Sea Level Rise Research at RSES
Sea level is a major key to understanding the system because it varies with the volume of the icecaps. Recently we found that sea level rises of 9-16 m accompanied Heinrich events during the period 30 - 65 ka, when abrupt climate changes were particularly strong. Based on evidence in coral terraces at Huon Penisula, Papua New Guinea sea level rises of 9-16 meters coincided with Heinrich events 40 - 60 ka.
Present-day melting of polar ice sheets increases the volume of the oceans, causing increases in sea level. Warming of the oceans causes thermal expansion which also causes changes in sea level. Variations in sea level can be measured through satellite altimetry and tide gauge measurements. Local land movements need to be considered and can be measured through GPS measurements and InSAR.
The following staff members are involved in Sea Level Rise research at RSES. |
Other Links: |
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| Dr. Kurt Lambeck | Web Page | |||
| Andrea Dutton | Web page | |||
| Paul Tregoning | Web Page | |||
Current Research projects
The History of Past Ice Sheets
Changes in geopotential (mm/yr) in Antarctica
caused by present-day glacial isostatic adjustment (from Ivins
and
James, 2005).
Understanding present-day melting patterns of polar ice caps and the contribution to present-day sea level variations requires first unravelling the puzzle of how the continents are still adjusting after the melting that has occurred since the Last Glacial Maximum around 10,000 years ago. This is a complicated field that involves field sampling to identify indicators of past sea levels, then generating numerical models that represent the amount and spatial locations of ice caps that can reproduce the observed sea level histories.
The mass of ice sheets deform the surface of the Earth - depressing the continents beneath the ice and causing slight uplift just outside the ice sheet perimeter - and reduce the volume of the oceans. Consequently, the melting of ice sheets has the reverse effect, causing uplift of continents and increases in ocean volumes. The processes can take thousands of years to reach equilibrium and the regions of the major ice sheets (Fennoscandia, Greenland, Laurentia, Antarctica) are still deforming as a result of melting that occurred thousands of years ago.
Dating of biological and geomorphologic samples yield information about the retreat of the ice caps and the rebound rates of regions previously covered with ice. Such data are inverted to generate models for the amount and timing of melting that has occurred.
The present-day glacial isostatic adjustment of Antarctica generates
uplift of the continent that is detectable by space-geodetic observing
techniques. The Gravity Recovery and Climate Experiment (GRACE)
is sensitive to the induced gravity changes, while satellite altimetry
and ground-based GPS can measure directly the vertical movement
of the surface. These effects must be removed from GRACE and altimetry
estimates of temporal changes in ice in order to estimate present-day
mass balance changes that might be occurring as a result of global
warming.
Present-day variations in sea level
Global sea level rise over the past decade as
measured
by two satellite altimetry missions (sealevel.colorado.edu).
Sea levels can vary if the volume of the oceans increases or if changes of the temperature of the oceans causes thermal expansion. Both of these would occur as a consequence of global warming. Accurate knowledge of variations in sea level is essential for our society.
How do we measure sea level? Relative sea level (the height of the water relative to the land) can be measured using tide gauges but it then becomes critical to know whether the land itself is moving before knowing whether the absolute height of the oceans is changing. Satellite altimetry measures the distance between an Earth-orbiting satellite and the surface of the ocean. Knowing accurately the position of the satellite we can know the height of the ocean relative to the centre of the Earth.
Are sea levels changing? Yes, they are. There is strong scientific
evidence that sea levels are rising and it has been suggested that
it may be accelerating (Church and White, GRL, 2006). But improving
the accuracy with which we can measure changes in sea level - and
hence understand better the processes of climate change - remains
an ongoing task.
Global network of GPS sites analysed at RSES to
monitor the vertical movement of tide gauges since 2000.
Vertical movement of tide gauges
Tide gauges are used to measure the rate of change of the ocean surface relative to the land (relative sea level). The vertical movement of the tide gauge itself needs to be considered before such observations can be used to estimate variations in regional or global sea level. Space-geodetic techniques such as the Global Positioning System can be used to measure movement of the continents - which may arise through natural processes such as isostatic adjustment, subsidence, deformation near locked plate boundaries - but to attain an accuracy of < 1 mm/yr great care must be taken in the analysis. Issues of significant importance include the effects of elastic deformation of the Earth's surface from atmospheric pressure and ocean tide loading, the retardation of the transmitted signals as they pass through the atmosphere,
modelling the trajectory of the satellites in their orbits, electrical phase properties of the ground-based antennae. Once accurate estimates of land motion can be assimmilated into the analysis of tide gauge records the long-term sea level trends can be estimated. Such estimates can then be used to compare/calibrate decadal-scale estimates from satellite altimetry.