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Abstract:
Interannual variations in the large-scale net ecosystem exchange (NEE) of
CO2 between the terrestrial biosphere and the atmosphere were estimated
for 1957–2017 from sustained measurements of atmospheric CO2 mixing
ratios. As the observations are sparse in the early decades, available records
were combined into a ‘quasi-homogeneous’ dataset based on similarity in
their signals, to minimize spurious variations from beginning or ending
data records. During El Nin˜ o events, CO2 is anomalously released from
the tropical band, and a few months later also in the northern extratropical
band. This behaviour can approximately be represented by a linear relationship
of the NEE anomalies and local air temperature anomalies, with
sensitivity coefficients depending on geographical location and season.
The apparent climate sensitivity of global total NEE against variations in
pan-tropically averaged annual air temperature slowly changed over time
during the 1957–2017 period, first increasing (though less strongly than in
previous studies) but then decreasing again. However, only part of this
change can be attributed to actual changes in local physiological or ecosystem
processes, the rest probably arising from shifts in the geographical area
of dominating temperature variations.
