The tree ring record used by scientists to map past global temperature changes appears to be losing its sensitivity and human actions might be to blame, according to new research.
Reduced ozone levels resulting in increased ultra-violet radiation may mean trees are speaking less clearly today, Keith Briffa, reader at the University of East Anglia's climatic research unit, told this week's Royal Society meeting on vegetation, climate and atmosphere interactions.
According to Dr Briffa, tree ring width and density have previously enabled scientists to pinpoint temperature changes, helping them chart years of volcanic eruptions when an increase in atmospheric particles leads to a reduction in global temperature.
But ultra-violet radiation, which damages plant growth, could make the rings unreliable.
"Over the past 100 years we see a good correlation between instruments and trees," said Dr Briffa. "But this only seems to last until the 1950s and 1960s." Then, he says, "the tree record starts diverging".
In samples taken from across northern Europe, Siberia and north America tree rings accurately chart the temperatures of the 1920s, 1930s and 1940s, but not beyond.
"A very large-scale, contemporaneous event" would be needed to explain the desensitising of the relationship, he said.
Dr Briffa sites ozone measurements taken in Switzerland since the 1930s, which show a reduction in ozone from the 1950s onwards, which, in turn, implies an increase in ultra-violet radiation.
CFCs, used in aerosols and fridges, are thought to be largely responsible for the reduction in atmospheric ozone.