Sea level is a very sensitive index of climate change and variability. As the ocean warms in response to global warming, sea waters expands and as a result, sea levels rise. When mountain glaciers melt in response to increasing air temperature, sea levels rise because more freshwater glacial runoff discharges into the oceans.
Similarly, ice mass loss from the ice sheets causes a rise in sea level. The increase of freshwater flowing into the oceans reduces its salinity, decreasing its density and affecting ocean circulation patterns that in turn affect the sea level and its spatial variability.
The global mean level of the oceans is an indicator of climate change. It incorporates the reactions from several different components of the climate system. Precise monitoring of changes in the mean level of the oceans is vitally important for understanding not just the climate but also the socioeconomic consequences of any rise in sea level.
Mean sea level is an average over all the oceans of sea surface height, with respect to a reference. However, what are really sought are the variations of this mean sea level along time. For almost 30 years now, the global mean sea level has been routinely measured over the whole oceanic domain with high-precision satellite altimetry, and such observations show clear evidence of global mean sea level rise (+3.4 +/-0.4 mm/yr).
This mean sea level can be global, but basins (e.g. Atlantic) or regional areas can be computed. The rise in the sea level of the oceans is far from uniform. In fact, while in certain ocean regions the sea level has risen (by up to 20 millimetres a year in places), in others it has remained unchanged or has even fallen. These regional differences, observed by altimetry satellites, mostly reflect sea level fluctuations over several years.
Consequences
A large proportion of the world’s population lives in coastal areas vulnerable to rising sea level. Permanent submersion, repeated flooding, faster erosion of cliffs and beaches and increasingly saline estuaries and salt contamination of groundwater are just some of the possible consequences of a big rise in the sea level near low-lying regions.
The main objective of the sea level CCI project is to produce and validate a Sea Level Essential Climate Variable (ECV) product.
To achieve this global objective, the specific objectives for the Sea Level ECV are:
- To involve the Climate research community and to improve the understanding of this community's needs and thus ensure a perfect consistency between the need and the future development and improvement of the altimeter processing system. A clear gap exists between the requirements expressed by the CGOS and the current status of the altimeter sea level error budget. Consequently, the user consultation carried out during the project is an important step to revisit and to precise these requirements in order to find a consensus between the Climate Research and the Earth Observation communities. The User Requirement Document presents a synthesis of this work.
- To develop, test and select the best algorithms and standards in order to produce high quality sea level products for climate applications. These activities have allowed the altimeter community to work on the development of climate optimised processing algorithms. A large spectrum of algorithms and methods have been assessed during the project and the more relevant for climate applications have been selected. The Round Robin Reports produced within these activities are all available for the users.
- Assess and collect information on the quality and error characteristics of the Sea Level ECV product through the involvement of independent climate research groups. The main objective here is to obtain relevant feedback from the user community in order to qualify the accuracy and suitability of the ECV products and to define new suggestions for improvement. It is also the opportunity to implement an in-depth dialogue between the climate research and Earth Observations communities which is fundamental to address climate change issues.
- Provide a complete specification of the operational production system.