We know that for the duration of reduced travel during the pandemic, these emissions will stay lowered. But what will happen when measures are eventually lifted?

In terms of routine trips like commuting, those miles left untravelled during the pandemic aren’t going to come back – you’re not going to travel to the office twice a day to make up for all the times you worked from home, says Nicholas. But what about other kinds of travel – might the cabin-fever of self-isolation encourage people to travel more when the option is there again?

“I can see arguments in both directions,” says Nicholas. “It may be the case that people who are avoiding travel right now are really appreciating spending time with families and focusing on those really core priorities. These moments of crisis can highlight how important those priorities are and help people focus on the health and wellbeing of family, friends and community.”

If this change in focus as a result of the pandemic sticks, then this could help to keep emissions lower, Nicholas suggests.

But there’s another way it could go. “It could also be that people are putting off long-distance trips but plan on taking them later,” Nicholas says. Frequent flying forms a large part of the carbon footprint for people who do it regularly, so these emissions could simply come back if people return to their old habits. (Read more about how to go on a “flight diet”.)

Historic epidemics

This is not the first time an epidemic has left its mark on atmospheric carbon dioxide levels. Throughout history, the spread of disease has been linked to lower emissions – even well before the industrial age.

Julia Pongratz, professor for physical geography and land use systems at the Department of Geography at the University of Munich, Germany, found that epidemics such as the Black Death in Europe in the 14th Century, and the epidemics of diseases such as smallpox brought to South America with the arrival of the Spanish conquistadors in the 16th Century, both left subtle marks on atmospheric CO2 levels, as Pongratz found by measuring tiny bubbles trapped in ancient ice cores.