Monitoring ocean change in the 21st century

Time series data sets, which contain measurements repeated over a span of decades, yield important insights into our oceans’ vital signs.

Ocean temperature and chemistry changes strongly influence the well-being of organisms and the composition of marine food webs. Since the preindustrial era, the oceans have absorbed at least one fourth of the carbon dioxide (CO2) that humans have put into the atmosphere, resulting in an increase in ocean acidity. The amount of oxygen dissolved in ocean water has changed as well, driven by warming and changes in circulation patterns. These effects are further compounded in coastal regions, where marine ecosystems face additional human pressures such as pollution, land use changes, and overfishing.

Long-term, seasonally resolved time series observations in the ocean have provided unprecedented insights into how ocean biogeochemistry and ecosystems are changing on a range of temporal scales. We now know that these observations must be sustained for a minimum of several decades to statistically distinguish natural climate variability from changes associated with human activities [e.g., Henson et al., 2016]. Here we present a scientific vision for the future of shipboard biogeochemical time series in light of new scientific findings and recent technological advances.

Ocean time series stations have documented major changes in the chemistry of ocean waters, in the functioning of ecosystems, and in oceanic carbon sequestration. These stations have increasingly served as test beds for the development of new sensors and methodologies. They provide valuable seagoing opportunities and hands-on training for the next generation of ocean scientists. Ocean time series efforts are also a forum for collaboration between nations, building scientific capacity. Efforts to establish a global network of observatories are underway: If these efforts are sustained into the future, they will allow us to monitor how global marine ecosystems are responding to a changing climate.

Chemistry Changes and Marine Ecosystems

The oceans have absorbed 25%–30% of the anthropogenic CO2 emitted since the preindustrial era [Sabine et al., 2004]. Recent analyses of ocean CO2 across multiple independent ocean time series sites show increasing acidity across ocean basins over the past 2–3 decades [e.g., Bates et al., 2014; Tanhua et al., 2015]. This widespread change in the ocean CO2 system is a direct consequence of ocean uptake of CO2 (Figure 1). These studies demonstrate the global extent of this phenomenon and underscore the importance of high-resolution data sets to remove seasonality and elucidate longer-term trends.

In addition to increasing acidity, the oceans have undergone warming, increased stratification, and circulation changes that have reduced dissolved oxygen levels, particularly in coastal regions like the southern California Current System. The California Cooperative Oceanic Fisheries Investigations (CalCOFI) ocean time series has documented a decline in dissolved oxygen levels and a shoaling of the oxygen minimum layer (a relocation of a region of the water column with very low dissolved oxygen levels to shallower waters) over the past 2–3 decades. These changes are likely to affect organisms living in the water column and on the seafloor [Bograd et al., 2008].

Neuer, S., Benway H. M., Bates N., Carlson C. A., Church M., DeGrandpre M., Dunne J., Letelier R., Lomas M., Lorenzoni L., Muller-Karger F., Perry M. J. & Quay P., 2017. Monitoring ocean change in the 21st century. Eos 98, doi: 10.1029/2017EO080045.  Published on 8 September 2017. Full article.

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