No, Ocean United is not a football team. It is a group of professionals leading international efforts to improve our understanding and management of the Ocean. Oh, how boring you'd think if you thought this was about football. Well, think again. The Ocean and it's implications on your life is way bigger than football! Water is essential to all life on Earth.

This last week in November, Ministers from more than 70 countries meet in Cape Town, South Africa, to discuss how we can build a global earth observing system including the oceans.

More than two third of our planet Earth is covered by oceans. Thus, Ocean United has put together an information package explaining in more detail why the ocean is so important to us and how we observe the ocean today. I'll let you in on a few secrets about measuring sea level that you will not find in this package.

Why the Ocean?

The environmental consequences of bad management of the ocean is incomprehensive. Monitoring of the ocean is in fact vital for our lives on this planet. To remind you about a few good reasons why we want to know as much as possible about the vast oceans, here is a list of various ocean impacts in societal areas

• from natural disasters
• on human health and well-being
• on energy resources
• from a changing climate
• on water resources
• on marine ecosystems 
• on marine biodiversity 

United Ocean member POGO has produced a video presentation of the ocean and its role in our lives. How the ocean observing system contribute to the earth observation system of systems is described by more members of Ocean United in GEO report The full picture. 

For those who take an interest in climate change have understood how important it is for us to understand how and what makes sea level change.

Sea Level

Sea Level is traditionally defined to be the intersection between ocean and land. So, end of story? When you place yourself on this intersection - in other words on the beach - you realize of course that it is hard to say exactly where that line should be drawn. You have waves that moves the water back and forth, you have the tidal differences and do you really know it is the sea that rises or is it the land that is sinking? Out on the open ocean we talk about sea surface. The sea surface is defined to be the intersection between the ocean and atmosphere. There are many other variations in the sea level or sea surface that makes these definitions pretty hard to make. It may then surprise you that we can define and measure this line - sea level - or sea suface, with millimeters accuracy. Let's see how.

On the intersection between ocean and land we can harvest and

have fun... 

Global monitoring of sea level

Along our coasts we use tide gauges to measure the sea level. By analysing the data collected from these tide gauges we can learn about the changes of sea level over time.  Global Level of the Sea Surface (GLOSS) is an international network of coastal tide gauges dedicated to retrieve climate change and coastal management data. The data is also being used for validating the satellite altimetry that can measure sea level on the open ocean. 

The GLOSS Core Network (GCN) is designed to provide an approximately evenly-distributed sampling of global coastal sea level variations. Another component is the GLOSS Long Term Trends (LTT) set of gauge sites (some, but not all, of which are in the GCN) for monitoring long term trends and accelerations in global sea level. These will be priority sites for Global Positioning System (GPS) receiver installations to monitor vertical land movements, and their data will contribute to long term climate change studies such as those of the WMO-UNEP Intergovernmental Panel on Climate Change (IPCC).

The GLOSS altimeter calibration (ALT) set consists mostly of island stations, and will provide an ongoing facility for mission intercalibrations. A GLOSS ocean circulation (OC) set, including in particular gauge pairs at straits and in polar area, complements altimetric coverage of the open deep ocean within programmes such as the World Ocean Circulation Experiment (WOCE) and Climate Variability and Predictability (CLIVAR).

Sea Level Change - Compared to What?

Sea level changes all the time for several reasons. When we talk about sea level rise in connection with climate change, we mean a permanent change of the mean sea level. The sea level varies with the amount of water, the temperature and salinity of the ocean and other effects like atmospheric loading and ocean currents.

This illustration from IPCC shows the water cycle and the reason why sea level change. 

When you measure something you need a reference. What do you do if this reference is constantly changing as well? This is were geodesy comes in and provide us with a reference frame that integrate the continuous changes of the Earth - like mass distribution (water condensing from the ocean, comes down as rain on land and gradually makes it back to the ocean again).

There are two ways of defining the origin or centre of such a reference system. The geometric origin is defined the same way you make a circle with a ruler except that for the Earth we use the satellite orbits. The geophysical origin is the center of mass of the planet. We need these two origins to be linked in a perfectly known fashion if we want to use satellites AND tide gauges to measure sea level. This linking is one of the major future goals of the Global geodetc observing system GGOS which they describe in more detail in the strategy document Global Geodetic Observing System: Meeting the requirements of a global society on a changing planet in 2020 (GGOS2020)

What's new?

