New theory blames cosmic rays for helping CFCs deplete ozone
Fellow scientists say Waterloo professor's hypothesis needs more study
The hole in the Earth's protective ozone layer above Antarctica will be very big this year — and it will be big again in 2020 — contrary to previous predictions, argues a Canadian researcher.
Qing-Bin Lu, a professor of physics and astronomy at the University of Waterloo, says his research suggests the long-held theory about how ultraviolet light combines with man-made chemicals to destroy the Earth's ozone layer is incorrect.
His results, published in the latest issue of Physical Review Letters, back up a new theory, he says, and call into question predictions about when the ozone layer will recover. They also suggest some ozone-depleting chemicals might need to be re-evaluated.
The ozone layer in the atmosphere, more than a dozen kilometres above the Earth's surface, absorbs most of the sun's ultraviolet radiation and prevents it from hitting the surface of the Earth, where it can cause sunburn and other skin damage that could eventually lead to skin cancer.
Damage to the layer has been blamed for the large ozone hole above the South Pole that forms each spring in the southern hemisphere, increasing exposure to ultraviolet radiation for people in Australia and New Zealand.
Scientists have long believed that chlorofluorocarbons (CFCs) — chemical compounds that used to be found in refrigerants, aerosol spray cans and fire extinguishers — released into the atmosphere undergo a photochemical reaction in the presence of ultraviolet light that breaks down the ozone. Many of those compounds were banned by the Montreal Protocol, an international agreement ratified by 180 countries in 1987.
Lu agrees that CFCs are responsible for destroying the ozone layer but argues it's not reactions induced by ultraviolet light that deplete the ozone but CFCs' reaction with electrons in the atmosphere. Those electrons are released when high-energy particles, mostly protons, from outer space known as cosmic rays hit the atmosphere and cause ionization.
Because the electrons are charged, they tend to be drawn toward the poles because of the Earth's magnetic field. The reaction is enhanced in the presence of ice, found in clouds at the level of the ozone layer.
Lu based his theory on measurements of cosmic rays and ozone made by NASA between 1980 and 2007. That data shows that when the amount of cosmic rays hitting the Earth is high — as it was in 2008 and 2009 — more ozone is lost from the atmosphere. The level of cosmic rays hitting the Earth rises and falls according to an 11-year cycle based on energy fluctuations from the sun. The sun's energy affects the level of cosmic rays hitting the Earth, with the amount of cosmic rays being highest when the level of solar radiation is lowest and vice versa, Lu said.
If the photochemical theory is correct, you would expect to see a smaller ozone hole when the level of solar radiation is lower, as in 2008 and 2009, Lu said.
"The fact is, actually, in 2008, we've already seen one of the biggest [ozone holes], and very likely, we will see one of the biggest in 2009, too."
Another large hole in 2019-2020: Lu
Lu's theory predicts another large ozone hole around 2019-2020. That is contrary to a prediction by the United Nations World Meteorological Organization, which predicts that the ozone hole will shrink five to 10 per cent between 2000 and 2020 because the amount of CFCs in the atmosphere is declining. That is true, Lu said, but the high level of cosmic rays at certain points in the solar cycle is more significant.
Lu also suggested that scientists will need to run more experiments on the impact of CFCs on ozone depletion.
"If you don't have [the] right understanding, probably you will not make a correct evaluation of the chemicals," he said.
Lab experiments show the reaction between chlorofluorocarbons and electrons breaks down ozone 10,000 to a million times more quickly than the similar reaction involving CFCs and ultraviolet light, Lu said.
Theory needs more study: researchers
Lu learned about the reaction between electrons and CFCs while doing unrelated experiments about 10 years ago and later wondered what implications it had for reactions in the atmosphere, where electrons are generated by cosmic rays. He first proposed it as a cause of ozone depletion in a paper in 2001 but now has far more data to back it up, he said.
Tom McElroy, an Environment Canada researcher who has been studying ozone depletion for decades, said there is still a lot scientists don't understand about ozone depletion and the Antarctic ozone hole, and it's possible that cosmic rays might be a contributing factor.
"However, this concept needs further study in order to quantify the effect, if any," McElroy said in an email.
Lu's paper does not offer conclusive proof about the role of cosmic rays in ozone depletion, he said. Nor does it offer enough evidence to discount the role of ultraviolet radiation, which has been "well studied for many years and is supported by strong scientific evidence," McElroy said.
Doug Degenstein, a physics and engineering physics researcher at the University of Saskatchewan, said the evidence to support the ultraviolet theory includes both experiments and models.
As for the cosmic ray theory, Degenstein said "there's no doubt" Lu's paper has demonstrated a correlation between ozone depletion and cosmic rays.
However, he said, there are a lot of other factors that could have affected the results that were not discussed in the paper, and while the correlation is interesting, it doesn't offer definitive proof that Lu's theory is correct.
"I think there's definitely something there," said Degenstein, whose own research involves satellite measurements of the atmosphere. "It's one of those things where [you might ask], 'How important is it, though?'"
University of Toronto physics professor Kimberly Strong said in an email that there has already been some discussion in the scientific literature of the past year about Lu's theory.
"I think the jury is still out on Dr. Lu's hypothesis," Strong said, "which I suspect will lead to further analysis and discussion in the scientific literature."