Global Sea Level: Past, Present and Future

The oceans are a central part of the global climate system.

As Roger Revelle, one of the founders of the Intergovernmental Oceanographic Commission of UNESCO, said many years ago : 'The oceans exert a profound influence on mankind and indeed upon all forms of life on Earth. The oceans are inexhaustible sources of water and heat, and control the climate of many parts of the world'.

One of the major consequences of climate change is rising sea levels.

Does rising sea level matter?

We love the ocean and coasts. Millions of people are crowded along the coastal fringes of continents, attracted by recreational opportunities, coastal and deep-sea fishing and rich fertile land. In excess of 150 million people live within 1 metre of high tide level; 250 million within 5 metres of high tide. Many of the world's megacities, cities with populations of many millions, are situated at the coast, in addition to coastal infrastructures worth billions of dollars.

The impacts of sea level rise include inundation of low-lying coastal regions, particularly during extreme sea level events, coastal erosion of beaches, saltwater intrusion into coastal aquifers, deltas and estuaries, damage to coastal ecosystems, water resources and coastal infrastructure.

Has sea level changed in the past?

Sea leval has varied by over 100 metres during glacial-interglacial cycles as the major ice sheets have waxed and waned. Sea level was about 4 to 6 metres above present day values during the last interglacial period, when Greenland was about 3°C warmer than today.

The rate of sea level rise over the last 20 years in 25 per cent faster than any rate during the previous 115 years, almost twice as fast as the average over the twentieth century, which was in turn an order of magnitude larger than the rate of rise over the two millennia prior to the eighteenth century.

Why is sea level rising?

The two major reasons for sea level rise are thermal expansion of ocean waters as they warm and an increase in the ocean mass, principally from land-based sources of ice (glaciers and ice caps and the ice sheets of Greenland and Antarctica). Global warming from increasing greenhouse gas concentrations is a significant driver of both contributions.

Projections of future sea level change

During the twenty-first century, sea level will continue to rise due to warming from both past (twentieth century and earlier) and twenty-first century greenhouse gas emissions. Ocean thermal expansion is likely to be the dominant contribution to twenty-first century level rise, with the next largest contribution coming from the melting of glaciers and ice caps.

Improving our ability to adapt to sea level rise

Adaptation requires local planning base on local scientific information. Providing this local information requires improving our understanding at local, regional and global scales and across a wide range of scientific disciplines.

Priorities include sustained, systematic observations of :

• Sea level (extension of the Jason series of satellite altimetres and completion of the Global Sea Level Observing System - GLOSS network of tide gauges, with absolute positioning and real-time data availability).
• Ocean volume (the Argo array of profiling floats for the upper ocean and its extension to ice covered regions and the design and implementation of a deep ocean observing system).
• Ocean and terrestrial mass (observations of the time-varying gravity field to contribute to estimating changes in terrestrial water storage, ice sheet mass balance and changes in oceanic mass).
• Ice sheet and glacier topography and thickness and ice velocity.
• Two-dimensional surface water levels on land.

 

Related links:
:: Special reprint of John Church's Roger Revelle Lecture (January 2008) (PDF format)
:: World Climate Research Programme 
:: Marine Sciences and Observations for Integrated Coastal Area Management
:: The state of the ocean climate
:: The Global Sea Level Observing System (GLOSS)
:: Intergovernmental Oceanographic Commission (IOC)

 

 

• Author(s):John Church, CSIRO, Oceanographer
• Source:Based on "Roger Revelle Lecture 2006"
• 16-02-2008

• © UNESCO/D. Roger - National Centre for Oceanographic Research Measuring equipment on the scientific boat, ocean preservation