What is phenology and how does it help us understand the impact of climate change?

We humans have calendars and clocks, but how do plants and animals identify the changing seasons and know when to bloom, migrate, or shed their leaves? They rely on internal and environmental cues, especially daylight hours and temperature.

And this is where phenology (from the Greek phainein , "to show or appear" and logia , "study or science") comes in. It is the science that studies how climate influences the life cycles and seasonal changes of living things.

When the climate changes, that clock goes out of sync. That's why phenology is a key tool for detecting the effects of climate change . At CREAF, we've been studying it for over twenty years: with the Sentinel-2 satellites, RitmeNatura citizen observatory, and with experiments in Mediterranean forests. This article summarizes our findings, ongoing projects, and resources for further exploration.

Phenological dates change earlier and more noticeably than climate averages. For example, in January, temperature two degrees above average may sound abstract, but an almond tree blooming in November is a phenomenon anyone can observe and verify. This is why phenology acts as an early warning system for the climate crisis, reflecting how the combined effects of temperature, rainfall, and sunlight affect nature.

In Catalonia, the Meteorological Service of Catalonia (Meteocat) has measured these long-term changes. Some of the findings show that, compared to fifty years ago, the crop cycle has been drastically altered: pear trees ripen 37 days earlier, apricot trees 28, apple trees 23, and olive trees 19 days earlier. Grapevines are harvested 11 days earlier, and butterflies fly in the meadows weeks earlier than expected.

The reason? Temperature anomalies confuse vegetation. On the one hand, unusual heat spikes can cause flowering to occur earlier than usual. On the other hand, without sufficient winter chill, species that need a period of cold to flower properly (vernalization) – almond, cherry, apple, grapevine, chestnut, winter wheat, lavender – may flower poorly and, consequently, produce less fruit. Both scenarios can leave pollinators, that arrive on time in spring, without enough resources.

Adrià Descals doctoral thesis, awarded by the CSIC in 2024 as one of the twenty most relevant of the year, reconstructed global vegetation phenology using Sentinel-2 images, resulting in the first high-resolution phenological map of the Arctic and global maps for the period 2001-2021. The most striking result: heat waves are causing leaves to fall earlier than previously thought in deciduous forests more, and with global warming, plants capacity to absorb CO₂ may become saturated.

A study by Josep Peñuelas, a CSIC researcher at CREAF, published in Nature Climate Change with Jian Wang from Ohio State University, showed for the first time that a lack of rain—not just heat—also advances the phenological spring in the Northern Hemisphere. Fewer clouds mean more hours of solar radiation, cooler nights, and greater temperature fluctuations that plants interpret as signs of spring. For this century alone, projected rainfall reductions showed spring advancing 1.2 to 2.2 days per decade.

A second study published in Nature Climate Change , also with the participation of Josep Peñuelas, analyzed 393,139 forest plots in the Northern Hemisphere. The results show that forests with greater species diversity are less sensitive to rising temperatures and do not experience earlier bud break. One possible explanation lies in the soil and plant characteristics of biodiverse areas. Greater species diversity may favour the presence of roots that explore deeper soil layers, allowing them to acces to more resources. These areas also tend to have more fertile soils with more diverse microbal communities. Together, these factors help buffer the effects of local climate changes.

Barn swallows ( Hirundo rustica ) return historically from Africa in late February or early March. For centuries, their low flight over the Catalan countryside has marked the arrival of spring. But that is no longer the case. General estimates confirm that the swallows now arrive twenty-four days earlier than in 1970. And this is not anecdotal; it is a consequence of climate change, and we can experience it firsthand.

Studying these changes is the goal of phenology. Today, this discipline goes far beyond academic theory: it has become an essential barometer of the climate crisis .

As seen with swallows, climate change can indeed alter bird migration patterns. On the one hand, some birds may no longer need to migrate in search of areas with food and favorable weather conditions. On the other hand, a  phenological desynchronization may occur; for example, when birds arrive in spring, peak caterpillar abundance may have already passed, and consequently, there may not be enough food for the chicks.

This phenomenon is being studied in multiple European projects, which precisely seek to map and understand how birds migration is changing, thanks to the data collected through the EuroBirdPortal project , which has its own viewer to follow migrations in Europe in real time .

At CREAF, we are involved in:

• LIFE EBP Reinforcement: A project coordinated by our researcher Sergi Herrando, who is also president of the European Bird Census Council, that aims to optimize and expand the quantity and quality of data in the EuroBirdPortal database.
• SPEAR: a project that wants to assess how climate change alters the abundance and distribution of birds in Europe, in order to redesign policies and protected natural areas accordingly.
• WildE: a project in which our researcher Ella Plumanns-Pouton, along with her team, studies a little-explored function: the ability of swallows and other migratory birds to transport seeds over long distances, helping to spread them to new and more climatically favorable areas.

In autumn, the leaves of deciduous trees change color before falling: as chlorophyll, the pigment responsible for their green colour, other yellow and orange pigments become visible, and in some species, red pigments are synthesized. At the end of the process, tannins, along with the degradation and oxidation of other compounds, give the leaves their brown hues. This colour change is part of the survival strategy of deciduous plants that, before losing their leaves, recover nutrients so they can reuse them in spring.

But not all shades of brown are created equal. Climate change and droughts can disrupt this phenomenon, causing leaves to dry out and fall prematurely. Unlike the gradual, vibrant color changes of autumn, drought often leads to more abrupt changes and duller hues. Moreover, if leaves fall prematurely, plants may have less time to replenish its nutrients. However, mild autumns can delay color changes and leaf fall, altering when organic matter reaches the soil, the nutrient recycling cycle, as well as the plants' water consumption, among other processes.

RitmeNatura is a citizen science phenological observatory coordinated by CREAF and led by Ester Prat since 2015, in collaboration with MeteoCat. In this project, citizens voluntarily collect observations of their surroundings across Catalonia using the iNaturalist app. Participating is as simple as regularly monitoring the growth cycle of a tree or shrub in your area and recording its changes. To learn how to participate, visit the project website . All the collected data is then validated by the project's technical team.

Among other things, these observations have allowed researchers to detect anomalies in vegetation cycles over the years, consistent with what is expected in this context of climate change. For example, between July 2023 and June 2024, RitmeNatura recorded 214 plant species that flowered outside their usual season in Catalonia. Also in 2023, during the autumn, orange, almond, plum, pomegranate, broom, hawthorn, myrtle, and many other species sprouted in a phenomenon known as "second spring." This phenomenon, as Ester Prat points out, represents a waste of energy for the plants, since these flowers die with the first cold weather, and the energy reserves consumed will no longer be available during the true flowering season.

Since 2021, RitmeNatura has also been active in the education sector, bringing science closer to students and promoting classroom participation in the project. Additionally, it has created the Phenocenters network, comprised of schools and institutes that regularly submit phenological data throughout the school year. Furthermore, the project collects and offers free educational materials on its website. This educational involvement culminated recently with the new learning project "Seguim el ritme de la natura", developed in collaboration with the Xarxa d'Escoles Verdes.

Internationally, RitmeNatura joined the phenological modeling network coordinated by the Lorentz Centre in the Netherlands, along with the University of Twente, the French INRAE, the Technical University of Munich, and the University of Córdoba. This collaboration led to the recent publication of a joint article outlining a roadmap, with 21 key questions, for the future of research in the field of plant phenology. Read the paper here.

If you want to stay up-to-date with RitmeNatura's activities, as well as learn about the center's other citizen science projects, you can subscribe to MOSAIC, THE quarterly citizen science newsletter. And if you don't want to miss any CREAF activities or content, subscribe to our newsletter!