# The Azimuth Project Sea level rise (Rev #23, changes)

Showing changes from revision #22 to #23: Added | Removed | Changed

# Contents

## Idea

One among the expected side effects of global warming is a global increase in sea level, or sea level rise. Sea levels are expected to increase for a number of reasons, including melting ice and the fact that warmer water occupies a greater volume. This is expected to affect low-lying land areas along coasts, including river deltas and barrier islands.

Sea levels have varied substantially over time. The CSIRO report on Historical sea level changes includes this graph.

## Estimates

According to the NRC climate stabilization targets report, global sea level has risen by about 0.2 meters since 1870. The sea level rise by 2100 is expected to be at least 0.6 meters.

Ice loss is also occurring in parts of Greenland and Antarctica. If all the ice in Greenland were to melt, it would cause an additional sea level rise of 7.2 meters, and if all the ice in the West Antarctic Ice Sheet were to melt, it would cause a further rise of 4.8 meters (see below). However, the amount of sea level rise in the next century remain uncertain, because the rate of melting of these bodies of ice is hard to predict.

Note: Due to the Archimedes’ principle the meltdown of ice that is freely floating will not increase the sea level, in a first appoximation. Therefore all estimates need to include ice residing on land only. See also:

However, melting sea ice does change the sea level, because fresh ice becomes fresh water, which is less dense than sea water. But this effect is very small.

A kinematical upper bound of 2 m/century (if you max out everything) and a more realistic upper bound of 1 m/century (at least for this century; it could accelerate later) is obtained by

• W. T. Pfeffer, J. T. Harper and S. O’Neel, Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise, Science 321 (2008) no. 5894 pp. 1340-1343

## Greenland

If the entire $2.85 \cdot 10^6$ km3 of ice in Greenland were to melt, it would lead to a global sea level rise of 7.2 meters. This would inundate most of the world’s coastal cities and remove several small island countries from the face of the Earth, since island nations such as Tuvalu and Maldives have a maximum altitude below or just above this level:

However, according to the abstract of the following paper, it appears that these authors predict only an 0.16 meter sea level due to Greenland ice melting by 2080:

• Sebastian H. Mernild, Glen E. Liston, Christopher A. Hiemstra, and Jens H. Christensen, Greenland Ice Sheet Surface Mass-Balance Modeling in a 131-Yr Perspective, 1950–2080,J. Hydrometeorology 11 (2010), 3–-25.

The following paper shows that

the ice loss, which has been well‐documented over southern portions of Greenland, is now spreading up along the northwest coast, with this acceleration likely starting in late 2005.

The ice sheets is not a constant, but accelerating with time, i.e., that the GRACE observations are better represented by a quadratic trend than by a linear one, implying that the ice sheets contribution to sea level becomes larger with time. In Greenland, the mass loss increased from 137 Gt/yr in 2002–2003 to 286 Gt/yr in 2007–2009, i.e., an acceleration of −30 ± 11 Gt/yr2 in 2002–2009.GRACE observations are better represented by a quadratic trend than by a linear one, implying that the ice sheets contribution to sea level becomes larger with time. In Greenland, the mass loss increased from 137 Gt/yr in 2002–2003 to 286 Gt/yr in 2007–2009, i.e., an acceleration of −30 ± 11 Gt/yr2 in 2002–2009.

## Antarctica

It is estimated that the volume of the Antarctic ice sheet is about $2.54 \cdot 10^7$ km3. The weight of the ice has caused the underlying rock to sink by between 0.5 and 1 kilometers.

The West Antarctic Ice Sheet (or WAIS) contains just under 10% of this, or $2.2 \cdot 10^6$ km3:

• Matthew B. Lythe and David G. Vaughan, BEDMAP: A new ice thickness and subglacial topographic model of Antarctica, Journal of Geophysical Research 106 (June 2001), .

Large parts of the West Antarctic Ice Sheet (or WAIS) sit on a bed which is below sea level and slopes downward inland. This slope, and the low isostatic head, mean that the ice sheet is theoretically unstable: a small retreat could in theory destabilize the entire WAIS leading to rapid disintegration. However, current computer models do not include the physics necessary to simulate this process, and observations do not provide guidance, so predictions as to its rate of retreat remain uncertain.

In January 2006, in a UK government-commissioned report, the head of the British Antarctic Survey, Chris Rapley, warned that this huge west Antarctic ice sheet may be starting to disintegrate. Rapley said a previous Intergovernmental Panel on Climate Change (IPCC) report that played down the worries of the ice sheet’s stability should be revised. “The last IPCC report characterized Antarctica as a slumbering giant in terms of climate change,” he wrote. “I would say it is now an awakened giant. There is real concern.”

