species involved. At lower latitudes, ozone is destroyed mainly by chlorine and bromine compounds derived from short-lived ODS, such as methyl bromide, which has an atmospheric lifetime of less than a year. Once emissions cease, the atmospheric burden of methyl bromide from human sources disappears in just a few years. On the other hand, ozone loss at high latitudes depends on chlorine and bromine released from long-lived species like CFC-11, CFC-12, and Halon-1301, whose atmospheric lifetime ranges from 45 to more than 100 years. In this case, removal is a slow process, lasting on the order of a century. These long-lived compounds are also responsible for much of the greenhouse warming due to ODS. As a consequence, the rise in global temperature in 2070 is almost as large in the calculation where ODS emissions are curtailed in mid-21st century as in the full World Avoided scenario.
“Our experiment provides new perspectives on the effects of the Montreal Protocol on global ozone depletion, as well as on climate,” Garcia says. “As regards climate change, it is also useful to bear in mind that while the impact of the long-lived ODS may last about 100 years, carbon dioxide is ‘eternal’ as far as human lifetimes are concerned.”
Some of the atmospheric carbon dioxide will be absorbed by the oceans on the relatively short time scale of 1,000 years. After that, carbon removal is a geological process and takes place on time scales of hundreds of thousands of years. In terms of taking effective action for dealing with climate change, was the Montreal Protocol a good thing? Ironically, perhaps not, suggests Garcia.
“Without the agreement, the world today would be heating up more rapidly, which might have encouraged us to do something, right now, about climate change,” Garcia says.
The paper by Garcia et al, “World avoided” simulations with the Whole Atmosphere Community Climate Model is available at http://onlinelibrary.wiley.com/doi/10.1029/2012JD018430/abstract