Global sea levels are currently rising at an average rate of 1.8 mm per year since 1961, and 3.1 mm per year since 1993. The main contributions for this rise are from melting glaciers and ice caps and thermal expansion of the ocean. In line with this, the extent of polar snow and ice cover has been receding. One of the main targets of glaciologists and glacial geologists is to better understand rates of sea level rise, to allow better predictions of future change. The IPCC future estimations of sea level rise do not take into account dynamic changes in glaciers (such as the impacts of ice shelf collapse or marine ice sheet instability). Understanding Holocene rates of sea level rise contextualises present rates of change. Finally, understanding the rate of sea level rise around Antarctica can be used to constrain past ice volumes.
Holocene sea level rise
When an ice mass grows on land, it depresses the crust, and raises the relative local sea level. As the ice melts, the crust rebounds. This is called isostatic uplift. For example, Scotland is still rebounding after the last great ice age in Britain[4, 5]. Places like these, which were depressed during the last glaciation, are called near field sites. We can use near field sites to reconstruct the past volume of ice (because we know the viscosity of the crust and how much mass is needed to depress it be a certain amount).
The story is more complicated, however, because when there is a lot of ice in the world (a high global ice volume, for example, during the Last Glacial Maximum, ~18,000 years ago), global sea levels are lower. This is eustatic sea level change: the water is locked up in ice sheets, instead of in the oceans. Tectonically stable places far away from places with high ice volumes during the Last Glacial Maximum are called far field sites, because they had no isostatic depression during the last glaciation. These places measure the global change in sea level over the last glacial cycle (sea levels were about 120 m lower during the last glaciation).
So, far field sites constrain global sea level changes, and near field sites constrain ice volumes. However, it is complex, and regional interactions between isostatic and eustatic sea level change gives us local rates of relative sea level change. Scientists can use raised beaches, dated with a variety of methods, to constrain local rates of relative sea level change[8, 9]. On islands, hollows can accumulate marine sediments and organisms. When these are uplifted above sea level, they accumulate lacustrine (fresh-water lake) organisms and sediments. Using radiocarbon dating and biostratigraphy, and taking into account global eustatic sea level rise, scientists can calculate when the region was uplifted, and by how much[10-12].
The IPCC currently estimates global sea level rise to be around 1.8 ± 0.5 mm per annum. The melting of mountain glaciers and ice caps accounts for quite a lot of this rise, and this may be because smaller glaciers, which also tend to be steeper, are more sensitive to climate warming. Over the last 15 years, glaciers around the Antarctic Peninsula and in southern South America together have contributed 0.19 ± 0.045 mm per year to sea level rise.
Predicting the future
The IPCC predicts future sea level rise based on presents rates of melting and predictions of future carbon emissions and warming. However, there are large uncertainties (visible on the graph), because the dynamic interaction of ice sheets to climate change needs to be better understood. Predictions to 2100 range from 20 cm to 2 m. The best estimate is 0.6 m, mostly from thermal expansion of the oceans and glacier melt. Accelerated ice velocities, marine ice sheet instabilities and ice shelf collapse all form part of the large uncertainties in estimating future global sea level rise.
A collapse of the West Antarctic Ice Sheet would raise sea levels by about 3.3 m. Although it is unlikely, if the entire Antarctic Ice Sheet melted, it would raise sea levels by about 60 m . You can read more about Antarctica’s contribution to global sea level rise in this blog post.
Impact of sea level rise
Explore the impact of sea level rise in the USA to 2100 using this cool interactive feature from Climate Central.
- Future sea level rise from ice sheets
- Antarctica’s contribution to global sea level rise
- Post-glacial rebound
- Glaciers and climate change
Further reading: This nice, open-access paper by Van den Broeke et al., 2011.
See the Climate Institute for more information.
You can make sea levels rise here, and see if your house gets flooded!
1. IPCC core writing team, 2007. Climate change 2007: synthesis report. IPCC, 52 pp.
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