Convergence of atmospheric and North Atlantic carbon dioxide trends on multidecadal timescales

Oceanic uptake of carbon dioxide substantially reduces the rate at which anthropogenic carbon accumulates in the atmosphere(1), slowing global climate change. Some studies suggest that the rate at which the oceans take up carbon has significantly decreased in recent years(2-8). Others suggest that decadal variability confounds the detection of long-term trends(9-11). Here, we examine trends in the partial pressure of carbon dioxide in the surface waters of three large biogeographic regions in the North Atlantic, using observational data collected between 1981 and 2009. We compare these oceanic observations with trends in atmospheric carbon dioxide levels, taken from a global observational network. We show that trends in oceanic carbon dioxide concentrations are variable on a decadal timescale, often diverging from trends in atmospheric carbon dioxide. However, when the entire 29-year period is considered, oceanic trends converge with atmospheric trends in all three regions; it takes 25 years for this long-term trend to emerge and overcome the influence of decadal-scale variability. Furthermore, in the southernmost biome, the data suggest that warming-driven by a multidecadal climate oscillation and anthropogenic forcing(12,13)-has started to reduce oceanic uptake of carbon in recent years.