Serendipity and Stratospheric Ozone
So, in addition to ERB, in 1978 Nimbus-7 carried two other new NASA sensors designed to measure the total amount of ozone in a given column of atmosphere over the entire globecalled the Solar Backscatter Ultraviolet (SBUV) instrument and the Total Ozone Mapping Spectrometer (TOMS). Sensitive to radiant energy in the ultraviolet region of the spectrum, these sensors took advantage of the fact that molecules and aerosol particles reflect certain wavelengths of ultraviolet rays while ozone absorbs others at different levels in the atmosphere. By analyzing the amount of ultraviolet energy reflected back up to the spacecraft, researchers could produce profiles of how thick or thin the ozone was at different altitudes and locations. Ironically, it wasn't until October 1985 that a British team of scientists found a significant reduction in ozone over Halley Bay, Antarctica. Using a ground-based Dobson ozone spectrophotometer, the team found that the amount of stratospheric ozone over Halley Bay was about 40 percent less than it had been the previous year. Their finding stunned the science community because they were expecting anthropogenic ozone depletion to occur first at upper levels in the stratosphere (30 to 50 km) and so they anticipated that the initial signal of depletion in a total column of ozone would be weak. NASA researchers hastily reviewed their TOMS data and found that it too had detected a dramatic loss of ozone over all of Antarctica. Why hadn't they discovered the phenomenon earlier? Unfortunately, the TOMS data analysis software had been programmed to flag and set aside data points that deviated greatly from expected measurements and so the initial measurements that should have set off alarms were simply overlooked. In short, the TOMS team failed to detect the ozone depletion years earlier because it was much more severe than scientists expected.
In the years following the discovery of the ozone hole, NASA and ESA satellites recorded depleting ozone levels over Antarctica growing worse with each passing year. In response, in 1987, 43 nations signed the Montreal Protocol in which they agreed to reduce the use of CFCs by 50 percent by the year 2000. This protocol was amended in 1990 to eliminate all CFC emissions by 2000.
ESAs second European Remote-Sensing Satellite (ERS-2) carries a new sensor called the Global Ozone Monitoring Experiment (GOME). GOME is a nadir-looking sensor with four bands ranging from 240 to 790 nm for measuring backscattered visible and ultraviolet solar radiation. Since the summer of 1996, ESA has routinely produced 3-day global measurements of total ozone and nitrogen dioxide using GOME data.
As recently as 1998, both TOMS and GOME data show that at its Austral spring low, Antarctic ozone concentrations had worsened to 80 percent less than early 1970s levels. Today there is some evidence that levels of chlorine in the stratosphere are leveling off. Is this a scientific success story in the making? Will stratospheric ozone concentrations return to pre-1970s levels as the abundance of stratospheric chlorine stabilizes? Only time and continued monitoring will tell. ESA launched its Environmental Satellite (Envisat) in November 2001 with a new sensor called Global Ozone Monitoring by Occultation of Stars (GOMOS).