Painters like William Turner created more than art when they captured a sunset, said scientists Tuesday who found that shades of paint can be historic pointers to atmospheric pollution.
Dusk scenes tended to be redder after major volcanic eruptions that spew Sun-reflecting sulphur particles, or aerosols, into the atmosphere, said the team from Greece and Germany.
The findings, based on analysing dozens of canvases from the year 1500 right through to 2000, offer a new way of measuring pollution levels by aerosols, which are also released by cars and power plants.
"We wanted to provide alternative ways of exploiting the environmental information in the past atmosphere in places where, and in centuries when, instrumental measurements were not available," said Christos Zerefos, a physicist at the Academy of Athens.
Unbeknown to them, European artists of the early 19th century witnessed, and captured, the effects of the largest volcano eruption in recorded history.
When Indonesia's Tambora volcano blew up in 1815, killing tens of thousands of people directly and in the freezing "Year without a Summer" that followed, it also spewed large amounts of ash and gas into the atmosphere that produced spectacular sunsets for up to three years.
Turner, a British landscape artist known for his love of colours, was one of those who reproduced the natural extravaganza for later generations.
His paintings and others at the Tate Gallery and National Gallery in London were measured for the red-to-green ratio on their horizons, which were in turn correlated with records of volcano outbursts and sulfate deposits in ice samples.
The more pollution, the redder the paintings, the team reported in the journal Atmospheric Chemistry and Physics.
They also found striking examples in paintings after the eruption of fellow Indonesian volcano Krakatau, in 1883, which caused magnificent sunsets halfway around the world.
Seeking to corroborate the findings, the team got Greek landscape artist Panayiotis Tetsis to paint a number of sunsets on the island of Hydra in the Aegean Sea, where he lives.
The sunsets were painted in periods of low aerosol concentrations, and in periods of higher pollution caused by Saharan dust clouds passing over the island like an invisible veil. Tetsis was not informed of the passing dust.
The tests had the same outcome.
"The main conclusion of the paper is that nature speaks to the hearts and souls of the artists," wrote the team.
"When colouring sunsets, the red-to-green ratios perceived by the brain contain important environmental information."
The European Geosciences Union, which publishes the journal, said the study could help researchers understand how aerosols affected the Earth's climate in the past.
"This, in turn, can help improve predictions of future climate change."
The authors acknowledge that uncertainties in their study included factors like colour degradation with age -- and, of course, the painter's mood.