Early summer is when the vast Boreal forests, the tundra, and the Taiga reawaken and drive the world's deepest intake of carbon dioxide.
This is the highest point of the year for photosynthesis across vast, mostly empty northern landscapes, savannahs, mountains and tablelands. Billions of trees, crops, and wildflowers drink in CO₂. Bees, insects, ants, and birds join in as spring and summer bring warmth and sunlight to the wilderness of the Northern Hemisphere.
During this yearly spring surge, photosynthetic plants and trees absorb vast quantities of CO₂ to rebuild foliage, wood, and root systems. This is recorded by the Keeling Curve: visual evidence of terrestrial biology actively reshaping atmospheric composition.
There is no single, fixed global CO₂ reading—just an endlessly shifting, dynamic biological system. Globally, this collective biological drawdown drops atmospheric CO₂ concentrations by roughly 6 to 9 parts per million every summer.
In autumn and winter, leaves drop, growth stops, and microbes decompose organic matter on the forest floor. Stored carbon returns to the atmosphere, and CO₂ climbs back up, completing an ancient biological cycle.
The Northern Hemisphere contains the vast majority (68%) of Earth’s landmass and forest cover—spanning North America, Europe, and Asia—meaning its seasonal growth cycles dominate global atmospheric chemistry.
The Southern Hemisphere is dominated by oceans rather than expansive vegetated landmasses, so its seasonal CO₂ swing is far smaller. Meanwhile, tropical systems like the Amazon photosynthesise continuously year-round, contributing to steady baseline absorption rather than the sharp, seasonal fluctuations seen in the far north.
Satellite data and observatory records over recent decades reveal that this annual peak-to-trough cycle has expanded in high northern latitudes. As tracked by NASA MODIS satellite data monitoring the Leaf Area Index, vegetated regions are expanding and growing seasons are lengthening, resulting in a larger seasonal CO₂ intake and increased biomass production compared to fifty years ago.
Photosynthesis reprocesses roughly 120 billion tonnes of carbon every year through sheer plant metabolic activity.
The biosphere is never a passive observer, it's a dynamic, driving biological engine.