When fire led to widespread cooling AF archive / Alamy Stock Photo

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’s 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, …