In 1987, the Montreal Protocol banned the emission of ozone-depleting propellants, and the earth’s ozone layer has been recovering ever since. But as a simulation now shows, the protocol could have saved us from even worse climate change. Had mankind continued to emit ozone-depleting substances unchecked, the greenhouse effect of these gases alone would have heated the climate by an additional 1.7 degrees by 2100. In addition, there is another 0.8 degrees, because the vegetation would have absorbed less carbon dioxide (CO2) due to the increased UV exposure. The CO2 values would be around 30 percent higher by 2100 and the warming would have increased by an additional 2.5 degrees, as the researchers have determined.
The earth’s ozone layer is our most important protection against harmful UV radiation. If the short-wave, high-energy radiation hits an ozone molecule, it splits it and is absorbed in the process. In this way, the ozone layer protects people, animals and plants from the cell and genetic damage caused by excessive UV exposure. We owe the fact that the ozone layer is still largely intact today to the Montreal Protocol adopted in 1987. At that time it was recognized that chlorofluorocarbons (CFCs) and other propellant gases containing halogens were ozone-destroying and that they were to blame for the formation of an ozone hole over the Antarctic. Therefore, with the Montreal Protocol, the international community banned the production and emission of a large number of these ozone-depleting substances. As a result, the ozone layer has partially recovered since then, but the longevity of some CFCs means that the ozone density is thinned to this day – even over northern latitudes.
But if the Montreal Protocol had not existed, the ozone situation would look much worse today, as so-called “World Avoided” simulations demonstrated some time ago. For them, scientists use geophysical data and atmospheric models to reconstruct how the ozone layer, climate and UV exposure would have developed if CFCs had continued to be emitted after 1987. Such scenarios show, among other things, that UV exposure in our latitudes would have increased by 15 percent and that there would also be a regular ozone hole over the North Pole. In addition, there is another effect of halogenated hydrocarbons: They are potent greenhouse gases, some of which are several thousand times more effective than CO2. The ban on these substances therefore also had a climate effect.
Focus on the climate effect of the Montreal Protocol
Paul Young from Lancaster University and his colleagues have now investigated exactly what this effect looks like and how the global climate and atmosphere would have developed by 2100 without the Montreal Protocol. One focus was on an aspect that was mostly not taken into account in earlier “World Avoided” studies: the effect of greatly increased UV exposure on vegetation. “Experimental data indicate that uncontrolled ozone depletion, and in particular an increase in UV-B radiation, has a substantial negative effect on plant growth,” the researchers explain. This reduces the net primary production and thus also the amount of CO2 that the plant world absorbs and stores as part of its photosynthesis. As a result, the buffer effect of the vegetation, which so far absorbs part of the CO2 emissions and reduces their climate effect, is reduced.
For their model scenarios, Young and his team simulated the development of vegetation, CO2 values and temperatures up to 2100 based on the ban on CFCs and a rather weak climate protection with a decrease in CO2 emissions only from 2075. The second scenario – Fixed1960 – is based on compensate for climate protection, but in a world without the Montreal Protocol, in which CFCs continue to be produced and emitted in the 1960s. As the third, they simulated the “World Avoided” as the worst scenario, in which FKCW emissions even increase by three percent per year. As expected, the ozone depletion would proceed in this scenario. “This thinning will accelerate into the 2040s, when the ozone layer will collapse worldwide,” the researchers write. “By 2100, the global mean ozone density would drop to 90 Dobson units, which corresponds to a reduction of 72 percent compared to the control scenario with the Montreal Protocol.”
Additional warming of 2.5 degrees
The effect of this greenhouse gas enrichment and ozone depletion on the climate would be considerable: With the Montreal Protocol, the simulations forecast an increase in global temperatures by 2100 by 3.2 degrees above the mean between 1986 and 2005. The additional greenhouse effect of CFCs alone would lead to the ” World Avoided ”add another 1.7 degrees warming – the effect on the vegetation is not yet taken into account here. If you add this, the scenario darkens further. “The increased UV exposure would massively reduce the ability of the plant world to absorb CO2, which would result in higher CO2 values and more global warming,” explains Young. Assimilating a three percent reduction in assimilation for every ten percent additional UV exposure, the vegetation would have bound around 580 billion tons (gigatons) less carbon by the end of the century. As a result, the CO2 content of the earth’s atmosphere would be 165 to 215 parts per million (ppm) CO2 higher than in the prohibition scenario – this corresponds to around 30 percent more. For the climate, this would mean a further 0.8 degrees warming.
All in all, the Montreal Protocol and the subsequent bans on ozone-depleting substances have saved the world from a hot period in two respects: The avoided greenhouse gas effect of CFCs would have heated the climate by an additional 1.7 Gad, and the buffering effect of the vegetation, which would have declined due to UV damage another 0.8 degrees. In the “World Avoided” scenario, we would therefore have to reckon with a temperature increase of 2.5 degrees from the thinned ozone layer and the CFCs alone. This would add up to a good three degrees of the already forecast warming due to our CO2 emissions on top of that. “Our research shows us that the success of the Montreal Protocol goes far beyond protecting humankind from increased UV exposure – it also preserves the ability of the plant world to absorb CO2,” says Young. “Of course we hope that our worst scenario would never have happened, but it reminds us once again of the importance of protecting the earth’s ozone layer.”
Source: Paul Young (Lancaster University, UK) et al., Nature, doi: 10.1038 / s41586-021-03737-3