Bomb pulse: Canada's sugar maples held Cold War carbon for 60 years

In Gatineau Park in Quebec, across the river from Ottawa, sugar maple (Acer saccharum) leaves hit their highest level in 1964: 842 parts per thousand above the reference level, according to the Geological Survey of Canada. That was about a year after the Limited Test Ban Treaty of 1963 ended atmospheric tests. By 2025 the figure had fallen to an estimated -13.9.

Why did the maples lag behind the atmosphere?

Earth's atmospheric carbon-14 returned to its pre-bomb baseline between 2021 and 2022. The maples only got there around 2024, a lag of roughly 2.2 years, give or take three to four months.

The reason is that trees recycle. "Plants often use older carbon, stored from previous years, to grow new leaves," said geoscientist Leonard Wassenaar. Small but steady offsets, averaging about 7.4 parts per thousand above reference between 1990 and 2019, showed the signal came from this biological recycling rather than from the air itself. The team published the work in Scientific Reports.

What does it change for climate research?

Studies that reconstruct past air from carbon-14 in leaves and tree rings have often assumed the plant matches the atmosphere of the year it grew. The researchers say that now has to be "corrected explicitly" for each species.

Fossil fuels also helped clear the bomb pulse. Burning coal, oil and gas releases carbon dioxide with almost no carbon-14, because it has decayed away over eons, and that diluted the nuclear leftovers. The oceans absorbed some too. The 2025 leaf samples confirmed that fossil-fuel dilution has now overwhelmed the bomb-pulse residue in the Ottawa-area maples.

Wassenaar, who runs the university's accelerator mass spectrometry facility, calls the leaves "a 65-year diary of the atmosphere."