The Roman Empire lit so many fires that the resulting air pollution cooled the climate in Europe.
The finding adds to the evidence that human societies have been affecting Earth鈥檚 climate for thousands of years. However, these past changes are dwarfed by the current global warming, which is caused by greenhouse gas emissions.
Complex societies like the Roman Empire affect the climate in many ways. People burn fuel like wood to heat their homes, releasing greenhouse gases and soot. They also cut down forests to grow crops, and burn the residues of crops.
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Previous studies have estimated the impacts past societies made by releasing greenhouse gases and converting forests to farmland. For instance, in 2016, a team including Joy Singarayer at the University of Reading in the UK concluded that, in Europe and South-East Asia, .
Burn everything
However, these studies all neglected the ways air pollution affects climate. Soot particles from fires trap heat and have a warming effect, while particles of organic carbon scatter sunlight and cool the climate. The new study estimates the impacts of these aerosols.
鈥淲e looked for the first time at whether anthropogenic aerosol impacts had an impact on climate a long time ago,鈥� says Anina Gilgen of ETH Zurich in Switzerland. She and her colleagues estimated the amount of pollution the Roman Empire emitted, based on existing studies on how much land was used for farms, homes and other purposes. The team then used this to drive a climate model.
Deforestation and other changes in land use had a small warming effect of up to 0.15掳C. However, the air pollution had a cooling effect, which led to an overall temperature drop. Gilgen estimates that the climate in Europe cooled by 0.17掳C to 0.46掳C, depending on the true scale of the pollution.
鈥淭he novelty here is in their thinking about what the aerosol contribution would be, which seems to be quite considerable,鈥� says Singarayer.
Decline and fall
Nevertheless, Gilgen says the cooling was probably too small to have a major impact on Roman society. In fact, at the height of the Roman Empire, the climate went through a warm spell that lasted roughly from 250 BC to AD 400. This Roman Warm Period was and most climatologists think it was a natural phenomenon. The cooling effect of the air pollution may have slightly countered this natural warming, says Gilgen.
However, the fires could have had another harmful consequence. 鈥淚t might rather be that air pollution was a problem for people living in cities,鈥� says Gilgen. Air pollution is known to cause a variety of health problems, including increasing the risk of heart attacks and strokes.
Singarayer says the pollution might also have affected rainfall because water droplets condense around the aerosol particles. 鈥淭here could have been an impact on precipitation patterns and therefore water availability,鈥� she says. 鈥淭here鈥檚 been similar things in the Sahara, where aerosols have impacted on where precipitation is falling and contributed to drought conditions in certain areas.鈥�
Other periods of Roman history may have been affected by shifts in climate. For instance, it has been suggested that volcanic eruptions around 250 BC disrupted the African monsoon, destabilising African societies that the Romans were then able to conquer. Later, rapid shifts in the climate have been linked to the collapse of the Western Roman Empire between AD 300 and 500.
Climate of the Past
