They say that Emperor Nero fiddled while Rome burnt — but now research suggests that the Romans lit so many fires that the resulting pollution cooled Europe.
Researchers estimated the amount of air pollution, including soot and organic carbon particles, that would have been produced by the ancient Empire.
From this they found that the dominant cooling effect of air pollution would have lowered the European climate of the time by 0.3°F (0.17°C).
But the decrease would have been more than countered by the warm climate at this period of time.
Experts also claim it is dwarfed by modern man-made climate change.
Nevertheless, the researchers said, the study adds to the evidence that shows how human activity has been altering the natural climate for millennia.
Anina Gilgen of ETH Zurich, Switzerland, and her colleagues estimated the impact of the air pollution produced by the Roman Empire on the climate of Europe.
‘We looked for the first time at whether anthropogenic aerosol impacts had an impact on climate a long time ago,’ Ms Gilgen told the New Scientist.
Researchers used existing studies of how much land the Romans used for farms, homes and other purposes to estimate the amount of air pollution the empire would have created and then factored this in to a model of European climate at the time.
They found that while deforestation and greenhouse gas release had a modest warming effect of up to 0.27°F (0.15°C), air pollution had a stronger cooling effect.
This lead to an overall average temperature drop of around 0.3°F (0.17°C), cooling Europe down to 32.3 (0.46°C), Ms Gilgen and colleagues estimated.
‘The novelty here is in their thinking about what the [Roman’s] aerosol contribution would be, which seems to be quite considerable,’ said Joy Singarayer, a past climate expert at the University of Reading who was not involved in this study.
The results highlight how human activities have been altering the Earth’s climate for thousands of years.
The temperature shifts caused by the Romans, however, were tiny in comparison with current man-made climate changes driven by the widespread emission of greenhouse gases.
In fact, the cooling was probably too minor to have had a significant impact on Roman society, researchers said.
At the Empire’s height, the climate underwent a hotter spell, known as the Roman Warm Period, which lasted from around 250 BC to 400 AD.
First identified in the nineties, climatologists believe that this warm period was a natural phenomenon.
As a consequence, the cooling from the air pollution may only have served to slightly counter the natural warming of the time.
The fires, however, may have had a different harmful consequence. Air pollution today is known to cause various health issues, including increasing the chance of heart attacks and strokes.
‘It might rather be that air pollution was a problem for people living in cities,’ said Ms Gilgen.
In addition, the pollution might have caused more rain locally, with water droplet condensing around the aerosol particles.
‘There could have been an impact on precipitation patterns and therefore water availability,’ Professor Singarayer said.
‘There’s been similar things in the Sahara, where aerosols have impacted on where precipitation is falling and contributed to drought conditions in certain areas.’
Advanced societies like the Roman Empire would have had the potential to impact the climate in various ways, from the burning of fuel and wood to heat homes to the cutting down of trees and the burning of crop residues.
Previous research has quantified the impacts that these kinds of activities can have through both their release of greenhouse gases and conversion of forests to agricultural land.
In fact, human-induced changes in temperatures from deforestation and greenhouse emissions can be seen in Europe and South-East Asia as early as 7,000 years ago.
However, these studies have typically overlooked how air pollution can also bring about changes in the climate.
For example, soot particles from fires trap heat in the Earth’s atmosphere, leading to warming, while organic carbon particles have a cooling effect thanks to how they reflect sunlight back into space.
The Roman Warm Period was not the only other climate shift faced by the ancient empire, however.
Researchers have proposed, for example, that the Roman conquest of northernmost Africa was aided by volcanic eruptions that took place around 250 BC.
These eruptions would have disrupted the African monsoon, negatively impacting the local societies and weakening them prior to Roman invasion.
Other experts have suggested that the collapse of the Western Roman Empire between 300 and 500 AD were associated with rapid changes to the climate.
The full findings of the study were published in the journal Climate of the Past.