(Note that the IPCC report did not use the words “slumbering giant”.)

Rapley said, “Parts of the Antarctic ice sheet that rest on bedrock below sea level have begun to discharge ice fast enough to make a significant contribution to sea level rise. Understanding the reason for this change is urgent in order to be able to predict how much ice may ultimately be discharged and over what timescale. Current computer models do not include the effect of liquid water on ice sheet sliding and flow, and so provide only conservative estimates of future behaviour.”

It has been argued that a collapse of the WAIS could raise global sea levels by approximately 3.3 meters:

• # J. L. Bamber, R.E.M. Riva, B.L.A. Vermeersen and A.M. LeBroq, Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet, Science324 (2009), 901.

However, these authors claim there would be important regional variations, with the maximum increase concentrated along the Pacific and Atlantic seaboard of the United States, where the value is about 25% greater than the global mean, even for the case of a partial collapse.

If the entire West Antarctic Ice Sheet were to melt, this would contribute 4.8 m to global sea level:

• J. L. Bamber, R.E.M. Riva, B.L.A. Vermeersen and A.M. LeBroq,, Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet (supporting online material), Science 324 (2009), 901.

• Rob Young, Orrin Pilkey, How High Will Seas Rise? Get Ready for Seven Feet, Yale Environment 360, 14 Jan 2010.

Indications that the West Antarctic Ice Sheet is losing mass at an increasing rate come from the Amundsen Sea sector, and three glaciers in particular: the Pine Island, Thwaites and Smith Glaciers:

Data reveals they are losing more ice than is being replaced by snowfall. Total ice discharge from these glaciers increased 30% in 12 recent years, and the net mass loss increased 170% from 39 ± 15 Gt/yr to 105 ± 27 Gt/yr. The melting of these three glaciers alone is now contributing an estimated 0.24 millimetres per year to the rise in the worldwide sea level (see the article by Jenny Hogan above).

More generally, there has been substantial increase in Antarctic ice mass loss in the ten years 1996-2006, with glacier acceleration a primary cause:

In 1996 the net mass loss was 78 ± 78 gigatons/year. By 2006 this had risen to 153 ± 78 gigatons/year.

Another estimate of West-Antartic ice loss:

• Sasgen, I., Z. Martinec, and J. Bamber, Combined GRACE and InSAR estimate of West Antarctic ice mass loss, J. Geophys. Res. 115 (2010).

Velicogna - see reference under Greenland - estimates:

In Antarctica the mass loss increased from 104 Gt/yr in 2002–2006 to 246 Gt/yr in 2006–2009, i.e., an acceleration of −26 ± 14 Gt/yr2 in 2002–2009. The observed acceleration in ice sheet mass loss helps reconcile GRACE ice mass estimates obtained for different time periods.

## CSIRO

The CSIRO or Commonwealth Scientific and Industrial Research Organisation of Australia has a project on sea level changes. Their report on Historical sea level changes is on our recommended reading list.

Here is some research from their project on sea levels:

Quoting:

Research by Australian climate scientists has shown that global sea level has been rising at an increasing rate over the past 130 years. Using information from tide gauges and measurements from satellites, Dr John Church and Dr Neil White estimated changes in global mean sea levels since 1870.

Their work, published in the science journal Geophysical Research Letters (6 January), indicates an acceleration in the rate of sea-level rise that had not been detected previously.

‘Although predicted by models, this is the first time a 20th century acceleration has actually been detected,’ Dr Church says. ‘Our research provides added confidence in sea-level rise projections published by the Intergovernmental Panel on Climate Change Third Assessment Report.

‘If the acceleration over the past 130 year period continues, we would expect sea level to be 280-340mm above its 1990 levels by 2100. This is consistent with the projections in the Intergovernmental Panel on Climate Change Third Assessment Report.’

## The Copenhagen Diagnosis

The Copenhagen Diagnosis, written in 2009, was intended to serve as an interim evaluation of the evolving science before the 5th IPCC report, which is not due for completion until 2013. Its executive summary says, among other things:

Current sea-level rise underestimates: Satellites show great global average sea-level rise (3.4 mm/yr over the past 15 years) to be 80% above past IPCC predictions. This acceleration in sea-level rise is consistent with a doubling in contribution from melting of glaciers, ice caps and the Greenland and West- Antarctic ice-sheets.

Sea-level prediction revised: By 2100, global sea-level is likely to rise at least twice as much as projected by Working Group 1 of the IPCC AR4, for unmitigated emissions it may well exceed 1 meter. The upper limit has been estimated as 2 meters sea-level rise by 2100. Sea-level will continue to rise for centuries after global temperature have been stabilized and several meters of sea level rise must be expected over the next few centuries.

