The oceans are vast, covering about 70% of the earth surface and large parts of the ocean are remote and seldom visited by ships. In addition the oceans are deep, with an average depth of about 3800 meters. The remoteness and the difficult conditions (for instance, the pressure a 2000 meters depth is 200 bars, the same pressure as in a full scuba tank) make observations of the ocean difficult. At the same time, the large scale of the ocean makes the ocean a fundamental part of the global climate system. It is interesting to note that the ocean is absorbing 93% of the extra heat to the Earth, making the ocean the most important buffer of heat on the planet. That means that even small variations in the way water are transported around by currents will have large effect on things like atmospheric temperature. The oceans also have a large buffer capacity for carbon dioxide that has absorbed about 30% of CO2 (the main greenhouse gas) emitted by human activities. In that sense, the ocean is doing a great service to us in absorbing heat and CO2, without the oceans the atmosphere would have warmed a lot more.  It is therefore important to make measurements of the ocean to understand how fast the ocean is heating and absorbing CO2.

The obvious way to observe the ocean is to sail out on a research vessel, take water samples and make measurements. Even today, that is the only way to measure a whole range of chemical and biological variables, and the only way to observe the ocean below 2000 m depth (about half of the ocean volume is below 2000 m depth). The drawback is that research vessels are expensive and so that scientists are trying to use robots to make measurements. The most important robotic system is the so called Argo system, also called profiling floats. These are instruments that can adjust its buoyancy so they either sink, stay or float. The floats are drifting with the currents at 1000 meters depth, and every 10 days they sink to 2000 meter and then ascend to the surface while taking measurements. Once at the surface the data are send home over satellite telephone and then the float sinks back to 1000 m depth to sleep for another 10 days. In the beginning these floats only measured temperature and salinity but recently scientists has developed sensors for measuring chemical variables such as oxygen, nitrate and the acidity (i.e. pH) of the water. The “new” variables are being measured on some of the almost 4000 Argo floats that are currently measuring the ocean at any time.

Other autonomous observing systems include moored observing platforms, where various sensors measure variables such as salinity, temperature, oxygen and currents, although several other variables can be measured from sub-surface mooring. The advantage of a mooring is the high temporal frequency that can be sampled, so that even short time fluctuations and variability can be observed and characterized; long term mooring observations are critically important to understand variability and trends in the ocean. The moorings are, obviously, not moving but measure at one particular spot, so that data from other, mobile, platforms can be used to extrapolate observed local signals to larger areas of the ocean. On several locations there are regular visits by research vessels, often combined with a mooring operation. This has the advantage of the feasibility to measure a wide range of variables that are not easily done with a sensor on a mooring. One example of such a site is the Cape Verde Ocean Observatory (CVOO, that is operated by GEOMAR Helmholtz Centre for Ocean Research Kiel in cooperation with the local partners INDP. It is from sites like this one that we can best monitor the acidification of the ocean related to the uptake of anthropogenic CO2.

Another important ocean observing system is satellites. From space one can determine both the temperature and salinity of the ocean surface and by analyzing the color of the ocean scientists can calculate the amount of phytoplankton and particles, for instance. Obviously, the satellites can only measure the surface ocean, for the interior ocean other instruments are needed. 

Although the satellite observations are organized across agencies and are well aligned with ocean observational requirements, the in-situ systems are more fragmented and funding for sustaining observations is fragile in most case. One problem is that a large fraction of ocean observations are funded on short-term (a few years) scientific projects, which aligns poorly with requirements for sustained observations. Scientists are trying to remedy these problems in different ways; one very exciting development is the multitude of new and accurate sensors that can be deployed on a range of autonomous or manned platforms.

Recently the Implementation Plan of the Global Climate Observing System (GCOS) 2016 was published, and approved by the UNFCCC during the COP-22 in Marrakech. This plan outlines observational needs for climate, and one part of the plan is focused on the ocean. Although we need to make sustained observations of the ocean for different purposes than climate, the needs for climate observations are well articulated. For instance, the Paris Agreement from 2015 identifies the need of using the best available scientific knowledge for effective and progressive response of climate change. One part of that is systematic observation of the climate system and early warning systems, in a manner that informs climate services and supports decision-making.

Text: Dr. Toste Tanhua GEOMAR, Helmholtz-Zentrum für Ozeanforschung, Kiel.

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