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| Titel |
The Rise and Fall of the North Atlantic CO2 Sink |
| VerfasserIn |
Paul Halloran, Ian Totterdell, Matthew Mennary, Ben Booth, Nick Dunstone |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054625
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| Zusammenfassung |
| Over the last decade numerous observational studies have alluded to a declining, or at least
highly variable, North Atlantic carbon sink. Analysis of ocean carbon cycle models has gone
some way to explaining the high CO2 variability observed in this region, but until
now has been unable to explain the postulated historical decrease in the strength
of the sink. Here we present results from the Met Office Hadley Centre’s latest
IPCC simulations which describe a strong increase in North Atlantic carbon uptake
throughout much of the 1900’s, rapidly switching to a decrease around the turn of the
century.
Whilst the rate of change of atmospheric CO2 concentrations continues to rise we broadly
expect the rate of ocean CO2 uptake to follow suit. Major marine physical and
biogeochemical change may however regionally modify this pattern, influencing
ocean CO2 storage, and leading to a disconnect between anticipated and realised
atmospheric CO2 concentrations occurring in response to specific CO2 emission scenarios.
Both observational studies and atmospheric model inversions have suggested that
changes are already occurring in the major marine CO2 sinks, including the North
Atlantic. If the postulated changes in ocean carbon storage are shown to be robust, it is
imperative that earth system models are developed which can capture, explain and
assess the future variability and impact of these changes. Using the Met Office
Hadley Centre’s latest Earth System model HadGEM2-ES we argue that the rise
in North Atlantic CO2 uptake since the preindustrial occurs in response to ocean
circulation changes following volcanic and anthropogenic aerosol forcing, and
that the recent decline in North Atlantic CO2 uptake occurs as a readjustment to
the reduction in European and North American aerosol emissions initiated in the
1980’s.
Although the mechanisms linking volcanic and aerosol emissions to North Atlantic
carbon uptake within the model are still under investigation, it appears that wind driven
changes in Arctic Ocean circulation influence the salinity of water reaching sites of deep
convection. Subsequent changes in overturning circulation impact the carbon-chemistry of
water moving up the North Atlantic, in particular, elevating surface alkalinity. The
elevated alkalinity and increased northwards advection of surface waters allow
increased North Atlantic carbon uptake. A readjustment of Arctic circulation to the
removal of aerosol-imposed forcings over recent years slows the northward flux of
surface waters and drives a rapid decline in the strength of the North Atlantic Carbon
sink.
The ensemble of model experiments in which we observe this response are not initialised
or forced with any surface-ocean heat, wind-energy or precipitation fluxes, instead
responding freely to reconstructed atmospheric greenhouse-gas and aerosol concentrations,
natural (volcanic and solar) forcings and historical land-use change. Model ensemble
members were initialised from periods in the free-running preindustrial climate
experiment representing disparate phases of the ocean’s internal variability. We
therefore suggest that the unexpected carbon-uptake changes recently observed
in the North Atlantic may represent a forced response to anthropogenic aerosol
emissions, rather than a carbon cycle response to natural variability in the climate
system. |
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