World leading expert on sea level, Prof. Hans-Peter Plag at University of Nevada, Reno, has introduced a new approach to sea level monitoring. Local Sea Level (LSL) is what you need to know when you are managing a coastal urban environment like New York and Venice. London, one of the world economic centres, risk flooding due to sea level rise even though it is situated off coast. With this approach you do not have to include the global geodetic reference frame in your equations, but you still need other geodetic products. 

In the GEO report The Full Picture , John Church et. al (p 177) give us the following advise and perspectives on how to relate to the sea level changes and its uncertainties: ...the current generation of climate models does not yet provide robust projections of regional patterns of sea level rise. As a result the global average of sea level rise should be considered for planning purposes with some allowence for a potentially larger contribution as a result of regional pattern of sea level rise...

Hans-Peter Plag and William Hammond from Nevada Bureau of Mines and Geology & Seismology Laboratory, University of Nevada, Reno went into more details at a symposium  “Chronic Risk of Global Climate Change to Urban Coasts & Economies,” held at Stevens Institute of Technology in Hoboken, New Jersey 14. - 15. November 2007 (Hoboken is just across the Hudson river opposite Manhatten, another world economic centre.) Plag and Hammonds conclusions are that we still lack a lot of information to narrow down sea level rise estimates. Because of the great uncertainties in the information we have today about the local sea level, they recommend that urban settlements withdraws gradually from coastal zones. While at the same time improving our capacity to get the necessary information about the different factors contributing to sea level rise like better understanding and information about the melting of glaciers and a much improved estimation of the geoid .

Humans live closer to the coasts and spend more time on the ocean as never before. This proximity can turn into an economic and human disaster if we do not address the risks in an educated and responsible manner. 

Small is bigger

We have to measure sea level with an incredible accuracy if we want to understand and predict climate change. How small changes do we have to be able to measure? Today we can in fact measure sea level down to an accuracy of millimeters! But this small is not small enough. The harsh requirements are submillimeter, thank you very much.

If you think of how the ocean behave, all the small and big constant changes, ranging from riples on the ocean surface, tidal differences of several meters, storm surges and tsunamis, how on earth (or ocean) can we measure that? With the combination of different observing networks ranging from in-situ measurements along the coasts, satellite altimeter scanning of sea surface on open ocean and the geoid. The geoid is a gravity reference surface. An improved calculation of the geoid is especially important for the oceans since we then can get better estimates of ocean currents. Measuring smaller entities means bigger understanding of our planet.

As you can see from this illustration the Earths surface is not smooth if you put on your "geoid glasses".

Illustration from DORIS. 

Conclusions

In-situ and space data must be integrated 

The in-situ network Argo  will provide global information about ocean temperature and salinity. Data from Argo alone will improve our knowledge about the ocean currents. But, if you combine this with space geodetic products like the geoid calculations resulting from the ESA GOCE satellite to be launched next year, our knowledge will sky-rocket! It is this kind of multidisciplinary cooperation and integration of the in-situ and space components of the global earth observing system that will result with a big leap in our understanding of the oceans.

Fill the gaps in the observing network

When Ocean United points out that we urgently need to improve our monitoring capacity of the ocean, this definitely goes for the polar regions. Not only are the tide gauges scarse on both poles, but the satellite altimetry that can provide us with very important information about ocean sea surface in these remote and harsh areas, does not cover the poles. We are even in danger of loosing altimetry capacity in the areas where we observe today because the politicians fight over who should pay for new satellites.  

Another region where it should be pretty obvious that we need good observations are in the Carribean. The yearly hurricanes affect a large number of people. The problem in these areas are both economic, as many of the countries are relatively poor, and political.  

Geodetic products are pivotal to meet the future requirements

Regardless of whether we use traditional methods for determining sea level or the new approach introduced by Hans-Peter Plag , we do need geodetic products like reference frames and the geoid to be determined with higher accuracy. Especially since the ocean is so vast, I believe that space geodesy will be particularly important for the advancement of our knowledge. 

Is this bigger than football? I think so. 

Stumble It!

Note: This article will be updated during the next couple of weeks.