In these days of rapidly escalating climate catastrophe, it can be hard to find shreds of good news. The results emerging from the Montreal Protocol, a 1987 treaty designed to protect Earth’s atmospheric ozone layer, are a welcome exception.
Often held up as a success story of international cooperation, the protocol’s global ban on ozone-damaging commercial compounds—such as chlorofluorocarbons (CFCs)—has led to the recovery of the ozone layer in the stratosphere, which absorbs dangerous ultraviolet (UV) radiation from the Sun. As it turns out, according to new research, the Montreal Protocol is even more important to life on Earth than previously quantified.
The protocol has shielded plants around the world from harmful UV rays that would have reduced their ability to store carbon, giving Earth more of a fighting chance against climate change, according to a study published on Wednesday in Nature.
Previous studies have assessed the Montreal Protocol’s power to mitigate climate change by limiting CFCs, which are a powerful greenhouse gas, but a team led by Paul Young, an atmospheric and climate scientist at Lancaster University, is the first to specifically investigate the treaty’s impact on the crucial role of plants as a global carbon sink.
Their results reveal that a world without the treaty would have led to UV-damaged plants that could have released anywhere from 325 to 690 gigatonnes of carbon into the atmosphere by 2100. As a result, this “world avoided” might have faced an additional temperature rise of 2.5°C—above and beyond the 2-to-6°C increase already projected for the end of the century—with 0.50–1.0°C linked specifically to UV-damage of plants.
“While it was originally intended as an ozone protection treaty, the Montreal Protocol has been a very successful climate treaty,” Young said in an email. “It has not only controlled the emissions of highly potent greenhouse gases but, as we show, through protecting plants and the land carbon sink, has led to avoided additional [carbon dioxide] increases.”
“The total avoided temperature increase is 2.5°C, with 1.7°C coming from the higher CFCs and 0.8°C coming from the additional CO2” linked to plants, he added.
The Montreal Protocol was ratified due to strong evidence that a depleted ozone layer would lead to an increase in UV light at Earth’s surface, which would boost the risk of cancers in humans, along with many other negative outcomes for public health and ecosystems.
Young and his colleagues modeled three possible worlds that focused specifically on the consequences for Earth’s plants, a biological kingdom that is critical for the stability of the global carbon cycle: A “world projected” model represents our current reality with the Montreal Protocol, while the “world avoided” model assumes that the treaty never existed, leading to an increase in CFC pollution of three percent each year. A middle ground model called “Fixed1960” assumes that CFC concentrations remain at the same levels that existed in 1960.
The team created a new modeling framework for the study that accounted for various levels of ozone depletion, UV-related plant damage, climate change, seasonal variations, and the global carbon cycle, among other factors.
The “world avoided” scenario offers a counterfactual nightmare scenario in which the ozone layer deteriorates by the 2040s. The consequent increase in UV radiation at Earth’s surface would damage botanical tissue around the world, reducing the photosynthetic ability of plants to assimilate carbon dioxide and turn it into oxygen and energy.
In this “scorched Earth” simulation, the capacity for plants to assimilate carbon would have decreased by up to 90 percent in northern mid-latitudes such as the United States, Europe, and central Asia by the 2050s. In the tropics, where rainforests currently soak up a huge amount of carbon, plants would lose up to 80 percent of their efficacy as a carbon sink by the 2090s.
“We had some understanding of how ozone affects UV and how UV damages plants, and our initial back-of-the-envelope calculations gave us an idea of what we might expect from the more complex model,” Young said.
“However, what was perhaps surprising was how plants' ability to absorb carbon declines so massively,” he continued. “Compared to a world where we enact the Montreal Protocol, net carbon uptake is 85 percent less in the ‘world avoided’ by the end of the century.”
The study offers yet another sobering glimpse of the catastrophic environmental bullet our civilization has dodged thanks to the Montreal Protocol. It’s intimidating enough to confront the escalating effects of climate change as they stand right now without the deleterious impacts of ozone depletion baked in as well.
To that point, the comprehensive new report from the Intergovernmental Panel on Climate Change (IPCC) is a blaring “code red” about the next set of challenges that demand urgent action from world governments and communities. The 2015 Paris Agreement laid out goals to limit global temperature rises to under 2°C from pre-industrial averages, but the IPCC has warned that we are currently not on track to meet those targets.
Young hopes that one day, we will be living in a future where the Paris Agreement goals are reached, and scientists like him are comparing Paris-related “world avoided” models to a much safer and more stable reality. But though the Montreal Protocol offers a powerful example of cooperation, he emphasized that addressing the climate crisis and eliminating fossil fuel emissions will require a far more expansive and complicated effort from world societies.
“While it’s tempting to see the parallels with climate negotiations, we must remember that the Montreal Protocol was dealing with a handful of chemicals, made by a handful of companies, and for which there were replacements readily available,” Young said. “Despite some resistance from the companies, and the existence of some cranks denying the evidence, the ozone layer issue was ultimately a tractable one.”
“Fossil fuels, on the other hand, are a far more complex issue, whose use permeates our lives more deeply and whose reduction will not just be a straight swap with another chemical,” he concluded. “Of course, that is not to say that we shouldn’t rise to the challenge—we must! Perhaps the hope from the Montreal Protocol is that it has been a tremendous success story: science identified a threat and the world agreed and acted on that threat.”