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Ban on ozone-depleting chemicals has protected plant photosynthesis

If not for an international treaty signed in 1987, researchers say the ozone layer would be nearly gone, leading to a scorched Earth scenario caused by a spike in harmful ultraviolet radiation.

 (CN) — If it weren’t for a global ban on chemicals known to destroy ozone, plants may have become unable to absorb carbon dioxide from the atmosphere, leading to a more quickly warming planet, a new study has found.

The Montreal Protocol, signed in 1987, is a worldwide agreement to phase out the use and production of ozone-depleting substances that were often used as propellants for bug sprays, paints and health care and beauty products.

The ozone layer in the stratosphere protects life on Earth from the sun’s ultraviolet rays, which can cause severe sunburns, skin cancer, cataracts and other health problems. 

At its worst point, 10% of the ozone’s upper layer had been depleted by the 1990s. But thanks to the Montreal Protocol, the ozone is slowly recovering and could be restored by the middle of the century, the United Nations reported in 2018.

Now, a new study published Wednesday in the journal Nature has found that the agreement has helped the planet avoid “catastrophic” effects for vegetation, which has in turn given the Earth a fighting chance against global warming.

“Our new modeling tools have allowed us to investigate the scorched Earth that could have resulted without the Montreal Protocol’s ban on ozone-depleting substances,” Paul Young, atmospheric and climate scientist at Lancaster University and lead author of the study, said in a statement.

If ozone-depleting substances were not phased out, it would have led to a worldwide collapse in the ozone layer by the 2040s, according to researchers. And by 2050, ultraviolet radiation from the sun in the Earth’s mid-latitudes — including the northern United States, most of Europe and central Asia — would be stronger than it is today in the tropics, which surround the equator.

That damaging increase in UV radiation could restrict plant growth by damaging their tissues and impairing their ability to photosynthesize.

In other words, it would likely make plants smaller, Young said.

“It’s not improbable that very high UV levels would – separate from the growth effect we simulate – severely ‘burn’ plant tissue,” he said.

Unable to undertake photosynthesis, plants would absorb less carbon — possibly to the tune of 580 billion tons less carbon stored in forests — which means far more carbon dioxide would remain in the atmosphere and cause more global warming.

CO2 is a greenhouse gas — it absorbs and radiates heat. Without it, Earth’s average annual temperature would be below freezing, according to the National Oceanic and Atmospheric Administration. But increases in those types of gases have tipped the balance, trapping additional heat and raising Earth's average temperature.

Scientists estimate there would be an additional 115 to 235 parts per million of CO2 in the atmosphere by the end of the century. Compared to today’s 420 parts per million CO2, that is an additional 40 to 50%, which could have led to additional warming of global surface temperature.

On top of carbon dioxide, ozone depleting substances on their own are potent greenhouse gases.

So in a world without the Montreal Protocol, temperatures would have risen by about 2.5 degrees Celsius overall, researchers estimate. That would have been in addition to the 1-degree increase the planet has already seen from pre-industrial temperatures, making the total rise 3.5 degrees. 

That is far beyond the maximum 1.5 degree rise above pre-industrial levels that many scientists see as the most global temperatures can rise in order to avoid some of the most damaging effects of climate change.

“Although we can hope that we never would have reached the catastrophic world as we simulated, it does remind us of the importance of continuing to protect the ozone layer,” Young said. “Entirely conceivable threats to it still exist, such as from unregulated use of [ozone-depleting substances].”

Manufacturing in eastern China accounted for a rise in global ozone-depleting substance emissions from 2010 to 2015, a 2018 study found.

Thankfully, some well-placed measurement stations alerted to the emissions and “this source was caught in time such that it did not and will not pose any serious threat to the ozone layer,” Young said

But, he said, “there is always the risk that some unscrupulous producer will emit more [ozone-depleting substances], and our monitoring networks may not be best placed to pinpoint that source next time.”

And a bigger potential threat to the ozone layer could come from certain geoengineering techniques, which are being suggested as mechanisms to mitigate climate change, Young added.

Stratospheric geoengineering with sulphate aerosols — an attempt to counter climate warming by emulating a large, constant volcanic eruption — is probably the most serious potential threat to the ozone layer, he said.

“While we certainly wouldn’t expect that to lead to the drastic ozone reductions we simulate in our study, we need to evaluate the possible side effects of such propositions, which would include the UV-vegetation-carbon dioxide feedback we simulate here,” Young said. “This is in addition to the other well studied potential shortcomings, such as disruption of rainfall and the rapid climate warming that could result if geoengineering was halted and CO2 emissions hadn’t been decreased.”

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