In the atmosphere, ozone acts as both an environmental savior and a toxic intruder. High up in the stratosphere, it shields the planet from the sun’s harmful ultraviolet rays and permits life on Earth to thrive. Yet down here in the troposphere, it acts as a dangerous air pollutant that harms human health.
In 1987, world governments responded to a weakening ozone layer with the Montreal Protocol—often hailed as one of the most prominent international efforts to protect the environment—which mandated countries worldwide to reduce the use of harmful chlorofluorocarbons (CFCs) that damaged stratospheric ozone. The ozone layer was projected to fully recover in the next few decades, but here is where the duality of ozone presents a twist.
As ozone functions to block ultraviolet radiation, it also promotes radioactive forcing, which is when Earth traps thermal energy from the sun. William Collins from the Department of Meteorology at the University of Reading, in his recent study in Atmospheric Chemistry and Physics, investigated how this forcing is altering climate projections about global warming. “The Montreal Protocol is successful in terms of the environment, but it was not so significant for the climate,” said Collins. To understand this, Collins created three types of approaches to measure radiative forcing: effective, instantaneous, and stratospheric temperature adjusted. All of these aim to generate a precise assessment of the future climate through different calculation methods.
By running these measures with multiple climate models, the team was able to capture a clearer picture of ozone’s role in the atmosphere. What the team found was startling. From 2015 to 2050, there would be an increase of 0.27 watts per square meter—which is much greater than that assessed by the Intergovernmental Panel on Climate Change, whose assessments guide international policy. Collins’ team further concluded that ozone is the second-largest contributor to global warming, trailing only carbon dioxide. Their study also highlighted a complex understanding of ozone-depleting substances (ODS). Before the Montreal Protocol, ODS destroyed the stratospheric ozone layer and partially cooled the climate, but as ODS were reduced, the ozone layer recovered and trapped more heat. This warming caused by ozone recovery possibly cancels out the cooling that ozone loss once provided and may in fact be three times stronger than scientists projected beforehand.
The findings from this study help us understand one thing: ozone’s role on our planet is complex. However, Collins makes clear that this discovery only accentuates the need for decreasing fossil fuel use. “It is essential to continue prioritizing global efforts to reduce carbon dioxide,” advises Collins.