Depletion Said to Increase Rainfall in Subtropical Regions
Depletion of ozone around the South Pole has altered global air-circulation patterns, bringing more rain to the subtropics. (Kang et al., 2011)
The hole in the Earth’s ozone layer over the South Pole has affected atmospheric circulation in the Southern Hemisphere all the way to the equator, and a new study says this has led to increased rainfall in the subtropics. The study, which appears in the April 21 issue of the leading journal Science, is the first time that ozone depletion has been linked to climate change over such a wide area.
Previous work has shown that the ozone hole has caused the dominant westerly jet stream in the mid-latitudes to move toward the pole, with an accompanying shift in precipitation patterns associated with the mid- to high latitudes. The new study demonstrates that the ozone hole has influenced atmospheric circulation all the way to the tropics and increased rainfall at low, subtropical latitudes in the Southern Hemisphere.
The paper used different computerized climate models to try to identify the impact of the ozone depletion, compared to other factors. The experiment found patterns of moistening in high latitudes, drying in mid-latitudes and moistening in the subtropics. Between 15 and 35 degrees south—roughly the level of Australia and southern Africa -- the researchers saw about a 10 percent increase in precipitation.
Most weather occurs in the lower atmosphere, between 8 to 10 miles above the surface of the earth. This layer, the troposphere, is marked by a gradual cooling of the air as you gain altitude. Higher up, in the stratosphere, the air stops cooling with height and becomes almost completely dry. The depletion of the ozone layer, from 8 to 25 miles up, causes severe cooling in the lower stratosphere, which expands the troposphere and alters patterns of air circulation. This leads to a pole-ward shift of the band of westerly wind circling the earth in mid-latitudes.
The study found these high-altitude shifts fed changes in the lower atmosphere that led to the higher levels of precipitation in the subtropics.
"It’s really amazing that the ozone hole, located so high up in the atmosphere over Antarctica, can have an impact all the way to the tropics and affect rainfall there — it’s just like a domino effect," said Sarah Kang, the lead author and a postdoctoral research scientist in Columbia Engineering’s Department of Applied Physics and Applied Mathematics.
The ozone layer absorbs most of the sun’s harmful ultraviolet rays. Over the last half-century, widespread use of manmade compounds, especially household and commercial aerosols containing chlorofluorocarbons (CFCs), has significantly and rapidly broken down the ozone layer, to a point where a hole in the Antarctic ozone layer was discovered in the mid 1980s. Thanks to the 1989 Montreal Protocol, now signed by 196 countries, global CFC production has been phased out. As a result, scientists have observed over the past decade that ozone depletion has largely halted, and they now expect it to fully reverse, and the ozone hole to close by midcentury.
Scientists earlier this month reported they have begun to see the Antarctic ozone hole slowly recovering for the first time, though other researchers are questioning that contention.
Together with colleagues at the Canadian Centre for Climate Modelling and Analysis in Victoria, British Columbia, Kang and Polvani used two different state-of-the-art climate models to show the ozone hole effect. They first calculated the atmospheric changes in the models produced by creating an ozone hole. They then compared these changes with the ones that have been observed in the last few decades. The close agreement between the models and the observations shows that ozone has likely been responsible for the observed changes in the Southern Hemisphere.
Kang and Polvani plan next to study extreme precipitation events, which are associated with major floods, mudslides and related phenomena. “We really want to know if and how the closing of the ozone hole will affect these,” Kang said.
According to Polvani and Kang, international agreements on mitigating climate change cannot be confined to dealing with carbon alone — ozone needs to be considered, too. “This could be a real game-changer,” Polvani said. “While the ozone hole has been considered as a solved problem, we’re now finding it has caused a great deal of the climate change that’s been observed.”
This study was funded by a grant from the National Science Foundation.