Carbon balance

Long growing seasons in a warmer world will not lead to carbon uptake in the terrestrial ecosystems of the Northern Hemisphere.

Credit: © Dustin M. Ramsey

The growing season in the Northern Hemisphere has lengthened over the past few decades, but the atmospheric carbon dioxide mopped up by plant growth has been offset by a parallel rise in soil carbon decomposition, finds a new study.

Shilong Piao and colleagues from the Laboratoire des Sciences du Climat et de l'Environnement in Gif-sur-Yvette, France¹ investigated changes in the onset and termination of the growing season and their impacts on air-land carbon exchange in the Northern Hemisphere between 1980 and 2002. Using a vegetation ecosystem model and observed data, they found that the growing season has lengthened by about three days per decade since 1980, mainly owing to earlier springs in Eurasia and later autumns in North America.

Total plant growth increased with longer summers, in agreement with satellite and ground station observations, and although the plants absorbed carbon dioxide from the atmosphere as expected, the warming climate activated soil microbes that decompose litter, returning a comparable amount of CO₂ to the atmosphere. The study suggests that the amount of carbon sequestered on land is not directly linked to the lengthened growing season and increased plant growth associated with global warming.