London, September 1 : A team of researchers has devised a way to reconstruct past levels of ozone by measuring levels of the chemicals that act as protective 'sunscreens' in the spores of plants.
The researchers were led by Barry Lomax from the University of Nottingham, UK.
Until now, atmospheric scientists have been limited to ozone measurements made by satellites that date back only to the late 1970s and data from ground-based spectrophotometers going back to the 1920s.
Lomax's team said that the levels of ultraviolet-absorbing compounds in plant spores can show how much of this radiation they were exposed to, and hence show what the ozone levels were in the atmosphere millennia ago.
"At the moment, it's very much an unknown how ozone has changed over recent and geological time," said Lomax.
"This method could help address issues of climate change and whether we're seeing recovery (in the ozone layer) now, or whether it's natural variation," he added.
In his research, Lomax detailed how spores can be use as a 'biological proxy' for ozone levels.
Plants subjected to increased levels of UV-B radiation make more of the natural phenolic 'sunscreen' compounds that absorb the potentially harmful rays.
By analysing the concentration of these UV-absorbing compounds in the walls of spores from herbarium collections, it is possible to work out how much radiation they have been subjected to.
"From this it is possible to work out how much ozone was in the atmosphere between the plant and the Sun," said Lomax.
Traces of these compounds are even preserved in fossils.
"We certainly should be able to go back into the Tertiary, about 55 million years ago, without a problem," said Lomax. "It will work on fossil spores provided they haven't been heated to over 200 degrees C," he added.
The research paper details analysis of spores of clubmosses from high- and low-latitude locations, which were found to contain concentrations of UV-absorbing compounds that strongly correlate with known historical changes in UV-B levels.
Spores from Ecuador, where there have been no historic changes in UV-B, showed no change in 'sunscreen' concentrations over the same time period.
Using spores from Greenland, Lomax reconstructed historical ozone levels between 1907 and 1993 and found strong correlations between their reconstructions and atmospheric ozone measurements made within this period.
According to Geir Braathen, an atmospheric chemist and senior scientific officer at the World Meteorological Organization in Geneva, Switzerland, "It would be extremely interesting if we could reconstruct the UV climates in the past."
"It would be very interesting to see how the ozone layer has developed," he added.