White butterflies filling Joburg skies earlier than usual due to climate crisis

Temperatures that used to signal the onset of spring for plants and animals are now occurring earlier in most parts of the world.
Each year around mid-summer, somewhere between December and mid-January, the skies of South Africa’s Gauteng province, including the city of Johannesburg, fill with small white butterflies. Some land in people’s gardens, allowing a closer look at the thin brown markings on their wings. Those markings give the butterflies their name: the brown-veined white butterfly (Belenois aurota).
Their annual migration takes between 80,000 and 155,000 butterflies per hour from South Africa’s Kalahari region to Mozambique, a journey of hundreds of kilometres via Gauteng. They are leaving the arid Kalahari in search of food and moisture.
The butterflies move in a huge group and their migration is relatively quick — it takes a week or so for most of them to move through Gauteng. The resulting clouds of butterflies are a beautiful spectacle, noticed not just by butterfly enthusiasts and scientists, but by residents.
This year, the butterflies have arrived early.
That may seem unimportant. But, to phenologists like myself, it’s evidence of changes in the environment that require close attention. Phenology refers to the timing of annually recurrent biological events: the blossoming of jacaranda trees, for instance, or a mass butterfly migration.
Across the world, phenological events are occurring increasingly earlier as a result of climate change. The temperatures that used to signal the onset of spring for plants and animals are now occurring earlier in most parts of the world. Simultaneously, the timing and amount of precipitation are changing too.
Climate change is intangible to many people. We know it is happening, but our larger surroundings look the same — for now. It’s difficult to feel the 1.1°C post-industrial global temperature increase. But we do notice when the jacarandas flower earlier or butterflies arrive in our gardens earlier. This is important in raising public awareness regarding climate change.
Media records
In an article published earlier this year, my students and I used media reports to quantify how the timing of the annual butterfly migration had changed over nearly 100 years.
The butterfly migration has featured in newspaper reports for many years. More recently, with the advent of social media, these butterflies have also been photographed and posted on Instagram, tweeted, and posted on a range of other social media platforms.
These print and social media records are a gold mine for phenologists. For our research, we recorded the ...