36 years of bike race footage reveals how Belgium’s climate is changing

Green-thumbed grandmothers the world over have noticed a disturbing trend: Spring seems to be arriving earlier every year. But without hard data, it’s difficult to back up those claims. Now, scientists have found a new source of data—old television footage of outdoor sporting events, which reveals how early trees and other plants are “leafing out” each year.

Scientists started with footage of the Tour de Flanders (above), a professional cycling race that’s taken place in Belgium nearly every April since 1913. The researchers watched more than 200 hours of video for races from 1981 to 2016, picking out 20 trees that they could see clearly in subsequent years. For each tree, they noted the presence—or absence—of leaves, along with their size.

The old footage reveals that spring has sprung earlier along the race course in the past decade. In the 1980s, hardly any trees had leaves during the race. But from 2006 to 2016, 45% of trees had at least started growing leaves, the team reports today in Methods in Ecology and Evolution .

It isn’t surprising that the Belgian trees are leafing out earlier, given previous studies in places with detailed leaf-growth records. But this is the first time researchers have used old sporting footage to come to similar results. The team says this latest study is just a start, and old television footage could be a valuable resource in the quest to understand how climate change is affecting plants.

*Correction, 5 July, 9:45 a.m.: This story has been updated to correct the percentage of trees that started growing leaves from 2006 to 2016.