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Abstract

The global land and ocean carbon sinks have increased
proportionally with increasing carbon dioxide emissions during the
past decades1. It is thought that Northern Hemisphere lands make a
dominant contribution to the global land carbon sink2–7; however,
the long-term trend of the northern land sink remains uncertain.
Here, using measurements of the interhemispheric gradient of
atmospheric carbon dioxide from 1958 to 2016, we show that the
northern land sink remained stable between the 1960s and the late
1980s, then increased by 0.5 ± 0.4 petagrams of carbon per year
during the 1990s and by 0.6 ± 0.5 petagrams of carbon per year
during the 2000s. The increase of the northern land sink in the
1990s accounts for 65% of the increase in the global land carbon
flux during that period. The subsequent increase in the 2000s is
larger than the increase in the global land carbon flux, suggesting a
coincident decrease of carbon uptake in the Southern Hemisphere.
Comparison of our findings with the simulations of an ensemble
of terrestrial carbon models5,8 over the same period suggests that
the decadal change in the northern land sink between the 1960s
and the 1990s can be explained by a combination of increasing
concentrations of atmospheric carbon dioxide, climate variability
and changes in land cover. However, the increase during the 2000s is
underestimated by all models, which suggests the need for improved
consideration of changes in drivers such as nitrogen deposition,
diffuse light and land-use change. Overall, our findings underscore
the importance of Northern Hemispheric land as a carbon sink.