Research suggests warming could be depleting ozone

Source: Chelsea Harvey, E&E News reporter • Posted: Friday, February 9, 2018

It’s been a hopeful couple of years for the planet’s ozone layer.

In 2016, scientists announced in the journal Science that the Antarctic “ozone hole,” first discovered in the 1980s, was finally in recovery, two decades after an international agreement, known as the Montreal Protocol, spurred large-scale reductions in emissions of ozone-depleting chlorofluorocarbons.

And just over a year ago, nations adopted an amendment to that agreement calling for additional reductions in hydrofluorocarbons, another ozone-depleting — and also climate-warming — class of chemicals.

But as the ozone hole continues to heal, scientists have discovered that ozone concentrations elsewhere in the atmosphere are still declining. And while the causes remain uncertain, climate change could be part of the problem.

New research published this morning in the journal Atmospheric Chemistry and Physics finds that ozone levels in the lower stratosphere — the part of the atmosphere where the ozone layer is densest — have continued to fall over the last few decades. A variety of factors could be contributing to the problem, the researchers note, including climate-driven changes in a major atmospheric circulation system that helps to distribute ozone around the globe.

“The ozone hole over the Antarctic got all the attention because it was so shocking and so easy to show,” said lead study author William Ball of the Swiss Federal Institute of Technology in Zurich. But at the time the “hole” was discovered in 1985, ozone levels were actually declining across the globe.

Since then, researchers have tracked ozone levels in the atmosphere to monitor the success of the Montreal Protocol. Since 1998, they’ve found that ozone levels in the upper part of the Earth’s stratosphere have generally been on the rise — a sign that slashing chlorofluorocarbons is working. But until now, there’s been little research on the lower part of the stratosphere, which is where the majority of ozone is actually found.

“Part of the reason for that is because it’s a notoriously hard region to analyze,” Ball said. But new satellite data on stratospheric ozone, and improvements in the statistical methods used to analyze those data, have helped pave the way in recent years.

Now, using these updated methods and data sets, Ball and nearly two dozen other researchers from institutions around the world have been able to focus on ozone trends in the lower stratosphere. They’ve found that the ozone there has declined by about a half percent since 1998, even as concentrations continue to rise in both the upper stratosphere above it and the troposphere below it.

Because ozone is on the rise in these other parts of the atmosphere, particularly the troposphere, ozone levels taken as a whole have remained more or less steady over the last two decades. But that doesn’t mean there’s no cause for concern about ultraviolet radiation and human health.

For one thing, ozone levels in different parts of the atmosphere can vary somewhat from one location to the next, Ball pointed out — this means it’s possible that some regions are being exposed to more radiation than others. More research will be needed to determine how the balance between growth and declines in different parts of the atmosphere is playing out over time in different regions of the world.

Additionally, Ball noted, gains in tropospheric ozone are not necessarily all good. Ozone is actually harmful to human health when inhaled, so accumulating ozone in the parts of the atmosphere closest to the Earth’s surface can actually be dangerous. The thick layer of ozone in the stratosphere, higher up in the atmosphere, is what’s typically thought of as the “good” ozone.

But some research suggests that climate change could cause continued declines in stratospheric ozone, by affecting the atmospheric currents that help to carry it around the world. Ozone naturally forms in the stratosphere when oxygen molecules react with solar radiation, and it forms most readily in the sunny tropics. From there, a system of atmospheric currents known as the Brewer-Dobson circulation carries ozone-rich air from the tropics up to the mid-latitudes and the poles.

Because the flow of air around the world is largely driven by differences in temperature, which can cause the atmosphere to be thicker or thinner in different places, climate change is expected to disrupt certain wind patterns and other atmospheric currents. And climate models suggest that global warming may cause parts of the Brewer-Dobson circulation to accelerate, changing the way ozone is distributed around the world.

For now, it’s just a theory, Ball cautioned. Other factors may be at play, as well — some recent research, for instance, has found that other human pollutants, not regulated by the Montreal Protocol, may still be depleting ozone in the stratosphere, particularly at the poles. More research will be needed to determine both the causes behind the newly discovered declines and their implications for human health.

Despite the concerns, though, Ball added that the new research by no means suggests that international efforts to curb ozone destruction aren’t working — just the opposite.

“It is clear that the Montreal Protocol is working,” he said. Recovering ozone levels in the upper stratosphere and over Antarctica are evidence of that. Continued declines in the lower stratosphere may be a sign of the growing influence of climate change on the atmosphere, or that some other factor is still involved — and should be investigated.

“This is something that needs to be understood,” Ball said.