August 9, 2013
Slight Global Warming May Come From Ozone Winds
April Flowers for redOrbit.com - Your Universe Online
Laypeople tend to mix up the ozone hole and global warming. The popular belief is that the hole is a major cause of the world’s increasing average temperature. On the other hand, scientists have long attributed the ozone shortage in the hole to a small cooling trend.
A new computer model study, led by the Lamont-Doherty Earth Observatory, suggests that there might be a slight warming influence from the ozone hole. The influence is due to the hole’s effect on winds, however, not temperatures. The shifting wind patterns caused by the ozone hole push clouds farther toward the South Pole, according to the findings. This reduces the amount of radiation the clouds reflect and possibly causing a bit of warming rather than cooling.
“We were surprised this effect happened just by shifting the jet stream and the clouds,” said Kevin Grise, a climate scientist at Lamont-Doherty Earth Observatory. This small warming effect may prove important in future predictions of the Southern Hemisphere climate, Grise notes.
The study findings, published in Geophysical Research Letters, are the result of a collaboration between Grise, Lorenzo Polvani of Columbia University, George Tselioudis of NASA Goddard Institute for Space Studies, Yutian Wu of New York University, and Mark Zelinka of Lawrence Livermore National Laboratory.
Three oxygen atoms bound together create an ozone molecule, and these molecules gather in the lower portion of the stratosphere – around 12 to 19 miles above ground. This is approximately twice as high as commercial airliners fly. The ozone layer shields life on the planet from some of the hazardous ultraviolet radiation barraging the atmosphere. Without that protection, UV rays can cause sunburns, eye damage and even skin cancer.
Researchers in the 1980s discovered that the ozone layer was thinning above Antarctica during the spring months in the Southern Hemisphere. The resulting “hole” was attributed to chlorofluorocarbons (CFCs), such as Freon, from cooling systems, aerosol cans and degreasing solvents, which break apart ozone molecules. The 1987 Montreal Protocol banned the use of CFCs globally, but the ozone hole persists decades later.
PERCEPTION VS. REALITY
Previous studies have found that people falsely equate the ozone hole to global warming. For example, a Yale University poll in 2010 found that 61 percent of those surveyed believed the ozone hole significantly contributed to global warming and 43 percent agreed with the statement “if we stopped punching holes in the ozone layer with rockets, it would reduce global warming.”
Perception aside, there are actual consequences to the ozone hole. One of these is its odd effect on the Southern Hemisphere polar jet stream. The polar jet stream is a group of fast flowing air currents that encircle the South Pole. The ozone hole only appears during the spring months, however, the summer after each appearance the high-speed jet stream swings south toward the pole.
“For some reason when you put an ozone hole in the Southern Hemisphere during springtime, you get this robust poleward shift in the jet stream during the following summer season,” said Grise. “People have been looking at this for 10 years and there’s still no real answer of why this happens.”
Grise and his team wondered if the hole’s impact on the jet stream has any indirect effects on the cloud cover. They used computer models to work out the clouds’ reaction to changing winds.
“Because the jet stream shifts, the storm systems move along with it toward the pole,” said Grise. “If the storm systems move, the cloud system is going to move with it.”
The researchers found that high- and mid-level clouds traveled with the shifting jet stream towards the Pole and the Antarctic continent. Low-level cloud coverage, in contrast, dropped in their models throughout the Southern Ocean. Due to the wide variety of factors that govern the formation and movement of clouds, modeling them is difficult. Grise notes, however, that their theory of migrating cloud coverage is supported by observational evidence from the International Satellite Cloud Climatology Project, a decades-long NASA effort to map global cloud distributions.
The amount of energy the clouds can reflect drops when the cloud cover moves poleward. This increases the amount of radiation that reaches the ground. “If you shift the reflector poleward,” Grise explained, “you’ve moved it somewhere there is less radiation coming in.”
The Intergovernmental Panel on Climate Change (IPCC) reported a direct cooling effect from the thinning ozone layer in 2007. This amounts to a reduction of about 0.05 watts per square meter’s worth of energy reaching the ground. The new study estimates the indirect effect of the shifting cloud cover to be an increase of approximately 0.2 watts per square meter, suggesting not only that warming rather than cooling would be taking place, but also that there’s a larger influence overall. The jet stream only shifts during the summer months, so the warming only takes place in those months.
“Theoretically this net radiation input into the system should give some sort of temperature increase, but it’s unknown if that signal could be detected or what the magnitude of it would be,” said Grise.
According to the George Washington University Solar Institute, an average of about 175 watts per square meter reaches the ground from sunlight globally.
Since predicting cloud behavior is so challenging, the model used in Grise’s study could be underestimating clouds north of the jet stream being pulled toward the equator and in turn reflecting more light, according to Dennis Hartmann, an atmospheric scientist at the University of Washington in Seattle who was not involved in the study. This increase reflection could potentially reduce or even negate the warming effect, Hartmann added. He also noted concerns with the modeling of the low-level cloud response.
Still, “this is certainly a very interesting topic and potentially important from a practical perspective of predicting Southern Hemisphere climate and even global warming rates,” he commented.
As the ozone layer above the South Pole recovers, the jet stream should show less and less of a shift to the south during the summer months. Increasing levels of greenhouse gases, however, could also cause a change in mid-latitude wind patterns and push the jet stream poleward, creating a complicated scenario in which Grise said he plans to study in future work.
“You have sort of this tug-of-war between the jet being pulled equator-ward during the summer because of the ozone recovery and the greenhouse gases pulling the jet further poleward,” said Grise. “What the clouds do in that scenario is an open question.”