summer in sunny birch forest

A London-smoke coloured moth

As in many of Arthur Conan Doyle’s stories, the London of “The Adventure of the Bruce-Partington Plans” is cloaked in a ghostly smog…

by Luca Longo
4 min read
by Luca Longo
4 min read

Sherlock Holmes may have been struggling to make criminals out in the late nineteenth century thanks to a constant blanket of fog, but the sky over the capital was already far from clean a hundred years before.
The pioneering climatologist Luke Howard was the first man to create a solid body of observations from experiments on London’s smog. His essay “The Climate of London”, published in 1818, was the fruit of years of minute meteorological observations carried out every day in the City. They show that the city was 2.1 °C hotter than the surrounding countryside on average.
Howard also understood that this was the fault of human activity. Smog developed from chimney smoke and from the very presence of buildings.

The hot City

Tarmac roads and air conditioners were still waiting to be invented. Now that they have been, and the city’s population has increased, the centre of London has reached an average temperature that is 6 °C higher than the ever more distant countryside. It has become the hottest place in the whole of Britain. The Met Office predicts that by the year 2100 the temperature in the city will be 10 °C higher than in the rest of the country.

This phenomenon is now so intense and widespread in cities that meteorologists have to include large urban conglomerations in their atmospheric simulations. Such cities can affect air circulation and create convection currents of warm air. They are almost like volcanoes, sucking in cool air from the countryside around them and defiling it before belching it out into the sky.
In a city powered mostly by coal, as London was from around 1750 to 1900, chimney smoke does not disappear. It gathers in convection currents above the city. Even the heat generated at street level did not go away. In fact, it was swollen by heat absorbed from the sun’s rays, which penetrated the blanket of dark clouds, was trapped as infrared radiation and further raised the temperature.

The white wings crisis

It was in London that one of the first examples of the Industrial Revolution changing the ecosystem was discovered. In the early eighteenth century, the capital was surrounded by thick woods of beech and birch, their trunks covered in white lichen. Here the peppered moth (Biston betularia), a small, nocturnal species, would hide by day, camouflaged by its white wings that mimicked the trunks.
But when the factories appeared, the smoke produced by the coal that fed them killed the lichen and covered the trunks with soot.
After this, the unfortunate white moths were easily picked out and devoured by birds. But luckily for a few of them, nature always finds a way to adapt to changes. Moths born with black wings thanks to a random genetic mutation did not get eaten. On the contrary, they found they had an evolutionary advantage over their white counterparts.

The peppered moth (Biston betularia)

In the soot-blasted woods around London, they were the ones with the camouflage to avoid ending up on some bird’s plate.
Two forms of the species therefore developed. The light moth (Biston betularia f. typica) carved out an ecological niche in Europe’s thinning forests, far from urban centres, while the dark moth (Biston betularia f. carbonaria) became widespread in the increasingly blackened woodland near large cities.

This process of environmental change and resulting adaptation has been called industrial melanism. It is the subject of a study by Olivia C Walton and Martin Stevens from the University of Exeter, published in Communication Biology. The study shows that environmental changes, especially man-made ones, can radically alter the process of evolution for organisms, with results that are hard to foresee. It is up to each of us to consider and change our actions, so that climate change does not mean that we all meet the same end as the white moth.