Human footprint on world's oceans

The increase in atmospheric CO₂ has multiple consequences for the ocean carbon cycle and marine life. The best known is ocean acidification resulting from the uptake of this excess CO₂ by the ocean and the subsequent chemical reactions that acidify the ocean. A much less well known, but potentially also important consequence is the increase in the amplitude of the seasonal cycle of CO₂ in the ocean. While the increase in the amplitude of the seasonal cycle of CO₂ in the ocean has been predicted by model studies, the observational evidence for this increase has been lacking on a global scale.

Fluctuations in CO2 concentration confirmed

Using the to-date largest collection of surface ocean CO₂ measurements in combination with a sophisticated data interpolation technique, Peter Landschützer, researcher at Max Planck Institut of Meteorology in Hambrug, and co-authors are now able to quantify this increase in amplitude for the global ocean surface waters from 1982 through 2015. Their results suggest that the added CO₂ has led to an increasing seasonal amplitude of 2.2±0.4 µatm / decade, which is in agreement with independent measurements from timeseries stations and basic thermodynamic considerations. While the study highlights the advances of the surface ocean observing networks for CO₂ in our ability to validate model predictions, the study also emphasizes the detectable imprint human emissions have already left on the global oceans over the past 3 decades.  

Sensitive balance at risk

«Our works shows once again that the large service to humanity provided by the ocean thanks to it taking up about 30% of the global human-made emissions has negative consequences for marine life», Prof. Nicolas Gruber, professor at ETH Zürich sums up. «To understand why an increase in the seasonality of CO₂could be detrimental for marine organisms, one can take the seasonal cycle of temperature as an example. Our detected increase in the seasonality of CO₂ is analogous to winters getting colder, and summers warmer. Thus, if an organism is sensitive to e.g., hot conditions, a trend towards warmer summer will greatly enhance its exposure to potentially deleterious conditions, e.g. induced by a heat wave.»

The results of the study are far from good news as the increasing seasonal variation amplifies the impact of ocean acidification on marine life by inducing the transition across critical thresholds that are harmful to marine organisms earlier in time.

Human footprint in the sea

«The increasing seasonality should not come as a surprise, since it has been predicted by several model-based studies before», says Peter Landschützer. What our study tells us, however, is that the observational network in place can be used to verify model predictions on the global scale, which is of increasing importance in light of the Paris climate agreement. In addition, the study shows that climate change is not worries for later and the increasing CO₂ seasonality in the global oceans is one more piece of evidence that we humans have already left a detectable imprint on our planet.»

Dorothee C. E. Bakker, researcher from the University of East Anglia, Norwich, UK adds: «The study highlights the importance of the ongoing data collection and synthesis efforts by seagoing scientists for detecting the pervasive influence of human activity on the oceans. Annual public releases of the external pageSurface OceanCO2 Atlascall_made, which this study has used, are a Voluntary Commitment to the 2017 UN Ocean Conference.»