There’s no greater challenge than the climate emergency we’re currently facing. Since 1906, the global average surface temperature has increased by more than 0.9 degrees Celsius, triggering the melting of glaciers, shifting precipitation patterns, and affecting wildlife. By 2050, global heating could pose an existential threat — the forecasted 3 degrees Celsius could displace as many as one billion people, according to a 2019 study. 

These gloomy outcomes are the result of human activities, mainly due to the release of greenhouse gas emissions. And although the scale of human-induced climate change is unprecedented today, we’ve been causing slight alterations of the climate since antiquity.

In a new study, a team led by researchers at ETH Zurich found that, at its height, the Roman Empire released so many aerosols from burning fuel (i.e. large quantities of organic matter like wood) and cleared so much land for agriculture that it effectively cooled all of Europe.

At its prime, the Roman empire stretched from the Atlantic coast to the Euphrates river, and from Scottland’s highlands to the Sahara desert.

The researchers employed the “climate model ECHAM-HAM-SALSA with land use maps and novel estimates of anthropogenic aerosol emissions from the Roman Empire.” They concluded that Roman air pollution produced a cooling effect which caused land surface temperatures to drop by as little as 0.23 degrees Celsius and as much as 0.46 degrees Celsius.

However, an exceptionally warm climate likely countered this effect. The Roman Warm Period, which lasted from around 250 BC to 400 AD, was an unusually hot period, which likely helped the empire expand to such great lengths. Previous studies have shown that this was a natural phenomenon.

“Cooling caused by aerosol emissions is largest over Central and Eastern Europe, while warming caused by land use occurs in parts of North Africa and the Middle East. Our results suggest that the influence of Roman Era anthropogenic aerosol emissions on European climate may have been as important as that of deforestation and other forms of land use,” the authors reported in the journal Climate of the Past.

All of this is not to say that the Romans had nearly as much influence on the global climate as our current civilization. Nevertheless, these findings show that even 2,000 years ago, humans already had the means to make important contributions to the climate, nevermind today when there are more than 7 billion people living on the planet (and with vastly superior technology).