The city lights are bringing spring early and postponing autumn.

Streetlights, neon signs, and LED displays are confusing plants: in cities, spring arrives several days earlier than in the countryside, and autumn is also delayed by almost two weeks. Research published in the scientific journal Nature Cities shows how light pollution acts like the Sun, accelerating fundamental processes in the phenology of plants, such as budding and leaf fall. Researchers believe that city lights are altering plant life, with consequences yet to be determined.

Light pollution already prevents us from seeing the stars. Recent studies have also shown how excessive artificial lighting in cities affects the behavior of urban animals and human health. Meanwhile, many studies have confirmed that spring arrives earlier in cities. Previously, climate change was pointed out as the cause, intensified in urban environments by the phenomenon known as the urban heat island effect: the concrete of buildings, the asphalt of streets, the height of constructions, and the entire urban design trap heat. Now, light pollution adds to the equation, resulting in a widespread disturbance of urban vegetation.

The work published in Nature Cities found that, on average, the city center greens up about 12.6 days earlier than plants in the surrounding countryside. Conversely, leaf aging begins in urban areas an average of 11.2 days earlier. The phenomenon is widespread. The study tracked the onset of the plant growth season in 428 major cities in the Northern Hemisphere. In 378 of them, 88.3%, these alterations in plant phenology were observed. To detect these changes, they used data from several satellites and information from 2014 onwards.

“Satellites detect differences in greenness by measuring the amount of light reflected from the Earth’s surface,” explains Lin Meng, a researcher at Vanderbilt University (USA) and co-author of the study. They carry onboard instruments that capture this reflection in different wavelengths, particularly in the visible and near-infrared parts of the spectrum (NIR). “Vegetation strongly reflects NIR light while absorbing red visible light for photosynthesis,” adds Meng. They use markers like the Normalized Difference Vegetation Index (NDVI). “Areas with denser vegetation or greater photosynthetic activity reflect more NIR light and less red light, resulting in higher NDVI values,” the researcher details.

By comparing these reflectance patterns in cities and nearby fields, the satellites were able to detect spatial changes in greenness. The study uses a gradient from the most distant areas to the city center. In the case of Barcelona, for example, the monitored rural area for comparison is located 32 kilometers from the center. In the Catalan capital, they found that the length of the growing season is 14 days longer in the urban core compared to the more distant rural areas.

Another significant contribution of this work is its ability to break down the causes of this city-country differential. The authors also used satellites to measure the degree of light pollution. Under natural conditions, on a cloudy night, luminosity hovers around 0.6 millilux (lux measures the amount of incident light). If there are no clouds, with a full moon at its zenith, the figure can reach up to 0.3 lux. But in cities, the light range goes from 7 to 65 lux. Previous studies have shown that the night has become almost 10% brighter each year for over a decade. However, until now, temperature was always cited to explain the phenological changes in urban flora.

“We used the statistical analysis method, that is, partial correlation analysis, to separate the impact of temperature from that of artificial lighting on plant phenology,” Meng explains. “Partial correlation measures the degree of association between two random variables while eliminating the influence of a set of control variables. This allows us to isolate the effect of each factor on the target while controlling for other factors,” she details. Thus, they found that both thermal and light variables are affecting the duration of the plant growing season. However, the latter has a greater influence in four of the seven major climate regions covered by the study, from dry and warm climates like Dallas to cold and dry climates like those in Canadian cities.

Meng elaborates on the cities: “Cities with higher artificial light intensity have a greater impact on phenology,” she says. Since levels vary seasonally, their influence differs between spring and autumn. “In particular, many American cities showed higher brightness during spring. Conversely, European cities have higher light pollution in autumn, thus showing greater influence at the end of the growing season, in autumn,” she adds.

Regarding the consequences for flora, Dunxian She, a professor at Wuhan University (China) and the study’s lead author, reminds us that the advancement of spring and the delay of autumn can have both positive and negative effects. “In natural ecosystems, a longer growing season could enhance productivity and carbon capture, thereby contributing to climate change mitigation. However, it could also affect local ecosystems if new species become more dominant, leading to changes in biodiversity, especially with the introduction of exotic species for urban landscape design,” he says.

Artificial light extends the photoperiod, acting as an extension of daylight. For the study’s authors, this extension could alter the accumulation of signals that initiate sprouting or senescence, such as reduced photosynthesis efficiency. But the disruption is more profound. Plants perceive light differently based on its wavelength. Phytochromes are proteins that function as photoreceptors, while cryptochromes respond to blue light. The LED revolution is being driven by the latter, and research is ongoing to understand the consequences of this.

Georg Wohlfahrt leads the biometeorology group at the University of Innsbruck (Austria). Although he did not participate in this work, he was the lead author of pioneering work that confirmed that spring was arriving earlier in cities. His work pointed to the urban heat island effect and environmental pollution; he also acknowledges the role of light pollution. “Aside from non-seasonal climates, such as the tropics, plant phenology depends on temperature and the quality/quantity of light,” he recalls in an email. “Artificial light in urban areas modifies the light exposure of plants and, consequently, their phenology. This is not new. The novelty of the article in Nature Cities is that it addresses the problem on a large scale in a number of cities and attempts to untangle the effect of artificial light from that of the urban heat island,” he adds.

If anyone knows about light pollution, it is Christopher Kyba. A professor at the Helmholtz Centre for Geosciences GFZ (Germany), he has been researching the impact of artificial light for over a decade. To this end, he has created a network of scientists, citizen science projects, and even an app to measure the loss of night. The results of his latest project were just published in the same edition of Nature Cities, unrelated to the study of the 428 cities. The citizen science work coordinated by Kyba collected data from 33 German municipalities, finding that after midnight, more than one light per inhabitant remains on. What is revealing is that for every streetlight on, there is a lit advertisement or display.

“I see reasons for both hope and fear,” says Kyba about the future of light pollution. “On one hand, there are examples of cities and even entire countries in wealthy, industrialized areas of the world where we see decreases in light emissions after midnight,” he explains in an email. “On the other hand, the vast majority of cities and countries worldwide continue to increase their lighting.”