## Impacts

Worldwide, the NRC Climate Stabilization Targets report estimates that a 0.6 meter sea level rise would displace 3 million people and raise the risk of flood for millions more.

According to the UNEP, 1.5 meters in sea level rise would displace 18 million people in Bangladesh:

## Fast ice dynamical effects

There are several major “fast ice” dynamical effects that could accelerate mass loss.

• R. B. Alley, P. U. Clark, P. Huybrechts and I. Joughin, Ice-Sheet and Sea-Level Changes Science 310 (2005) no. 5747 pp. 456-460

There is paleoclimatic evidence for abrupt sea level rise, see e.g.

• J. T. Overpeck et al , Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise, Science 311 (2006) no. 5768 pp. 1747-1750.

### Zwally effect

This effect is a mechanical instability of the entire ice sheet due to basal lubrication from meltwater. It is mostly mentioned in the context of the Greenland ice sheet.

### The Larsen B scenario

For the West Antarctic ice sheet (WAIS), the important effects are often associated with floating ice shelves. Ice shelves can disintegrate due to warm ocean water underneath, or surface melt ponds “drilling” down throught the shelf and fragmenting it. When a shelf is removed, the land ice sheet it was “buttressing”, or holding back, can slide more rapidly into the ocean. The latter is called the Larsen B scenario.

• A. Shepherd, D. Wingham, T. Payne and P. Skvarca, Larsen Ice Shelf Has Progressively Thinned Science 302 (2003) no. 5646 pp. 856-859;

• T. A. Scambos, C. Hulbe; M. Fahnestock and J. Bohlander, The link between climate warming and break-up of ice shelves in the Antarctic Peninsula, Journal of Glaciology 46 (2000) no. 154 pp. 516-530

• E. Rignot et al , Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf, Geophysical Research Letters 31 (2004)

### Instability due to underlying topography

Another concern is the underlying bed topography. The WAIS rests on an “upsloping”, or “foredeepened” bed, meaning that the bed slopes up toward the sea, or down toward the center of the ice sheet. There has been a debate for almost 40 years about whether ice sheets resting on such beds are particularly unstable, which is still unresolved.

• D. Goldberg, D. M. Holland, C. Schoof, Grounding line movement and ice shelf buttressing in marine ice sheets, Journal of Geophysical Research 114 (2009)

### Significance?

Summarizing, there are mechanical effects by which ice loss can be accelerated other than simply melting the whole ice sheet from the top down, i.e. by dumping ice directly into the sea. If you hear about GIS or WAIS disappearing within centuries, it’s because these effects are being invoked. But it’s still unknown how significant these effects are. For example, the Zwally effect in the GIS may be compensated by more efficient subglacial drainage relieving pressure at the base of the ice sheet, as described in

• A. V. Sunkal et al, Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage, Nature 469 (2011) pp. 521–524

Although there is paleoclimate evidence for abrupt sea level rise, we heard from Nathan Urban that he doesn’t know glaciologists who think that GIS or WAIS could disappear within one century.

The SeaRISE project (see references below) is an attempt to determine how fast you could lose an ice sheet if all of these effects are strong. Maybe SeaRISE will get a much higher number of sea level rise with more explicit ice sheet modeling, but Nathan Urban doesn’t consider it to be probable.

## Questions

How much sea level rise should we expect from Greenland and the West Antarctic Ice Sheet within the next, say, 10 years?

What are the best adaptive measures? Floating cities?

Has there been a time in history when the sea level was significantly higher than today?

## References

Abstract: We propose a simple relationship linking global sea-level variations on time scales of decades to centuries to global mean temperature. This relationship is tested on synthetic data from a global climate model for the past millennium and the next century. When applied to observed data of sea level and temperature for 1880–2000, and taking into account known anthropogenic hydrologic contributions to sea level, the correlation is >0.99, explaining 98% of the variance. For future global temperature scenarios of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report, the relationship projects a sea-level rise ranging from 75 to 190 cm for the period 1990–2100.

The following paper aims to estimate the sea level rise with a greater variety of factors included in the analysis than is generally done:

• S. Jevrejeva, J. C. Moore and A. Grinsted, How will sea level respond to changes in natural and anthropogenic forcings by 2100?, Geophysical Research Letters 37 (2010), L07703.

The authors say their estimates are in line with past sea level responses to temperature change, and they suggest that estimates based on ice and ocean thermal responses alone may be misleading. With six different IPCC radiative forcing scenarios they estimate a sea level rise of 0.6–1.6 m, and are confident the rise will be between 0.59 m and 1.8 m.

category: climate, oceans