Susan welcomed the participants to NCAR and kicked off the day's proceedings with a half-hour overview of the experiment. As she noted in her chronology, much of the knowledge we now take for granted about the ozone hole and its workings had yet to be uncovered a decade ago.
For instance, the 1985 analyses that first brought the hole to widespread attention showed results only for February and October. Thus, it wasn't yet clear that the depletion happened in the austral spring (mainly September and October). "We did know that the depletion had intensified from 1978 to 1983 and that it was centered over Antarctica," says Susan. She and her colleagues, including ACD's Rolando Garcia, foresaw that it was likely a relationship between atmospheric chlorine and polar stratospheric clouds (PSCs) at work. A paper they published in Nature in 1986 expounded on this hypothesis. But, she says, scientists were also considering the 11-year solar cycle and dynamic uplifting as important factors. More data were urgently needed, and a field program was the only way to get them.
"By the time we had the meeting to discuss ground-based measurements in March," says Susan, "a lot of people were interested in Antarctica." Four groups were especially involved:
A University of Wyoming (UW) team, led by Dave Hofmann (now director of NOAA's Climate Monitoring and Diagnostics Laboratory), which carried out balloon-based ozone and aerosol particle measurements from August to November 1986
The State University of New York at Stony Brook (ground-based microwave emission measurements)
The Jet Propulsion Laboratory (ground-based solar infrared absorption measurements)
The NOAA Aeronomy Laboratory (ground-based visible absorption measurements)
A number of sponsors and supporting organizations came to the fore, including NSF, NASA, NOAA, the Chemical Manufacturers Association, the U.S. Navy, and ITT Antarctic Services. In particular, says Susan, "There was a tremendous effort on behalf of NSF to get this project going. It was really difficult and challenging to get it together between March and August."
Once the project, based at McMurdo Station on the Antarctic coast, was under way, the depletion process began to come into focus. For instance, the dynamical hypotheses for the ozone hole's formation lost favor when the measurements of aerosols (tracers of atmospheric motion) showed little change between 28 August and 16 October, even while stratospheric ozone was being drastically depleted. The solar-cycle hypothesis could not be reconciled with observations of low nitrogen oxide (NO2).
All four of the major teams at NOZE-1 successfully measured the formation and strengthening of the ozone hole, confirming for even the most ardent skeptics that the phenomenon was real. And the ozone changes happened quickly--mostly between late August and late September. Furthermore, the changes were confined primarily to the lower stratosphere, between about 12 and 20 kilometers. These findings all pointed toward stratosphere-based chemical processes prevalent in the Antarctic springtime.
Ultimately, Susan's hypothesis involving chlorine and PSCs evolved (in conjunction with other work) into the now-accepted theory by which chlorofluorocarbons (CFCs) export chlorine to the stratosphere, where it interacts with PSCs in the sunlight of the Antarctic spring to erode the ozone layer. Through a more recently elaborated process involving heterogeneous chemistry apart from PSCs, stratospheric ozone depletion is now understood to occur on a lesser scale at other latitudes and during other parts of the year.
NOAA's Hofmann discussed the onset and disappearance of the ozone hole and trends in that perennial cycle over the past few years. He believes the hole may have had its genesis somewhat earlier than we think. Even in 1971, he noted, an early October sounding from the South Pole reveals a suppression of the ozone concentration that's correlated with a cold temperature profile (also indicative of ozone depletion). "This could be the first signal of the hole's appearance at the vortex center," Hofmann says. Retrospective analyses from stations farther from the pole show no signs of depletion until 1980.
The long-range forecast for the ozone layer is bright, predicts Hofmann. Thanks to the international agreements limiting CFC use, the chemical's presence in the stratosphere is expected to begin decreasing by 2000. In turn, recovery of the continent's average amounts of total ozone should first be detected around 2017, and the recovery should reach 1980's ozone levels by 2045. Balloon measurements at the pole may show signs of recovery as early as 2007, although the pole's vortex recovery is not expected to be complete until 2085.
One number that Hofmann sees as a useful index of the ozone layer's recovery is the date at which the Dobson level of ozone between 14 and 16 km at the South Pole drops below 20. From 1986 to 1995, that date has crept back from late September to early September. "The amount of ozone present on 15 September each year also has dropped systematically," Hofmann says, "and it will be used as an ozone recovery index."
Things may worsen even more before they get better. On 21 August 1996, the day before the symposium, Dobson levels at the South Pole already had fallen to 231, said Hofmann. "This is about two weeks early. It's probably the earliest we've ever seen the ozone hole develop." Even during the austral winter just past, the levels hung around 250 Dobson, as low as ever measured for that time of year.
Sessions during the rest of the NOZE retrospective dealt with observations of PSCs and chlorine monoxide, satellite data, spectroscopy, and aircraft campaigns, among other topics. The two sessions on PSCs revealed a dearth of hard facts about them. "We still don't really know what these clouds are made of," says Susan, "which is ironic when you realize how important they are in this whole problem."
The symposium closed with a slide/video show that took participants back to those halcyon below-zero days at McMurdo. Ryan Sanders (NOAA) noted that the base's phone system was called Southern Belle--"The next best thing to walking there." Recalls Susan: "You got to know people by the shape of their parka." BH
"It's been a real pleasure to be part of NCAR," says Susan. "NCAR is not this beautiful building, although it is beautiful. It is not these great computers and airplanes, although they are great resources. NCAR is its people. Thanks for the opportunity to be part of that for this year." BH
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