​In 2015, Pieter De Frenne was watching archived footage of the 1980 Liège–Bastogne–Liège, an annual one-day road cycling race that occurs every April in Belgium, when he noticed something: the weather. It appeared much colder than it is today. The trees were bare.

Aside from being a cyclist himself, De Frenne is a scientist who studies climate change and botany at Ghent University's faculty of bioscience engineering in Belgium. He wondered if he could use archived footage of annual cycling road cycling races to chart the changing climate.

So he turned to the most regular of the races: the Tour of Flanders, an annual race that takes place in early April that had television coverage going back for decades.

In a new study published in the British Ecological Society's journal Methods in Ecology and Evolution, De Frenne and his colleagues found that, through archived footage, the leaves of trees could be seen coming out earlier each year, which indicated a response to climate change.

Leafing through the footage

Co-author Lisa Van Langenhove sifted through more than 200 hours of television data of the race shot between 1981 and 2016. Though the route had changed over the years, the team selected 12 climbs and landmarks where they could pinpoint individual trees. They studied 46 trees in particular. Most of them were not native to the area and included magnolia, hawthorn and forsythia.

The researchers found that in the 1980s, there were virtually no leaves on trees. After 1990, however, many trees were already in full leaf.

The change was significant. When leaves begin to emerge on a tree branch, it's referred to as flushing. The study found that between 2006 and 2016, 45 per cent of trees had begun to grow leaves. That's compared to nearly zero in the 1980s.

The shifts also corresponded with climate data that showed temperatures in the area rose by 1.5 C since 1980.

Early spring has its drawbacks

While people may enjoy the early arrival of warmer weather, there are downfalls. For one, trees play an important part in the ecosystem, as many animals and insects depend on them as a food source.

That timing change has a trickle-down effect. For example, studies have shown earlier springs can affect migratory birds. They can arrive at their breeding site after a bloom where their food source — insects — have already hatched.

And other plants also feel the effects.

"Many of the attractive spring flowers start their life cycle before the trees produce leaves, so if the trees shift their leaf flushing, it means these flowers will be growing more in the shade, if their timing stays the same," De Frenne says. "Which means they will have less energy to complete their own life cycle."

A 2013 study showed that plant flowering in Canada has advanced by nine days for every degree Celsius of warming. (Graeme Roy/Canadian Press)

Alemu Gonsamo, a research associate at the University of Toronto's geography department who studies phenology and climate change, but who was not involved with the study, says there are other consequences that scientists are trying to better understand.

For example, with earlier leafing and flowering comes a longer growing season. Trees take in carbon from the atmosphere. While having more carbon uptake may seem like a positive in a rapidly warming world, Gonsamo notes it will cause the soil to dry out. And because there will also be more decomposition, more carbon will be released. It's unknown what the long-term, large-scale effects will be.

Different approach

De Frenne says that the study illustrates a different method with which scientists can track climate shifts.

"The objective of this particular study was to show that this method is actually working, and it is, apparently," De Frenne says. "So this method can be applied to archived footage of other races as well."

Gonsamo says the method is a novel way of approaching a field that, to date, has relied mostly on citizen science and satellite data. In a 2013 study that relied on citizen science, he and his colleagues found that plant life cycles are responding to a changing climate: plant flowering in Canada has advanced by nine days for every degree Celsius of warming.

"The method is good, smart. I've never seen this kind of study before," Alemu says. "This is very creative. Part of the problem with phenology is that we need data which is over a long time series and for larger geographic areas ... It's only three decades of data, but it fills the gaps. So I think it's a very good idea."