Long-Term Atmospheric CO2 Data Show Increase in Carbon Absorption by Northern Hemisphere Vegetation

​Based on long-standing data on atmospheric carbon dioxide (CO₂) concentrations, an international team of researchers coordinated by the Laboratory of Climate and Environmental Sciences (CEA/CNRS/UVSQ) have found that vegetation in the Northern Hemisphere is capturing increasing quantities of the CO₂ generated by human activities, thus partially counterbalancing the effects of global warming. The study was published in Nature on April 3^rd, 2019.

Origin of the discovery

In the early 1990s, a few atmospheric observatories and ocean data resulted in the discovery that terrestrial biomes in the Northern Hemisphere absorb considerable amounts of CO₂. This discovery, originally disputed, was later proven by forest biomass inventories and the formation of new atmospheric measurement stations.

Three decades later, the precise magnitude and trend of the terrestrial carbon sink in the Northern Hemisphere remain undefined. The outcomes of carbon cycle models vary from one another. The rich variety of ecosystems, ranging from Mediterranean shrubs to Arctic tundra, the broadly varying approaches of managing these environments, especially forest harvesting and farming practices, and events such as fires and droughts make it extremely difficult to estimate carbon budgets on a continental scale.

An international team of researchers, coordinated by the Laboratory of Climate and Environmental Sciences (CEA/CNRS/UVSQ), reconstructed the evolution of continental carbon sinks using atmospheric CO₂ content records between 1958 and 2016. These exclusive records, which encompass a period of more than 50 years, emanate from the two oldest atmospheric CO₂ measurement stations: Mauna Loa in Hawaii for the Northern Hemisphere and the South Pole in Antarctica for the Southern Hemisphere.

Northern Hemisphere vegetation: essential to slowing global warming

The difference between CO₂ records in the Northern and Southern Hemispheres reveals that the concentration of CO₂ in the atmosphere remains higher on average in the north. This is because of CO₂ emissions from fossil fuels, which are mostly formed by industrialized regions situated in the Northern Hemisphere. However, this difference in concentration is not as noteworthy as the difference between fossil-fuel emission levels in the two hemispheres put forth. This can only be described by an intensification of the carbon sink each year, partially in the oceans but mostly in continents in the Northern Hemisphere.

Since 1958, Northern Hemisphere vegetation has continued to absorb a significant amount of CO₂, with two significant increases in uptake: once in the 1990s and then again in the 2000s. On the other hand, the carbon uptake in southern continents seems to be stagnating. The models of the carbon cycle in vegetation and soils used to assess future CO₂ projections and consider climate change have not been able to replicate the intensification of CO₂ uptake observed in the 2000s.

Philippe Ciais, Researcher, Laboratory of Climate and Environmental Studies.

Philippe Ciais guided the analysis related to the research. A number of hypotheses have been put forward to explain this real marvel of increased carbon uptake in the Northern Hemisphere, which does not match model simulations: the growth and comparatively young age of forests, predominantly in North America and China, in addition to the fertilization of ecosystems in Asia that have been subjected to increased amounts of nitrogen from the atmosphere and to variations in soil management practices.

Reconstructing the CO₂ balance of Northern Hemisphere ecosystems over approximately sixty years thereby aids climatologists to properly understand the carbon cycle and fixed a reference base for conservation or carbon sequestration actions in biomass and soils in the future.