The Seagull Effect and global warming

GUILLEM CHUST, researcher at Global changes in marine ecosystems

Lying in the shade of a Mediterranean pine tree during this long, leaden summer, I traced the outline of the fluttering wings of a pearly butterfly and wondered how this delicate lepidopteran could cause a tsunami on the other side of the world. Yes, I am certainly referring to the ‘butterfly effect’, an expression coined by the mathematician Edward Lorenz, reflecting the unpredictability of weather and its sensitivity to small initial variations. Chaotic processes such as climate or the dynamics of a predator and its prey are often associated with tipping points or points of no return that determine the change from a stable state to a completely different one. One of the most relevant planetary limits is associated with the global temperature threshold, the exceeding of which can lead to a climatic point of no return. In 2023, the monthly temperature of the planet in the 12 consecutive months was 1.5ºC higher than pre-industrial times, a threshold that scientists still consider to be within the limits to avoid irreversible consequences. And on 22 July this year, 2024, the highest global temperature was reached according to the Copernicus Service since records began in 1940. What will the climate in Europe be like if this global temperature threshold continues to be exceeded? And what effect will it have on the Atlantic’s meridional overturning circulation? Recent observations have already detected a weakening of this circulation and a cooling of surface waters extending beyond the Labrador Sea, called the ‘cold blob’. According to studies by Peter and Susanne Ditlevsen, a future collapse of this circulation could occur this century and would have serious repercussions on the climate of the North Atlantic.

Identifying the planetary boundaries is one of society’s greatest challenges in anticipating the effects of global warming. But at the local level, it is just as important to identify the points of no return at which an ecosystem ceases to be what it was. Take, for example, the European seas, which have experienced gradual warming over the last 50 years. As a result, the study of more than 1800 species in collaboration with European experts has enabled us to observe a northward shift of marine populations and the tropicalisation of their algae, invertebrate and fish communities. That is, there is an increase in the abundance of warmer water species, such as the chavo, while cold water organisms are declining, such as the Mediterranean coralligenous. The sea is acidifying, the Atlantic is less saline and fish are getting smaller. Cantabrian anchovy and horse mackerel are advancing their spawning, bonito and mackerel are changing their migration patterns. Similar responses can be seen first hand in the early arrival of the stork around Christmas and its return to Africa at the beginning of August. Heat waves are now more frequent and can produce effects such as the bleaching of tropical corals or the mass mortality in 2023 of many tree species due to the combined effect of heat and drought, especially in Catalonia. When heat waves occur in winter, they can have other consequences such as throwing off some trees that flower earlier and then late frosts affect the flowers, leaving the crop without fruit, known as false springs. Each species also has its own particular strategy of adapting to the new environment, cascading through the food web, making it unpredictable which species will gain and which will lose. The ecosystem of the Bay of Biscay has already begun to reorganise itself, and is there an abrupt change in its biodiversity that is the result of a point of no return?

If we maintain this rate of greenhouse gas emissions, the key to understanding future climate lies in the ocean and the role of ocean conveyor belts as a sink for carbon and heat. No wonder Edward Lorenz, in his early talks on the unpredictability of climate, spoke of the flapping of a seagull, until a colleague later suggested he change the metaphor to that of the butterfly, as Thor Hanson recounts in his magnificent book ‘Hurricane lizards and plastic squid’.

The hunter-gatherer groups that followed the retreat of the ice during the last ice age to northern Europe and Asia and eventually crossed over to America did so because of a climate-induced range shift, acclimatisation and taking advantage of the new habitat available. In this 21st century, the human species must go further and confront global warming with all its technological and knowledge potential, and thus avoid the ‘Seagull Effect’.

This article was originally published in El Diario Vasco