The study results are published online in the journal Science Express. The research was funded by the National Science Foundation (NSF), NCAR’s principal sponsor, as well as by NASA and other agencies.

Our research indicates that trying to artificially cool off the planet may be a perilous endeavor. While climate change is a major threat, this solution could create severe problems for society.

—Simone Tilmes

Climate scientists, concerned that society is not taking sufficient action to prevent significant changes in climate, have studied various geoengineering proposals to cool the planet and mitigate the most severe impacts of global warming. One of the most-discussed ideas is to regularly inject large amounts of sun-blocking sulfate particles into the stratosphere. The goal would be to cool the climate, much as sulfur particles from large volcanic eruptions have cooling impacts.

Since volcanic eruptions temporarily thin the ozone layer in the stratosphere, Tilmes and her colleagues looked into the potential impact of geoengineering plans on ozone. They used an empirical relationship between ozone depletion and chlorine activation to estimate how sulfate particle injection might influence polar ozone.

The study concluded that an injection of sulfur large enough to compensate surface warming due to the doubling of CO₂ would cause a drastic increase in the extent of Arctic ozone depletion during the next century for cold winters and would cause a considerable delay, between 30 and 70 years, in the expected recovery of the Antarctic ozone hole.

This study highlights another connection between global warming and ozone depletion, which had been thought of as separate problems but are now increasingly recognized to be coupled in subtle, yet profoundly important, ways.

—co-author Ross Salawitch, the University of Maryland

To determine the relationship between sulfates and ozone loss, the authors used a combination of measurements and computer simulations. They then estimated future ozone loss by looking at two geoengineering schemes—one that would use volcanic-sized sulfates, and a second that would use much smaller injections.

The study found that injections of small particles over the next 20 years could reduce the ozone layer by 100 to 230 Dobson Units. The average thickness of the ozone layer in the Northern Hemisphere is 300 Dobson Units. (A Dobson Unit is a common measure of ozone.) For large particles, the loss would range from 70 to 150 Dobson Units. The larger figure is correlated with colder winters.

In the Antarctic, the sulfate injections would not significantly reduce the thickness of the already depleted ozone layer. Instead, they would significantly delay the recovery of the ozone hole.

The authors caution that the actual impacts on ozone could be somewhat different than estimated if atmospheric changes led to unusually warm or cold polar winters. They also warn that a geoengineering project could lead to even more severe ozone loss if a volcanic eruption took place at the same time.

Clearly much more research needs to be conducted to determine the full implications of geoengineering before we may discuss seriously the injection of sulfate aerosols into the stratosphere.

—co-author Rolf Müller, the Jülich Research Center, Germany

Recovery of Antarctic ozone hole may amplify warming

A separate study by CU-Boulder, the National Oceanic and Atmospheric Administration (NOAA) and NASA suggests that a full recovery of the stratospheric ozone hole could modify climate change in the Southern Hemisphere and even amplify Antarctic warming.

While Earth’s average surface temperatures have been increasing, the interior of Antarctica has exhibited a unique cooling trend during the austral summer and fall caused by ozone depletion, said Judith Perlwitz of the Cooperative Institute for Research in Environmental Sciences, a joint institute of CU-Boulder and NOAA.

If the successful control of ozone-depleting substances allows for a full recovery of the ozone hole over Antarctica, we may finally see the interior of Antarctica begin to warm with the rest of the world.

—Judith Perlwitz

Perlwitz is lead author of a new study on the subject to be published 26 April 26 in Geophysical Research Letters. Co-authors include Steven Pawson and Eric Nielson of NASA’s Goddard Space Flight Center in Greenbelt, Md., and Ryan Fogt and William Neff of NOAA’s Earth System Research Laboratory in Boulder. The study was supported by NASA’s Modeling and Analysis Program.

The authors used a NASA supercomputer model that included interactions between the climate and stratospheric ozone chemistry to examine how changes in the ozone hole influence climate and weather near Earth’s surface.

The study authors calculated that when stratospheric ozone levels return to near pre-1969 levels by the end of the 21st century, large-scale atmospheric circulation patterns now shielding the Antarctic interior from warmer air masses to the north will begin to break down during the austral summer. The circulation patterns are collectively known as a positive phase of the Southern Annular Mode, or SAM.

The scientists found that as ozone levels recover, the lower stratosphere over the polar region will absorb more harmful ultraviolet radiation from the sun. This could cause air temperatures roughly 6 to 12 miles above Earth’s surface to rise by as much as 16 degrees Fahrenheit, reducing the strong north-south temperature gradient that currently favors the positive phase of SAM, said the research team.

The supercomputer modeling effort also indicated that ozone hole recovery would weaken the intense westerly winds that currently whip around Antarctica and block air masses from crossing into the continent’s interior. As a result, Antarctica would no longer be isolated from the warming patterns affecting the rest of the world.

NASA’s Pawson said ozone recovery over Antarctica would essentially reverse summertime climate and atmospheric circulation changes that have been caused by the presence of the ozone hole.

It appears that ozone-induced climate change occurred quickly, over 20 to 30 years, in response to the rapid onset of the ozone hole. These seasonal changes will decay more slowly than they built up, since it takes longer to cleanse the stratosphere of ozone-depleting gases than it took for them to build up.

—Steven Pawson

The seasonal shift in large-scale circulation patterns could have repercussions for Australia and South America as well. Other studies have shown that the positive phase of SAM is associated with cooler temperatures over much of Australia and increased rainfall over Australia’s southeast coastline.

During late spring and early summer, the positive phase of SAM also is associated with drier conditions in South America’s productive agricultural areas like Argentina, Brazil, Uruguay and Paraguay, said Perlwitz. If ozone recovery induces a shift away from a positive SAM, Australia could experience warmer and drier conditions while South America could get wetter, she said.

But just how influential a full stratospheric ozone recovery will be on Southern Hemisphere climate largely depends on the future rate of greenhouse gas emissions, according to the GRL authors. Projected increases in human-emitted greenhouse gases like carbon dioxide will be the main driver for strengthening the positive phase of SAM.

In running our model simulations, we assumed that greenhouse gases like carbon dioxide would double over the next 40 years and then slowly level off. If human activities cause more rapid increases in greenhouse gases, or if we continue to produce these gases for a longer period of time, then the positive SAM may dominate year-round and dwarf any climatic effects caused by ozone recovery.

—Judith Perlwitz

NASA’s High-End Computing Program provided the Columbia supercomputer resources at the NASA Ames Research Center in Moffett Field, Calif.

Resources

• Simone Tilmes, Rolf Müller, Ross Salawitch; The Sensitivity of Polar Ozone Depletion to Proposed Geoengineering Schemes; Science Express DOI: 10.1126/science.1153966